Symposium F.NM02—Advanced Linear/Nonlinear, Tunable and Quantum Materials for Metasurfaces, Metamaterials and Plasmonics (Includes Symposium S.EL03—Novel Approaches and Material Platforms for Enhanced Light–Matter Interaction, Plasmonics and Metasurfaces)

Nanolasers with plasmonic mode confinement represent a critical area of photonics, as new scientific phenomena such as low power consumption and fast switching are accessible because of the accompanying very small mode volumes and laser thresholds. That will be beneficial for its application in future on-chip ultracompact integrated circuits for imaging, data storage, quantum information technology, and optical communication. While plasmonic nanolasers are well established, with demonstrations in several gain materials and cavity geometries, notably missing among this important family of photonic devices is an upconverting nanolaser.
In this study, we demonstrate the first known example of a subwavelength plasmonic upconverting nanolaser. This organo-lead trihalide perovskite based plasmonic upconverting nanolaser, realized via a two-photon nonlinear optical pumping process, features a record-small mode volume (~0.06 λ³) and an ultralow lasing threshold (10 μJcm^–2). In addition, we observed temporal coherence of the emission, which is an important feature of lasing. This result was made possible by replacing the conventional noble metal plasmonic cavity structure with one composed of titanium nitride, whose material properties can be tailored during the fabrication processes to co-optimize the pump photon absorption and the upconverted photon emission rate. Moreover, the results show a very close agreement between the experiment and modeling, which provides detailed insight into the novel working mechanism of a plasmonic upconverting nanolaser. In addition, we demonstrate a continuous wave lasing from single lead halide perovskite (CsPbBr₃) quantum dot integrated with plasmonic nanocaviy with undetectable threshold (power density lower than 90 mWcm^-2) and ultra-sharp single mode lasing (linewidth of 1.4 nm) under a cryogenic temperature at 4 K. We will discuss the outlook for perovskite-based plasmonic nanolasers as well as quantum-dot based plasmonic nanolasers as robust on-chip light sources for bio-imaging, optical communication applications.

One of the main obstacles limiting potential applications of semiconductor nanolasers is the high threshold carrier density required for lasing, which makes them difficult to realize electrically driven lasing and integrate into optoelectronic devices. Organo-lead halide perovskite materials have recently received considerable attention for achieving an economic and tunable diode laser, owing to the use of solution-processable materials and the exceptional optical attributes of long carrier lifetimes and diffusion lengths, high fluorescence quantum yields, wavelength tunability and high optical gain coefficients1 (>104 cm?1). However, reducing the volume of such lasers to the nanoscale is the challenge nanophotonics, with potential applications in arrays of ultra-compact lasers on a chip. In this work, we demonstrate a continuous wave lasing from single lead halide perovskite (CsPbBr3) quantum dot integrated with plasmonic nanocaviy with undetectable threshold (power density lower than 90 mWcm-2) and ultra-sharp single mode lasing (linewidth of 1.4 nm) under a cryogenic temperature at 4 K. In our design, a silver nano-cube was placed on a gold substrate, with a single CsPbBr3 quantum dot placed in between. By finite-difference time-domain simulation method to calculate the electric field distribution of the designed structure, we theoretically observed a strong localized optical confinement, plasmonic gap mode, formed between the silver nanocube and the gold substrate acts as plasmonic nanocavity and contributes a large Purcell enhancement (Purcell factor ~200) which can be determined via time-resolved photoluminescence measurement. In order to avoid quenching effect, a 6 nm Al2O3 spacer was used to separate the quantum dot and the gold substrate, and the silver nano-cube was covered by 1 nm Polyvinylpyrrolidone to isolate the emitter. In order to measure the lasing threshold, the temperature dependent light-in-light-out curve was performed and we observed a very small lasing threshold (power density lower than 188 mWcm-2) at 80 K. We will also discuss the outlook for lead halide perovskite plasmonic nanolasers in applications including, on-chip coherent light sources for of bio-imaging, optical communication applications.

Harvesting non-equilibrium carriers at semiconductor-metal interfaces offers an opportunity to modify the rates and pathways for photochemical reactions at the nanoscale. The carrier generation, transport and interface dynamics can be addressed with a combination of first-principles theory for carrier excitation and relaxation, observation of carrier dynamics via hot-carrier photocurrent spectroscopy, ultrafast transient absorption spectroscopy, and photoelectrochemical measurements that assess current transport, product yield and selectivity. We use as an example the gold/gallium nitride interface and compare results of theory to excited carrier transport, transient absorption and photocatalysis measurements. We also report the first known example of photoexcited hot hole injection at a metal-p-type wide bandgap semiconductor interface. Despite the presence of an interfacial Schottky barrier to hot-hole injection of more than 1 eV across the Au/p-GaN heterojunction, plasmonic Au/p-GaN photocathodes exhibit photoelectrochemical properties consistent with the injection of hot holes from Au nanoparticles into p-GaN upon plasmon excitation. The incident photon-electron conversion efficiency spectrum for the plasmonic photocathodes faithfully follows the surface plasmon resonance absorption spectrum of the Au nanoparticles, and a sustained photovoltage during plasmon excitation of plasmon-induced hot-hole capture and conversion, using a gold/p-type gallium nitride (Au/p-GaN) interface conditions tailored for studying photoelectrochemical CO2 reduction. Whereas the Au/p-GaN structure has an interfacial Schottky barrier to hot-hole injection of more than 1 eV, the Au/p-NiO has an approximately zero Schottky barrier height. Taken together, our results offer experimental validation of photoexcited hot holes in high-barrier Au/p-GaN structures and also in low-barrier structures, such as Au/NiO. These results, together with other recent advances, form an outlook for the use of photoexcited carriers as a tool for enhancing chemical product selectivity in photon-driven catalytic processes.

Great progress has been made in the last 10+ years since the first demonstrations of plasmonic nanolasers or spasers. This talk intents to present more recent progress and points to certain existing problems that remain unresolved.

Plasmonic antennas are capable of enhancing the coupling of light and matter due to their strong electric field confinement. At room temperature, where dephasing is the dominant contribution to the linewidth of a solid-state emitter, studies of plasmonic coupling have usually concerned incoherent interactions, e.g. a Purcell enhancement [1]. At low temperatures, however, the resonant coherent scattering from the emitter becomes significant and another regime of plasmonic interactions can be observed: here, not only the plasmonic particle changes the interaction of the emitter with light, but the emitter can influence the scattering properties of the plasmonic particle. In an extreme case, a single molecule can fully suppress scattering from a (much larger) plasmonic particle, rendering it effectively transparent [2]. In this presentation, I discuss an experiment, where a single organic molecule partially cloaks a large gold nanoparticle [3].

It has recently been reported that a nanogap antenna formed by a gold nanoparticle separated from a metallic substrate by a thin dielectric gap can achieve ultra-strong enhancement, even at room temperature [4]. When the enhanced emission rate is fast enough to compete with the room temperature dephasing and the plasmon losses rates, the emitter starts to interact coherently with the plasmon mode, bringing the system into the strong coupling regime. In this presentation, we also demonstrate a significant enhancement of a single quantum dot coupled to a plasmonic nanogap antenna. We observe an ultrashort fluorescence lifetime less than 38 ps limited by the instrumental response function. Moreover, by controlling the position of the quantum dot with respect to the nanogap antenna, we can tune the system from the weak coupling to the strong coupling regime leading to spectral splitting in the fluorescence spectra [5].

[1] K. Matsuzaki, et al., Sci. Rep 7, 42307 (2017).
[2] X-W. Chen, V., Sandoghdar, M., Agio, Phys. Rev. Lett., 110, 153605 (2013).
[3] J. Zirkelbach, et al., Phys. Rev. Lett. under review (2020); arXiv:2004.08156
[4] R. Chikkaraddy, et al., Nature 535, 127 (2016).
[5] Liu, et al., in preparation.

In nanostructured metals, the coherent coupling of electromagnetic radiations to plasmons creates a nonequilibrium distribution of electrons at an elevated temperature, which is termed hot electrons. The dynamics of such hot carriers has been wide employed for various applications, ranging from the detection of sub-bandgap photons in semiconductors to the production of hydrogen gas via photocatalytic processes.

The generation, transfer, and relaxation of hot carriers also provide a novel route to nonlinear optical effects. For example, the optical Kerr nonlinearity of plasmonic metals provides enticing prospects for developing reconfigurable and ultracompact all-optical modulators. Although enhanced nonlinear responses of metals are known to enable the optical control of light, the intrinsically slow relaxation dynamics of photoexcited carriers, primarily governed by electron-phonon interactions, impedes sub-picosecond modulation speeds. In this talk, we present femtosecond all-optical modulation in plasmonic systems via the activation of relaxation pathways for hot-electrons at the interface of metals and electron acceptor materials. We show that the relaxation kinetics and the optical nonlinearity can be tuned by leveraging the spectral response of the plasmonic system in the linear regime. Our findings introduce a generic scheme for achieving sub-picosecond modulation speeds in plasmonic systems, suitable for the ultrafast control of the intensity, polarization, and phase of light upon exchange of energetic hot carriers.

The dynamics of hot carriers in hybrid plasmonic system is further exploited for the creation of optically induced second-order nonlinear materials. Second-order optical processes are pivotal to the active modulation and wave mixing of light waves. The inversion symmetry in most materials, however, prevents achieving a bulk chi-2 effect, thereby limiting the portfolio of second-order nonlinear materials. We propose and demonstrate ultrafast conversion of a statically-passive dielectric to a transient second-order nonlinear medium upon the generation and transfer of plasmonically induced hot electrons. Triggered by an optical switching signal, the amorphous dielectric with vanishing intrinsic chi-2 develops dynamically tunable second-order nonlinear responses, which can be leveraged to address the critical need for all-optical control of second-order nonlinearities in nanophotonic systems. The methodology developed here can be readily extended to any electron reservoir/acceptor pairs, enabling a new class of “material-by-design” for second-order nonlinear optical effects.

Nanometallic structures, such as resonant plasmonic nanoantennas, are useful for the conversion of light to surface plasmon-polaritons (SPPs) localized to ultra-small volumes [1]. Such structures can provide highly enhanced fields, strong confinement, high surface sensitivity, and can double as a device electrode for applying voltages or passing currents to active regions in optoelectronic devices. Here we review some of their many applications, in the areas of plasmon-enhanced nonlinear optics [2-5] and optoelectronics [6-9].
In nonlinear optics, plasmon enhancement can be applied to the pump [2], or to the nonlinear emission [3-5], through spectral alignment. An advantage of the former is that all nonlinear processes involving the pump are enhanced, but a disadvantage is that the pump, typically of high intensity, can damage the nanostructures. Nonetheless, such enhancement was exploited in high-harmonic generation via re-collision radiation in Si [2], where a 10x enhancement was observed for all harmonics emitted up to the 9^th. Alternatively, exploiting the latter is advantageous because the structures are resonant with the nonlinear emission which is typically weak, and non-resonant with the pump so are less susceptible to damage. Such enhancement was exploited in Raman scattering experiments [3-5], where the nanoantennas were spectrally-aligned with the Stokes wavelengths of graphene. Under this scenario, single-antenna scattering cross-section enhancements of over 500× were observed [5].
In optoelectronics, plasmon enhancement can be exploited to improve the performance of, e.g., electro-optic modulators [6], photodetectors [7,8], and beam-steering devices [9]. A key requirement is for the plasmonic enhancement to overlap strongly with the active region of the device, which requires that the nanostructure also operate as a device electrode. Such a structure consists of an array of Au nanoantennas contacted perpendicularly by electrical contact lines. The contact lines may be aligned on the nanoantennas such that they are minimally invasive optically. Structures such as these are very promising for beam steering, as each row can be routed to an individual contact such that a 1D phase gradient is applied over the array [9]. Nanostructures that consist of holes in a metal film are also of significant interest [10], especially arrangements of nanoholes (e.g., heptamers), which can be fabricated via focussed ion beam milling because they exhibit interesting and useful Fano resonances [11].

References
[1] Bharadwaj, P., Deutsch, B., Novotny, L., Adv. Opt. Photonics 1, 438-483, 2009
[2] Vampa, G., Ghamsari, B. G., Siadat Mousavi, S., Hammond, T. J., Olivieri, A., Lisicka-Skrek, E., Naumov, A., Villeneuve, D. M., Staudte, A., Berini, P., Corkum, P. B., Nature Physics 13, 659, 2017
[3] Ghamsari, B. G., Olivieri, A., Variola, F., Berini, P., Nanophotonics 3, 363, 2014
[4] Ghamsari, B. G., Olivieri, A., Variola, F., Berini, P., Phys. Rev. B 91, 201408, 2015
[5] Al-Shehab, M., Ghamsari, B. G., Berini, P., J. Opt. Soc. Am. B, 36, F49, 2019
[6] Olivieri, A., Chen, C., Hassan, S., Lisicka-Skrzek, E., Tait, R. N., Berini, P., Nano Letters 15, 2304-2311, 2015
[7] Alavirad, M., Olivieri, A., Roy, L., Berini, P., Optics Express 24, 22544-22554, 2016
[8] Alavirad, M., Olivieri, A., Roy, L., Berini, P., Chin. Opt. Lett. 16, 050007, 2018
[9] Calà Lesina, A., Goodwill, D., Bernier, E., Ramunno, L., Berini, P., IEEE J. Sel. Top. Quant. Electr., DOI: 10.1109/JSTQE.2020.2991386
[10]Gordon, R., Brolo, A. G., McKinnon, A., Rajora, A., Leathem, B., Kavanagh, K. L., Phys. Rev. Lett. 92, 037401, 2004
[11] Hahn, C., Hajebifard, A., Berini, P., Nanophotonics 9, 393-399, 2020

Topological insulators (TI) are a class of quantum materials that exhibit metallic surface states and bulk insulating states. Electrons occupying these surface states are 2D massless, linear dispersed, and spin-momentum locked Dirac Fermions. The optical transitions among these surface states, for example Dirac plasmon polariton, are in the hardly accessed far-IR and THz spectral ranges by tradtional semiconductors. In TI thin film, Dirac plasmons are excited simultaneously on the top and bottom surfaces. They are electrostatically coupled and generate an optical mode and an acoustic mode. Only the former can be observed in optical measurements because of a non-zero dipole. In addition, for the 3D TI Bi₂Se₃, the continuum plasmon state interacts with discrete bulk phonon states, namely the α phonon at 2THz and the β phonon at 4THz, giving rise to plasmon-phonon polariton modes. These unique properties enable TI a promising platform for far-IR sensing, security screening, and other optoelectronic devices that function in the THz.
Similar to the two surfaces coupling in a single layer TI film, the multiple surfaces introduced by stacking alternating TI and band insulator (BI) layers will also couple and produce additional plasmon modes. The optical responses are expected to be dependent on the structural parameters and bulk properties of both the TI and BI material. To create multiple topological edge states, we stack Bi₂Se₃ with (Bi_0.5In_0.5)₂Se₃ (BIS), which is topologically trivial band insulator. It has the same van der Waals strucutre and close lattice constant to Bi₂Se₃. We build a five layer structure comprising BIS /Bi₂Se₃/BIS/Bi₂Se₃/BIS on a c-plane sapphire substrate via molecular beam epitaxy (MBE) and explored the plasmonic polariton coupling of such layered TI heterostructures by varying the structural parameters: spacer BIS thickness and Bi₂Se₃ thickness. To excite localized surface plasmons polariton, the films were patterned into periodic microribbon arrays. The wavevector of the polariton is defined by q=π/a, a is the width of a microribbon, so we can map out the dispersion by measuring polariton frequencies in films with different microribbon widths.
An asymmetric Fano lineshape extinction curve is observed in the FTIR measurement due to the plasmon hybridization with both α phonon and β phonon. Three plasmon-phonon polariton modes are generated: two of them (ω_l for lower frequency, ω_h for higher frequency) show visible peaks in the range of 2-8THz where detector is sensitive and the sapphire substrate is transparent; the other one is pushed below 2THz due to the Fano-resonance. With increasing wavevector, ω_l and ω_h both blue shift, in accordance with the dispersion predicted by transfer matrix (T-matrix) modelling. Fixing the wavevector, when the BIS spacer thickness d_s<50nm, ω_l blueshifts and ω_h redshifts, which is not captured by T-matrix modelling; when d_s>50nm, they both stay nearly constant, agree well with modelling. With increasing Bi₂Se₃ thickness d_TI, ω_l always blueshifts while ω_h shows a redshift. All the modes shifts can be explained within the frame of plasmon-phonon hybridization except the behavior for thin d_s. These changes can only be explained by the quantum correlation of the surface states across the band insulator layer, or in other words, coupling of two Bi₂Se₃ layer. This has revealed that the Dirac surface states can also couple across a topologically trivial band insulator even though the spin-orbit coupling strength is weak inside the bulk. A thorough understanding of how these surface states in multilayer TIs are coupled will provide a deep insight to the many-body interaction in low-dimensional strong spin-orbit coupling systems.

Plasmonic nanostructures enable to confine light into deep subwavelength scales and at the same time significantly increase the local field intensity. They have been utilized to increase the device performances in a variety of applications, such as plasmon-enhanced energy harvesting, magneto-plasmonics and heat-assisted magnetic recording (HAMR). In the HAMR technique, the plasmonic nanoantenna first locally heat up the recording media within a data bit. Then an external magnetic field (generated by electric currents) is applied to write data within that heated area. This technique can boost the recording areal density to the petabyte per meter square level^1,2. On the other hand, recent research work has demonstrated the control of magnetization by femtosecond (10^-15 s) lasers, paving the way towards all-optical recording with the writing speed three orders of magnitude higher than that of the current magnetic data storage technologies. In all-optical recording, laser is utilized to directly record data on the magnetic media, and the external magnetic field is no longer needed. The phenomenon was firstly found in rare-earth transition-metal (RE-TM) ferrimagnetic alloys, and later demonstrated in frromagnetic thin films^3-5.

Combining plasmonics with all-optical recording is of great research interest, which can potentially be developed into a magnetic recording technique with low power, high intensity and ultrafast speed. Such plasmonic-enhancement effect in all-optical recording has been demonstrated in the RE-TM materials⁶. People have been pursuing similar enhancement effects in ferromagnet thin films, but no experimental demonstrations were reported so far. In this talk, we will present the experimental observation of the plasmon-enhanced all-optical magnetic manipulation in ferromagnet thin films⁷. We have integrated the gold nanoislands on top of the Co/Pt thin films (with a SiO₂ separation layer). The gold nanoislands were fabricated by annealing a thin film of gold layer under appropriate temperature and time. We have measured the minimum laser power required for magnetic manipulation under the on- and off-resonance conditions, and have observed an 18.5% decrease in the threshold power. The full-wave simulations indicated an averaged 14.6% field enhancement of the electric field intensity in the Co/Pt thin films, which quantitatively explains the observed reduction in the threshold. Our research findings demonstrate the possibility to achieve all-optical magnetic recording with low energy consumption, low cost, and high areal density by integrating plasmonic nanostructures with magnetic media.

References
1. Challener, W. A., et al. "Heat-assisted magnetic recording by a near-field transducer with efficient optical energy transfer." Nature photonics 3.4 (2009): 220-224.
2. Stipe, Barry C., et al. "Magnetic recording at 1.5 Pb m^-2 using an integrated plasmonic antenna." Nature photonics 4.7 (2010): 484-488.
3. Stanciu, C. D., et al. "All-optical magnetic recording with circularly polarized light." Physical review letters 99.4 (2007): 047601.
4. Lambert, Charles-Henri, et al. "All-optical control of ferromagnetic thin films and nanostructures." Science 345.6202 (2014): 1337-1340.
5. Mangin, Stéphane, et al. "Engineered materials for all-optical helicity-dependent magnetic switching." Nature materials 13.3 (2014): 286-292.
6. Liu, Tian-Min, et al. "Nanoscale confinement of all-optical magnetic switching in TbFeCo-competition with nanoscale heterogeneity." Nano letters 15.10 (2015): 6862-6868.
7. Cheng, Feng, et al. "Plasmon-Enhanced All-Optical Manipulation of Magnetization." Nano letters (2020), submitted.

We use transient optical spectroscopy to study a molecular singlet fission system that is strongly coupled to an optical microcavity to probe the interconversion of exciton-polaritons, singlet excitons, and triplet excitons. We find that transient spectra in the optical microcavity are influenced by modifications of the exciton-photon coupling conditions in addition to the population dynamics of “reservoir” or uncoupled molecular excited states. For example, though our exciton-polaritons themselves are short-lived (< 1ps), weak interactions with long-lived molecular excited states give rise to exciton-polariton emission signatures that persist for the lifetime of the singlet exciton (~ 50 ps). Similarly, we observe long-lived transient signals corresponding to optical transitions of the exciton-polariton, even in the absence of a real polariton population. Following singlet fission, polariton transient features persist for > 10us, consistent with the excited state lifetimes of dark triplet excitons. We conclude that in addition to the usual singlet fission population dynamics, microcavity transient spectra reveal information pertaining to the dynamics of (1) exciton-polaritons populated via direct excitation, (2) exciton-polaritons populated via scattering from “reservoir” states, and (3) transient modification of the exciton-polariton transition energies due to changes in the population of molecular excited states.

Metal nanoantennas have been under intense investigation due to their strong light-matter interactions and significant polarization sensitivities determined by their nano-structure. For applications seeking to realize on-chip polarization-discriminating nanoantennas, efficient energy conversion from surface waves to far-field radiation is desirable. However, the response of individual nanoantennas to the particular polarization states achievable in surface waves, such as evanescent fields, has not yet been thoroughly studied. Here, we report the giant modulation of the visible light scattering predicted from gold half-ring, pinwheel, and other nanoantennas excited through total internal reflection of left- and right-handed circularly polarized light, by exploiting the distinct polarization properties of surface evanescent waves1. This result provides a fundamentally different mechanism for chiroptical responses requiring a phase delay between transverse and longitudinal electric field oscillations, not found in free-space light2. Specifically, we focus on the insight provided by theory, and in particular by the electromagnetic simulations, performed with COMSOL Multiphysics software, of the systems of interest and aspects of their chiroptical response. References (1) Smith, K.W.; et al. ACS Nano 2018, 12, 11657-11663. (2) McCarthy, L.A.; et al. submitted.

Metal nanostructures are key elements in nanooptics due to their strong resonant interaction with light through plasmonic charge oscillations1. Their ability to shape light at the nanoscale have made them critical in a multitude of areas, including biosensing, energy conversion and ultrathin flat metaoptics. Yet another dimension of avenues is foreseen for dynamic nanoantennas2, ranging from tuneable metalenses for miniaturized medical devices to adaptable windows that control radiation flows in and out of buildings. However, enabling nano-optical antennas to be dynamically controllable remains highly challenging and particularly so for traditional metals with fixed permittivity1. Here, I will present our recent work that introduces conductive polymers as a new class of redox-tuneable plasmonic materials3. We show that nanodisks of highly conductive polymers can provide clear optical extinction peaks via excitation of dipolar localised surface plasmon resonances. The resonances redshift with increasing nanodisk aspect ratio, in agreement with simulations and calculations based on dipolar polarizability theory. We furthermore demonstrate complete and reversible switching of the optical response of the organic nanoantennas by chemical tuning of their redox state, which modulates the permittivity between plasmonic and dielectric regimes. Our results show that conductive polymer nanostructures can act as redox-tuneable plasmonic nanoantennas, based on polaronic charge carriers rather than electrons or holes as in conventional metals. Future research may investigate different polymers and geometries and explore their use in various applications. References 1 Maier, S. A. Plasmonics: Fundamentals and Applications. (Springer Science & Business Media, 2007). 2 Shaltout, A. M., Shalaev, V. M. & Brongersma, M. L. Spatiotemporal light control with active metasurfaces. Science 364, eaat3100 (2019). 3 Chen, S., Kang, E. S. H., Shiran Chaharsoughi, M., Stanishev, V., Kuhne, P., Sun, H., Wang, C., Fahlman, M., Fabiano, S., Darakchieva, V., & Jonsson, M. P. arXiv: 1907.11453 (2019).

Localized Surface Plasmons (LSPs) of metal nanoparticles have a distinguished characteristic, confining the electromagnetic fields on the nanometer length scale. This feature fascinates many researchers who are interested in surface-enhance Raman spectroscopy, biosensing, and nano- optics. Recently, the interaction between LSPs and dielectric matter as well as the LSPs themselves have drawn attentions from many people due to its unique energy dispersion and potential applications. However, the fundamental process in plasmon-matter interaction is not well understood. The clear understanding the physics of the phenomena is essential for tailoring them to specific applications, which can be achieved by correlating measurements with theoretical simulations. In this study, a silver nanoparticle on a ZnO nanorod is used as a model system, in which the plasmon-exciton strong coupling was reported [1]. For characterization, Scanning Transmission Electron Microscopy (STEM) – monochromated Electron Energy Loss Spectroscopy (EELS) approach is chosen to investigate the spatial distributions of plasmonic or plasmon-matter coupled modes. In addition, Cathodoluminescence (CL) measurement in a STEM mode was also employed for complementary information on electronic structures. We discuss the differences in EEL spectra in the Ag/ZnO nanorod system compared to the spectrum of a single silver nanoparticle. Boundary Element Method (BEM) simulations are performed for more correct theoretical interpretation of EEL spectra with MNPBEM developed by Hohenester, Ulrich [2]. Various factors such as distorted shape of a silver nanoparticle, symmetry breaking of dielectric environment, and plasmon-exciton coupling are considered to find the origin of the differences. The work will help to figure out the fundamental principle for light-matter interaction. It also highlights the power of STEM-EELS and STEM-CL for studying plasmonics. [1] Wei, Jiake, et al. Nano letters 15.9 (2015): 5926-5931. [2] Hohenester, Ulrich. Computer Physics Communications 185.3 (2014): 1177-1187.

A promising trend in plasmonics involves shrinking the size of plasmon-supporting structures down to a few nanometers, thus enabling control over light-matter interaction at extreme-subwavelength scales. In this limit, quantum mechanical effects, such as nonlocal screening and size quantization, strongly affect the plasmonic response, rendering it substantially different from classical predictions. For very small clusters and molecules, collective plasmonic modes are hard to distinguish from other excitations such as single-electron transitions or photonic modes. Using rigorous quantum mechanical computational techniques for a wide variety of physical systems, we describe how an optical resonance of a nanostructure can be quantitatively classified as either plasmonic or nonplasmonic. More precisely, we define a universal metric for such classification, the generalized plasmonicity index1 (GPI), which can be straightforwardly implemented in any computational electronic-structure method or classical electromagnetic approach to discriminate plasmons from single-particle excitations and photonic modes. The GPI relates to experimentally accessible quantities and it is equivalent–at resonance–to the Q-factor of the corresponding harmonic oscillator. Using the GPI, we investigate the plasmonicity of optical resonances in a wide range of systems including: classification and characterization of features in single and hybrid classical systems; the emergence of plasmonic behavior in small jellium spheres as the size and the number of electrons increase; atomic-scale metallic clusters as a function of the number of atoms; and nanostructured graphene as a function of size and doping down to the molecular plasmons in polycyclic aromatic hydrocarbons (PAHs). Further, the GPI identifies molecular plasmonic excitations of selected charged PAHs, in the visible region of the spectrum, whose excited-state lifetime dynamics is then studied2. The GPI metric deepens our fundamental understanding of what is a plasmon down to the molecular limit of plasmon-supporting nanostructures and provides a rigorous foundation for their further development based on molecular plasmonics. References (1) Zhang, R.; et al. ACS Nano 2017, 11, 7321–7335. (2) Chapkin, K.D.; et al. Proc. Natl. Acad. Sci. USA 2018, 115, 9134–9139.

Colloidal quantum dots (QDs) are attractive gain materials for coherent light sources because of their bright photoluminescence, size-controlled color tunability, and solution processability. Engineering directionality of QD lasing is important for applications in remote sensing, laser projection, and on-chip optical network. This talk describes how the emission angle of a QD nanolaser can be manipulated by leveraging the high-symmetry points in plasmonic nanoparticle lattices. We built nanolasers by conformally coating CdSe-CdS core-shell QDs on two-dimensional square arrays of Ag nanoparticles. With waveguide-surface lattice resonances (W-SLRs) near the Δ point in the Brillouin zone as optical feedback, we realized lasing emission at off-normal emission angles. Varying the periodicity of the plasmonic lattices enables other high-symmetry points (Γ or M) of the lattice to overlap with the QD gain, which facilitates controllable lasing direction. We also increased the thickness of the QD film to access higher order W-SLR modes with additional avoided crossings in the band structure, thereby expanding the selection of cavity modes for any desired lasing emission angle.

Active modulation of a material’s permittivity makes it possible to control the phase, amplitude, and polarization of light interacting with the material. The ideal material for tunable nanophotonics would exhibit large optical changes and fast response times, in addition to being compatible with industrial fabrication techniques [1].

Extraordinary optical phenomena like ultrafast reflection modulation [2], extraordinary enhancements of nonlinearities [3], and broadband frequency shifts [4] have been demonstrated with doped conducting oxides. Thus, the ultrafast time-scales of optical modulation make it a promising avenue to study optical non-reciprocity [5], control topology protection [6], and implement photonic time-crystals [7]. Yet the effect of optical pumping on their permittivities tuning limits, optical losses, and their effective carrier masses remain largely unexplored.

In this work, we demonstrate unity-order permittivity changes in low, loss, undoped zinc oxide films, induced by optically generated free-carriers. Because of this permittivity modulation, metal-backed ZnO films demonstrate broadband reflectance modulation up to 70% at the telecommunication wavelengths, with picosecond-scale relaxation times. The epsilon near zero points can be dynamically tuned from 8.5 microns down to 1.6 microns with an optical pump, with picosecond-scale relaxation times. The modulation can be selectively enhanced at specific wavelengths by designing hybrid plasmonic resonators while maintaining the picosecond switching times. The modulation is limited by the increasing effective mass of the photoexcited carriers, saturable absorption of the material at the pump wavelength, and the increased Drude damping of the photoexcited electrons.

This work provides insights into the free-carrier-assisted permittivity modulation in zinc oxide, and demonstrates reflectivity modulation of both lithography-free and patterned nanostructures, with large modulation depth and picosecond response times. The techniques employed in this work can be used to study doped conducting oxides employed in optically switched nanophotonic devices and will facilitate the realization of active devices for beam-steering, polarizers, and spatial light modulators.

References:
[1] S. M. Choudhury, D. Wang, K. Chaudhuri, C. Devault, A. V. Kildishev, A. Boltasseva, and V. M. Shalaev, Nanophotonics 7, 959 (2018).
[2] M. Clerici, N. Kinsey, C. DeVault, J. Kim, E. G. Carnemolla, L. Caspani, A. Shaltout, D. Faccio, V. Shalaev, A. Boltasseva, and M. Ferrera, Nat. Commun. 8, 15829 (2017).
[3] M. Z. Alam, I. De Leon, and R. W. Boyd, Science 352, 795 (2016).
[4] V. Bruno, S. Vezzoli, C. DeVault, E. Carnemolla, M. Ferrera, A. Boltasseva, V. M. Shalaev, D. Faccio, and M. Clerici, Appl. Sci. 10, 1318 (2020).
[5] A. M. Shaltout, V. M. Shalaev, and M. L. Brongersma, Science (80-. ). 364, eaat3100 (2019).
[6] Z. A. Kudyshev, A. V. Kildishev, A. Boltasseva, and V. M. Shalaev, ACS Photonics 6, 1922 (2019).
[7] E. Lustig, Y. Sharabi, and M. Segev, Optica 5, 1390 (2018).

With sixteen million colors available in the realm of natural and artificial palette, we are still yearning for new shades of colors. Common approaches to achieve a wide color range involve synthetic dyes that expel toxic chemicals that are not only non-biodegradable and carcinogenic, but also are capable of altering the physical and chemical properties of soil, deteriorating water bodies and causing harm to the environment. With a growing demand for higher resolution and shrinking of the ink-jets, current color printing technologies generate ultrasmall particles that cause adverse health effects such as cardio-pulmonary ailments and complex disorders in cellular processes.
Recently, plasmonic colors have emerged as a no-fade, environmentally-friendly alternative for artificial dyes. Their color filtering effects are a consequence of the resonant coupling of light and free electrons in metal nanoparticles, known as surface plasmons that confine light at the nanoscale and lead to the enhancement of the local electromagnetic field. Semicontinuous metal films (SMF), which are randomly distributed metal nanoparticles at the near-percolation regime, can offer both plasmonic sub-wavelength color resolution and large-scale, low-cost fabrication. Random, fractal-type island SMFs are comprised of nanoparticles of diverse sizes and shapes resonating at different wavelengths. When illuminated with high-intensity laser light, thermal effects modify nanoparticles in SMFs, leading to variations in the optical response, and hence, generation of different colors.
In this work, we demonstrate a wide range of plasmonic colors generated by a silver SMF on a silver mirror with a dielectric spacer (SMF/M). This structure utilizes the so-called gap-plasmon modes and generates colors in the reflection mode. The thickness of Ag SMF layer is varied, and the dependence on a dielectric overcoating layer to increase the stability of Ag SMF is analyzed. An ultrafast Ti:Sapphire femtosecond laser (1 kHz, 80 fs, 800 nm, linear polarization) is used to structurally modify the Ag nanoparticles for respective color generation through varying the laser parameters. We obtain intricate artistic designs with the in-house built laser scanning setup.
The proposed approach offers the possibility of producing CMYK colors in an easy and less expensive manner compared to commonly used e-beam lithography. Potentially, SMF-based coloring can be applied to flexible substrates. In addition to color printing and arts, the demonstrated sub-micrometer resolution technique could pave the way to security and optical data storage applications while successfully addressing the challenges of scalability and toxicity across multiple technologies.

The study of doped semiconductor nanocrystals exhibiting localized surface plasmon resonances (LSPR) has been growing over the last decade, with the main interest being their intrinsic tunability through dopant concentration. Among these materials, degenerately doped metal oxide nanocrystals are of particular interest because of their ability to absorb light in the near-infrared (NIR) range while remaining non-absorbing at visible frequencies. These properties open many opportunities for applications in the fields of solar shielding, laser optics, and biomedical treatments [1].

Hexagonal tungsten bronzes (HTB) have a unique NIR-LSPR property originating from their highly anisotropic crystal phase. It was shown that the crystalline anisotropy of HTB nanocrystals influences the LSPR ‘band-splitting’ in conjunction with the effect of shape anisotropy. This results in a double peak feature of isotropically shaped particles and a band overlap for elongated particles [2]. However, synthesis of HTB nanocrystals with a good control over both their crystalline structure and their morphology has been challenging. Specifically, high aspect ratio HTB rods have not been reported, however producing them could confirm the theorized ‘bands-crossover’ effect and the polarization selectivity of the absorbance.

We investigated the limitations of the previous chloride-salt-based synthetic process and found them to be linked to a competitive precipitation of alkaline chloride salt. Using alternative reactants, we achieved synthesis of highly anisotropic HTB nanorods, with aspect ratios up to 6.5 and lengths up to 100 nm. Extending the tunability of the particles’ aspect ratio also extended the LSPR tunability further in the NIR range. We present a full LSPR band dynamics where particles grow from strongly colored HTB platelets to long HTB rods invisible to the eye. Furthermore, control of the particle size allowed us to measure LSPR light scattering for the first time in doped semiconductor nanoparticles, despite carrier densities in these materials being lower than in the well-studied noble metal nanoparticles. These experimental measurements are supported by numerical simulations based a finite-element model and using an anisotropic Drude-Lorentz dispersion for the dielectric constant of HTB. Our simulations are in good agreement with the experimental measurements. These findings extend our insight into LSPR phenomena, allowing for tunable dual-band NIR absorbers and scatterers.

[1] Kriegel, I., Scotognella, F., & Manna, L. (2017). Plasmonic doped semiconductor nanocrystals: Properties, fabrication, applications and perspectives. Physics Reports, 674, 1-52.

[2] Kim, J., Agrawal, A., Krieg, F., Bergerud, A., & Milliron, D. J. (2016). The interplay of shape and crystalline anisotropies in plasmonic semiconductor nanocrystals. Nano letters, 16(6), 3879-3884.

The high surface-to-volume ratio as well as the typically high crystallinity and directional charge carrier transport of metal oxide NWs are beneficial for the use in chemo-resistive gas sensors. Semiconducting n-type SnO₂ nanowire networks have been site-selective deposited on gas sensor platforms by chemical vapor deposition approach based on a gold catalyst-mediated vapor-liquid-solid (VLS) growth mechanism. The selective detection of target gas molecules in a complex gaseous composition is quite challenging. We improved the lack of selectivity toward gaseous species through surface decoration with metal oxides (VO₂) and noble metals (Rh) nanoparticles by a second CVD step. For this approach, we developed novel molecular precursors based on designed multidentate organic ligands in order to tailor the gas-phase conversion into targeted nanoobjects with defined size and shape. An enhanced gas detection capability towards reducing gases and oxidizing gases were demonstrated, due to electronic and chemical sensitization via spill-over effects. Moreover, optical reflectance spectroscopy revealed plasmonic resonance absorption in visible (465 nm) and UV (350) region resulted from the gold catalyst on the tip on the SnO₂ nanowire and distinctive rhodium nanopyramids. In addition, surface decoration with thermochromic VO₂ exhibits a plasmon resonance absorbance in the near-infrared region. The potential as a multimodal optical sensor towards adsorbed complex chemical species will be discussed.

The search for metamaterials that support negative permittivity and permeability towards the visible frequency has led to the exploration of various architectures that support coupled plasmonic responses. While standard split ring resonators (SRR) have been extensively studied and shown to be extendable to near infrared frequencies, pushing to higher frequencies is challenging. Thus ring-like structures of nanoparticles and nanorods have become a viable solution for higher frequency applications. While coupled nanosphere assemblies have been more extensively studied for their unique optical and magnetic properties, rods have stronger polarizability and plasmon resonances. High-aspect-ratio nanorods, in particular, provide an additional route for engineering plasmonic responses, however, have not been studied as extensively as spherical nanoparticles. Although studies on individual nanoparticle/nanorod arrangements have been studied, a comprehensive investigation on the geometric effects on the hybridization of plasmonic nanorods is still lacking. In this study, the hybridization of the electric and magnetic dipoles for a series of polygons were lithographically patterned, where the number of edges (n=3-6), gap length, and edge length were studied. High-resolution EELS spectrum images were measured, and the combined spectra and spectrum images clearly reveal the nature of the various resonance modes that are active. Electron discrete dipole approximation simulations were performed to clarify the resultant consequent electric and magnetic field distributions. This work elucidates the hybridization trends of arranged polygons to provide the opportunity for negative permeability and permittivity.

High conductivity metals have long been the material of choice for traditional electronics, and over the past two decades low-loss metals have led to many novel optical devices and structures as a result of plasmonic confinement. As these fields merge, metal optoelectronics using hot carrier effects may pave the way for new device architectures with improved flexibility, frequency response, and ultrafast time-dynamics. In this presentation, we will discuss our recent work building hot electron photodetectors for NIR detection using Si and metal oxides [1], improved response using nanoscale metal alloys [2], and time-resolved ultrafast detection via pump-probe techniques exploiting surface plasmon excitation [3]. Further, we will show how the incorporation of index near zero (INZ) substrates can lead to nearly 100% absorption in thin metal films, providing a new platform for hot electron devices [4]. We will conclude with an outlook and discuss future possibilities with these novel material systems.

[1] Kim et al, “Interfacial Defect-Mediated Near-Infrared Silicon Photodetection with Metal Oxides,” ACS Appl. Mater. Interfaces, 11, 47516–47524 (2019)
[2] Krayer et al, “Enhanced near-infrared photoresponse from nanoscale AgAu alloyed films,” ACS Photonics (in press)
[3] Memarzadeh et al, “Surface plasmon assisted control of hot-electron relaxation time,” Optica (in press)
[4] Krayer et al, “Optoelectronic devices on index-near-zero substrates,” ACS Photonics, 6, 2238-2244 (2019)

Nanoscale photothermal effects enable important applications in cancer therapy, imaging, and catalysis. They also induce substantial changes in the optical response experienced by probing light, thus suggesting their application in all-optical modulation. These important applications have been extensively investigated in conducting materials due to the substantial enhancement produced in the strength of nanoscale photothermal processes as a result of the excitation of their collective electron oscillations, known as plasmons. In this work [1], we theoretically investigate the thermal control of plasmons assisted by light absorption in graphene, thin metals and graphene/metal hybrid films.

Graphene displays stronger photothermal effects than noble metals. A small amount of heat deposited on graphene electrons causes large temperature variations and changes in its optical response because the electronic heat capacity of this material is remarkably small. On the other hand, because plasmon resonances are deep subwavelength excitations, extra momentum is required to couple external light into them. Recently, an alternative route to excite graphene plasmons has been proposed [2], where two interfering optical pump beams create a periodic spatial temperature profile in graphene to assist the coupling of a probe beam to plasmons. Here, we further demonstrate that not only graphene plasmons but also metal plasmons or even sub-wavelength acoustic plasmons can be excited using this approach. We assume a periodic pattern of electron temperature created by the optical grating pump only inside the graphene sheet with a modulated temperature range extending from 300 to 8000 K. Due to the strong temperature dependence of the graphene conductivity, this is spatially modulated and serves as an effective grating.

Lateral patterning of the structures considered above directly enables light coupling to their plasmons. We study the thermo-optical properties of these types of structures by considering a thin metal grating placed underneath a graphene sheet. We first investigate metal-like plasmons occurring at a higher energy and supported primarily by the metal. Importantly, we consider graphene doped to some Fermi level E_F and choose a silver grating period such that the plasmon energy is slightly smaller than 2E_F, so that a large modulation is expected by thermally activating interband transitions. At room temperature, 2μ≈2E_F is above the plasmon energy, so we have well-defined modes observed in the reflection spectrum of the patterned 1-nm silver film. However, when graphene electrons are heated to a high temperature via ultrafast optical pumping, the chemical potential can be reduced, enabling Landau damping of the plasmon, whose quality factor strongly drops. The observed dip in peak reflectance is observed also for graphene acoustic plasmons and even for metal plasmons in the absence of graphene, although in these cases the effect is due to the increasing of the electronic inelastic damping. Depending on the material composition and thickness, this modulation effect can be as strong as >70%.

The remarkably small electronic heat capacity of atomically thin systems, such as graphene and few-atomic-layer noble metal films, allows us to elevate their electron temperature via ultrafast optical pumping in such a way that their optical responses are dramatically modified with moderate pump intensities below the damage threshold. We have shown that the addition of a noble metal film in close proximity to graphene enables similar levels of modulation >70% to be reached over a wider spectral range extending down to the visible domain.

[1] E.J.C. Dias et al. Thermal manipulation of plasmons in atomically thin films. Light. Sci. Appl. 9, 87 (2020).
[2] R. Yu et al. Photothermal engineering of graphene plasmons. Phys. Rev. Lett. 121, 057404 (2018).

Cadmium oxide (CdO) is one of the most promising material systems for low-loss infrared (IR) plasmonics. Through reactive co-sputtering using high power impulse magnetron sputtering (HiPIMS) and radio frequency (RF) sputtering, doped CdO demonstrates high mobility films with tunable carrier concentrations across a range corresponding to plasmonic light-matter interactions coupling to wavelengths spanning the mid-IR and reaching into the near-IR. Specifically, indium-doped CdO (In:CdO) yields carrier densities ranging between 2x10¹⁹ cm^-3 and 5x10²⁰ cm^-3 while maintaining mobilities between 300-400 cm²/Vs across this range, thus allowing for the widest accessibility of the IR spectrum (1650-5325 cm^-1) of a single plasmonic material grown by sputtering.

Fully accessing the spectrum of plasmonic applications, including surface plasmon oscillations, epsilon-near-zero (ENZ) modes, and strong coupling phenomena requires that one maintains these appealing transport properties over a film thickness range spanning a few tens to a few thousands of nanometers. Extensive CdO fabrication experiments using high-power impulse magnetron sputtering reveal a strong dependence of carrier density and mobility on film thickness when thickness drops below 100 nm, a particularly significant range for ENZ modes. We attribute this behavior to well-reported surface charge accumulation and depletion layers caused by Fermi level pinning. In this presentation, we demonstrate the reversal between surface charge accumulation and depletion in In:CdO. Further, we show how these surface effects can influence optical properties, i.e. perfect absorber ENZ modes, and how they can be controlled by suppressing acceptor defect formation and sandwiching ENZ layers between unintentionally doped CdO to eliminate interfacial defects.

Plasmonic materials have garnered a lot of interest due to their ability to confine light below the diffraction limit which enables applications in energy harvesting, light localization and detection, and for environmental sensing in the mid-infrared. The vast majority of plasmonic materials used at visible and infrared wavelengths are either metals or doped inorganic semiconductors. Most metals exhibit their plasma frequency in the UV or visible wavelength regions and both localized plasmon resonances and surface plasmon polaritons can be supported at wavelengths spanning from the UV to near infrared (IR). Inorganic semiconductor compounds cannot support visible-wavelength plasmonic modes; however, they can exhibit strong plasmonic behavior in the infrared if their conductivity is high enough. Many of these inorganic compounds are expensive and involve high processing and fabrication costs that make devices that include these materials very expensive. Doped organic semiconductors, such as conjugated polymers, could act as cheaper and more versatile IR plasmonic materials that enable advancement of IR localization and sensing. Numerous studies have shown that highly-doped conjugated polymer materials such as polyacetylene, polypyrrole and polythiophene can exhibit conductivities ranging from ~100 S/cm to 10,000 S/cm. In that conductivity range, a plasmonic response could be observed at mid-IR wavelengths because highly-doped polymers can exhibit metal-like behavior. However, very few studies have demonstrated plasmonic responses in the IR region for highly-doped conjugated polymers.
In this study, we investigate the use of doped conjugated polymers as potential IR plasmonic materials. Drude model calculations were performed to estimate the theoretical conductivity that is required for a material to exhibit a plasma frequency in the mid-IR region. A correlation was established between plasma frequency and the conductivity of the thin films was established using the calculations. Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) thin films were doped multiple times with para-toulenesulfonic acid (PTSA) in dimethyl sulfoxide (DMSO). These doped films were then treated with sulfuric acid to further increase the conductivity of the PEDOT:PSS films. IR reflectivity measurements will be performed across mid-IR wavelengths to check for a non-linear increase in reflectivity from the highly doped PEDOT:PSS films. This is expected to provide evidence for the occurrence of a plasma frequency in the IR region. Subsequently, the surface of the PEDOT:PSS will be imprinted with gratings using nano imprinting lithography. These gratings will then be excited at a range of incident angles to investigate if surface plasmon polaritons can be launched on the doped polymer films.

The optical phonons of many oxides manifest through atomic oscillations at infrared frequencies. Often polar, these oscillations create electric fields that themselves have infrared frequencies. When exposed to IR-photons, the electromagnetic and vibration-induced fields can be of similar energy and momentum causing interactions that dominate the material's infrared properties within the Reststrahlen band bounded by the transverse- and longitudinal-optical phonon modes. Thus, by controlling, modifying, and even creating "designer" polar phonons, a route is opened to induce infrared phenomena. Here, the infrared properties of composite oxides composed of lamellar combinations of common dielectrics and ferroelectrics—MgO/CaO and SrTiO3/CaTiO3specifically—are examined to this end. Using a suite of vibrational spectroscopies in combination with materials modelling, the optical response of these "Reststrahlen laminates" extends beyond just a summation of the constituents. The resulting optical properties are examined in light of "synthetic" phonon dispersions that emerge with the variations in periodicity implicit in lamellar structures. Together, the work suggests a means of creating arbitrary infrared materials via composite phonon responses. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE's National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. DOE or the United States Government.

Au nanobipyramids are anisotropic nanostructures with tunable plasmon resonance frequencies and well-defined electromagnetic hot spots, both of which are highly desired for quantitative surface-enhanced Raman spectroscopy (SERS). Here I will talk about how to fine-tune the plasmon resonance frequencies and fine-tailor the tip sharpness to achieve the optimal SERS enhancements on colloidal Au nanobipyramids. Taking full advantage of their wide plasmon tuning range and narrow plasmon bandwidths, we were able to systematically tune the longitudinal plasmon resonance frequency of Au nanobipyramids with respect to the excitation laser wavelength and the Raman scattering frequency of the molecular adsorbates. We found that the highest SERS enhancements were achieved when the plasmon resonance energies of the nanobipyramids matched the energies of the inelastic scattering photons rather those of the excitation photons. We also discovered that the tip sharpness of Au nanobipyramids could be tailored in a highly controllable manner through plasmon hot electron-driven photocorrosion, which allowed us to establish a detailed correlation between the surface curvature at the nanobipyramid tips and the Raman enhancements.

The great capacity of plasmonic nanoparticles to efficiently absorb, scatter and emit light has recently garnered attention for application of these materials in plasmon-based photocatalysts, solar cells and thermo emitters. Particularly, gold and silver have been used to successfully drive hydrogen dissociation and CO oxidation reactions by injecting hot electrons into molecules adsorbed to their surface. However, due to the low thermal stability and the high cost of both materials, the need of studying alternative plasmonic materials, that potentially will expand the field towards more cost-effective applications, has been growing in the last years. Titanium nitride (TiN) is a conductive ceramic with high hardness and bulk melting point (2930 °C). Its plasmon resonance located in the visible-NIR region, low cost (relative to gold and silver), and the well-understood properties as a thin film in the semiconductor industry, make it a strong alternative to plasmonic metals. The present work encompasses a comprehensive study of the plasma-based synthesis of TiN nanocrystals and highlights its potential as a high-temperature-resistant photothermal absorber. TiN particles were synthesized via a scalable, modular, non-thermal plasma method. Titanium and nitrogen precursors were transported into a RF frequency plasma where TiN particles nucleate and grow. Platinum nanoparticles were subsequently deposited on the TiN by photo-induced reduction of an aqueous solution of chlorplatinic acid (H2PtCl6). The reduction of the precursor metal was driven by electron hole pair generation via plasmon decay. The addition of methanol as a hole scavenger increased the electron lifetime, obtaining metallic platinum. This reaction occurred at temperatures below 40°C under visible light illumination. In addition, a novel TiN@SiOxNy core-shell structure was produced by taking advantage of the modular capabilities of the non-thermal plasma synthesis method. The synthesized core shell particles displayed a 60% higher plasmon peak in the extinction coefficient with respect to the uncoated TiN particles. To probe the potential of these heterostructures, core-shell and uncoated TiN particles were deposited on SiOx substrates by coupling the plasma reactor with a nozzle. This simple modification enables to deposit a thin film by jet impaction method. The absorptivity (1 - reflectivity) of both samples were measured under vacuum conditions, demonstrating the high temperature resistance of TiN@SiOxNy core-shell films, as their optical properties at 700 °C remained stable. This simple experiment demonstrates the stark promise of this material as an alternative to gold and silver nanoparticles.

Recently developed plasmonic metasurface approach contributes an extraordinary way to manipulate the light-matter interaction at nanoscale, which plays a vital role in various fields such as biological and chemical sensors, surface enhanced Raman spectroscopy (SERS), nonlinear optics, and manipulation of quantum states. Titanium nitride is a promising material due to excellent plasmonic properties from visible to near-infrared. In this work, epitaxial TiN (111) thin films were grown on c-sapphire via plasma-assisted molecular beam epitaxy technique (PAMBE) under ultra-high vacuum conditions. By tuning stoichiometry of TiN via controlling nitrogen flux from lower nitrogen flux to nitrogen rich condition, the spotty reflection high-energy electron diffraction (RHEED) pattern became streakier, indicating a change in surface morphology. The smooth stoichiometric TiN provides excellent optical properties which can replace gold as a plasmonic material platform. The dielectric functions of TiN have been determined via Drude-Lorentz oscillator model fitting and ellipsometry measurement. It also reveals the plasmonic frequency has a blue shift from 2.6 eV (475 nm) to 2.7 eV (452 nm). Therefore, not only can replace TiN for gold in visible to near- infrared regions, but also TiN outperforms gold in the short wavelength region (<500 nm). XRD omega scan indicated the full width at half maximum is ~200 arcsec and the high-resolution atomic image revealed TiN crystalline arrangement. Furthermore, the epitaxial relationships of TiN and sapphire are determined to be TiN (111)|| sapphire (0001), TiN (1-10)|| sapphire (10-10) and TiN (11-2)|| sapphire (11-20). Reference: 1. W. P. Guo et al., Titanium Nitride Epitaxial Film as a Plasmonic Material Platform: Alternative to Gold, ACS Photonics 6, 1848-1854 (2019). 2. R. P. Sugavaneshwar et al., Fabrication of Highly Metallic TiN Films by Pulsed Laser Deposition Method for Plasmonic Applications, ACS Photonics 5, 814-819 (2018).

Bifunctional nanocrystals with integrated plasmonic and catalytic activities embrace great promise for monitoring chemical reactions by in situ surface?enhanced Raman spectroscopy (SERS). In this talk, I first introduce two distinctive strategies to achieve dual functionality. The first strategy involves the fabrication of Ag-based bimetallic nanostructures with an integrated SERS activity from Ag and catalytic activity from the platinum group metals such as Pt or Pd. The second strategy relies on tuning the passivation of Ag surface with an isocyanide molecule for revitalizing the catalytic activity of Ag toward hydrogenation reaction. Next, I use the reduction of 4?nitrothiophenol as a model system to evaluate the capabilities of these two different types of bifunctional nanocrystals for monitoring catalytic reactions and elucidating reaction mechanisms.

Active meta-materials offer an appealing way to create high speed modulators in the mid- and far- infrared which cannot be realized with currently available technology. This kind of device typically relies on a material with optical properties which change upon application of an electric and/or optical field, resulting in changes to their refractive index and/or loss. Whilst conventional characterization techniques (such as FTIR) are capable of accurately characterizing large changes in the properties of an active metamaterial, measuring small absolute changes is often problematic. This is particularly important for understanding the properties of devices which are functioning off-resonance, or trying to determine changes to any weak resonances in the spectral response. In this paper, we describe how the use of photoreflectance implemented in a FTIR microscope acts as a powerful tool to understand the properties of active metamaterials in the infrared with high dynamic range. In particular, we study the modulated behaviour of VO2 films and coatings integrated onto SiC nano resonator arrays. Photoreflectance is a technique which was originally used to study semiconductor quantum wells in the visible to near infrared, capable of measuring optically dark transitions. It is based on using a modulated pump laser to modulate the optical constants of a material, and then using a lock-in amplifier to monitor the corresponding change in sample reflectance. Here, we extend this technique to FTIR microscopy using broadband cooled MCT detectors, providing modulated information across an extremely broad spectral range. Our approach is an expansion of earlier implementations in the mid-infrared, where we use a combination of amplitude modulated step-scan spectroscopy and visible pump lasers, in order to build a photoreflectance system. In order to characterize the properties of our photoreflectance system we study films of vanadium dioxide (VO2), a correlated oxide with a volatile metal-insulator phase transition near room temperature. VO2 acts as an exemplary material for such studies, as when fully switched it shows an extremely large change in its optical properties across the mid-infrared, and has been widely integrated into metamaterials. First, we consider thin (35nm) planar films of VO2 in order to determine the sensitivity of the photoreflectance technique. By fully undergoing the MIT in VO2 we can observe the change in optical properties using both conventional FTIR microscopy, as well as photoreflectance. However, with photoreflectance we are able to characterize much smaller changes in the optical properties, in particular close to the phonon energies of VO2. Furthermore, we are able to probe the spectral regions close to the optical phonons of the film. To implement these studies for studying metamaterial devices, we then study silicon carbide nanopillars which are conformally coated with a layer of VO2. We are able to quantify the change in device response across the full Reststrahlen band, and the modulation speed of the device, which would normally be extremely difficult without infrared lasers. We envision that this technique could act as a platform to understanding the coupling of metamaterials to active materials, including those with small changes in optical constants such as in piezoelectric materials.

Plasmonic nanomaterials absorb light extremely well due to a localized surface plasmon resonance (LSPR) that is correlated with the density of free charge carriers in nanomaterials. Plasmonic nanomaterials have received interest in a broad variety of fields, such as photocatalysis, photochemistry, photovoltaics, biophotonics, spectroscopy, sensing, and wave-guiding. We present a novel technique for the synthesis of plasmonic zirconium nitride (ZrN) nanoparticles using a scalable non-thermal plasma process.1 Cost, production concerns and most importantly high thermal stability motivate the search for plasmonic materials alternative to gold and silver, like Group IV transition metal-nitrides such as TiN2 the relatively unexplored ZrN. Our ZrN nanoparticles display a plasmonic peak around 620 nm and from XRD and TEM we infer the crystallinity of the particles to be a cubic rock salt structure and a tunable size distribution below 10 nm. To prevent oxidation and the subsequent loss of plasmonic effect, we developed a modular non-thermal plasma system that coats the particles with amorphous silicon nitride in flight. This coating acts as an oxygen-sink when the material is exposed to atmosphere and yields blue-shifted and increased-intensity absorption. An attractive application of these plasmonic particles is the reduction of metals like platinum and chromium (VI) species in water which are extremely toxic.3 Here we have provided evidence of plasmon driven photo catalytic activity within visible wavelengths to reduce Platinum in solution.4 An aqueous solution of ZrN, methanol, and chloroplatinic acid (H2PtCl6) was illuminated using a monochromator set to various wavelengths in the visible regime. Energy dispersive X-ray spectroscopy (EDS) was then used to determine the ratio of reduced platinum to zirconium at a given wavelength. A similar method was used to reduce Chromium (VI) in water. [1] S. Exarhos, A. Alvarez Barragan, E. Aytan, A. A. Balandin, and L. Mangolini, “Plasmonic Core-shell Zirconium Nitride–Silicon Oxynitride Nanoparticles,” ACS Energy Lett., no. September, p. acsenergylett.8b01478, 2018. [2] A. Alvarez Barragan, N. V. Ilawe, L. Zhong, B. M. Wong, and L. Mangolini, “A Non-Thermal Plasma Route to Plasmonic TiN Nanoparticles,”J. Phys. Chem. C, vol. 121, no. 4, pp. 2316–2322, 2017. [3] M. Valari, A. Antoniadis, D. Mantzavinos, and I. Poulios, “Photocatalytic reduction of Cr(VI) over titania suspensions,”Catal. Today, vol. 252, pp. 190–194, 2015. [4] A. Alvarez Barragan et al., “Photochemistry of Plasmonic Titanium Nitride Nanocrystals,” J. Phys. Chem. C, p. acs.jpcc.9b06257, 2019.

Organic small molecules comprising π-conjugated moieties and functional groups have been the subject of considerable applications in organic opto-electronics, however, π-conjugated small molecules have yet to play a significant role as substrate for Surface Enhanced Raman Spectroscopy (SERS) studies. Very recently we demonstrated that metal-free nanostructured semiconductor films based on a π-conjugated small molecule, (DFH-4T diperfulorohexyl-quaterthiophene), could be SERS-active with a remarkable enhancement factor (EF) surpassing 103.13 Prompted by this breakthrough, we questioned whether molecular engineering of the SERS-active organic semiconductors, and thus the corresponding optoelectronic properties and solid-state microstructure/morphology, can be tuned to further advance Raman enhancement performance. Here we report that a fluoroarene-modified oligothiophene, (DFP-4T, diperfluorophenyl-quaterthiophene) semiconductor, which includes an electron-rich quaterthiophene π-core similar to that of DFH-4T but end-capped with π-electron-deficient perfluorophenyl (-C6F5) units. Thus, this molecule employs a quite different design approach as compared with DFH-4T, wherein the π-insulating perfluoroalkyl substituents present in DFH-4T are removed and replaced with -C6F5 substituents leads to a fully π-conjugated backbone featuring small intramolecular torsions. Hydrophobic and nanostructured DFP-4T films prepared via PVD are found to exhibit unprecedented EF values of >105, indicating that the SERS performance of metal-free organic semiconductor films now approach those of the current plasmonic metal and inorganic semiconducting platforms. Furthermore, a very low detection limit of 10-9 M is demonstrated for methylene blue (MB) as the probe molecule.

The early observation of strong circular dichroism for individual nanoparticles (NPs) and their assemblies have developed into a rapidly expanding field of chiral inorganic nanostructures. They encompass a large family of mirror-asymmetric constructs from metals, semiconductors, ceramics, and nanocarbons with multiple chiral geometries with characteristic scales from Ångströms to microns. Versatility in dimensions and polarizability of the inorganic materials enables their multiscale engineering to attain a broad range of optical and chemical properties. These capabilities as chiral materials enabled their fast technological translation for biosensing and optoelectronics, which, in turn, opened new venues for scientific inquiry into the unifying role chirality at the interface of materials science, biology, chemistry, and physics. This talk will address (1) the origin of the uniquely high values of optical anisotropy; (2) mechanisms of chirality transfer in inorganic materials; and (3) differences/similarities with chiral supramolecular, liquid crystal, and biological assemblies. Special attention will be given to the relationship between chirality and complexity traced over a large family of nano-, meso,- and microscale particles. Strict enumeration of complexity was accomplished by developing graph theory methods to describe NPs assemblies resulting in phase diagrams relating chirality and complexity. Non-uniform rise of complexity depending on temperature and enantiomeric excess in the system was examined in relationship with the phase transition between different hierarchical assemblies. The highest complexity was observed when high chirality coincided with the triple critical point in the phase diagram. If time permits, emerging venues for practical realizations of chiral inorganic nanoassemblies related to circular polarization spectroscopy in terahertz spectral window with chiroplasmonic kirigami composites will be introduced and discussed in relation to long-standing questions intermolecular interactions of chiral supramolecules.

Light is an efficient tool to activate and control a wide range of chemical reactions, from heterogeneous photocatalysis, to molecular switching, and pharmaceutical drug production. The advent of nanophotonics has further advanced these fields by allowing to focus, control and steer light at the nanoscale. In particular, the use of localised surface plasmon resonances, which are light-driven oscillations of free electrons in metal nanoparticles, is currently being explored to drive chemical processes such as nanoparticle syntheses, photocatalytic reactions, and nanolithograpic patterning. These plasmon resonances can activate chemical reactions thanks to non-equilibrium "hot" charge carriers, near-field enhancement of the electromagnetic fields, and nanoscale temperature gradients.1 The relative contribution of these different mechanisms is often quite difficult to disentangle due to the complex light propagation, poorly-understood quantum mechanical charge carrier generation and ejection, and complicated macroscopic and microscopic heat dissipation in irradiated reaction vessels.2 Single nanoparticle studies can be used to elucidate the mechanism at play, as parameters such as illumination intensity, light polarization, and sample and illumination geometry can be accurately controlled.3 So far, however, it has been challenging to control the synthesis of individual nanostructures with desired chemical compositions and controlled morphologies. Here, we show how we can selectively use plasmonic photothermal heating to activate and control the synthesis of individual Au@semiconductor core@shell nanoparticles.4 Under plasmon excitation of individual gold nanoparticles in a reactive flow cell, we activate the formation of conformal metal oxide and metal sulfide shells with growth rates that scale with the nanoparticle surface temperature. Given the large spectral sensitivity of plasmon resonances to the nanoparticle morphology, the shell growth reaction is self-limited in nature and can be monitored in-situ by tracking the photoluminescence spectra of the growing plasmonic nanostructure under laser irradiation. We demonstrate the versatility of plasmonic heating as a chemical activation mechanism by synthesizing CeO2, ZnO and ZnS shells on gold nanoparticles of different sizes and shapes. The use of light as a tool to activate and control chemical reactions at the nanoscale can lead to the synthesis of shape- and size-controlled hierarchical nanostructures which are inaccessible with classical colloidal synthetic methods, with potential applications in nanolithography, catalysis, energy conversion, and photonic devices. 1 M.L. Brongersma, N.J. Halas, and P. Nordlander, 10, 25 (2015). 2 R. Kamarudheen, G.W. Castellanos, L.P.J. Kamp, H.J.H. Clercx, and A. Baldi, ACS Nano 12, 8447 (2018). 3 J. Olson, S. Dominguez-Medina, A. Hoggard, L.-Y. Wang, W.-S. Chang, and S. Link, Chem. Soc. Rev. 44, 40 (2015). 4 R.Kamarudheen, G. Kumari, G. Baffou, and A.Baldi (submitted)

Plasmons have emerged as one solution to optically control chemistry at the nanoscale. Plasmonic photocatalysts have not only demonstrated increased reactivity, product selectivity, and new reaction pathways for heterogeneous catalysis, but also nanolocalized chemistry. However, in plasmonic photocatalytic systems, the overall reactivity is dictated by both the regions of localized enhanced electromagnetic fields (“hot spots”) and regions of high innate chemical reactivity (i.e. undercoordinated sites), which often coincide. To gain a systematic understanding of the influence of light on chemical reactions, one needs to be able to decouple the optical response from the surface chemistry preferred in the dark. Here, we spatially separate the electromagnetic hot spot from the catalytically favorable sites to demonstrate how plasmons can modify a reaction. As a proof of concept, we visualize the dehydrogenation of Pd nanorods coupled to Au nanobars in a cross-bar geometry. We use electron-beam lithography to pattern 75 nm wide Au nanobars onto a Si3N4 membrane, followed by the deposition of a 2.5 nm SiO2 spacer layer. Colloidally synthesized Pd nanorods of median length 450 nm are then dropcast on top. This geometry supports a strong gap plasmon mode between the Au nanobar and the Pd nanorod, creating an electromagnetic hot spot that is localized to the center of the Pd nanorod and well separated from the catalytically more favorable Pd tips. Using a light-coupled environmental transmission electron microscope, we track the plasmon-induced dehydrogenation of individual Pd nanorods with sub-particle, nanometer-scale resolution. We map the nucleation site of the new phase as a function of hydrogen pressure, illumination power, and wavelength. Without illumination, the Pd nanorods dehydrogenate from their tips. In contrast, upon resonant illumination (around 640nm), the Pd nanorods nucleate their new phase in the middle of the nanorod near the plasmonic hot spot. We confirm this behavior across six Au-Pd antenna-reactor systems for H2 pressures spanning 49Pa-89Pa. By varying the illumination wavelength, we further confirm that this induced phase transition depends on exciting the gap plasmon resonance as no phase transition happens with off-resonance illumination. Our direct visualization of a plasmon-driven, spatially-selective reaction demonstrates optical control of reactive sites, an important result towards designing next-generation, multifunctional photocatalysts.

Immunogold staining is the current standard for characterizing cellular ultra-structures using the sub-nanometer resolution afforded within an electron microscope. However, despite nearly 50 years of use, immunogold staining only yields binary information based on image contrast between the heavy gold atoms and carbonaceous cellular structures and therefore cannot provide multiplexed imaging capabilities. This limit restricts the potential of immunogold staining when imaging critical biological phenomena such as protein-protein interactions or to map the extensive ensemble of transmembrane proteins across a cell’s surface. Here, we report on the development of a library of small, electron-beam stable, and bright cathodoluminescent nanophosphors based on an alkaline-earth rare-earth fluoride host lattice. The base composition of each nanophosphor is BaYF5, where the Y3+ cations can be substituted with lanthanide cations (Ln3+) between 0 and 100% doping to act as optical emitters with distinct spectral emissions under electron beam excitation. The nanophosphors were colloidally synthesized using a strategy based on the thermal decomposition of mixed metal trifluoroacetate salts in a solvent system composed of oleic acid, octadecene, and oleylamine, which resulted in the nucleation of a narrow size distribution of 5 nm cores. Following a series of subsequent shelling steps, the nanophosphors were grown systematically until cubic nanoparticles with median edge lengths of 20 nm and 25 wt% loading of Ln3+ are incorporated into the nanophosphor where Ln3+ = Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, or Yb. These nanophosphors result in distinct and narrow spectral emissions that are characteristic of the atomic identity of the lanthanide dopant. Using an optically coupled scanning transmission electron microscope, ensemble cathodoluminescent spectra were obtained for eleven distinct nanophosphors using a focused electron beam at low currents (<100 pA) and short acquisition times (greater than or equal to 10 seconds). Focusing on the brightest nanophosphors, dozens of single nanoparticle cathodoluminescence spectra were obtained for three BaY0.75Ln0.25F5 species, where Ln3+ = Ho, Tb, Er. Each of these nanophosphors produce distinct cathodoluminescent spectra with maximum spectral emission at lEr= 675 nm, lHo= 635 nm, and lTb= 555 nm. The distinct emission of each nanophosphor allows for the realization of multiplexed spectrum imaging, and line scan analysis on samples containing both BaY0.75Tb0.25F5 and BaY0.75Ho0.25F5 opening the door to true nanoscale multiplexed imaging based on optical signatures from lanthanides. Through future work on surface modification, individual cathodoluminescent nanophosphors can be selectively attached to specific proteins of interest using similar chemistry pioneered through 50 years of immunogold staining research. This development opens the possibility for multiplexed imaging of biomolecules at the single-molecule level within the cellular ultra-structure, providing insight into the distribution and expression of proteins within cells at the high resolution afforded by electron microscopes.

Among the various studied shapes of gold nanoparticles, nanobipyramids are one of the most interesting ones. Their anisotropic structure enables the tuning of their LSPR even further than gold nanorods, with higher local electric field enhancement, and their morphology (aspect-ratio, roughness, size, etc.) can be controlled with more freedom. We developed in our lab a high yield process to make such nanostructures, in the classic submillimolar concentration range. While this process was very efficient, its low concentration makes the production of concentrated solutions quite cumbersome and generates lots of waste. Therefore, we developed a new versatile process enabling the direct preparation of gold nanobipyramids and several other pentatwinned nanostructures at concentrations range between 15 and 30 mM (3 to 6g per liter). This novel synthesis has nearly quantitative yields, while allowing an easy tuning of LSPR wavelength. A specific influence of the nature of the reducing agent on the morphology has been observed, and this can be further controlled or enhanced by variating experimental conditions (surfactant, temperature, co-reagents). Those breakthroughs open further uses for gold pentatwinned nanostructrures by simplifying their preparation at gram scale and tuning their morphology for specific uses. In order to illustrate the potentialities of such gold nanostructures, we will show our recent investigations on their use for optical applications. We developed an easy and general gold surface modification by a functional thiolated silicon polymer. In addition to be an alternative to the use of surfactants or long-chain polymers in order to provide efficient colloidal stabilization in a wide range of solvent, this surface modification allows us to prepare homogeneous hybrid materials like gold-doped sol-gel silica. Association with various photoactive compounds have been explored and enhancement for example of photocatalytic properties of TiO2 or non-linear optical absorption of dye-doped glasses. Finally, we will present our current efforts regarding core-shell AuBP@SiO2 NPs for enhanced photocatalysis based on singlet oxygen generation.

Detection of trace amount molecules is crucial for analytical studies and inspection of environmental contaminants. Of many detection systems, surface-enhanced Raman spectroscopy (SERS) stands out as an outstanding option not only for its rapid process but also for its non-destructive properties and molecular-specific signals. Previous studies have reported up to attomolar concentration detection via SERS by utilizing “hot spots” produced from diverse nanostructures and fabrication methods. “Hot spots” are regions with localized surface plasmon resonance (LSPR) formed between nanostructured gaps that significantly enhance the Raman signal of analytes. Yet, despite advances in fabricating noble metal nanostructures with high-signal enhancement factors, trace-amount detection remains problematic as the delivery of adequate amounts of analytes to the proximity of “hot spots” is required for sufficient enhancement effects. Therefore, a platform to gather enough target molecules within the vicinity of “hot spots” is important for trace-amount molecule detection.
Here, we introduce a hydrophobic Au nanowire substrate with a hydrophilic center for the collection and detection of low-concentration analytes. The hydrophilic center acts as a concentrating point during the evaporation of highly-dilute analyte solutions and allows easy location of the measurement point during direct Raman spectroscopy. The SERS substrate is fabricated via solvent-assisted nanotransfer printing (S-nTP) to form the highly dense Au nanowire surface. The center is then selectively coated with a thin layer of SiO₂ to provide hydrophilicity while the surrounding area is subsequently functionalized with 1H,1H,2H,2H-Perfluorodecanethiol (PFDT) to amplify the hydrophobicity of the remaining Au nanowires. When an analyte solution is dropped and dried on the substrate, the solution is guided towards the hydrophilic center with minimum liquid contact line pinning on the hydrophobic surroundings. As a result, Raman measurements at the hydrophilic area show clear spectra of the dye and pollutant molecules up to attomolar concentrations compared to non-functionalized Au nanowire substrates with analytes spread over the entire surface.

Plasmonic chiral metamaterials has attracted significant attention at it provides a new route to intriguing optical properties such as negative refractive index, light polarization filters, and phase modulation. However, the fabrication regarding with limited resolution in conventional synthesis and complexity of asymmetric synthesis pose a major hindrance for further development. In this study, a new class of three dimensional chiral plasmonic nanostructures was successfully fabricated using molecular shape modifiers and crystallographic control of nanoparticle. Previously, we developed three-dimensional chiral metamaterials based on photolithography overlay and electron-beam lithography overlay for terahertz and near-infrared, respectively. We found scaling down them further to visible frequency is extremely difficult. As an alternative solution, we discovered a novel system that characteristic of molecule is transformed into distinctive gold nanoparticle shape. On the basis of this system, chirality transfer between molecular modifier and gold surface allow us to achieve numerous chiral morphologies of gold nanoparticle, named plasmonic helicoids. Particularly, enantiospecific interaction of molecule and high index plane plays pivotal role to provide asymmetric structuring process on the gold surface, forming distinct chiral morphology in single nanoparticle level. One of the representative shapes of helicoid structure showed gammadion-like structure, consisting of four highly curved arms of increasing width, in all six faces of cubic geometry. The unprecedented chiral morphology of plasmonic helicoid has remarkable optical activity (dissymmetry factor ~ 0.2 at 622 nm) even in a randomly dispersed solution, substantiated by direct visualization of macroscopic color transformation. Changes in molecular recognition and growth parameter led to different morphological evolution, and structural alterations provided a straightforward means of tailoring optical response, such as optical activity, handedness, and resonance wavelength. Also, our aqueous phase synthesis is readily scalable without losing exquisite chiral structure at nanoscale. In these aspects, our approach, chirality evolution in single nanoparticle, provides a truly new paradigm and valuable insight for chiral metamaterial fabrication. Such unique fabrication technique will provide the opportunity to achieve the significant step making metamaterials from science to technology. References 1. S. Zhang et al., Phys. Rev. Lett. 102, 023901 (2009) 2. S. Zhang et al., Nat. Commun. 3, 942 (2012) 3. G. Yoon et al., Sci. Rep. 7, 6668 (2017) 4. J. Mun et al., Opt. Lett. 43, 2856-2859 (2018) 5. M. Kim et al., Opt. Express 26, 14051-14057 (2018) 6. H. Lee et al., Nature 556, 360-365 (2018) (with News and Views Article, Nature 556, 313-314 (2018)) 7. J. Mun et al., Nanophotonics 8, 941-948 (2019) 8. H. Lee et al., Nature Commun. (in revision) 9. J. Mun et al., Light: Sci. Appl. (in review)

We have demonstrated an ultrafast, ultra-compact, and all-solid-state metaphotonic spatial light modulator (M-SLM) which can dynamically shape light in reflection. The M-SLM is an array of electrically controlled plasmonic nanostrip cavities, and it can continuously adjust the phase of reflected light from 0 to 360 degrees and, at the same time, independently modulate the amplitude over its full range. The fabricated M-SLM contains 50 active pixels and measures 250×250 ?m2 in dimensions. By tuning the voltages applied to the M-SLM, we have demonstrated beam steering and beam splitting with a steering angle up to 8°, the side mode suppression ratio of +2.7 dB (the highest value reported in the field), and the deflection efficiency up to 48%. The experimentally measured modulation speed marks 5.6 MHz while the energy consumption per pixel is calculated to be as low as 354 pJ per switching. Accomplishing arbitrary light manipulation using active metaphotonics has remained challenging, especially due to coupling between the phase and amplitude during modulation which resulted in unwanted background noises. To overcome this limitation, we have delved into the fundamental modeling of the light wave and observed that it would require minimum two variables, for example, the real and imaginary coefficients, to properly express the phase and amplitude of the light wave. Based on this observation, we developed the two-control-parameter algorithm and translated the concept into the hardware design. In the array of the designed M-SLM, each plasmonic nanostrip cavity is a stacked double-gated structure with an indium-tin-oxide layer as an active intermediate channel, which allows an application of two independent control voltages to the top and bottom gate metals. We have also performed extensive numerical investigation and confirmed that the highly versatile light-manipulating capability of the designed M-SLM, including arbitrary phase and amplitude modulation, originates from the superposition of the effects generated by the voltages applied independently to the top and bottom metals. To our knowledge, the M-SLM is the first all-solid-state, electrically controlled metasurface that accomplishes the 360-dgree full phase sweep and independent amplitude modulation. Unlike previously reported work, our M-SLM does not involve the use of slow, temperature-sensitive liquid crystals, stiction-and-vibration-prone micro-electromechanical systems (MEMS), or an array of pre-fabricated size-varying structures to achieve phase changes. The M-SLMs extremely low power consumption, ultrafast modulation speed, ultra-compact form factor, outstanding predicted reliability, and the use of standard IC-fabrication processes promises wide adaptation and proliferation of the ultrafast and extremely power-efficient SLMs for 3-dimensional real-time holographic display, light detection and ranging (LiDAR), and wearable display for virtual reality. We look forward to sharing and discussing the details of our M-SLM design philosophy, hardware design & fabrication, and testing results with the audience at the MRS 2020 Spring.

In this talk, I will present our recent efforts to drastically enhance the efficiency of high-harmonic generation (HHG) and broadband terahertz (THz) generation in ultra-thin transparent conducting oxide films leveraging the epsilon-near-zero (ENZ) effect. HHG has been used to generate extreme ultra-violet (EUV) light sources to probe fast electron dynamics in the attosecond time scale. While traditionally observed in rare-gas atoms, HHG has also recently been reported in solids with reduced threshold pump field. Here, we use the ENZ effect to greatly boost the efficiency of the HHG process in solids. We report high-harmonic emission up to the 9th order directly from a low-loss, solid-state ENZ medium: indium-doped cadmium oxide, with an excitation intensity at the GW cm-2 level. Furthermore, the observed HHG signal exhibits a pronounced spectral red-shift as well as linewidth broadening, resulting from the photo-induced electron heating and the consequent time-dependent resonant frequency of the ENZ film. THz technology has undergone much development in the past few decades, yet it still remains a grand challenge to realize THz sources on an integrated platform. The intensity and spectral bandwidth of typical nonlinear THz sources are limited by the phase matching condition in the nonlinear medium. Here, we demonstrate broadband THz emission, both in transmission and reflection configurations, from a commercially available 10-nm-thick indium tin oxide (ITO) film, leveraging the ENZ effect in ITO. The bandwidth of the THz signal generated from the ultrathin ITO film is unrestricted by the phase matching condition.

Optical nanomaterials offer the ability to bend, twist, guide, and confine light in nanoscale dimensions. Among these materials, chiral nanostructures particularly show promise for applications ranging from polarization manipulation to 3D displays, sensing, and spin-selective transport. Compared to their molecular counterparts, chiral nanomaterials exhibit orders of magnitude stronger dissymmetry factors, but have only been realized in a limited set of materials systems. We have recently demonstrated different strategies to tune and manipulate the polarization response of chiral metamaterials that combine metallic nanostructures, dielectric materials, and semiconductor nanocrystals. By controlling the refractive index of the dielectric components in different architectures, we show that the sign of the circular dichroism can be reversed, and the polarization and directionality of the outcoupled luminescence from nearby nanocrystals can be controlled. To create light-emitting metamaterials, it is additionally useful to create nanostructured elements comprised entirely of photoluminescent materials. We have recently developed patterning methods to transform semiconductor nanocrystals into patterned nanocrystal solids, realizing lateral feature sizes as small as 30 nm and heights in excess of 100 nm without degradation of the photoluminescence. We show that by designing the shape of the nanocrystal solid at this length scale and controlling connectivity between the nanocrystals, the refractive index and nanostructure absorptivity can be tailored. This work points to new strategies to design dynamically tunable metamaterials and control nanoscale light-matter interactions.

Heteropolar or polar covalent semiconductors can have their opposite-phase transverse phonon vibrations activated by an impigning infrared (IR) light beam. Thus, strong coupling occurs between the incoming light and the matter vibrations resulting in a hybrid mode known as phonon polariton (PhP). This hybrid PhP mode has a frequency gap in its photonic dispersion, typically refered to as the reststrahlen band, which signifies a regime of forbidden light propagation inside the material; hence bulk PhP material are near-perfect reflectors. This is why PhP material have been largely overlooked as the material of choice to control the flow of infrared light for modern photonic applications. However, in the recent years there has been a heightened attention around PhP materials which now are prominent candidate materials for infrared photonics. PhP materials owe this recent growing interest to the versatile photonic responses within and around the reststrahlen band which include plasmonic-like responses similar to those of noble metals in the visible spectrum as well as epsilon-near-zero (ENZ) responses. In this talk, we discuss how to bring judiciously together material patterning and PhP photonic responses for extreme light control in the mid- and far-IR. We present paradigm PhP-based systems exhibiting unconventional infrared effects such as uni-directional, almost all-forward, thermal emission and ultrathin near-perfect PhP absorbers. These effects can have a vast applications range such as for bolometric detectors, high-intensity one-way THz/infrared sources, or thermal-management devices.

For efficient nonlinear processes, the engineering of the nonlinear optical properties of the used materials becomes an important task. The most well-known technique for spatially engineering nonlinear optical properties is the quasi-phase-matching scheme for second-order processes like second-harmonic generation. However, the widely used technique of periodic polling of natural crystals only provides a binary state for the nonlinear material polarization, which is equivalent to a discrete phase change of π of the nonlinear polarization. The continuous tailoring of the phase of the nonlinear susceptibility can greatly enhance the flexibility in the design and reduce parasitic effects, like the generation of other undesired frequency components.
Metasurfaces have the potential to address some of the issues with the continuous phase tailoring in nonlinear processes. In the linear optical regime, they have shown great promise for tailoring the wavefronts of beams for a large variety of applications, ranging from lenses for imaging and vector beam generation to optical holography. For that different concepts were used to address the issue of tailoring the spatial phase and amplitude. One promising concept is the Pancharatnam-Berry phase that appears for a polarization conversion of the light. However, this concept is not directly transferable to nonlinear metasurfaces, where each meta-atom performs also a nonlinear optical process, like a harmonic generation. The reason is the coupling of different spatial field components through the nonlinear susceptibility tensor, which complicates the design and can give even rise to different phase factors at the same time. Furthermore, the optimization for a particular phase factor might counteract the goal for high conversion efficiency.
Here, we will discuss the concepts of enhancing nonlinear processes with simultaneously engineering the phase for the manipulation of third-harmonic generation from metasurfaces and the arising restrictions for symmetry and geometry of meta-atoms. We theoretically and experimentally studied the nonlinear geometric-phases associated with the THG process occurring in silicon metasurfaces. The THG process involves the wave coupling among the different components of the fundamental field excited inside the dielectric nanoresonators. This gives rise to different geometric-phases carried by the THG signal. We show that the dielectric nanofin structures with various in-plane rotational symmetries generate circularly polarized TH signals carrying the geometric-phases as predicted by the selection rule for nonlinear processes. However, additional phase terms can arise due to the complexity of the mode field inside the nanoresonators. The experiment results agree with our theoretical model.
Furthermore, by tailoring the geometry of high index resonators can result in the excitation of a particular magnetic dipole or higher-order modes that result in relatively strong fields and high conversion efficiencies. We analyze how such modes can be used for tailoring the nonlinear phase utilizing the Pancharatnam-Berry phase approach. By using the recently introduced concept of bound states in the continuum in nanoresonators, we demonstrate that a relatively high conversion efficiency can be obtained for a third-harmonic generation while preserving the spatial phase control possibility using the geometrical phase effect. Controlling the properties of nonlinear metamaterials has fundamental significance in nonlinear optics and for tailored nonlinearities, as such materials provide a further degree of freedom in the design of nonlinear materials.

Manipulation of waves is mostly achieved by materials with spatial inhomogeneities. However, exploiting the dimension of time, in addition to the three dimensions of space, for variation of material parameters can bring additional functionalities in such tailoring and sculpting the wave. The spatiotemporal metastructures in which the material parameters are engineered with spatial and/or temporal inhomogeneities exhibit special features that enrich the functionality of light-matter interaction. In my group, we are investigating a variety of scenarios in which such four-dimensional (4D) spatiotemporal material platforms can be exploited. These scenarios include 4D materials that provide us with temporal aiming, temporal anisotropy, anti-reflection temporal coatings, frozen waves, phase conjugation, asymmetric diffusion, temporal effective medium methods, etc. In this talk, I will give an overview of our most recent results in this area

We report on the study of some metamaterials whose properties, including dispersions and topological characteristics, are determined by the connectivity of a network rather than the resonance of individual resonators.

Metamaterials are described using effective medium theories in the long-wavelength limit in the sense that their optical properties can be described using local permittivity and permeability tensors. We will see that some metallic structures cannot be described using such a language. Their properties depends on the network connectivity, where braiding can create effective sythetic gauge fields that induce shift in momentum space. The consequence is that such materials can have their index ellipsoids centered not at k=0, and they can possess multiple index ellipsoids centered at arbitrary nonzero k-points. The number of modes that are allowed at zero frequency can be described by Betti numbers. Such systems differ from conventional metamaterials because the functionality does not come from the resonances of individual resonators and as such, their functionalities tend to be broadband.

We will also discuss network structures that have non-trivial topology in momentum space and these materials can couple orbital angular momenta with the wave vector. The topological character originates from the connectivity of the network and goes beyond typical space group descriptions. Such systems can exhibit angular momentum-dependent one-way edge states, and their properties can be described using local Chern numbers. Such systems can also be used to realize other interesting topological systems such as high order topological insulators and topological Anderson insulators, and topological systems in which the topological order cannot be described by integers.

Rationally assembled nanostructures exhibit distinct physical and chemical properties beyond their individual units. This talk will describe fundamentals and applications of metal nanoparticle lattices that show long-range diffractive interactions when the lattice spacing is close to the wavelength of incident light. These collective excitations are known as surface lattice resonances (SLRs) or lattice plasmons. We will discuss advances in nanofabrication techniques that have enabled the patterning of a wide range nanoparticle materials over macroscale (> in²) areas and with units that are highly uniform in size, shape, and crystallinity. Such patterned substrates support SLRs with wavelengths that approach the theoretical limit. We will also describe how different lattice symmetries can result in band-edge modes at high symmetry points in reciprocal space. The confined electromagnetic fields of SLRs can manipulate processes such as exciton-plasmon coupling, nanolasing, and imaging via tunable, flat optics.

Optical metasurfaces [1] can be fabricated in flexible substrates [2,3]. Flexibility brings additional advantages to MSs respect to their rigid counterpart, in terms of scalability, conformability, and tunability [4-7]. When metasurfaces are used as supports to encode computer generated holograms, flexibility allows to harness the shape and topology of the target substrate as an additional degree of freedom to encode the holographic information [8,9].

Here we show that this platform can be scaled from the visible to the mm-wave range. This in turn opens the way to the exploitation of flexible holographic metasurfaces for a large number of applications, from security holograms that display the correct image only when the substrate has a determined shape, to the possibility to interface the metamaterials technology to biophotonics systems or to create holographic patches to retrofit existing antennas infrastructures for telecommunication applications [10].
References
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[2] A. Di Falco, M. Ploschner, and T. F. Krauss, New J. Phys. 12, 113006 (2010).
[3] A. Di Falco, Y. Zhao, and A. Alú, Appl. Phys. Lett. 99, 163110 (2011).
[4] X. Li, C. Rizza, S. A. Schulz, A. Ciattoni, A. Di Falco, APL Photonics 4, 056107 (2019).
[5] S. Song, X. Ma, M. Pu, X. Li, K. Liu, P. Gao, Z. Zhao, Y. Wang, C. Wang, X. Luo, Advanced Optical Materials 5, 1600829 (2017).
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Realization of active optical metasurfaces capable of tunable phase modulation of light in response to variations in an external stimulus has recently stimulated a great interest in nanophotonics community as it can pave the way toward achieving flat ultrathin optical components offering reprogrammable functions with real-time control. In this talk, we will give an overview of our works on realization of active metasurfaces with robust voltage-controlled phase response. We will demonstrate that the integration of electro-optical tuning mechanisms based on free-carrier and quantum-confined effects into the constituent building blocks of metasurfaces can be exploited for achieving wide phase tuning in reflection and transmission modes. We will discuss the routes toward attaining optimal tunable performance based on field effect modulation of carrier density in degenerately doped semiconductors such as indium-tin-oxide. We will also explore novel photonic design paradigms based on extreme light-matter interactions for enhancing the tunability of optical response arising from the weak electro-refractions due to double carrier injection in p-i-n junctions and carrier accumulation in p-n junctions of moderately doped semiconductors, as well as quantum-confined Stark effect in multi-quantum well structures by leveraging high-Q resonant modes such as cavity modes, guided mode resonances, and quasi-bound states in the continuum. As the next frontier of active metasurfaces, we will examine time-modulated optical metasurfaces which render a four-dimensional design space and give rise to a myriad of novel physical phenomena through generation of higher-order frequency harmonics. We will illustrate that the phase of higher-order frequency harmonics generated by a time-modulated metasurface can be tuned with uniform amplitude via a non-resonant dispersionless geometric phase shift induced by the modulation phase delay. This dispersionless phase elevates time-modulated metasurfaces beyond their quasi-static counterparts relying on resonant phase delays in terms of dynamic wavefront engineering capabilities in that it increases the functionality bandwidth and minimizes spurious spatial scattering by providing access to the full phase span (2π) with uniform amplitude. Moreover, we will show that frequency conversion operations in space-time gradient metasurfaces can break the time-reversal symmetry and lift the Lorentz reciprocity constraint. We will highlight the required conditions toward achieving strong nonreciprocal responses and will demonstrate high-Q time-modulated metasurfaces can serve as free-space optical isolators by providing access to the non-adiabatic modulation regime with small electro-optical modulation frequencies.

Abstract:
Discovering novel, unconventional optical designs in combination with advanced machine-learning assisted data analysis techniques can uniquely enable new phenomena and breakthrough advances in many areas including on-chip circuitry, imaging, sensing, energy, and quantum information technology. Topology optimization, which has previously revolutionized aerospace and mechanical engineering by providing non-intuitive solutions to highly constrained material distribution problems, has recently emerged as a powerful architect for advanced photonic design. Compared to other inverse-design approaches that require extreme computation power to undertake a comprehensive search within a large parameter space, topology optimization can expand the design space while improving the computational efficiency. This talk will highlight our most recent findings on 1) merging topology optimization with artificial-intelligence-assisted algorithms and 2) integrating machine-learning based analysis with photonic design and quantum optical measurements. Particularly, we will discuss our studies on implementing deep-learning assisted topology optimization for advanced metasurface design development, focusing on highly efficient thermal emitter/absorber development for thermal-photovoltaics applications. We will outline our recent work on merging topology optimization technique with quantum device design for achieving ultrafast single-photon source that offers efficient on-chip integration. Finally, we will outline our recent works on implementing a novel convolutional neural network-based technique for real-time material defect metrology at the quantum level that outperforms all existing approaches in terms of speed and fidelity. This new method rapidly extracts the values of the single-photon autocorrelation function at zero delays from sparse data and ensures one order speed up on solving “bad”/”good” emitter classification problem in comparison with conventional techniques.

References:

[1] Zh. Kudyshev, A. V. Kildishev, V. M. Shalaev, A. Boltasseva, Machine-learning-assisted metasurface design for high-efficiency thermal emitter optimization, Applied Physics Reviews, 7, 021407 (2020); https://doi.org/10.1063/1.5134792
[2] Zh. A. Kudyshev, S. Bogdanov, T. Isacsson, A. V. Kildishev, A. Boltasseva, V. M. Shalaev, Rapid classification of quantum sources enabled by machine learning, arXiv preprint at https://arxiv.org/abs/1908.08577 (2019)

The use of reconfigurable metasurfaces to dynamically control the complex amplitude modulation of scattered light has stimulated the interest of the research community. Despite an extensive cumulative body of knowledge in metasurfaces, insights and observations for the most intriguing possibilities have remained elusive due to the lack of design methodology. All proposed designs thus far can only control the optical phase over a fraction of a wavelength, and, furthermore, they are highly inefficient, and couple the phase and amplitude of the scattered light. In short, the current strategies offer very low prospects of creating a transformative optical device. This fundamental problem poses severe performance limitations – particularly for applications relying on accurate spatiotemporal complex field modulation, which includes dynamic holography, high-resolution imaging, optical tweezing, and optical information processing.

Tunable plasmonic modes offered by graphene provide new opportunities to create electo-optically active devices with novel characteristics that have thus far been impossible to be realized by using conventional media. In this talk, we present a new strategy for dynamically controlling the local complex amplitude of light. We devise “metamolecules” consisting of a pair of subwavelength graphene plasmonic metaatoms. Independent tuning of the two metaatoms secures the complete control of both amplitude and phase of light. Our proposed metasurface allows for 2π phase shift as well as large amplitude modulation including perfect absorption. The capabilities of the proposed metasurface were proven by demonstrating active beam steering and holographic wavefront reconstruction with an unprecedented figure-of-merit. Our proposed metamolecule represents a conceptual advancement to metasurface design, allowing for complete amplitude and phase control of light, and should find its application in active thermal engineering and real-time hologram systems beyond the mid-infrared.

Optical metasurfaces have emerged as promising candidates for multi-functional devices. Dynamically reconfigurable metasurfaces have been introduced by employing phase-change materials and/or by applying voltage, heat, or strain. While existing metasurfaces exhibit appealing properties, they don’t express any significant nonlinear effects due to the negligible nonlinear responses from the typical materials used to build the metasurface. In this work, we propose and experimentally demonstrate one kind of Kerr metasurfaces that shows strong intensity-dependent responses. The Kerr metasurface is composed of a top layer of gold antennas, a dielectric spacer, and a ground layer of metallic quantum wells (MQWs) made by TiN/Al₂O₃multilayer stack. Due to the large Kerr nonlinearity supported by the MQWs, the effective optical properties of the MQWs can change gradually from metallic to dielectric with increasing of the input intensity, leading to dramatic modifications of the metasurface responses. This opens up new routes for tunable metasurface applications in the fields of nonlinear optics and extremely fast meta-optics.

Metamaterials are artificially engineered structures designed to shape electromagnetic responses not found in nature including superlensing, cloaking and communications. They traditionally comprise an array of fabricated subwavelength anisotropic structures on a planar geometry, which inevitably limit their responses in certain directions. We explore a novel type of soft metamaterial to overcome such limitations, in which building blocks and overall configurations that can be freely tailored, even after formation. In particular, we explored the complex soft metamaterial nanoarchitectures with tailored symmetries that achieved isotropic negative index at the optical frequency. Moreover, we expand such soft metamaterial design that created the first Casimir quantum trap in solution by manipulating electromagnetic waves from quantum fluctuations. These results provide new insights that facilitate and expand novel metamaterial design and applications.

Metasurfaces and flat optics with sub-wavelength structures fabricated on a thin film have shown their strong power in light manipulation and potentials in compact optical device applications. Metasurfaces normally are constructed by sub-wavelength meta-atoms such as disks or pillars spatially arranged according to the design and the function of the device. Meta-optics having a flat surface with meta-atoms embedded within the film would facilitate multi-layer integration for an optical system and ease the electrical control layout for active devices besides the benefits of robustness. Tunable flat optoelectronics allowing dynamic control of the light wave is highly desirable to increase the functionality and flexibility of the optical device and system design. On the other hand, the emergence of two-dimensional (2D) transition metal dicharcogenides (TMDCs) semiconductors offer great opportunities for optoelectronic applications for their strong quantum confinement and the easiness in forming heterostructures enabled by the out-of-plane van der Waals bonding. The intriguing exciton characters in 2D TMDCs and their heterostructures render new dimensions to realize tunable flat optoelectronic devices operable at a wide spectral range from ultra-violet (UV) to mid-far-infrared (IR).

In this talk I will introduce several of our work on tunable flat optoelectronics including programmable flat optics through continuously reversible refractive index change in the phase change materials GST by laser pulse writing, electrical and thermal tuning of terahertz response and complete reflection suppression in an atomically smooth Si metasurface produced by CMOS compatible process, and the gate tunable absorption in TDMC heterostructures by manipulating the interlayer excitons for room temperature high responsivity mid-far IR photodetection, and the 2D flat lens made on 10 nm thick TMDCs for sub-diffraction limit focusing and imaging at 450 nm wavelength.

Since the development of diffractive optical elements in the 1970s, major research efforts have focused on replacing bulky optical components by thinner, planar counterparts. The more recent advent of nanophotonic metasurfaces has further accelerated the development of flat optics through the realization that resonant optical antenna elements can be utilized to facilitate local control over the light scattering amplitude and phase. In this presentation, I will highlight recent efforts to realize electrically-tunable metasurfaces employing nanomechanics, electrical gating, microfluidics, and phase change materials. I will pay particular attention to the opportunity of realizing dynamic metasurfaces from atomically-thin semiconductors. Such elements are capable of active wavefront manipulation for optical beam steering and dynamic holography. I will illustrate how the proposed optical elements can be fabricated by scalable fabrication technologies, opening the door to many commercial applications.

High quality factor (“High Q”) cavities have revolutionized information processing, communications, sensing, and nonlinear optics by increasing photon storage times and significantly enhancing light-matter interactions. However, when the size of dielectric cavities is reduced to the nanoscale, resonant modes start to resemble point sources, scattering an incident wave in many different directions. While this scattering has been leveraged to create remarkable metasurfaces that precisely control the phase, amplitude, and polarization of light in an ultrathin footprint, metasurfaces generally exhibit high radiative loss rates and thus low Q-factors. Indeed, to date, resonant features in the far-field transmission of metasurfaces have Q-factors on the order of ten.

Here, we describe a general strategy for generating high quality factor resonances in a phase gradient metasurface. Resonances are created by including subtle structural changes in individual resonators to weakly couple free-space light into otherwise bound modes. We experimentally demonstrate control over the quality factor and resonant wavelengths in this scheme, achieving record metasurface Q’s greater than 2500. We also show via simulations that Q-factors can exceed millions. We highlight this scheme’s general applicability by designing and fabricating metasurfaces that act as beamsteerers to different angles, beam splitters, and lenses. We also show how this platform can be integrated with electro-optic materials for low-power metasurface modulation. Finally, we describe applications of high-Q metasurfaces spanning i) free-space optical sensors of COVID-19 antigens and antibodies; ii) all-optical magnetic switching; and iii) an all-optical self-isolating laser.

Metasurfaces significantly widen the parameter space for control over light, mainly via the many ways phase control can be achieved and varied within the plane of the surface. Here, we take a closer look on how to further widen these possibilities via introducing elements such as control over amplitude and phase simultaneously, i.e. the complex amplitude of the impinging light field, and secondly via disorder.
As a first application, we examine the use of beams of higher orbital angular momentum coupled to metasurfaces in a context of holography. We demonstrate the design and implementation of metasurface phase holograms with strong orbital angular momentum selectivity, based on nanostructured gallium nitride. This opens up a new degree of multiplexing, allowing lensless reconstruction of distinct holographic images in each orbital angular momentum channel. We further show how metasurfaces with additional variation in the height of the underpinning nanostructures allow for complete control over the complex amplitude of the impinging wave. We demonstrate that this significantly reduces cross talk between different orbital angular momentum coding channels, enabling up to 200 independent orthogonal image frames for realistic parameters.
The second application is in a context of energy harvesting. Based on a disordered array of plasmonic nanoresonators, we show how deterministic coupling of this effective photonic layer to an external cavity can lead to two different regimes of operation — broadband absorption and narrow-band reflection. As an example, we demonstrate the use of such coupled disordered-layer / cavity systems for colour generation.

The interfacing of nano-structures or meta-surfaces with optical fibers represents a promising approach to unlock novel types of functionalities within a multitude of fields including biophotonics, quantum technologies or optical sensing. Here commonly used top-down implementation strategies are hard to employ in case fiber end-faces are considered due to the intrinsic mismatch of the fiber geometry with wafer-based fabrication technology.
Within this presentation, we would like to report on two implementation pathways to circumvent the mentioned bottleneck, namely (i) 3D nano-printing and (ii) modified electron-beam lithography. Both approaches allow for the implementation of sophisticated nano-structures on the end-face of optical fibers leading to significant performance improvements compared to fibers with unstructured end faces. These improvements are shown here on the examples of (i) optical trapping using only one single-mode fiber via the integration of dielectric meta-surfaces and (ii) boosting in-coupling efficiencies into fibers at almost grazing incidence via dielectric ring-type nano-structures .

(i) A tight focus of light plays an essential role in a vast number of applications including optical imaging, optical manipulation, material processing, biophotonics, microscopy and quantum technology. Here optical fibers show fundamental limitations due to the divergence of light emerging from the fiber facet which for instance has limited the use of single fibers within optical trapping, demanding numerical apertures >0.8. In this presentation we show that 3D nano-printing via direct laser writing allows for the integration of ultra-high numerical aperture meta-lenses on the facet of single-mode fibers that include beam expansion sections. Record-high numerical apertures of up to 0.9 and diffraction-limited spots have been reached. The capabilities of this device have been demonstrated by optically trapping microbeads and biologically relevant bacteria (E. coli) for the first time with one single-mode fiber, solving a serious limiting factor within fiber optics research.

(ii) Optical fibers are particularly difficult to use within the context of wafer-based implementation technologies due to the large aspect ratios of fibers, being complementary to wafers. Using an additional planarization step prior to nano-fabrication we open up a novel pathway to implement dielectric nano-structures onto the end-face of optical fibers via electron-beam based technology. Here, we show that by integrating dielectric gratings consisting of concentric rings located on the core of single-mode fibers, in-coupling efficiencies can be increased by more than four orders of magnitude at almost grazing incidence compared to fibers with unstructured end faces. Specifically, application relevant in-coupling efficiencies of several percent at angles of more than 75° have been reached, suggesting a clear pathway to solve one serious bottleneck of optical fibers which has imposed limitations on applications of optical fibers.

Weyl points are linear band crossings at discrete points in the momentum space, corresponding to three-dimensional (3D) extensions of the two-dimensional (2D) Dirac nodes, and functioning as monopoles of Berry flux with topological charges defined by the Chern numbers. In photonics, Weyl points have been realized in systems with broken inversion symmetry such as photonic crystals and metamaterials, with broken time reversal symmetry such as magnetized plasma and in synthetic dimensions. As a key signature of Weyl systems, Fermi arcs that connect between the projections of Weyl points of opposite topological charges have been observed. However, the manifestation of the topological nature of Weyl points in their interaction with free space radiations remain obscure in both quantum and classical systems.
Scatterings and transport in Weyl semimetals have caught growing attention in both condensed matter and classical physics. In condensed matter systems, the chiral zero modes and the associated magnetoresistance and chiral magnetic effects underlie some highly unusual scattering and transport properties of Weyl semimetals. It has also been proposed that Weyl semimetals may have handedness dependent Imbert–Fedorov shift within the bulk Weyl material. Electrical conductance is usually measured in the experiments, which has contribution from both the bulk Weyl nodes and the fermi arcs. However, it is generally difficult to perform momentum resolved scattering and transport studies, since the leads that transport the electrons to the Weyl semimetals, by their nature, cannot control the momentum of the input electrons. Therefore, previous attempts to associate the topology to scattering matrix mostly remain theoretical.
In this work, vortical phase profile in the momentum space is demonstrated for elliptically polarized electromagnetic waves reflected by a photonic Weyl system, which represents a key signature of the topological nature of the Weyl points and could lead to alignment-free phase plates for vortex beams generation. The nontrivial topology of Chern insulator can not only be revealed by the number of surface states, but also by the scattering matrix S, i.e. the total winding number in 2π of the phase of the eigenvalues of the scattering matrix. It has been rigorously proven that the topological classification of the Hamiltonian and the scattering matrix are equivalent. The scattering matrix method however, are more accessible for investigation in photonics with the readily available optical components from visible to microwave wavelengths to perform the angle resolved measurements.
We further discover a novel type of spiraling guided mode in a hybrid waveguide system formed by the Weyl metamaterial and a metal plate, enabled by the vortical phase profile in the momentum space. we demonstrate that the fermi arcs possessing arbitrary number of windings around the projected Weyl points can be realized in a photonic Weyl semimetal-waveguide hybrid system, revealing a new intriguing physical properties of photonic Weyl systems. The metamaterial-waveguide hybrid system is formed by an air layer sandwiched between a metallic ground plane and an ideal Weyl metamaterial. In such a system, the surface arc states exponentially decay into the Weyl metamaterial, while forming waveguide modes in the thin air layer between the Weyl metamaterial and the confining metallic plate. By measuring the electromagnetic field distribution inside the waveguide, we are able to visualize the Fermi arc’s iso-frequency contour in the momentum space through Fourier transformation, and demonstrate the windings of the Fermi arcs in the vicinity of the Weyl frequency.

Plasmonic, photonic and excitonic states at interfaces govern the optical response of tunable nanophotonic structures such as active metasurfaces, photodetectors and photocatalysts. I will discuss how we can harness plasmonic and excitonic states at interfaces to enable power-efficient phase modulation of active metasurfaces and discuss implications for time-modulated flat optics. Specifically, I will discuss avenues for increasing the power density and power efficiency of plasmonic conducting oxide metasurfaces for temporal phase and amplitude modulation and requirements for time-modulated photonic applications, such as multi-frequency generation and realization of non-reciprocal metasurfaces. I will also report on how time-resolved measurements of plasmonic and excitonic states at interfaces can yield insights about fundamental light-matter interactions in these systems. Specifically, I will discuss i) how electron-hole correlations affect the dynamics of hot carrier relaxation in plasmonic Au/GaN photodetectors and photocatalysts and ii) how time-resolved measurements of excitons generation inform our understanding of carrier dynamics and transport in transition metal dichalcogenides such as WS₂ and MoS₂.

Inverse design algorithms enable metasurfaces and other nanophotonic devices to achieve new functionalities and high efficiencies. We will discuss recent advances in inverse algorithms based on global topology optimization networks (GLOnets), in which global optimization is framed as the training of a generative neural network. We will show how reparameterization methods, in which device layouts are recast in a latent space, can be used to strictly enforce design constraints such as minimum feature size and robustness to fabrication imperfections. We will also show how multi-objective problems, such as those involving the selection of materials from a materials database, can be solved using GLOnets. We apply these concepts to a range of devices, from metasurfaces and thin film stacks to grating couplers, showing how GLOnets can produce nanophotonic devices operating near the physical limits of structured media.

The electromagnetic near-field enables subwavelength applications such as near-field microscopy and nanoparticle manipulation. Present methods to structure the near-field rely on optical antenna theory, involving nanostructures that locally convert propagating waves into confined near-field patterns. We develop a theory of remote rather than local near-field shaping, based on cascaded mode conversion and interference of counterpropagating guided waves with different propagation constants. We demonstrate how to structure at will the longitudinal and transverse variation of the near-field, allowing for distributions beyond the conventional monotonic decay of the evanescent field. We provide an experimental realization that confirms our theory. Our method applies to fields with arbitrary polarization states and mode profiles, providing a new path towards unprecedented three-dimensional control of the near-field.

All-dielectric nanostructures composed of high refractive index host Mie resonances that yield resonant light scattering and enhanced internal field [1-4]. Mie resonances result from the existence of complex eigen-frequencies. In the framework of the multipolar theory, the Mie scattering coefficients have an infinite set of complex eigen-frequencies. The scattering matrix can be expanded with respect to the eigen-frequencies of the nanostructure [5,6] which allows to calculate the internal and scattered fields with respect to the eigen-frequencies of the cavity. A thorough convergence analysis shows that the convergence is faster for the internal field and that the field can almost be fully retrieved by taking into account a very limited number of eigen-frequencies [6]. This theoretical framework turns out to be of high interest to explain the formation of anapoles that correspond to non-radiating current distributions [7]. Anapoles have attracted a strong interest to enhance light matter interactions in subwavelength dielectric nanostructures with applications in second and third harmonic generation or enhanced Raman signal [8]. Anapoles can be well identified in the spectrum of the dipolar and electric Mie coefficient as a strong dip that closely follows a peak. It turns out that the anapole is spectrally close to the maximum of the electric dipolar polarizability. Two eigen-frequencies can be found in this spectral range, with a common real part and a large and a weak imaginary part. In other words, two modes associated with high and small quality (Q) factors will influence the optical response. The low Q-mode does not influence the internal field that only results from the high Q-mode. However, the low Q-mode contributes to the external field. The external field is the result of an interference between three terms: the fields associated with the low and high Q-modes plus an non resonant term. Depending on the phase of these terms, the external field can feature a maximum of a minimum. The important result is that neither the maximum nor the minimum correspond to the real parts of the poles, due to the contribution of the non resonant term. As a result, the spectral location of the anapole does not correspond to the maximum of the internal field that is given by the real part of the high Q-mode [7].
In a second step, a Fourier transform of the modal scattering matrix (S- or T- matrix) provides a time dependant scattering matrix. The impulse response function (IRF) of the S-matrix can be found by considering an incoming field described by a Dirac distribution. This modal IRF allows us to retrieve the outgoing field when the Mie scatterer is excited by any kind of pulse. A case of special interest is the case of a causal sinusoidal excitation field, consisting of a sinusoidal excitation behind a Heaviside. The sharp excitation in the time domain of the dielectric cavity yields a transient field. Once the sharp excitation is over and the excitation driven exclusively by the sinusoidal field, the transient regime vanishes and is replaced by a permanent regime. The modal analysis of this problem reveals that the eigen-modes of the cavity contribute only to the response of the cavity during the transient regime, and not during the permanent regime [9]. This study shows that how quasinormal modes turn out to be excellent tools to unveil the complex time dynamics of dielectric cavities.

Electromagnetic waves are essential to our lives, due to the information they carry, be it through the sense of vision in the visible domain or in our everyday wireless communications within the lower frequency range of the microwaves. Controlling them dynamically is hence of paramount interest. Several approaches have been proposed in the past, using large numbers of independently controlled sources, such as in the concept of phased arrays originally used in radars and now in massive MIMO. Yet these systems are extremely complex and costly, especially for large apertures containing high number of sources, if not impossible to design for instance in the optical domain. In this domain, a paradigm shift has been proposed 10 years ago with the introduction of wavefront shaping, where one does not control the emission of waves, but rather their reflections [1]. This is achieved using large arrays of tunable reflectors, namely spatial light modulators, that are based on micrometric liquid crystal cells or movable micro-mirrors. Due to its simplicity and versatility, wavefront shaping has recently opened avenues for applications, from imaging inside scattering media to communicating with multimode fibers [2].
In this talk I will show that the concept of wavefront shaping can be transposed to the microwave domain using tunable metasurfaces that are controlled by real time electronics. Starting from conventional approaches to beamforming in the radiofrequency domain such as phased array or time reversal, I will first discuss the drastic hardware simplifications that allow the use of tunable metasurfaces alongside algorithms inspired from wavefront shaping [3]. To do so I will introduce the concept of spatial and temporal degrees of freedom that are levers to the physicist to control the propagation of waves. This will allow me to connect the various concepts introduced before.
Further, I will briefly introduce various applications we develop at Greenerwave using wavefront shaping in the radiofrequency domain, from the low frequency of RFID to the millimeter wave range. More specifically, I will first demonstrate how any box such as a fridge or a cabinet can be made smart using wavefront shaping of microwaves, ensuring error-free RFID detection at very low power and cost. Then, I will focus on the millimeter wave range and prove that a leaky cavity dressed with a tunable metasurface can turn a simple electromagnetic dipole into a dynamically reconfigurable antenna whose properties are unique, paving the way to low cost and fully electronically steerable antennas for 5G and satellite applications. Finally, I will explain how the concept can also turn a cheap detect and avoid system into a powerful imaging device. This tool is an ideal complement to camaras and lidars that are currently used in the context of autonomous vehicles, notably in difficult weather conditions.
[1] Focusing coherent light through opaque strongly scattering media. I.M. Vellekoop and A.P. Mosk. Optics Letters 32, 2309-2311 (2007).
[2] Controlling waves in space and time for imaging and focusing in complex media. A.P. Mosk, A. Lagendijk, G. Lerosey and M. Fink. Nature Photonics 6, 283–292 (2012)
[3] Shaping complex microwave fields in reverberating media with binary tunable metasurfaces. N. Kaina, M. Dupré, G. Lerosey and M. Fink. Scientific Reports 4, 6693 (2014).

As car is one the most important transportation for each individual, cooling private vehicles has highly demanded. In particular, the extreme temperature can develop in stationary automobile under the greenhouse effect, when the windows are transparent for incoming solar radiation but opaque for outgoing long-wave thermal radiation. The temperature inside the cabin tremendously increased to approach 60 °C, which is causing human discomfort and car aging problem. Current car cooling methods mostly rely on vapor compression and fluid-cooled systems. However, 28.4% of the total vehicle power is consumed for space cooling and place enormous stress on fuels consumption and CO₂ emission. Passive cooling strategy, which is sustainable alternatives to active cooling technologies, can release heat without energy consumption or pollutant emission.

Recently reported passive radiative coolers have shown outstanding sub-ambient cooling during the daytime. Those coolers are attached on exterior materials, roof, or human skin to draw heat from the periphery through conduction and convection. However, this attractive solution has explicit limitation in thermally sealed space (e.g., stationary automobiles). Since most of conventional radiative coolers only consider emissivity of the side exposed to the sky, they cannot thermally access to inside of the enclosure. Emissivity of the inner side could open the path to inside of the enclosure and allow the trapped heat to escape by increasing the inner radiation flux. Accordingly, strategic design of emission spectra on both sides of the cooler is vital for releasing the trapped heat.

Here, we propose a Janus cooler that acts as a selective emitter on the top side and a broadband emitter on the bottom side. This design effectively draws heat from enclosure because the bottom side can absorb thermal input in a broad spectral range while the top side emits heat to space without disturbing ambient radiation. We first design a Janus cooler comprising of an Ag-polydimethylsiloxane layer on micro-patterned quartz substrate. The induced spoof surface plasmon polariton helps overcome inherent emissivity loss of the polymer, and creates near-ideal selective and broadband emission on the separate sides. Next, we demonstrated several modification methods to improve adaptability as a car roof, such as scalability and mechanical strength, while maintain its optical properties. Consequently, we obtain large-scaled Janus cooler with affordability by replacing quartz substrate to PMMA and PDMS and high elastic modulus and scratch resistance by using harder material.

Last, we theoretically and experimentally demonstrate the remarkable cooling performances of Janus cooler for enclosures. We numerically solved the steady-state heat transfer model of enclosed space to estimate the impact of emissivity of each layer on cooling. The Janus cooler was more effective in drawing heat away as compared to materials with no emissivity and one-directional emissivity. Using Janus bi-directional emission characteristics, the Janus cooler lowered the temperature of a radiative object inside an enclosure by ~4 °C compared to the Conventional radiative cooler in experiments simulating a stationary automobile environment. The significance of the Janus cooler for migrating the high temperature of the automobile is illuminating the importance of emissivity of the substrate itself meanwhile designing the selective emitter in an interpretive way. With the superior ability to passively cool the enclosure, this advanced design leads to comfortability and cost effectiveness in unconscious way, which are critical demands for situation where heat entrance is present and exit is absent.

Recent studies of nanoresonators with strong confinement of light opened the way towards efficient nonlinear processes at the subwavelength scales [1]. All-dielectric nanostructures are particularly promising to enhance the efficiency of nonlinear processes. Such nanostructures can support nontrivial optical modes, including optically induced magnetic dipole resonances, higher-order multipoles, and composite resonances [2]. High field enhancements at the magnetic dipole modes and composite resonances increase the efficiency of nonlinear processes by orders of magnitude. Being assembled in 2D layouts such nanoresonators may form ultra-thin flat optical components – metasurfaces – in which the nonlinear response enables functionalities beyond their linear counterparts. [3]

Here we demonstrate experimentally a novel class of nonlinear metasurfaces with an asymmetric response. We fabricate translucent dielectric metasurfaces that generate images in transmission at third-harmonic frequency. Compared to conventional holograms, such metasurfaces generate completely independent images being illuminated from the opposite sides.

We employ cylindrical dielectric nanoresonators as our building blocks for the metasurface. The resonators consist of two layers of materials with different optical constants: Si and SiN, which introduce asymmetry in the direction normal to the metasurface. The resonators further support two Mie resonances – electric and magnetic dipoles overlapped spectrally. Asymmetric bi-layer design leads to magneto-electric coupling between the two dipole modes, where the coupling constant (positive or negative) depends on the direction of excitation as well as on the resonator geometry. In the linear optics regime, such bi-layer nanoresonators demonstrate identical forward-scattering for the illumination from both sites. However, for the two illumination cases, the near-field distributions are drastically different due to positive/negative magneto-electric coupling between the two Mie resonances hosted within the resonators. For high intensities of light, the nonlinearities of the constituent dielectric materials start playing a role, whereas the different near-field distributions lead to different nonlinear changes of the material parameters when the transmitter and receiver positions are exchanged. Such changes generally have small effects on the light scattering at the fundamental frequency in particular for low-loss materials as well as within the approximations of perturbative nonlinearities and the nondepleted pump approximation. These processes however lead to a striking contrast in the efficiency of the nonlinear generation of optical harmonics.

We employ a set of four dissimilar resonators that allows us to assemble metasurfaces generating independent images for two opposite directions of excitation due to asymmetry in the nonlinear light generation. We fabricate such metasurfaces with standard electron-beam lithography and conduct their optical diagnostics with a tuneable short-pulse laser system.

We discuss connections between asymmetric third-harmonic generation in our metasurfaces and nonlinearity-induced optical nonreciprocity [4]. We believe that the ability to engineer optical interactions at the subwavelength scale beyond the electric dipole type to induce a strong artificial magnetic response and strong magnetoelectric coupling pave the way towards optical nonreciprocity at the nanoscale.

[1] K. Koshelev et al., “Subwavelength dielectric resonators for nonlinear nanophotonics.,” Science 367, 288, 2020.
[2] S. Kruk and Y. S. Kivshar, “Functional Meta-Optics and Nanophotonics Govern by Mie Resonances,” ACS Photonics 4, 2638, 2017.
[3] G. Li, “Nonlinear photonic metasurfaces,” Nat. Rev. Mater. 2, 17010, 2017.
[4] C. Caloz, “What is Nonreciprocity? Part II,” arXiv:1804.00238, 2018.

Giant Kerr-type optical nonlinearity of transparent conducting oxide (TCO) thin films in their epsilon-near-zero (ENZ) region (i.e. the region where the real part of their dielectric permittivity approaches zero), have been reported recently [1,2,3]. Under excitations of plasmon-polariton modes near the ENZ wavelength (?ENZ), effective nonlinear optical properties of these films can be further boosted. Due to ultra-fast response time of these nonlinear processes, the ENZ TCO films could open distinct functionalities to the path to revolutionary nanoscale nonlinear optics and ultrafast on-chip optical applications. In this work, we present a method to engineer the nonlinear refraction coefficients (n2) and the nonlinear absorption coefficients (?) of Al-doped zinc oxide (AZO) ENZ thin films synthesized by atomic layer deposition (ALD) technique. To engineer the effective nonlinear optical properties, we first design the linear absorption and electric field intensity enhancement (FIE) associated with ENZ modes in these AZO nanolayers via control over the ENZ wavelengths and optical losses by ALD deposition parameters such as dopant ratio and number of macro cycles. The dopant ratio determines the material dispersion and ENZ wavelength whereas the marco cycle regulates the film thickness. Since the optical Kerr nonlinearity has cubic dependence on the electric field confined inside the TCO films, the controllable and enhanced FIE of the ENZ mode could lead to an enhancement of the ENZ nonlinearity. AZO thin films with ENZ wavelengths varying between 1400-1600 nm and thicknesses ranging from 55 to 205 nm are fabricated via ALD technique to examine the ENZ nonlinearity [4]. The nonlinear optical properties of the fabricated films are measured using Z-scan and degenerate pump-probe spectroscopy techniques with linearly polarized femto-second laser pulses (pulse duration ~70 fs, repetition rate 1 KHz, peak wavelength ?p tunable between 1200-1600 nm). Experiments are performed for normal and oblique incidences of TE/ TM polarized light pulses and for different pulse peak wavelengths (?p= ?ENZ, ?p> ?ENZ, and ?p< ?ENZ). Wavelength and angular dependence of n2 and ? values of the films are evaluated upon fitting the close and open-aperture Z-scan data. Results suggest that ENZ nonlinear refraction and nonlinear absorption properties of AZO films are enhanced by excitation of the ENZ modes and can be engineered by changing the mode-strengths via control over either the excitation angles or by ALD engineering of material dispersion/ film thickness. Measured values as large as n2(eff)?10-9 mm2/Watt and ?2(eff)?-10-5 mm/Watt are obtained along with approximately an order of magnitude tunability. The temporal dynamics of the nonlinear processes are also measured. The rise and fall-times of the nonlinear processes are evaluated to be sub-picosecond. We believe that giant ultra-fast Kerr nonlinearity of these ALD AZO films in their ENZ region and their controllability will be important for efficient all-optical signal processing. [1] a) M. Z. Alam, I. De Leon, R. W. Boyd, Science 2016, 352, 795; b) L. Caspani, R. Kaipurath, M. Clerici, M. Ferrera, T. Roger, J. Kim, N. Kinsey, M. Pietrzyk, A. Di Falco, V. M. Shalaev, Physical review letters 2016, 116, 233901; c) M. Clerici, N. Kinsey, C. DeVault, J. Kim, E. G. Carnemolla, L. Caspani, A. Shaltout, D. Faccio, V. Shalaev, A. Boltasseva, Nature communications 2017, 8, 15829. [2] A. Anopchenko, S. Gurung, L. Tao, C. Arndt, H. W. H. Lee, Materials Research Express 2018, 5, 014012. (This work is supported in part by the Young Faculty Award Program from Defense Advanced Research Projects Agency (grant number N66001-17-1-4047) and Robert A. Welch Foundation (Award number: AA-1956-20180324)

Nanophotonics has incredible potential for developing new mid-infrared technologies for applications including free-space communications and spectroscopic-based sensing. Phonon-polaritons, electromagnetic modes of a polar material resulting from the coupling of light with optical phonons, are a particularly promising area of mid-infrared nanophotonic research because these modes can enhance light-matter interactions through their ability to concentrate light to nanoscale dimensions. We report on the confinement of phonon-polaritons in calcite, a ubiquitous polar material. We fabricated calcite nanopillars with varying aspect ratio using focused ion beam etching. In the mid-infrared reflection spectra of the nanopillars, we observe multiple phonon-polariton modes with frequencies that depend on the aspect ratio. The modes are supported in the spectral region where calcite's permittivity is hyperbolic, i.e., where the principal components of the permittivity tensor have opposite sign, and the modes are identified as volume-confined modes. Our interpretation of the modes as aspect-ratio dependent volume modes is reinforced by the good agreement with FDTD simulations. The results demonstrate the basic properties of phonon-polaritons in calcite and highlight the material's potential for developing mid-infrared nanophotonic devices.

Indium tin oxide (ITO) has become a very useful plasmonic and nonlinear material for NIR and MWIR applications in that it has highly tunable electric and optical properties, and has exhibited strong optical nonlinearity. In particular, ITO exhibits interesting properties at and around its epsilon-near-zero (ENZ) wavelength. We present here the detailed process parameters of a high temperature reactive sputter process to finely tune the ENZ wavelength throughout the NIR up to the MIR spectrum. Films are characterized by spectroscopic ellipsometry, X-ray diffraction and energy dispersive spectroscopy. Electrical characterization is also reported. Films are designed to be integrated into a NIR perfect absorber that exhibits second harmonic generation.

Gratings and holograms are patterned surfaces that tailor optical signals by diffraction. Despite their long history, variants with remarkable functionalities continue to be discovered. Further advances could exploit Fourier optics, which specifies the surface pattern that generates a desired diffracted output through its Fourier transform. To shape the optical wavefront, the ideal surface profile should contain a precise sum of sinusoidal waves, each with a well-defined amplitude, spatial frequency, and phase. However, because fabrication techniques typically yield profiles with at most a few depth levels, complex "wavy" surfaces cannot be obtained, limiting the straightforward mathematical design and implementation of sophisticated diffractive optics. Here we present a simple yet powerful approach to eliminate this design-fabrication mismatch by demonstrating optical surfaces that contain an arbitrary number of specified sinusoids. We combine thermal scanning-probe lithography and templating to create periodic and aperiodic surface patterns with continuous depth control and sub-wavelength spatial resolution. Multicomponent linear gratings allow precise manipulation of electromagnetic signals through Fourier-spectrum engineering. Consequently, we immediately resolve an important problem in photonics by creating a single-layer grating that simultaneously couples red, green, and blue light at the same angle of incidence. More broadly, we analytically design and accurately replicate intricate two-dimensional moiré patterns, quasicrystals, and holograms, demonstrating a variety of previously impossible diffractive surfaces. Therefore, this approach provides instant benefit for optical devices (biosensors, lasers, metasurfaces, and modulators) and emerging topics in photonics (topological structures, transformation optics, and valleytronics).

Computer-generated holography (CHG) involves iterative numerical algorithms to obtain the phase and/or amplitude profiles needed to physically realize holograms. Metasurfaces consist of arrays of subwavelength nanoresonators that can control the wavefront of light in a desired way. They recently proved themselves to be an effective platform for CGH by surpassing the quality of traditional holograms in terms of image resolution and field-of-view. These metasurface holograms showed prospects not only in imaging and display but also in security applications [1]. In particular, applying metaholograms to anticounterfeiting applications requires not only the technology of encoding multiple pieces of information, but also the manufacturability of highly efficient devices. To meet these complex needs, we have implemented a highly efficient metahologram based on hydrogenated amorphous silicon (a-Si:H) [2]. Compared to well-known titanium dioxide (TiO2) and gallium nitride (GaN)-based metasurfaces, a-Si:H metasurfaces offer easier nanofabrication thanks to a low aspect ratio of 3~5 (cf. TiO2/GaN: 10~15) and is more suitable for mass production using CMOS fabrication methods. In this abstract, I will discuss our efforts in realizing multifunctional a-Si:H metaholograms that can encode multiple pieces of information in a monolayer device for anticounterfeiting applications. First, I will present a spin-multiplexed visible metahologram [3]. A straightforward method for encoding multiple pieces of information in a single metahologram device is using polarization. To obtain significant birefringence for the control and reversal of photon spin, two sets of nanorods are designed, and depending on their orientation, they imprint inversed spin photons along their corresponding geometrical phase. As a result, this allows switching between two different images by simply flipping the handedness of the circularly polarized light on the transmission-type metahologram with 61% diffraction efficiency. Second, I will introduce a direction-multiplexed visible metahologram [4]. This approach is to multiplex two distinct pieces of information onto a monolayer metahologram operating in the forward and backward directions depending on the direction of light incident on the device. Particularly, in this part we will reveal underlying physics of high transmission efficiency (around 75%), which is the antiferromagnetic resonances in the a-Si:H nanorod. Finally, I will propose a wavelength-multiplexed visible/NIR metahologram. The device consists of a-Si:H and gold (Au) metasurfaces in a monolayer device, which is fabricated by the electron beam lithography overlay process. The a-Si:H metasurfaces generate a visible hologram and the Au metasurfaces produce a NIR hologram simultaneously with low crosstalk. I believe our efforts for making a multiplexed metahologram will lead to pragmatic anticounterfeiting applications. [1] I. Kim et al., ACS Photonics 5, 3876-3895 (2018) [2] I. Kim* et al., ACS Nano 11, 9382-9389 (2017) [3] I. Kim* et al., Laser and Photonics Reviews 13, 1900065 (2019) [4] I. Kim* et al., Nanoscale Horizons (2019) (DOI:10.1039/c9nh00460b)

Controlling magnetism and spin as a state variable in devices for high-performance computing has received increased attention as an energy-efficient alternative to processing and storing data with electric charge. This approach circumvents increasing power requirements that accompany Moore's Law scaling of transistors. Deterministic reversal of ferromagnetic domains using circularly polarized, ultrafast light pulses arising from magnetic circular dichroism promises an all-optical approach towards high-performance data storage, non-volatile memory technologies, and quantum computation. However, domain-size criteria haave limited helicity-dependent switching in ferromagnet films, placing restraints on available materials and film thicknesses/coercivities in devices for deterministic magnetization reversal with light. Moreover, using light to control magnetization for high-density recording requires reduction of the size of optically switched areas below the diffraction limit. Here, we show how metasurfaces can locally confine, tune, and enhance these magneto-optical effects, leading to more precise control over magnetic ordering for energy-efficient memory and logic architectures. Metasurfaces amplify both electric and magnetic field strengths while preserving the complex phase angle between them. In particular, periodic nanoscale dielectric disks can enhance the electromagnetic density of chirality, C, controlling magnetic circular dichroism in ferromagnetic films. We fabricated Si nanodisk arrays via sputter deposition and electron-beam lithography on ferromagnetic [Pt (0.7 nm)/Co (0.6 nm)]N multilayers, where N = 1,2, or 3. The Co/Pt films are designed to exhibit perpendicular magnetic anisotropy, with the easy-axis of magnetization out-of-plane. The radii of the Si nanoantennas are varied between 130-190 nm in 5 nm increments, supporting electric and magnetic resonances that overlap and cross with increasing radius; transmission measurements confirm spectral overlap at wavelengths spanning 700-1000 nm depending on Si disk radius. Magnetic circular dichroism of the films, detected by asymmetry in transmission and reflection, is measured using left- and right-handed circularly polarized light for substrates magnetized parallel vs antiparallel to the direction of light propagation at normal incidence. Increasing disk radii modulates the magnetic circular dichroism of the Co/Pt multilayer films with local maxima and minima of increasing amplitude as N is increased from 1 to 3. We find a maximum enhancement of transmission asymmetry of 2x over unpatterned films, correlating with overlap of electric and magnetic dipole resonances as predicted by full-field electromagnetic simulations. To deconvolute near-field enhancements from Fabry-Pérot cavity effects, we vary the separation distance between the ferromagnetic metal layers and Si disks using an SiO2 spacer region with thickness of 0, 30, and 100 nm. This modulation and enhancement in magnetic circular dichroism is translatable to diverse materials including metamagnetic films, chiral magnetic nanostructures, and single-molecule magnets, and can be utilized directly in the design of waveguide magneto-optical modulators and non-reciprocal device elements. Our presentation will also describe ultrafast magnetization dynamics for Co/Pt films patterned with Si metasurfaces showing how they can enhance all-optical control over magnetic domain switching.

Reaching the true potential of nanophotonic materials devices requires the arbitrary control of spectral and angular selectivity in the absorption and emission of electromagnetic waves. To this end, previously investigated design methods for nanophotonic structures and have encompassed both conventional inverse-optimization approaches as well as nascent machine-learning (ML) strategies. However, inverse-optimization processes are computationally-intensive, and image generation-based ML design techniques which can facilitate the generation of complex geometries require exhaustive investigation to produce stable results. Moreover, the possibility of using machine learning approaches to enable forward design (that is, to effectively function as an electromagnetic simulator) has largely been unexplored. Here, we demonstrate that stable and well-established deep learning architectures such as convolutional neural networks (CNN) can effectively enable "forward design" and thereby replicate the functionality of a traditional numerical electromagnetics solver, for a limited class of nanophotonic structure. We then show that the same networks can also be utilized for "inverse design" by leveraging neural network interpretability and visualization methods. To illustrate this capability, we trained a CNN model with 10,000 images of selective mid-infrared thermal emitters and their corresponding absorption spectra. The trained CNN established the relationships between nanophotonic structures and their responses, then predicted the responses of new and unknown designs with over 95% accuracy. After training the CNN, we applied the Shapley Additive Explanations (SHAP) algorithm to the model to determine features that made positive or negative contributions towards specific spectral points, thereby informing which features to create or eliminate in order to meet a target spectrum. These explanations, for the first time, allow us to systematically understand the underlying physical relationship between a given nanophotonic structure and its electromagnetic response. We demonstrate a concrete example of the CNN and SHAP algorithms recovering a previously known analytical relationship between the length of a metal-insulator-metal metamaterial resonator and its resonance wavelength. We also show how this approach can yield insights into why more complex, optimized designs are able to perform well, for instance in terms of their spectral peaks or shapes of their electromagnetic modes. Furthermore, using this strategy, we show that a starting electromagnetic metasurface design can be selectively manipulated to create or remove spectral peaks, thus demonstrating that inverse design can be achieved by exposing the valuable information that is hidden within a neural network. More broadly, our results highlight that emerging approaches to opening the 'black box' of neural networks can yield real physical insights into the relationship between a given nanophotonic structure and its electromagnetic response. Deep learning approaches such as the one we implement can thus enable physics discovery and elucidate the design of broad classes of nanophotonic materials and devices.

Thermal emitters always have absorption losses and hence are open systems. Open systems are non-Hermitian and are best described by non-Hermitian physics. Here, we develop a non-Hermitian description of resonant thermal emitters and thereby take advantage of absorption loss in the system. Further, the non-Hermitian description provides new design tools such as symmetry, phase, and topology to control the properties of thermal emission. We demonstrate such a thermal emitter using coupled plasmonic and photonic resonators. A lossless silicon photonic resonator is coupled to a lossy tungsten plasmonic resonator via a spacer. As the spacer thickness is increased, the thermal emission from the device held at 1000 K exhibits a transition from PT-symmetric to symmetry-broken phase through an exceptional point. The thermal emission from the device breaks the trade-off between emission brightness and spectral selectivity and simultaneously achieves both. Further, we show that the internal phase of resonators is a powerful tool to control the thermal emission from this device. Overall, this work is an unorthodox approach towards designing not only thermal but also other nanophotonic light sources.

Mie-resonators scatter when its size is comparable to the wavelength of the incident light. The scattering modes of Mie-resonator are determined by its size and refractive index. The nanostructures composed of high index material such as silicon can have multipole scattering modes in visible regime. When Mie-scatterers are arrayed, the scattering response can be greatly amplified; the subwavelength arrays of the resonators are called metasurfaces. To properly design metasurface, the hybridization of radiation mode of scatterer and lattice effect, i.e. guided-mode resonance (GMR), must be understood. Nevertheless, the exact mechanism behind the scattering response of metasurface has not been fully addressed so far. A precise understanding of the scattering mechanism may enable to design metasurface as spectral filters, i.e. metasurface structural coloration [1-3].
Herein, we would like to provide the scattering mechanisms behind the hybridization between individual scattering mode and lattice effect, and use them to realize full and grey-scale structural coloration by silicon-based metasurface [4]. The array of silicon resonators play three roles: (i) Diffraction grating for momentum matching to allow lattice coupling, (ii) waveguide of laterally propagating light through lattice, (iii) outcoupler which determines radiation modes to free-space. Through multipole expansion and RCWA simulation, we have successfully verified our design method to modulate bandwidth of lattice coupling and resonant peaks of radiation. For experimental demonstration, we chose two types of silicon for metasurface: silicon(P25) and (P45). Silicon(P25) has a higher refractive index and extinction coefficient than silicon(P45) so that radiation modes coupled to lattice differ from each other under the same geometry. As an application, silicon(P25)- and (P45)-based metasurface has been utilized for full and grey-scale structural colors, respectively. We believe that a solid understanding of the coupling between individual Mie resonators and the lattice resonances can be a strong basis for designing all-dielectric structural colors for applications in advanced imaging, ultrahigh-definition television (UHDTV), and other display technologies.

[1] Kim, I.+; Yoon, G.+; Jang, J.+; Genevet, P.; Nam, K. T.; Rho, J. Outfitting Next Generation Displays with Optical Metasurfaces. ACS Photonics 2018, 5, 3876–3895.
[2] Jang, J.+; Jeong, H.; Hu, G.; Qiu, C.-W.; Nam, K. T.; Rho, J. Kerker-Conditioned Dynamic Cryptographic Nanoprints. Adv. Opt. Mater. 2018, 7, 1801070.
[3] Kim, M.+; Kim, I.+; Jang, J.+; Lee, D.; Nam, K. T.; Rho, J. Active Color Control in a Metasurface by Polarization Rotation. Appl. Sci. 2018, 8.
[4] Jang, J.+; Badloe, T.+; Yang, Y.+; Lee, T.; Mun, J.; Rho, J. Full and grey-scale structural colration using engineered silicon metasurface. (submitted)

Metasurfaces sculpt the phase, amplitude, and polarization of light at subwavelength scales, allowing for unprecedented control of the far field. For applications spanning LIDAR, LiFi, and AR/VR, it is imperative that metasurfaces be reconfigurable, for example able to steer light to particular directions, change their focal lengths, and otherwise react to their environment. However, the subwavelength size of metasurface constituents makes modulation and reconfigurable designs challenging to realize, as the interaction of light and matter is generally very weak. Here, we introduce an electro-optic scheme for reconfigurable and modulatable metasurfaces. Using Lithium Niobate (LNO) as part of the nanoantenna design, we utilize an electric field to change the refractive index, which modifies the phase gradient response. While the electro-optic effect is generally quite weak (r₃₃ ~30 pm/V), we employ high quality factor resonances to increase the environmental sensitivity of the diffraction, significantly reducing the required DC electric field strength. We provide designs in the near infrared based on 1 dimensional arrays of 600 nm tall silicon nanobars on top of a 200 nm layer Lithium Niobate. A design consisting of bar widths of 175 nm, 236 nm, and 350 nm repeated every 1.8 microns leads to beamsteering metasurfaces with 70% diffraction efficiency of 1400 nm light into the +1^st diffraction order. Subtle periodic purturbations in the form of grooves into the largest bar couples normally incident light into guided mode resonances. On resonance, at wavelengths near 1400 nm, the beamsteering efficiency drops from 70% to less than 7%, with an increase in the -1^st diffraction intensity (~20%). In addition to this strong contrast (~10 dB), light is trapped within the silicon bar and leaks into the LNO layer. We capitalize on both of these effects to create an efficient electro-optically tunable metasurface with an applied DC electric field. We show with full-field electromagnetic simulations that applying a DC electric field modifies the refractive index of the LNO, shifting the spectral position of the guided mode resonance. At 1400nm, this manifests in the far field as an obvious change in the diffraction spectrum from beamsteering in the +1^st order to the -1^st order with an applied electric field. Using full device simulations we show that, due to strong modal overlap with the electro-optic material and the high quality factor of the guided resonance, electrically tunable beamsteering and beamsplitting metasurfaces can be achieved with low (less than 15V) operating voltages. Via preliminary simulations, we also demonstrate methods to push this dynamic metasurface design principle to visible wavelengths using lithium niobate, rather than silicon, as the metasurface element. Our work shows the broad applicability of high-Q resonant LNO-based metasurfaces as reconfigurable optical devices, for applications spanning ultra-compact LIDAR, efficient and dynamic focusing elements for ultrathin cameras and displays, and dynamic manipulation of light at the nanoscale.

In recent years, the idea of using structures of subwavelength scale has led to the birth of what is now called “Flat Optics” or “Metasurfaces.” Metasurfaces locally impart phase shifts, amplitude modulations, and polarization changes to the outgoing light, and therefore achieve the resulting wavefront that performs one or more optical functions. As the metasurfaces can be as thin as the wavelengths with which they interact while achieving near-ideal optical functionalities, the reduction of volume and weight compared to those of the traditional refractive optical elements poses a great advantage in applications where footprint and payload are of concern.
Many experimental demonstrations of metasurfaces were designed to work in the visible or near-infrared wavelengths and therefore were required to use optically transparent materials. Their structure dimensions range from tens to hundreds of nanometers and require high-resolution lithography techniques such as electron beam lithography. When such fabrication methods limited size scalability and mass-producibility, other widely used CMOS compatible, wafer-based fabrication techniques, such as deep-ultraviolet (DUV) lithography and nano-imprint lithography (NIL), were also introduced as promising approaches.
In this presentation, we depart from the conventional wafer-based approaches and instead use roll-to-roll (R2R) fabrication methods to realize optically functional metasurfaces. As demonstrated by the many film manufacturing industries, R2R methods provide scalability in size, and throughput, while reducing the per-unit cost for large volume applications. An example of an industrial R2R film format would be rolls of film with lengths on the order of 1 km.
However, the transition from the wafer-based approach to a R2R compatible process has introduced a pair of challenges to overcome. First, as the R2R fabricated products are wound into rolls of film, free-standing nanostructures are undesirable for reasons related to robustness and integration. We therefore design metasurfaces comprising high-refractive index nanostructures embedded in an optically transparent, low-refractive index material for process and end use compatibility. Second, embedding the nanostructures comes at a cost: The overall height of the nanostructures needs to be increased from that of the free-standing metasurface elements to compensate for the refractive index contrast loss and to achieve the same optical functionality. This requires a departure from direct replication techniques of NIL, which have intrinsic limitations on aspect-ratio.
Successful implementation of a R2R process to fabricate a metasurface requires careful reconsideration of the optical design step. The optical design not only needs to consider the optical properties of the constituting materials, but also constraints arising due to process demands, such as sidewall draft angles. In this presentation, we discuss both design and fabrication strategies to create embedded metasurfaces using a R2R process and also show resulting fabricated examples. In detail, we present a R2R fabricated, TiO2-based embedded metasurface with optical functionality in the visible spectral region. We believe that the presented method paves the way to industrial-scale production of metasurfaces.

Optical fiber is a well-established waveguiding technology that has been employed in a wide variety of applications such as data communications, spectroscopy, and imaging, remote sensing, surgeries, seismic activity monitoring, etc. However, the defining attributes of an optical fiber (for instance, mode intensity and propagation loss) are fixed after drawing fabrication due to the dielectric properties of the constituent materials. Zero-index photonics, with vanishing permittivity (epsilon) and permeability values, have been studied for unique and extreme optical properties [1]. Ultrathin epsilon-near-zero (ENZ—the real part of permittivity approaches zero) layer can support plasmon polariton mode with enhanced and highly confined optical field [2]. Recent studies on ultrathin transparent conducting oxide (TCO) ENZ films have reported various optical applications such as enhanced nonlinearity [3, 4] and electrically tunable meta-devices [5, 6] among others.
In this work, we demonstrate for the first time the active nonlinear optical and electrical control of hybrid ENZ resonances in thin aluminum-doped zinc oxide (AZO) film integrated onto D-shaped optical fiber. The AZO film is deposited on the polished side of the fiber via atomic layer deposition (ALD) and possesses ENZ property in the near-infrared regime. The guided core mode of unpolished fiber is launched at the input. At the D-shaped portion of the fiber, the evanescent optical field interacts with the AZO layer on the polished side, coupling to the ENZ mode in the ultrathin film which can be detected at the output as a dip in the transmission. Meanwhile, a pump beam from an ultrafast femtosecond laser at the ENZ wavelength is incident on the AZO film from the air. The enhanced pump field due to the ENZ effect inside the AZO layer induces enhanced Kerr nonlinearity, leading to the modulation of complex refractive index and the shift in the ENZ resonance of the coupled fiber mode which is detected at the output as modulated transmission. As a result, the polarization-dependent propagation loss of AZO-coated fiber can be tuned with modulation strength of ~3.5 dB/mm near the ENZ wavelengths with the fs laser intensity of 150 GW/cm². On the other hand, due to the tunable optical properties of the AZO film, the mode propagating in the ENZ optical fiber can be dynamically manipulated by introducing the AZO-HfO₂-Au gate structure and applying bias. Our preliminary results show that the transmission dip of the hybridized epsilon-near-zero resonance redshifts ~20 nm with an applied bias of 5 V. The dynamic control of optical fiber modes by hybridization with ENZ resonance offers an alternative way of manipulating the ENZ property and functionality of optical fiber, which can find applications in tunable attenuator, polarization-controller, and other active optical fiber devices.
This work was supported in part by the Air Force Office of Scientific Research (AFOSR, Award number: FA9550-19-1-0274), Defense Advanced Research Projects Agency (grant number N66001-17-1-4047), and Robert A. Welch Foundation (Award number: AA-1956-20180324).
References
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At present deep space exploration is limited by transit time and overall mission costs. Conventional spacecraft make use of chemical or electric engines, which have fundamental limitations on the possible speed one can achieve. Light driven propulsion offers a conceptually different approach to space travel. Without a need to carry propellant and power on board, spacecraft propelled by the forces of radiation pressure can attain very high velocities, potentially close to relativistic speeds. In this talk we will discuss our recent work on solar and laser driven spacecraft propulsion.

Specifically we will discuss the use of high power laser radiation for space exploration within our solar system. We outline key dynamical equations and solve for possible trajectories. We show that with realistic systems parameters month long planetary missions are possible. We then discuss laser requirements, such as power and aperture. And show that relatively moderate power lasers (~1MW) may be utilized. Next we also study materials and photonic designs. We analyze a tradeoff between mass, reflectance and a need for a radiative cooling. Finally, suggest a range of materials and designs that can yield a desired performance.

In the talk, we will also touch upon the use of solar radiation pressure to propel a spacecraft. We show that solar sails with high area-to-mass ratio may be propelled to >30 AU/year velocities (i.e., more than 8 times faster than the Voyager – fastest space probe we’ve ever built). Achieving such velocities will pave the way to a new era of affordable and scalable deep space exploration. At the same time such velocities are achieved by a very close perihelion slingshot, where the spacecraft may face extreme conditions. We perform a feasibility study. Specifically, outline conditions needed for maximizing excess velocity and, hence, minimizing mission time, while at the same time ensuring spacecraft durability over the course of the mission. Our estimates show that sails based missions with >30 AU/year are feasible.

We present a study of room-temperature photoluminescence (PL) of MoS₂ monolayer flakes on TiN thin films. The TiN films of several thicknesses are grown on sapphire substrates by molecular beam epitaxy (MBE) [1] and magnetron sputtering [2]. The permittivity and thickness of TiN films are obtained by spectroscopic ellipsometry that shows the presence of a region of near-zero permittivity (ENZ) in visible wavelengths. Monolayer MoS₂ flakes, grown on silicon substrates using chemical vapor deposition, are transferred onto TiN and sapphire substrates using the dry-stamping method [3]. Before transferring MoS₂ monolayers, the TiN films were coated by a thin layer of aluminum oxide (Al₂O₃ spacer) using atomic layer deposition to avoid PL quenching due to metallic TiN.
PL spectra are obtained using a confocal laser scanning microscope with 405, 445, 488 and 561 nm excitation wavelengths. PL signal has neutral A- and B-exciton peaks and charged A^- trion peak. The observed PL is stronger for the MoS₂ monolayers on TiN than sapphire substrates for all used excitation wavelengths. The PL enhancement factors are measured as the ratio of areas under the PL peaks for TiN and sapphire substrates and calculated using a luminescence modelling method based on the dyadic Green’s functions [4-6]. A good agreement is observed between measured and calculated enhancements. For small Al₂O₃ spacer thickness, the PL enhancement has the maximum at the excitation wavelength that matches the ENZ wavelength of TiN films. An 8-fold PL enhancement is observed for a thicker spacer due to a positioning of MoS₂ monolayer in the anti-node of the standing wave above the TiN film. Larger PL enhancements are reported for MBE-grown than sputtered TiN films because of the lower optical losses of single-crystalline TiN films.
We also calculated absorptance and electric field intensity enhancement in MoS₂ monolayer as a function of the excitation wavelength. The increase in absorptance and field enhancement around the ENZ wavelength of TiN films is observed. Thus, the ENZ substrate enhances absorption and electric field of light excitation in MoS₂ monolayer and hence enhances its spontaneous emission. This study will enrich the fundamental understanding of spontaneous emission on ENZ substrates that might find applications for the development of advanced nanophotonic light sources.
This work was supported in part by the AFOSR-AOARD (Award number: FA2386-18-1-4099), Robert A. Welch Foundation (Award number: AA-1956-20180324), and the Ministry of Science and Technology in Taiwan (Grant No.: MOST 107-2923-M-007-004-MY3).

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In the last two decades, several technological breakthroughs have been closely linked to material discovery and development. Just as development of gallium nitride, high purity silica, and high-k dielectrics provided the foundation for the progress in solid state lighting, optical fibers, and integrated circuits, near-zero-index materials (NZI) have great potential to drive advancements in nanophotonics, nonlinear optics, and quantum optics. In addition to exhibiting enhanced light-matter interaction due to slow light effects, these materials offer excellent electrical and optical tunability. Due to these unique properties, NZI materials have already led to demonstrations of dynamic metamaterials, ultrafast photonic modulators, and extreme all-optical processes. Among a wide range of NZI materials explored for these applications, Al doped ZnO, which has a vanishing index in the telecommunication bands, is particularly attractive combining high electrical conductivity and transparency in the visible range, well-established ZnO fabrication technology, high solubility limits of Al, and the abundance of Zn in nature. To realize the potential of ZnO as a material platform for NZI photonics, there is a need for scalable deposition methods yielding high-quality material. Atomic layer deposition (ALD) is a versatile technique due to its ability to provide conformal, uniform, and pin-hole free thin films with sub-nanometre thickness control over large area at low deposition temperatures. However, NZI films grown by ALD have typically suffered from poor optical quality, exhibiting reduced free-carrier density and higher loss when compared to those grown by other methods.
In this work, we overcome the limitations impeding the deposition of high quality ZnO:Al by improving the doping efficiency (ratio of electron concentration to total Al content in the matrix) of the ALD method from 13% commonly reported in literature to 54%. As a result, we demonstrate films with low losses (ε" < 0.35), high conductivity (ρ ~2.8×10^-4 Ω-cm) and sufficiently high carrier density (8.5×10²⁰ cm^-3) to enable Al doped ZnO films with NZI regions tunable across both O and C telecommunication bands, for the first time. This significant material improvement became possible by moving away from the conventional ALD process of depositing doped films in a “digital” manner, wherein one dopant layer of AlO_x is incorporated periodically into the ZnO matrix and Al:ZnO is formed by the out-diffusion of Al into ZnO. Intuitively, such a process is very inefficient and leads to the creation of an inhomogeneous film where dopant atoms do not contribute to the free carrier density, but rather form clusters that increase carrier scattering and reduce the metallicity. However, moving away from the surface saturation condition during the Al precursor pulse, which is characteristic of the conventional ALD approach, leads to enhanced Al incorporation in each dosing cycle. This has been realized by utilizing H₂O/DEZ/TMA pulse sequence to increase competition between Zn and Al precursors (diethyl zinc, DEZ, and trimethyl aluminium, TMA) for the surface sites, thus reducing the amount of Al precursor supplied in each pulse to decrease surface coverage of Al precursor, and by concurrently increasing the substrate temperature to lower the density of OH groups on the surface, which act as active sites. Such growth conditions allowed us to incorporate the dopant cycle 4x more frequently in the growth process (every 0.5 nm, as opposed to every 2 nm for conventional ALD) and ensure a more uniform distribution of Al throughout the film, simultaneously increasing the doping efficiency and reducing density of clusters. Furthermore, this process provides a pathway to improve the electrical and optical properties of other emerging doped binary semiconductors such as doped GaN, CdO, and Ga₂O₃ grown using ALD.

Plasmonic nanostructures can significantly enhance electric and magnetic fields at nanoscale dimensions. Fields localized at nanosized plasmonic nanostructures can be several orders of magnitude higher than the incident light field, and this field enhancement strongly affects light-matter interaction processes in plasmonic nanostructures. Here, we study field enhancement in nanoparticle clusters and periodic arrays and analyze surface plasmon amplification by stimulated emission (spaser) in such nanostructures. We consider both sphere and disk plasmonic nanoparticles and employ full-wave numerical simulations for modeling field enhancement. Planar optical elements with efficient light control at the nanoscale can be designed based on transdimensional photonic lattices that include three-dimensional-engineered nanoantennas supporting multipole Mie resonances and arranged in the two-dimensional arrays to harness collective effects in the nanostructure. Periodic arrays of nanoparticles have gained special attention because of extraordinary lattice resonances in proximity to the wavelength of diffraction, the so-called wavelength of Rayleigh anomaly. Using full-wave modeling, we numerically observe plasmonic modes with even and odd field distributions in nanoparticle clusters and identify spatial regions with nodes and anti-nodes of the field enhancement. We use quantum density matrix (optical Bloch) equations for the spaser, calculate the plasmon amplification, and show its correspondence to the total mode characteristics. Because of the strong field enhancement in the plasmonic nanostructures, amplification of plasmons in nanoparticle clusters and periodic arrays is more efficient than ones in the proximity to a single nanoparticle and can be used an ultra-compact light source in optical devices and medical applications. Acknowledgment. This material is based upon work supported by the Air Force Office of Scientific Research under Grant No. FA9550-19-1-0032.

Titanium dioxide (TiO2) has attracted a lot of attention in plenty of applications varying from batteries, photo/electro-catalysis to optoelectronics and sensors. Although there are many different procedures for synthesizing sound TiO2 layers, methods leading to one dimensional (1D) structures are of interest due to having superior properties. Electrospinning is one of those methods to synthesize 1D mesoporous TiO2 fibers. Having the advantages of high surface area and ease of functionalization, those fibers can be quite beneficial in optoelectronic applications. Here in this study, nanoparticle (NP) decorated TiO2 fibers were formed with a modified electrospinning method and used as an active layer in an electrochromic (EC) device. A coaxial nozzle was used to form a core/shell type of TiO2/NP fibers. Structural and morphological properties were characterized by various techniques, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM) and x-ray diffraction (XRD). Fibers of 198.66±21.47 nm average diameter were confirmed to have the anatase phase of TiO2. The effects of different NPs such as silver (Ag), gold (Au) and copper(II) oxide (CuO), on optical properties were also investigated. The ultraviolet/visible/near-infrared spectroscopy (UV/Vis/NIR) revealed that depending on the absorption wavelength of the decorative NPs used, it is possible to tune the band gap of TiO2/NP final composite structure. Governing effects of the composite structures on electrochemical properties were evaluated in half-cell configuration in which EC layers were cycled in between ±3V. It was observed that the color attained can also be altered with different NPs. For instance, Ag shows a black-brown color while Au yields dark blue and CuO leads to a dark green color under negative bias. In addition, fast switching times below 10 seconds were possible in each TiO2/NP configuration. This is attributed to the faster kinetics of electrospun fibers in redox reactions.

We use surface plasmon polaritons (SPPs) as the foundation for examining two important phenomena. First, SPPs were used to generate energy-tunable hot carriers that can be injected into semiconductors or molecular systems, opening a new pathway to drive photo-induced chemical reactions. [1] Unlike metal nanoparticles that suffer from resonance inhomogeneity and limited spectral tunability, metal films support SPPs that are homogeneous in-plane and accessible by simply tuning an illumination angle in a broad spectral range. We measured light-induced electrochemical currents of a metal/semiconductor heterofilm (Ag/TiO2) and a bare metal film (Au) interfaced with a sodium hydroxide:methanol solution. We observed plasmonic electrochemical currents and extracted a strong photon-energy dependence of the photon-to-carrier conversion efficiency. This phenomenon was described using a Schottky transport model. Secondly, we examined strong optical coupling between the SPP mode and a thin molecular dye film. The dye, methylene blue (MB), is redox active enabling examination of the electrochemical response under strong coupling conditions. Our results demonstrate full modulation of the Rabi splitting, a diffusion-limited redox process, and imaging of molecular adsorption/desorption on the metal surface. All of these results have important implications for chemistries enhanced or enabled by light-matter interactions. [1] Ahn, W., Vurgaftman, I., Pietron, J. J., Pehrsson, P. E., and Simpkins, B. S. "Energy-Tunable Photocatalysis by Hot Carriers Generated by Surface Plasmon Polaritons" J. Mater. Chem. A., 2019, 7, 7015-7024.

Cuprous oxide (Cu2O) is an earth-abundant, p-type semiconductor of interest to photovoltaic and photoelectrochemical applications. Cu2O can be synthesized direclty into its semiconducting phase by electrodeposition, rather than requiring a high-temperature calcining step as is necessary for most other electrodeposited metal oxides. as a result, Cu2O electrodeposition rates can be enhanced by illumination during growth. in recent work, we showed that photoelectrodeposition forms a composite of Cu2O doped by Cu nanocrystals throughout, resulting in a black film with high abosorptivity across the visible spectrum. Continued growth is enhanced even for illumination with energies smaller than the band gap of Cu2O due to the sensitivity of the Cu2O electrodeposition to local temperatures at the electrode-electrolyte interface. Here, we describe experiments to discriminate the local photothermal and photoelectrochemical effects in the photoelectrodeposition of Cu2O. Identifying the role of each effect will lead to more robust control over the composition and structure of Cu2O-based interfacial heterostructures. In addition to standard spectroscopic and electrochemical characterization, we will describe novel experiments that use in situ high-speeed microscopy to unveil the complex interplay between light and heat on the electrodeposition of Cu2O.

Noble metal nanoparticles find applications in a wide range of fields, from (photo)catalysis, to bioimaging, and surface enhanced Raman spectroscopy. One of the most striking characteristics of these nanoparticles is that they can sustain localized surface plasmon resonances (LSPRs), light-driven oscillations of conduction electrons that give rise to strong scattering and absorption of light. These LSPRs can be used to activate a wide range of chemical and physical processes at the metal surface. For example, plasmon resonances have been used to perform the light-driven synthesis of hierarchical metal nanoparticles in solution with high yields and precise morphologies.1,2 The growth of these nanostructures can be linked to the decay of LSPRs via several different mechanisms: 1) plasmonic enhancement of the electromagnetic field at the nanoparticle surface that activates photosensitive precursors, 2) photothermal heating that accelerates kinetically limited reactions following the Arrhenius law, and 3) plasmonically-generated (intraband) and photo-generated (interband) electrons and holes that can catalyze redox reactions. In most plasmon-driven chemical reactions, however, these mechanisms are challenging to disentangle and typically occur simultaneously. Furthermore, the relative weight of their contributions is heavily dependent on the illumination and sample geometries.3 For these reasons, it would be ideal to develop a plasmon-driven process in which all the possible activation mechanisms can be assessed independently. Here, we develop a plasmon-assisted synthesis of Au@Ag core@shell nanorods in which interband “hot” holes are the only driving force of the reactions. Using a combination of extinction spectroscopy, transmission electron microscopy, thermal characterization, and finite-difference time-domain (FDTD) simulations, we can unequivocably exclude any effects due to photothermal heating, near-field enhancement, and intraband electrons and holes. In our experiments, we compare the growth rate of Ag shells while exciting either the transverse or the longitudinal resonances of Au nanorods using 532 nm and 730 nm continuous wave lasers, respectively. The laser powers in the two cases are modulated to obtain the same total amount of absorbed optical power in the solution and therefore the same magnitude of photothermal effects. We find that the Ag shell growth rate under 730 nm irradiation equals the one in the dark and therefore shows no plasmonic enhancement. On the contrary, the rate is strongly enhanced under 532 nm irradiation, although such enhancement slowly decreases and becomes negligible as the Ag shell approaches ~5 nm in thickness. The striking rate disparity under identical total absorbed power demonstrates the non-thermal nature of the enhancement mechanism. Furthermore, using FDTD we show that the intensity of the scattered electromagnetic fields at the nanorod surface increases with increasing Ag shell thickness. Near-field enhancements cannot therefore explain the observed time evolution of the reaction rate when using 532 nm light. Finally, the absence of any enhancement rate under 730 nm irradiation, which corresponds to an energy just below the interband transition of gold, shows that d-band “hot” holes in gold are the main responsible for the effects observed under 532 nm irradiation. Strikingly, these holes have typical mean free paths of a few nanometers, corresponding to the Ag shell thickness at which plasmon-driven effects are quenched. Understanding the mechanism of plasmon-activated chemistry at the surface of metal nanoparticles is of paramount importance for a wide range of applications, from the rational design of novel light-assisted nanoparticle syntheses to the development of plasmonic nanostructures for catalytic and therapeutic purposes. 1. Jin, R. et al. Science 294, 1901–1903 (2001) 2. Zhai, Y. et al. Nat. Mater. 15, 889–895 (2016) 3. Kamarudheen, R. et al. ACS Nano 12, 8447–8455 (2018)

Sustainable and industrial photochemical and photocatalytic reactions, including artificial photosynthesis, organic synthesis, and pollutant degradation, require nanoparticle catalysts in liquid environments for physically significant reactivity. Plasmonic nanostructures have emerged as materials which promise enhanced efficiency and selectivity at milder conditions compared to traditional metal catalysts. However, developing more efficient plasmon catalysts requires investigating them at the single and sub-particle level, monitoring their properties at particular spatial and temporal coordinates throughout a reaction. This approach both prevents the averaging of plasmonic responses across an ensemble of particles as well as enables the capture of variations in plasmonic and catalytic behavior across and within a single particle. In-situ transmission electron microscopy (TEM) has seen recent use in vacuum and gaseous environments to obtain real-time information of plasmonic nanostructures and monitor plasmon-driven reactions at sub-particle spatial and millisecond time resolution. Here, we bring these techniques to aqueous- and organic-phase environments via in-situ liquid TEM. We perform combined scanning TEM and monochromated electron energy loss spectroscopy (EELS) on a model system of individual Au nanoparticles, measuring and comparing their plasmon resonance energies for distinct chemical environments. We use a variety of particle shapes ranging from 30–100 nm in size, synthesized both through colloidal (reduction of AuCl4 in CTAB solution) and lithographic (e-beam lithography and metal lift-off) methods onto a specially designed liquid cell chip. Two such chips are used to enclose a small pocket of liquid for stable imaging and spectroscopy in the TEM. Selecting particles located in thinner regions of liquid (< 100 nm thick), we demonstrate that plasmon resonances can be measured with sub-second total acquisition times while maintaining sufficient signal-to-noise ratios, circumventing the dominant liquid background signal and without any interfering electron-induced side reactions. Furthermore, we observe that enough spectral resolution is maintained to differentiate resonances corresponding to spatially dependent visible modes in each shape of particle. For example, across colloidally synthesized Au rods bound to a SiN membrane, we measure longitudinal and transverse resonances which normally lie at 700-900 nm and 500-600 nm in vacuum but experience a 30-50 nm and 5-10 nm redshift, respectively, in water. Likewise, for lithographically fabricated triangular prisms we map edge and tip modes around 490 and 520 nm, respectively, in vacuum, and we observe a similar redshift to that of the rods when immersed in water. These trends quantitatively agree with calculated results from BEM methods for the geometries examined. We then change the solvent in the liquid cell via flow tubing, showing that Au particles experience a shift in plasmon resonance arising from the refractive index change of varying liquid environments vs. vacuum, agreeing with simulations. This work provides the foundation for in-situ TEM characterization of plasmonic catalysts for critical reactions in solution such as biosynthesis and water splitting.

Ultrafast laser spectroscopy is a versatile technique that is commonly used to both understand and manipulate carrier scattering mechanisms in semiconductors. We report the observation of excited carrier relaxation times in GaAs using optical pump mid IR probe spectroscopy. We find a sharp change in relaxation times occurs at the intervalley transitions of GaAs. Our results also show the relaxation times becomes faster with increasing photoexcitation fluence. These results provide direct evidence that excited carrier decay at greatly different rates based on their energy relative to the conduction band minimum. These findings provide additional insight into the energy-dependent nature and rate of phonon emission during electronic relaxation.

Automated particle-tracking techniques are important for studying biological processes ranging from tissue deformation to cellular uptake. Existing tracking methods, however, only focus on visualizing the translational motion but not rotational motion, which can also provide critical information on cellular activities such as membrane protein diffusion. The development of next-generation particle-tracking platforms that can characterize rotational dynamics requires concurrent advances in imaging strategies and computational analyses.
This presentation reports a deep-learning rotation-tracking platform based on 3D nanoparticle imaging probes and differential interference contrast (DIC) microscopy. We obtained libraries of DIC images at each sampled nanoparticle structure and orientation for training the prediction models based on convolutional networks. These models predicted the in-plane orientation of gold nanorods based on simulated and measured DIC images with an accuracy only limited by the inherent resolution of the technique. Using low-symmetry gold nanostars as imaging probes, the method could resolve particle rotations in the full 0-360° range by accounting for DIC patterns at multiple wavelengths simultaneously. This deep-learning method could make consistent orientation predictions that are insensitive to varying imaging backgrounds. Finally, we demonstrated the simultaneous prediction of in-plane and out-of-plane orientations from a 3D nanostar by analysis of DIC images collected at multiple wavelengths.

We introduce cathodoluminescence holography to directly retrieve the phase of light scattered from nanoscale objects. We use 30-keV electrons to coherently excite with a nanoscale spatial precision surface plasmon polaritons (SPP) that subsequently scatter from either nanoholes made in a Ag film or single-crystalline Ag nanocubes. We use transition radiation excited at the electron impact position as a broadband reference to create a far-field interference pattern that is collected in both angular and spectral domain. From the holography data we directly resolve the SPP dispersion and probe the phase of resonant coupling of SPPs to individual nanoscatterers. From the far-field wave fronts we derive the full basis of scattering dipoles using a modal decomposition analysis and unravel the relative strength and phase of all SPP-excited dipolar modes. We also show that scattering from a SPP off a helical nanoaperture leads to chiral light in the farfield. We observe experimentally a topological charge of -1. Cathodoluminescence holography takes advantage of the nanoscale electron excitation source and can find a broad range of applications to probe surface wave scattering by individual nanoscatterers.

In recent years, infrared (IR) optical systems have been increasingly used in imaging and sensing applications, as evidenced by a surge in demand for infrared sensors during the COVID-19 pandemic. However, cost and ease of fabrication of germanium-based optical elements used in IR systems remains a major barrier to widespread adoption, particularly in low-resource settings.

In this talk, we will demonstrate spectrally selective plasmonic surfaces as scalable and low-cost IR optical components that absorb ultraviolet-to-shortwavelength IR radiation, while showing a specular behavior in the longer infrared wavelengths. These surfaces can be conveniently manufactured at large scales, and alone or in combination with other materials, used as reflective or transmissive IR optical components.

The talk will present FDTD simulation results that validate the concept, and present experimental demonstration of the optical capability (selectivity, specularity and integrability into optical systems) using a specific plasmonic design. Since the fabrication, physical operation and application of our designs span different fields, a short, interdisciplinary overview of related challenges and opportunities will also be presented.

Acknowledgments: Jyotirmoy Mandal was supported by Schmidt Science Fellows, in partnership with the Rhodes Trust.

Metal electrodes are playing an increasingly important role in controlling photon absorption and in promoting optimal light management in thin-film semiconductor devices. For organic optoelectronics devices, it is challenging to control the surface of the metal electrode to promote good light management because metal electrode deposition is used as the last step in the conventional device fabrication approach. However, an inverted fabrication approach that builds the device on top of a metal electrode, makes it possible to control the morphology of the metal surface, independently of the organic semiconductor active layer, to achieve a variety of photonic and plasmonic behavior useful for optoelectronic devices. Silver (Ag) is one of the most suitable metals for fabrication of nanostructured electrodes with plasmonic behavior (i.e., plasmonic electrodes) because of its low parasitic absorption loss and high reflectivity.

In this work, we study the physical and optical characteristics of Ag nanoparticle (AgNP) aperiodic plasmonic metasurfaces. Then, we investigate the photonic and electrical behavior of the AgNP plasmonic metasurfaces when interfaced with thin films of the organic semiconducting polymer, poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT), using the inverted fabrication approach. The luminescence quantum yield of F8BT thin films increases from 29% on planar silver up to 66% on AgNP metasurfaces due to the Purcell Effect and the improved extraction of emission coupled to surface plasmon polariton (SPP) modes. In particular, we show that plasmonic enhancement can overcome ohmic losses associated with metals and metal-induced exciton quenching. According to the current-voltage characteristics of F8BT hole-only devices with and without plasmonic metasurfaces, we conclude that AgNP plasmonic metasurfaces have comparable electrical behavior to planar metal electrodes while having superior light management capability.

In quantum mechanics, Hamiltonians exhibiting real eigenvalues are often Hermitian. However, non-Hermitian systems that respect parity-time (PT) symmetry can also possess real eigenvalues. PT-symmetric Hamiltonians display unconventional behavior, such as the coalescence of eigenmodes at exceptional points. This unconventional behavior allows optical devices to take advantage of symmetry, topology, and phase, enabling new functionalities in ultra-sensitive sensing and lasing. There are two approaches to satisfying PT-symmetry in optics: active and passive. Active PT-symmetry requires a system with balanced optical gain and loss. Because of the difficulty of adding gain in nanophotonic systems, passive PT-symmetry is an ideal method for demonstrating PT-symmetry at the nanoscale. In comparison, passive PT-symmetry requires both nearly zero and large optical losses. While plasmonics has enabled many new optical devices, there have been no demonstrations of PT-symmetry in all plasmonic systems due to the large optical losses of plasmonic materials, which limit loss asymmetry. Here we will present an experimental demonstration of an all plasmonic system demonstrating passive PT-symmetry. To achieve this, we engineer a coupled plasmonic antenna system exhibiting large loss asymmetry. The system undergoes a PT phase transition as the coupling, or separation, between the antennas is varied. At high coupling, the system is in a PT-symmetric phase with two resonant absorption peaks. With decreased coupling, the peaks coalesce at the exceptional point. Further decreasing the coupling results in an optical phase transition to a broken PT-symmetric phase with a single resonant absorption peak. Using our nanophotonic PT-symmetric system, we will conclude with a discussion on guidelines for implementing PT-symmetry in nanophotonic devices.

Strongly correlated materials could greatly respond to a stimulus due to their collective behavior. Thus, strongly correlated materials are well studied in the past for their tunable properties, especially electrical and thermal. However, their optical properties remain to be explored. In this work, we study the optical properties of 1T-TaS2, a quasi-2D material supporting charge density waves (CDW). 1T-TaS2 is the only material to exhibit a nearly commensurate CDW phase at room temperature and its electrical and thermal properties are shown to exhibit a large change with temperature, DC and AC biases, pressure, and light. Here, we study the optical properties of 1T-TaS2 under DC and AC biases, different intensities of optical illumination, and at different temperatures. Under all these conditions, we observe a unity-order change in the refractive index of thin films of 1T-TaS2. We hypothesize that the stacking of CDWs in this material changes with stimulus resulting in its optical constants to change. ARPES and STM measurements are underway to verify the hypothesis. Our discovery of the large tunability of optical constants of 1T-TaS2 presents opportunities in studying light-matter interaction in strongly correlated materials and its application to nanophotonic devices.

The study of band structures of different types of Metal-Organic Frameworks (MOFs) is indispensable for the advancement of nanoelectronics. We designed a novel approach to predict and tailor the band structure of MOFs nanocrystals in a one-dimensional periodic array through the variation of structural and materials parameters. From the Bloch condition of the Schrodinger equation, we could predict the amplitude of potentials, ? and ? for periodically arranged MOFs only in one-dimension using experimental values of dielectric constant, absorption and emission peaks of MOFs. Predicting energy band structures of the one-dimensional array of an isoreticular MOF, IRMOF-8A using a theoretical method combined with experimental values of dielectric constants and amplitude of potentials provide insight into electronic band energy to tailor optoelectronic properties of MOFs and understanding their energy transfer capabilities with respect to the different ligand type. The dielectric constants of three different IRMOFs synthesized from the same metal precursor (Zn2+) and three different organic ligands with varying conjugation length were predicted from this novel approach. Band gap depends on the amplitude of potentials, which is a function of the conjugation length of the ligand and its dielectric constant. Thus, the effective conjugation length of the ligand in MOFs have a significant impact on the jumps between two allowed bands in energy band structures of the 1-D array of MOFs. This design method can widely applicable for predicting band energy for any designed MOFs and serves as a versatile tool to tailor and predict the electronic properties of MOFs with different ligand units.

Metal nanostructures are well known for various ultra-sensitive spectroscopic applications because of their ability to concentrate and enhance the local electromagnetic field due to a phenomenon called localized surface plasmon resonance (LSPR). The intense light localization, also known as "hotspot", becomes more pronounced near a narrow plasmonic gap. In this work, we report tailored hybrid metal nanostructures for LSPR sensing and surface enhanced Raman spectroscopy (SERS) of serum albumin. Using a combination of physical vapor deposition and thermal annealing, different hybrid metal nanostructure configurations have been fabricated, which create well defined arrangements of hotspots. The hybrid metal nanostructures have multiple resonance peaks, which demonstrate the capability of the identification of ultra-low differences in refractive index (0.001) when combined with machine learning algorithms. A detailed FEA simulation of these plasmonic nanostructures provides a quantitative estimate and the spatial distribution of hotspot closely match our experimental observations. The simulations also help us understand the various interactions between the nanostructures via their near field. Finally, these structures also show potential for sensitive label-free SERS detection. Overall, our study explores an integrated multimodal biosensing platform featuring LSPR combined with machine learning and surface enhanced Raman spectroscopy (SERS) for quantitative determination of concentration and molecular fingerprinting.

We investigate theoretically the unidirectional optical scattering properties of random monolayer particle films composed of dielectric particles that support overlapping electric and magnetic dipoles. The particle films exhibit dramatic reflection and transmission behavior which are manifest in the forward-to-backward scattering ratio of the nanoparticle building block. We also show that these films do not exhibit polarization dependent scattering and lack a traditional Brewster's angle. Our theoretical analysis uses a generalized version of Mie theory, which accounts for particle coupling effects, and was validated by full-wave simulations. For fill fractions up to 40% the average scattering response of an embedded particle is shown to accurately predict the behavior of the random film, and the overlap of the electric and magnetic dipole resonances are found to be robust to effects of particle coupling. Continuing research will extend this analysis to multi-layer particle films. This work indicates potential for the design of optical filters and anti-reflective coatings using single materials, without the need for self-assembly.

We use single particle spectroscopic imaging and statistical analysis to assess the plasmonic enhancement of NIR-to-visible upconversion luminescence (UCL) from single ?-NaYF4:Yb3+:Tm3+ upconverting nanoparticles (UCNPs) supported on substrates consisting of random arrangements of Ag nanowires (NW) and Au nano-cavity arrays. We measure both apparent luminescence enhancement, which is power dependent due to the nonlinear kinetics of energy transfer upconversion (ETU), and the excitation enhancement, defined as the ratio of the excitation intensity needed to produce a given UCL emission, divided by the equivalent power needed to produce the same emission on the plasmonic substrate1. By examining the effects at the single particle level, and accumulating a statistical sampling of single particle emitters, both on and off the plasmonic substrates studied, we can eliminate the effects of particle fluctuations on the apparent UCL emission enhancement and map out the statistical distribution of excitation and luminescence enhancement on the plasmonic substrates. The distributions obtained are reproduced by Finite Difference Time Domain (FDTD) calculations of the fields near the plasmonic substrate, assuming the variations are due to variations in UCNP coupling to the plasmonic substrate based on an exponential decay with distance. We compare these results to statistical analysis of ensemble measurements made of Er3+ doped UCNPs supported on random Ag NW substrates2 and both wide field and confocal spectroscopic imaging of single UCNPs on and off the plasmonic substrates. (1) Fisher, J.; Zhao, B.; Lin, C.; Berry, M. T.; May, P. S.; Smith, S. Spectroscopic Imaging and Power Dependence of NIR to Visible Upconversion Luminescence from NaYF4:Yb3+,Er3+ Nanoparticles on Nano-Cavity Arrays. J. Phys. Chem. C 2015, 119, 24976–24982. (2) Hor, A.; Luu, Q.; May, P.; Berry, M.; Smith, S. Non-Linear Density Dependent Upconversion Luminescence Enhancement of ?-NaYF4: Yb3 : Er3 Nanoparticles on Random Ag Nanowire Aggregates. MRS Adv. 2016, 1, 2677–2682.

The optimization of nanostructures for a specific application is a rather complex task, often akin to shooting in the dark with the hope of hitting the right target, viz. the best performing nanostructure. This difficulty stems from the usually large number of experimental parameters that can be changed in a nanotechnology process. This is the case for example in the fabrication of plasmonic nanostructures for surface enhanced Raman scattering (SERS): the choice of metal, its thickness, the shape of the nanostructures as well as their distribution on the substrate are all parameters that determine the overall Raman enhancement. Even for a relatively simple design, exploring all possible combinations of parameters is a prohibitive task both in terms of nanofabrication time and costs. In that contribution, we focus on the large scale fabrication of plasmonic nanostructures using solid-state dewetting and demonstrate how a full factorial design technique can efficiently optimize the process. In that approach, a target response for optimization and two specific values for the n fabrication parameters are first chosen, leading to an initial set of 2n experiments [1]. Here, as experimental parameters we consider the gold film thickness, the annealing temperature and time, and the heating rate; with the objective of maximizing the intensity of the SERS signal, specifically the 1363 cm-1 vibrational band of Rhodamine 6G. The corresponding 16 experiments provide a model of the dependency of the Raman enhancement on the four experimental parameters considered here. In the classical optimization procedure by full factorial design, this dependency is chosen linear. The set of experimental parameters that is obtained in this first fit of linear model does not necessarily lead to the strongest Raman enhancement and another set of experiments may be required. However, the full factorial approach also provides the direction in the parameter space in which one needs to move these parameters to reach a better solution. After such a solution is found, the linear model may need to be refined using second order functions to clarify the optimum region. This is usually done with the central composite experimental design [2]. The optimization process for the fabricated samples is documented in detail by studying the morphology and optical properties of each sample by scanning electron microscopy and UV-Vis spectroscopy. The gold nanostructures we fabricated this way have typical dimensions in the 50 nm range, which is in good agreement with data reported in the literature [3]. Furthermore, the optimized plasmonic substrate can be used to identify non-resonant bioorganic macromolecules such as bovine serum albumin. This work reveals an effective approach to optimize nanofabrication processes for large-scale and cost-effective production of uniform and highly sensitive SERS substrates. [1] Leardi, R. Analytica chimica acta 2009, 652(1-2), 161-172. [2] Box, G. E., & Wilson, K. B. J. R. Stat. Soc. Series B Stat. Methodol. 1951, 13(1), 1-38. [3] Krug et al. J. Am. Chem. Soc. 1999, 121, 9208-14.

As global temperatures rise, permafrost in the Arctic is thawing, stimulating increased release of methane, a key greenhouse gas. Accurate measurements of the dissolved methane concentration in seawater and freshwater are important for finding and quantifying the release of natural seabed and Arctic methane seeps to better understand how these sources are contributing to increasing global methane levels. In surface waters, the methane concentration can be as low as 3-10 nM (atmospheric equilibrium) while concentrations as high as 800-1000 nM are found in methane saturated deep seawater and near thawing permafrost. To measure the concentration of dissolved methane, changes in the refractive index (RI) of polymers functionalized to selectively trap methane molecules can be measured via an optical readout mechanism. However, the range of the RI change is very narrow, from 1.41198 to 1.41358 for atmospheric to saturated methane concentration levels ranging from 0 nM to 300 nM, which requires the use of highly sensitive optical sensors. This paper analyzes and compares three simple and low-cost planar nanophotonic and plasmonic structures as optical transducers for measuring refractive index change of polydimethylsiloxane (PDMS) polymer films doped with cryptophane-A molecules, which selectively trap methane. These structures include (i) a Bragg reflector with a defect layer, (ii) a hybrid plasmonic-photonic multi-layer stack that supports a Tamm plasmon mode, and (iii) a hybrid plasmonic-photonic stack that acts as a magnetic mirror, and can be used in a simple intensity-based measurement scheme with low cost light sources and detectors. Through numerical simulations, we evaluate the sensitivity of the proposed structures in both the angular-resonant-mode shift readout mode and in the reflectance intensity readout mode and compare them to the standard surface-plasmon-polariton-mode Spreeta sensor as a reference. Our results show that the planar Bragg reflector with a defect layer exhibits the largest intensity response and is the most promising design for a low cost and robust photonic chip for dissolved methane sensing. A practical implementation of this chip with a simple intensity-based measurement scheme is proposed. Integration of this planar structure into a small, portable, and low-cost dissolved methane sensor offers a way to make climate monitoring more widespread and accessible to researchers. The structures evaluated here all show promise for other chemical and biological sensing applications that require monitoring very small RI changes. Significantly, each structure has been designed to support resonant modes in both s and p polarizations, which allows enhanced sensitivity by calculating the ratio of the spectral responses of orthogonal polarizations, similar to the complex reflectance ratio measured with ellipsometry. Importantly, the response in both polarizations only needs to be monitored in the reflectance intensity readout mode, avoiding the use of expensive and large ellipsometry equipment.

Noble metal nanoclusters containing dozens to hundreds of metal atoms are of great interest because of their unique optical properties. Classical electrodynamics fails to describe the optical properties of clusters smaller than a few nm, so quantum mechanical models are needed to describe these clusters. However, it is challenging to identify which features of the computed excited states indicate plasmon-like character, particularly in a way that is generalizable to the ligand-protected clusters that are commonly studied in experiments. We present an analytical method to identify plasmon-like excited states using three quantifiable indicators that must be considered in combination: (1) large superatomic character, (2) large collectivity among single-particle excitations, and (3) large additivity of contributions of these single-particle excitations to the transition dipole moment. Visualizing these three indicators on a single plot enables rapid classification of hundreds of excited states into plasmon-like, collective, single-particle, or interband categories, or as intermediate between these categories. This method is used to identify excited states with plasmon-like character in both bare and ligand-protected Ag clusters at the TDDFT level. Using these three nearly orthogonal indicators in combination provides more information than any one criterion can provide.

Alternative plasmonic titanium nitride nanoparticles (TiN) are attractive photothermal agents because of their ability to strongly absorb a broad spectrum of light ranging from visible to near-infrared and convert that into heat. Additional advantages of low cost and stability of these refractory nanomaterials make them attractive for different applications ranging from photothermal cancer therapy to solar energy mediated catalytic reactions. In this work, we are developing a method to obtain a stable and monodispersed TiN nanoparticles solutions, which is an important prerequisite for most of their applications. Moreover, our goal is to optimize a solution based coating method to obtain a uniform well dispersed thin film of TiN nanoparticles on the substrate, which will be useful for their application in the device. However, preventing aggregation of TiN nanoparticles in a solvent has been a challenge. We have optimized protocols based on ultra-probe sonication and centrifugation, to obtain relatively monodisperse and stable TiN nanoparticles in an aqueous solution. We have applied our method to commercially available polydisperse TiN nanoparticles of average size 80nm and 20 nm. Using the optimized method we were able to decrease the polydispersity index (PDI), the measure of the distribution of nanoparticles in a solution, from 0.351 to 0.18 indicating a significantly more monodispersed solution. Moreover. we used a sol-gel based method for incorporating nanoparticles in transparent silica gels. There was no aggregation of the TiN nanoparticles observed during the sol-gel transition. UV-Vis spectroscopy and dynamic light scattering method confirm that incorporating TiN nanoparticles in silica gel preserved both of their optical properties and the dispersibility as in the original solution. The nanoparticles stay stable in the solution for several days. A biocompatible silica coating on TiN nanoparticles can be useful for drug delivery and photothermal therapy. In summary, we are developing a simple commercially viable method to efficiently disperse and stabilize titanium nanoparticles in solution and subsequently coating them on the substrate which can easily be applied for large scale applications.

Directed assembly has been the subject of extensive research over the years due to the desire to grow structures at specific locations. This is accomplished by increasing the probability of nucleation at the desired location. Mechanisms such as reduction of the diffusion of species, reduction of the strain to lower the bulk free energy to reduce the critical nuclei size, or a lowering of the surface energy which also reduces the critical nuclei size have previously been used. In all these cases, nucleation of the desired phase occurs directly on the substrate. Here we propose and demonstrate a scheme for directed assembly in which we confined the nucleation of the desired material to the region of a nanostructure formed on the surface. We accomplish this through the use of a dilute alloy containing the desired material and subsequent removal of the major constituent of the alloy inducing nucleation at some point after the dilute species exceeds it solubility limit. In particular, we use this approach to control the nucleation of ErAs nanoparticles from Er:InAs quantum dots by evaporation of InAs. We studied ErAs NP formation from InAs QD nominally doped to concentration of 1.25% and 2.5% on GaAs (001). The Er:InAs quantum dots were grown using normal conditions for the growth of undoped InAs and produced similar quantum dot densities (near 100/square micron). We explain these observed results in terms of normal nucleation and growth given the mesoscopic concentration of Er atoms expected in the InAs QD. A comparison of the undoped and Er-doped InAs quantum dots before and after InAs evaporation show the number of the ErAs nanoparticles is less than the original quantum dot density showing nanoparticle do not form in every quantum dot and suggests a critical number of Er atoms needed for nucleation. We have used our confined nucleation process in conjunction with normal direct assembly processes for InAs QDs to produce hybrid structures composed of InAs QDs and ErAs NP. We have formed vertically stacked Er:InAs QD over undoped InAs QDs to form ErAs NP over InAs QDs. We have formed Er:InAs quantum dots formed in ErAs divots to form ErAs nanoparticle clusters and have used Er:InAs to place ErAs in patterned surface. Transmission electron microscopy of these structures show that at times multiple nucleation sites are possible in the structure and suggest the potential of forming rich hybrid structures using this approach.

Cephalopods have the ability to take advantage of pigmented and structural coloration when camouflaging, signaling, or otherwise communicating through their adaptive coloration. The chromatophore, which is the actuating organ responsible for pigmented color within cephalopods, is populated with 500-600 nm diameter particles composed of pigment and protein called pigment granules. The pigment, identified as xanthommatin, has been found to have a uniquely high refractive index and contributes highly to the scattering of light with broadband optical properties. Developing metamaterials out of synthetic pigment granules, where the particle periodicity is smaller than the wavelengths of light with which they interact, was the next logical choice for us to not only further our understanding of the complex photonic system of cephalopods, but to explore its application in adaptable optical systems and possibly chemical sensing.

In this work, we aim to leverage our knowledge of biomaterials (namely the biopigment xanthommatin) to generate sub-monolayers of synthetically made pigment granules. Xanthommatin is encapsulated within polystyrene-co-maleic anhydride (PSMA) nanoparticles with good physical retention while maintaining its coloration. Sub-monolayer arrays are prepared via Chumanov-style roll-coating on substrates within a vial containing the solution of particles, and are physically characterized by scanning electron microscopy. We then show the arrays’ optical properties measured by integrating sphere spectrophotometry and broadband multi-angle ellipsometry. Depending on the spacings of the particles, the arrays will resonate at certain wavelengths within the visible. Our updated models, taking into account the particle’s refractive index and contribution from the pigment, will show a dependence of spacing and material on wavelength resonance. With the pigment as a functional feature within the particles, we explore the retained chemical properties (such as absorption changes with pH) with the possible inclusion of chemical sensing.

Optical matter can be formed by electrodynamic interactions of metal nanoparticles in an optical field. This light-mediated self-organization of nanoparticles offers a route to study mesoscale electrodynamics interactions in many-body systems. By controlling the intensity, phase, and polarization of light, the optical field can be tailored to assemble stable optical matter with up to 101 nanoparticles. Here we report that the spatial inhomogeneity of circularly polarized optical binding in a many-particle system. The spatial stability of self-organized nanoparticles is strongly influenced by the laser intensity and polarization state. In particular, the nanoparticles in the central area of an optical matter array are more stable than those on the edge. Moreover, larger arrays are self-protected from fluctuation-induced instability by incorporation more nanoparticle constituents. This study provides important information about the spatial inhomogeneity of optical binding interactions in mesoscale nanoparticle arrays.

Graphene's high mobility, linear band structure, and two-dimensional density of states result in tunable optical properties within the infrared (IR) that present new opportunities for dynamic multifunctional detectors and filters. Being atomically thin, however, necessitates field confinement to achieve practical utility. Graphene plasmons serve this purpose by both concentrating the electric field while "carrying" the tunability, which is dependent upon the charge concentration within graphene. In addition to graphene's electronic properties, its surrounding optical environment has a profound effect on the plasmonic response. Thus, design of the dielectric layers used to facilitate the electrostatic doping of graphene- required for active tunability- is of critical importance. Recognizing this fact, we examine here how dielectric layers can be chosen to enhance the spectral tunability of graphene plasmonic devices. To this end, the tunability of graphene's plasmonic dispersion is quantified relative to the dielectric atop of which it sits by solving for poles in the imaginary part of the reflection coefficient in a multilayer model. By solving for the dispersion at varying Fermi-levels (i.e., charge concentrations), the extent of tunability is quantified. Tunability varies appreciably between several materials typically employed in graphene plasmonic devices-HfO2, SiO2, and Al2O3. The causes for this material variation are then examined by assessing the dependence of tuning relative to parameters making up the transverse-optical longitudinal-optical (TOLO) dielectric function, which typically describes the optical properties of the dielectrics employed. Paths are highlighted by which synthetic multilayer oxides can be designed to enhance tuning in graphene devices beyond that presently attainable. Acknowledgements: Sandia National Laboratories is a multi-mission laboratory managed and operated by the National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract No. DE-NA0003525.

Co3Sn2S2 is an ideal Weyl semimetal in its magnetic phase [1]. Its ground state is antiferromagnetic and electromagnetic fields can induce a phase transition between the two states [2]. Using ab-initio tools, we analyse the axion field [3] and the linear and non-linear optical responses in the two phases of this time-reversal breaking compound. Our calculations include both electron-electron scattering events, as well as interactions with phonons [4]. We discuss how the phases can be distinguished from each other by their optical and electrical response, how the axion field in the Weyl semimetal state provide a way to realise non-reciprocal thermal emitters [5] and finally, present results on the shift-current generation and second-harmonic emission.

[1] Liu et al., Nature physics 14, 1125 (2018)
[2] Soh et al., Phys. Rev. B 100, 201102(R) (2019)
[3] Nenno et al., Nat. Rev. Phys. (under review, 2020)
[4] Garcia et al., Phys. Rev. Res. 2, 013073 (2020)
[5] Zhao et al., Nano Lett. 20, 1923 (2020)

Recently, interest in organic-inorganic perovskites has increased due to their application in photovoltaics, photonics and optoelectronics. They have also been used as a strong light emitters in the microcavities, due to the possibility of obtaining strong light-matter coupling regime, and the occurrence of coherent macroscopic effects, such as Bose-Einstein condensation of exciton-polaritons. Compared with multi quantum wells heterostructures produced by epitaxial growth of inorganic semiconductors, 2D perovskites displays stronger dielectric confinement in the inorganic layers, whereas excitons in such materials are characterized by a higher binding energy and exhibits strong nonlinearities. It indicates that the devices based on polaritons in 2D perovskites layers can work stable at the room temperature.

Optical and electronic properties of perovskites can be easily tuned by changing the thickness, which makes them an ideal system for the investigation of strong light-matter coupling phenomena. In this direction, we studied the unusual photoluminescence (PL) behavior in function of temperature of a 2D-layered perovskite-type semiconductor, (C₆H₅(CH₂)₂NH₃)₂PbI₄ deposited on a dielectric
mirror prepared from a solution using spin-coating technique as well as free-standing layers of 2D-perovskite with substrate from solution P2611:NMP. The low-temperature (〈110 K) photoluminescence of 2D perovskite reveals two distinct emission peaks. Furthermore the bandgap exhibits an unusual blueshift (about 7 nm) with increasing temperature from 4.5 to 295 K. The temperature-driven additional photoluminescence peak can be attributed to the presence of molecularly disordered domains.

We demonstrate a strong light-matter coupling regime between the excitonic resonance in 2D-perovskite and cavity photon in an open, tunable microcavity at room temperature. Exciton-polaritons in this material are expected to show unusually strong non-linear effects due to significant third order Kerr nonlinearity. At strong excitation conditions (up to 1 mW) we observe emission blueshift up to 8 meV from polariton mode. We demonstrate that this blue-shift can be attributed to the non-linear polariton interaction and the change of exciton-photon interaction strength. Exploring emission characteristics of a 2D-perovskites in wide range of temperatures and under strong light-matter coupling conditions provides a lot of important information regarding the performance of a light-emitting device, solar cell based on perovskites and physics phenomena related to non-linear effects observed previously only at cryogenic temperatures.

Owing to advancements in related technologies over the last few decades, terahertz waves possess the potential to be utilized in fields related to imaging, non-destructive testing, analysis, and next-generation telecommunication, among others.
Optical elements are also currently being explored as tools that can extend the applicability of terahertz (THz) waves. One obstacle in the development of THz optical elements is that there are fewer feasible materials that have different refractive indices in THz frequencies than there are such materials in other frequency ranges. Specifically, the options for transparent optical materials are limited to polymethylpentene, other olefin-based polymers, and highly resistive silicon.
In this study, we developed a highly transparent composite material composed of polymer and silicon nanoparticles (SiNPs) for THz optics; the refractive index can be controlled by varying the fraction of SiNPs in the composite. We also demonstrated its applicability as an anti-reflection (AR) coating for Si optical elements prone to reflection loss due to the large discrepancy between the refractive indices of Si and air.
Highly pure SiNPs with a typical diameter of 100 nm were employed to control the refractive index of the composite. The SiNPs were fabricated via a laser pyrolysis method. This method entails laser irradiation to decompose semiconductor-grade monosilane gas; hence, the SiNPs have extremely low metal impurity (i.e., on the order of ppb) and consequently, high resistivity. The utilization of highly resistive Si creates favorable conditions for THz waves as it allows for the composite material to be transmitted without absorption. Furthermore, the problem of wave scattering is eliminated, as the typical diameter of the SiNPs is smaller than the wavelength of THz waves by a factor of 1000.
We first characterized the basic optical properties of the SiNPs and its composite with a polymer chosen for present study. THz-wave transmission was characterized via THz time-domain spectroscopy of the SiNP films deposited on highly resistive Si (HR-Si) wafers. The transmission spectrum was observed to be in agreement with the spectrum calculated from the thickness, porosity of the film, and refractive index of Si; this indicated that the THz waves were not significantly absorbed or scattered by the film.
The refractive indices of the SiNP-polymer composites were also evaluated from the transmission spectrum of the composite films on HR-Si. The refractive index was controlled between 1.52 and 1.95 under a Si-content range of approximately 0–30 vol%.
Subsequently, for demonstration, we designed and fabricated bilayer AR coatings to apply to the Si plates. The SiNP-polymer composite layer (n = 1.95) and polymer layer (n = 1.52) were stacked on both sides of the Si plates. The amplitude transmittance of the Si plates with AR coatings exceeded 90%, with a bandwidth of 0.8 THz at around 1 THz, whereas in the case of Si plates without AR coatings, the transmittance was expected to be 70%.
In conclusion, we developed a novel refractive-index-controllable SiNP/polymer composite material for THz-range optics. The SiNPs demonstrated negligible absorbance and scattering of THz waves, confirming their suitability for THz-range optics applications. Additionally, the refractive index of the composite was controlled between 1.52 and 1.95. This range addresses several existing limitations regarding THz-range optical element designs. Furthermore, the AR coating developed using the proposed composite material successfully increased the transmittance of THz waves through Si. Thus, the proposed composite material can be applied in the design of various types of THz-range optical elements and has the potential to be used for developing optical elements with higher functionality.

The material class of transparent conductive oxides features a variety of properties that are interesting and useful for optical and opto-electronic applications. ITO belonging to this class has shown to be either processed (Gui et al, Nat. Sci. Rep. 2019), or electro-statically-biased (Sorger et al., Nanoph. 2012) into the ENZ regime. Here, I review our latest work including; (1) ITO ENZ material deposition control, (1) the first ITO-based phase-shifters in silicon photonics with VpL=0.5V-mm (Amin et al, APL 2018) and 0.06V-mm performance (Amin et al, arXiv 1907.11131 2019), (2) electro-optic nonlinear activation function in feed-forward photonic neural networks (Amin et al, APL Mat. 2019), (3), optical-phase array in silicon-ITO photonics for GHz-fast beam steering, (4) a metatronic circuit board towards an analog photonic computer for PDEs, and (5) strong nonlinearity in ENZ ITO showing ~10dB of optical limiting.

Low cost, light-weight, low-power, large-format, room-temperature, mid-wave infrared (MWIR) detectors are needed for drones, unmanned aerial systems (UAS), unmanned aerial vehicles (UAV), and other reduced-scale aircraft. An opportunity, suggested by direct-read X-radiography systems, is the use of thin-film transistor (TFT) array as read-out integrated circuit (ROIC) for low-cost sensors deposited directly and unpatterned onto this ROIC. TFTs have already been thoroughly optimized for power, weight, large-format and cost by the flat panel display industry. We present experimental investigation of aqueous-spray-deposited, mid-wave-IR, metal-chalcogenide photoconductors and heterojunction photodiodes for this application. Measured responsivity, detectivity D*, and photoresponse spectra are reported for ternary PbSSe photoconductors and PbS/CdS photodiodes.

Doped transparent conducting oxides (TCOs) have recently found utility as potential alternative plasmonic materials. One highly promising TCO which has been demonstrated to exhibit broad tunability while maintaining exceptionally low optical losses is highly-doped CdO. This is due to CdO possessing both a low effective mass (ranging from 0.12-0.26 in epitaxially-grown films with carrier densities ranging from 1019-1020 cm-3) as well as electronic mobilities extending upwards to 500 cm2/V-s with carrier densities ranging from 1019 to 1020 cm-3. Unique to this material is a range of carrier densities where increasing values result in increasing mobilities. Due to the low optical losses and broadly tunable plasma wavelength, CdO has been shown to support, carrier tunable surface plasmon polaritons (SPP) and epsilon-near-zero (ENZ) modes throughout the mid-infrared (MIR). These ENZ modes are associated with extreme electric field confinement, enabling control over light-matter interactions such as resonant perfect absorption and strong nonlinear interactions. Here we show how vertically heterogeneous films of doped CdO can be leveraged to create different nanophotonic phenomena across the infrared. Careful control over the carrier density during film growth has opened the door to achieving multilayer homoepitaxial CdO films. Despite the homogeneity of the crystal structure, the elevated carrier density and carrier density dependent changes to Fermi energy and effective mass confine carriers to their constitutive layers. Owing to this material's low carrier diffusivity multilayer stacks composed of deeply subwavelength thickness CdO films, each supporting ENZ modes at different frequencies, exhibit multifrequency absorption/emission. Further, prior work has shown that epitaxially-grown bilayer stacks of CdO, with bottom and top layers supporting SPP and ENZ modes respectively, exhibit strong coupling between the ENZ and SPP modes. The combined ENZ-SPP dispersion of these bilayer films displays a prominent anti-crossing with a separation that is dependent on both the spectral and spatial overlap of the modes. Here we have extended our focus to trilayer CdO films, where an unintentionally-doped (Nd ~ 1x1019 cm-3, much less than Nd of the SPP or ENZ layers) CdO film is grown between the SPP and ENZ layers. The spacer layer thickness modifies the spatial overlap of the constituent electric fields as well as alters the effective index of the region above the SPP layer, decreasing the degree of spectral splitting and shifting the anti-crossing position to higher momentum. The strong carrier confinement in doped CdO also allows for the engineering of extreme anisotropy in multilayer CdO films. By growing CdO layers of subwavelength thickness and alternating carrier densities (low-doped acting as a dielectric layer, high-doped as a metallic layer) hyperbolic metamaterials can be realized in a fully plasmonic, homoepitaxial system. In these metamaterials, the sign of the in- and out-of-plane components of the dielectric function are opposite in sign. This leads to an open hyperbolic dispersion relation which allows for the propagation of high-momentum, and therefore highly-confined, optical modes. Through careful control over the carrier densities and thicknesses of the CdO layers, tunable hyperbolic resonances can be supported throughout the MIR, allowing for the potential realization of next generation emission devices and subwavelength imaging.

There is a growing interest on the compact XFELs to be used in applications such as medical science (surgery, fat removal), material science and military. In order to build a compact XFEL, constructing an undulator with sub-millimeter period (?u) is mandatory. Many problems arise due to miniaturization of undulators; such as keeping the magnetic field high while still having a considerable gap to minimize beam scraping. In this work, RADIA program has been employed for the modeling of sub millimeter period undulators (20 µm < ?u < 400 µm) with three different magnet configuration; namely Up-Down(↑↓↑↓…), Halbach (Wiggler) (↑→↓←↑…) and Hybrid(→↑←↓→…). Effects of sub-millimeter undulator period and gap on the magnetic field pattern of the undulator have been discussed. This work was supported by TUBITAK project: 118F319

All dielectric nanophotonics has opened new pathways for engineering optical metamaterials, enabling a wide range of optical devices including perfect reflectors, magnetic mirrors, and flat optics. One method to decrease the size of nanophotonic resonators and increase the quality factor of resonances is to use materials with larger refractive index. However, it is commonly stated that the bandgap and refractive index of semiconductors are related to a constant through the empirical rule n⁴×E_g≈95 eV referred to as the Moss rule. From this rule, it is expected that any increase in refractive index comes at the cost of decreased bandgap. Because of the corresponding increase in optical losses, this is detrimental to optical devices operating in the sub-bandgap region. While the constant of 95 eV works for a variety of materials, this constant only works for materials with similar band structures. Here, we experimentally demonstrate iron pyrite (FeS2) as a high index material platform for nanophotonics with refractive index greater than 4.5 in the mid-IR. This high refractive index is obtained with a bandgap of 0.95 eV, resulting in FeS2 surpassing the common form of the Moss rule by over 40%, making iron pyrite a super Mossian optical material. Furthermore, we will present experimental results of an optical bound state in the continuum (BIC) in nanoscale FeS2 resonators showing the potential for high index super Mossian materials in nanophotonics. Finally, we will conclude with a discussion on the electronic and optical properties to screen for to aid in the identification of additional super Mossian materials.

III-Nitride based nanostructures can be designed to create photonic crystals or metamaterials that operate in the visible frequency regime. Nanophotonic devices that are important for many applications like solid-state lighting and quantum information science and flat optics can be enabled using such structures. A key issue in creating optical nanostructures in III-nitrides has been in creating optical isolation from the high refractive index GaN growth template due to a lack of compatible natural low index materials. In silicon photonics this is mitigated by using silicon-on-insulator substrate that provide a low index silicon oxide cladding, while in III-V semiconductors such as GaAs this is achieved by oxidation of high Al content AlGaAs buffer layer. While there are some examples in III-nitrides such as observation of low threshold lasing in nanowire array geometry photonic crystals [1], achieving simple optical isolation for membrane PhC, metasurfaces and VCSEL structure has been a challenge. A recent work [2] has demonstrated electrochemically etched nanoporous layers can provide lower index isolation. Here we will discuss achieving optical isolation in III-nitride nanophotonic devices using electrochemical/ photo-electrochemical [3] etching techniques. We will describe the fabrication of GaN nanowire array with optical isolation and present optical response to demonstrate the effectiveness of this approach.
Sandia National Laboratories is managed and operated by NTESS under DOE NNSA contract DE-NA0003525..

[1] Wright, J. B.; Liu, S.; Wang, G. T.; Li, Q.; Benz, A.; Koleske, D. D.; Lu, P.; Xu, H.; Lester, L.; Luk, T. S.; Brener, I.; Subramania, G., Multi-Colour Nanowire Photonic Crystal Laser Pixels. Sci. Rep. 2013, 3, 2982.
[2] Mishkat-Ul-Masabih, S.; Luk, T. S.; Rishinaramangalam, A.; Monavarian, M.; Nami, M.; Feezell, D., Nanoporous distributed Bragg reflectors on free-standing nonpolar m-plane GaN. Applied Physics Letters 2018, 112 (4), 041109.
[3] Anderson, P. D.; Fischer, A. J.; Koleske, D. D.; Gunning, B. P.; Subramania, G., III-nitride photonic crystal emitters by selective photoelectrochemical etching of heterogeneous quantum well structures. Optical Materials Express 2018, 8 (11), 3543-3550.

Metaphotonic materials and systems can create unusual optical properties through the interaction between light and artificial nanostructures. Such unusual properties can allow color imaging devices over the diffraction limit level (~100,000 dpi) that cannot be simply implemented with conventional imaging technology. Although these ultra-high resolution imaging technologies have shown substantial potential for applications that handle static images, such as color filters and printing, the feasible options for dynamic imaging technology using passive metaphotonic elements have not been fully demonstrated, impeding their application in active display devices.
Motivated by these issues, we introduce electrically controllable active color imaging metaphotonic devices consisting of metal/insulator/metal nanodisk stacks; the resonance modes of these stacks can be reversibly changed by structure deformation. To achieve the function of electrically driven mechanical deformation, disk elements of inconsistent size are designed, and electric potential is applied between the upper and lower metal disks, where the large upper metal disk can be bent by columbic force of induced reciprocal charges. The tunable gap between the stacked metal disks provokes resonance mode change, thereby resulting in variation of the transmission/reflection spectra. The fabricated metaphotonic nanodisk stacks exhibited a reasonable shift in reflectance spectra when sufficient electrical voltage was applied, which demonstrated the feasibility of sub-wavelength imaging technology for application in display devices.

Plasmonic, photonic and excitonic states at interfaces govern the optical response of tunable nanophotonic structures such as active metasurfaces, photodetectors and photocatalysts. I will discuss how we can harness plasmonic and excitonic states at interfaces to enable power-efficient phase modulation of active metasurfaces and discuss implications for time-modulated flat optics. Specifically, I will discuss avenues for increasing the power density and power efficiency of plasmonic conducting oxide metasurfaces for temporal phase and amplitude modulation and requirements for time-modulated photonic applications, such as multi-frequency generation and realization of non-reciprocal metasurfaces. I will also report on how time-resolved measurements of plasmonic and excitonic states at interfaces can yield insights about fundamental light-matter interactions in these systems. Specifically, I will discuss i) how electron-hole correlations affect the dynamics of hot carrier relaxation in plasmonic Au/GaN photodetectors and photocatalysts and ii) how time-resolved measurements of excitons generation inform our understanding of carrier dynamics and transport in transition metal dichalcogenides such as WS₂ and MoS₂.

Since the development of diffractive optical elements in the 1970s, major research efforts have focused on replacing bulky optical components by thinner, planar counterparts. The more recent advent of nanophotonic metasurfaces has further accelerated the development of flat optics through the realization that resonant optical antenna elements can be utilized to facilitate local control over the light scattering amplitude and phase. In this presentation, I will highlight recent efforts to realize electrically-tunable metasurfaces employing nanomechanics, electrical gating, microfluidics, and phase change materials. I will pay particular attention to the opportunity of realizing dynamic metasurfaces from atomically-thin semiconductors. Such elements are capable of active wavefront manipulation for optical beam steering and dynamic holography. I will illustrate how the proposed optical elements can be fabricated by scalable fabrication technologies, opening the door to many commercial applications.

Optical metasurfaces have emerged as promising candidates for multi-functional devices. Dynamically reconfigurable metasurfaces have been introduced by employing phase-change materials and/or by applying voltage, heat, or strain. While existing metasurfaces exhibit appealing properties, they don’t express any significant nonlinear effects due to the negligible nonlinear responses from the typical materials used to build the metasurface. In this work, we propose and experimentally demonstrate one kind of Kerr metasurfaces that shows strong intensity-dependent responses. The Kerr metasurface is composed of a top layer of gold antennas, a dielectric spacer, and a ground layer of metallic quantum wells (MQWs) made by TiN/Al₂O₃multilayer stack. Due to the large Kerr nonlinearity supported by the MQWs, the effective optical properties of the MQWs can change gradually from metallic to dielectric with increasing of the input intensity, leading to dramatic modifications of the metasurface responses. This opens up new routes for tunable metasurface applications in the fields of nonlinear optics and extremely fast meta-optics.

Manipulation of waves is mostly achieved by materials with spatial inhomogeneities. However, exploiting the dimension of time, in addition to the three dimensions of space, for variation of material parameters can bring additional functionalities in such tailoring and sculpting the wave. The spatiotemporal metastructures in which the material parameters are engineered with spatial and/or temporal inhomogeneities exhibit special features that enrich the functionality of light-matter interaction. In my group, we are investigating a variety of scenarios in which such four-dimensional (4D) spatiotemporal material platforms can be exploited. These scenarios include 4D materials that provide us with temporal aiming, temporal anisotropy, anti-reflection temporal coatings, frozen waves, phase conjugation, asymmetric diffusion, temporal effective medium methods, etc. In this talk, I will give an overview of our most recent results in this area

Metamaterials are artificially engineered structures designed to shape electromagnetic responses not found in nature including superlensing, cloaking and communications. They traditionally comprise an array of fabricated subwavelength anisotropic structures on a planar geometry, which inevitably limit their responses in certain directions. We explore a novel type of soft metamaterial to overcome such limitations, in which building blocks and overall configurations that can be freely tailored, even after formation. In particular, we explored the complex soft metamaterial nanoarchitectures with tailored symmetries that achieved isotropic negative index at the optical frequency. Moreover, we expand such soft metamaterial design that created the first Casimir quantum trap in solution by manipulating electromagnetic waves from quantum fluctuations. These results provide new insights that facilitate and expand novel metamaterial design and applications.

Plasmonic chiral metamaterials has attracted significant attention at it provides a new route to intriguing optical properties such as negative refractive index, light polarization filters, and phase modulation. However, the fabrication regarding with limited resolution in conventional synthesis and complexity of asymmetric synthesis pose a major hindrance for further development. In this study, a new class of three dimensional chiral plasmonic nanostructures was successfully fabricated using molecular shape modifiers and crystallographic control of nanoparticle. Previously, we developed three-dimensional chiral metamaterials based on photolithography overlay and electron-beam lithography overlay for terahertz and near-infrared, respectively. We found scaling down them further to visible frequency is extremely difficult. As an alternative solution, we discovered a novel system that characteristic of molecule is transformed into distinctive gold nanoparticle shape. On the basis of this system, chirality transfer between molecular modifier and gold surface allow us to achieve numerous chiral morphologies of gold nanoparticle, named plasmonic helicoids. Particularly, enantiospecific interaction of molecule and high index plane plays pivotal role to provide asymmetric structuring process on the gold surface, forming distinct chiral morphology in single nanoparticle level. One of the representative shapes of helicoid structure showed gammadion-like structure, consisting of four highly curved arms of increasing width, in all six faces of cubic geometry. The unprecedented chiral morphology of plasmonic helicoid has remarkable optical activity (dissymmetry factor ~ 0.2 at 622 nm) even in a randomly dispersed solution, substantiated by direct visualization of macroscopic color transformation. Changes in molecular recognition and growth parameter led to different morphological evolution, and structural alterations provided a straightforward means of tailoring optical response, such as optical activity, handedness, and resonance wavelength. Also, our aqueous phase synthesis is readily scalable without losing exquisite chiral structure at nanoscale. In these aspects, our approach, chirality evolution in single nanoparticle, provides a truly new paradigm and valuable insight for chiral metamaterial fabrication. Such unique fabrication technique will provide the opportunity to achieve the significant step making metamaterials from science to technology. References 1. S. Zhang et al., Phys. Rev. Lett. 102, 023901 (2009) 2. S. Zhang et al., Nat. Commun. 3, 942 (2012) 3. G. Yoon et al., Sci. Rep. 7, 6668 (2017) 4. J. Mun et al., Opt. Lett. 43, 2856-2859 (2018) 5. M. Kim et al., Opt. Express 26, 14051-14057 (2018) 6. H. Lee et al., Nature 556, 360-365 (2018) (with News and Views Article, Nature 556, 313-314 (2018)) 7. J. Mun et al., Nanophotonics 8, 941-948 (2019) 8. H. Lee et al., Nature Commun. (in revision) 9. J. Mun et al., Light: Sci. Appl. (in review)

We report on the study of some metamaterials whose properties, including dispersions and topological characteristics, are determined by the connectivity of a network rather than the resonance of individual resonators.

Metamaterials are described using effective medium theories in the long-wavelength limit in the sense that their optical properties can be described using local permittivity and permeability tensors. We will see that some metallic structures cannot be described using such a language. Their properties depends on the network connectivity, where braiding can create effective sythetic gauge fields that induce shift in momentum space. The consequence is that such materials can have their index ellipsoids centered not at k=0, and they can possess multiple index ellipsoids centered at arbitrary nonzero k-points. The number of modes that are allowed at zero frequency can be described by Betti numbers. Such systems differ from conventional metamaterials because the functionality does not come from the resonances of individual resonators and as such, their functionalities tend to be broadband.

We will also discuss network structures that have non-trivial topology in momentum space and these materials can couple orbital angular momenta with the wave vector. The topological character originates from the connectivity of the network and goes beyond typical space group descriptions. Such systems can exhibit angular momentum-dependent one-way edge states, and their properties can be described using local Chern numbers. Such systems can also be used to realize other interesting topological systems such as high order topological insulators and topological Anderson insulators, and topological systems in which the topological order cannot be described by integers.

We have demonstrated an ultrafast, ultra-compact, and all-solid-state metaphotonic spatial light modulator (M-SLM) which can dynamically shape light in reflection. The M-SLM is an array of electrically controlled plasmonic nanostrip cavities, and it can continuously adjust the phase of reflected light from 0 to 360 degrees and, at the same time, independently modulate the amplitude over its full range. The fabricated M-SLM contains 50 active pixels and measures 250×250 ?m2 in dimensions. By tuning the voltages applied to the M-SLM, we have demonstrated beam steering and beam splitting with a steering angle up to 8°, the side mode suppression ratio of +2.7 dB (the highest value reported in the field), and the deflection efficiency up to 48%. The experimentally measured modulation speed marks 5.6 MHz while the energy consumption per pixel is calculated to be as low as 354 pJ per switching. Accomplishing arbitrary light manipulation using active metaphotonics has remained challenging, especially due to coupling between the phase and amplitude during modulation which resulted in unwanted background noises. To overcome this limitation, we have delved into the fundamental modeling of the light wave and observed that it would require minimum two variables, for example, the real and imaginary coefficients, to properly express the phase and amplitude of the light wave. Based on this observation, we developed the two-control-parameter algorithm and translated the concept into the hardware design. In the array of the designed M-SLM, each plasmonic nanostrip cavity is a stacked double-gated structure with an indium-tin-oxide layer as an active intermediate channel, which allows an application of two independent control voltages to the top and bottom gate metals. We have also performed extensive numerical investigation and confirmed that the highly versatile light-manipulating capability of the designed M-SLM, including arbitrary phase and amplitude modulation, originates from the superposition of the effects generated by the voltages applied independently to the top and bottom metals. To our knowledge, the M-SLM is the first all-solid-state, electrically controlled metasurface that accomplishes the 360-dgree full phase sweep and independent amplitude modulation. Unlike previously reported work, our M-SLM does not involve the use of slow, temperature-sensitive liquid crystals, stiction-and-vibration-prone micro-electromechanical systems (MEMS), or an array of pre-fabricated size-varying structures to achieve phase changes. The M-SLMs extremely low power consumption, ultrafast modulation speed, ultra-compact form factor, outstanding predicted reliability, and the use of standard IC-fabrication processes promises wide adaptation and proliferation of the ultrafast and extremely power-efficient SLMs for 3-dimensional real-time holographic display, light detection and ranging (LiDAR), and wearable display for virtual reality. We look forward to sharing and discussing the details of our M-SLM design philosophy, hardware design & fabrication, and testing results with the audience at the MRS 2020 Spring.

Metasurfaces significantly widen the parameter space for control over light, mainly via the many ways phase control can be achieved and varied within the plane of the surface. Here, we take a closer look on how to further widen these possibilities via introducing elements such as control over amplitude and phase simultaneously, i.e. the complex amplitude of the impinging light field, and secondly via disorder.
As a first application, we examine the use of beams of higher orbital angular momentum coupled to metasurfaces in a context of holography. We demonstrate the design and implementation of metasurface phase holograms with strong orbital angular momentum selectivity, based on nanostructured gallium nitride. This opens up a new degree of multiplexing, allowing lensless reconstruction of distinct holographic images in each orbital angular momentum channel. We further show how metasurfaces with additional variation in the height of the underpinning nanostructures allow for complete control over the complex amplitude of the impinging wave. We demonstrate that this significantly reduces cross talk between different orbital angular momentum coding channels, enabling up to 200 independent orthogonal image frames for realistic parameters.
The second application is in a context of energy harvesting. Based on a disordered array of plasmonic nanoresonators, we show how deterministic coupling of this effective photonic layer to an external cavity can lead to two different regimes of operation — broadband absorption and narrow-band reflection. As an example, we demonstrate the use of such coupled disordered-layer / cavity systems for colour generation.

Plasmonic antennas are capable of enhancing the coupling of light and matter due to their strong electric field confinement. At room temperature, where dephasing is the dominant contribution to the linewidth of a solid-state emitter, studies of plasmonic coupling have usually concerned incoherent interactions, e.g. a Purcell enhancement [1]. At low temperatures, however, the resonant coherent scattering from the emitter becomes significant and another regime of plasmonic interactions can be observed: here, not only the plasmonic particle changes the interaction of the emitter with light, but the emitter can influence the scattering properties of the plasmonic particle. In an extreme case, a single molecule can fully suppress scattering from a (much larger) plasmonic particle, rendering it effectively transparent [2]. In this presentation, I discuss an experiment, where a single organic molecule partially cloaks a large gold nanoparticle [3].

It has recently been reported that a nanogap antenna formed by a gold nanoparticle separated from a metallic substrate by a thin dielectric gap can achieve ultra-strong enhancement, even at room temperature [4]. When the enhanced emission rate is fast enough to compete with the room temperature dephasing and the plasmon losses rates, the emitter starts to interact coherently with the plasmon mode, bringing the system into the strong coupling regime. In this presentation, we also demonstrate a significant enhancement of a single quantum dot coupled to a plasmonic nanogap antenna. We observe an ultrashort fluorescence lifetime less than 38 ps limited by the instrumental response function. Moreover, by controlling the position of the quantum dot with respect to the nanogap antenna, we can tune the system from the weak coupling to the strong coupling regime leading to spectral splitting in the fluorescence spectra [5].

[1] K. Matsuzaki, et al., Sci. Rep 7, 42307 (2017).
[2] X-W. Chen, V., Sandoghdar, M., Agio, Phys. Rev. Lett., 110, 153605 (2013).
[3] J. Zirkelbach, et al., Phys. Rev. Lett. under review (2020); arXiv:2004.08156
[4] R. Chikkaraddy, et al., Nature 535, 127 (2016).
[5] Liu, et al., in preparation.

Weyl points are linear band crossings at discrete points in the momentum space, corresponding to three-dimensional (3D) extensions of the two-dimensional (2D) Dirac nodes, and functioning as monopoles of Berry flux with topological charges defined by the Chern numbers. In photonics, Weyl points have been realized in systems with broken inversion symmetry such as photonic crystals and metamaterials, with broken time reversal symmetry such as magnetized plasma and in synthetic dimensions. As a key signature of Weyl systems, Fermi arcs that connect between the projections of Weyl points of opposite topological charges have been observed. However, the manifestation of the topological nature of Weyl points in their interaction with free space radiations remain obscure in both quantum and classical systems.
Scatterings and transport in Weyl semimetals have caught growing attention in both condensed matter and classical physics. In condensed matter systems, the chiral zero modes and the associated magnetoresistance and chiral magnetic effects underlie some highly unusual scattering and transport properties of Weyl semimetals. It has also been proposed that Weyl semimetals may have handedness dependent Imbert–Fedorov shift within the bulk Weyl material. Electrical conductance is usually measured in the experiments, which has contribution from both the bulk Weyl nodes and the fermi arcs. However, it is generally difficult to perform momentum resolved scattering and transport studies, since the leads that transport the electrons to the Weyl semimetals, by their nature, cannot control the momentum of the input electrons. Therefore, previous attempts to associate the topology to scattering matrix mostly remain theoretical.
In this work, vortical phase profile in the momentum space is demonstrated for elliptically polarized electromagnetic waves reflected by a photonic Weyl system, which represents a key signature of the topological nature of the Weyl points and could lead to alignment-free phase plates for vortex beams generation. The nontrivial topology of Chern insulator can not only be revealed by the number of surface states, but also by the scattering matrix S, i.e. the total winding number in 2π of the phase of the eigenvalues of the scattering matrix. It has been rigorously proven that the topological classification of the Hamiltonian and the scattering matrix are equivalent. The scattering matrix method however, are more accessible for investigation in photonics with the readily available optical components from visible to microwave wavelengths to perform the angle resolved measurements.
We further discover a novel type of spiraling guided mode in a hybrid waveguide system formed by the Weyl metamaterial and a metal plate, enabled by the vortical phase profile in the momentum space. we demonstrate that the fermi arcs possessing arbitrary number of windings around the projected Weyl points can be realized in a photonic Weyl semimetal-waveguide hybrid system, revealing a new intriguing physical properties of photonic Weyl systems. The metamaterial-waveguide hybrid system is formed by an air layer sandwiched between a metallic ground plane and an ideal Weyl metamaterial. In such a system, the surface arc states exponentially decay into the Weyl metamaterial, while forming waveguide modes in the thin air layer between the Weyl metamaterial and the confining metallic plate. By measuring the electromagnetic field distribution inside the waveguide, we are able to visualize the Fermi arc’s iso-frequency contour in the momentum space through Fourier transformation, and demonstrate the windings of the Fermi arcs in the vicinity of the Weyl frequency.

The interfacing of nano-structures or meta-surfaces with optical fibers represents a promising approach to unlock novel types of functionalities within a multitude of fields including biophotonics, quantum technologies or optical sensing. Here commonly used top-down implementation strategies are hard to employ in case fiber end-faces are considered due to the intrinsic mismatch of the fiber geometry with wafer-based fabrication technology.
Within this presentation, we would like to report on two implementation pathways to circumvent the mentioned bottleneck, namely (i) 3D nano-printing and (ii) modified electron-beam lithography. Both approaches allow for the implementation of sophisticated nano-structures on the end-face of optical fibers leading to significant performance improvements compared to fibers with unstructured end faces. These improvements are shown here on the examples of (i) optical trapping using only one single-mode fiber via the integration of dielectric meta-surfaces and (ii) boosting in-coupling efficiencies into fibers at almost grazing incidence via dielectric ring-type nano-structures .

(i) A tight focus of light plays an essential role in a vast number of applications including optical imaging, optical manipulation, material processing, biophotonics, microscopy and quantum technology. Here optical fibers show fundamental limitations due to the divergence of light emerging from the fiber facet which for instance has limited the use of single fibers within optical trapping, demanding numerical apertures >0.8. In this presentation we show that 3D nano-printing via direct laser writing allows for the integration of ultra-high numerical aperture meta-lenses on the facet of single-mode fibers that include beam expansion sections. Record-high numerical apertures of up to 0.9 and diffraction-limited spots have been reached. The capabilities of this device have been demonstrated by optically trapping microbeads and biologically relevant bacteria (E. coli) for the first time with one single-mode fiber, solving a serious limiting factor within fiber optics research.

(ii) Optical fibers are particularly difficult to use within the context of wafer-based implementation technologies due to the large aspect ratios of fibers, being complementary to wafers. Using an additional planarization step prior to nano-fabrication we open up a novel pathway to implement dielectric nano-structures onto the end-face of optical fibers via electron-beam based technology. Here, we show that by integrating dielectric gratings consisting of concentric rings located on the core of single-mode fibers, in-coupling efficiencies can be increased by more than four orders of magnitude at almost grazing incidence compared to fibers with unstructured end faces. Specifically, application relevant in-coupling efficiencies of several percent at angles of more than 75° have been reached, suggesting a clear pathway to solve one serious bottleneck of optical fibers which has imposed limitations on applications of optical fibers.

The electromagnetic near-field enables subwavelength applications such as near-field microscopy and nanoparticle manipulation. Present methods to structure the near-field rely on optical antenna theory, involving nanostructures that locally convert propagating waves into confined near-field patterns. We develop a theory of remote rather than local near-field shaping, based on cascaded mode conversion and interference of counterpropagating guided waves with different propagation constants. We demonstrate how to structure at will the longitudinal and transverse variation of the near-field, allowing for distributions beyond the conventional monotonic decay of the evanescent field. We provide an experimental realization that confirms our theory. Our method applies to fields with arbitrary polarization states and mode profiles, providing a new path towards unprecedented three-dimensional control of the near-field.

High quality factor (“High Q”) cavities have revolutionized information processing, communications, sensing, and nonlinear optics by increasing photon storage times and significantly enhancing light-matter interactions. However, when the size of dielectric cavities is reduced to the nanoscale, resonant modes start to resemble point sources, scattering an incident wave in many different directions. While this scattering has been leveraged to create remarkable metasurfaces that precisely control the phase, amplitude, and polarization of light in an ultrathin footprint, metasurfaces generally exhibit high radiative loss rates and thus low Q-factors. Indeed, to date, resonant features in the far-field transmission of metasurfaces have Q-factors on the order of ten.

Here, we describe a general strategy for generating high quality factor resonances in a phase gradient metasurface. Resonances are created by including subtle structural changes in individual resonators to weakly couple free-space light into otherwise bound modes. We experimentally demonstrate control over the quality factor and resonant wavelengths in this scheme, achieving record metasurface Q’s greater than 2500. We also show via simulations that Q-factors can exceed millions. We highlight this scheme’s general applicability by designing and fabricating metasurfaces that act as beamsteerers to different angles, beam splitters, and lenses. We also show how this platform can be integrated with electro-optic materials for low-power metasurface modulation. Finally, we describe applications of high-Q metasurfaces spanning i) free-space optical sensors of COVID-19 antigens and antibodies; ii) all-optical magnetic switching; and iii) an all-optical self-isolating laser.

Nanometallic structures, such as resonant plasmonic nanoantennas, are useful for the conversion of light to surface plasmon-polaritons (SPPs) localized to ultra-small volumes [1]. Such structures can provide highly enhanced fields, strong confinement, high surface sensitivity, and can double as a device electrode for applying voltages or passing currents to active regions in optoelectronic devices. Here we review some of their many applications, in the areas of plasmon-enhanced nonlinear optics [2-5] and optoelectronics [6-9].
In nonlinear optics, plasmon enhancement can be applied to the pump [2], or to the nonlinear emission [3-5], through spectral alignment. An advantage of the former is that all nonlinear processes involving the pump are enhanced, but a disadvantage is that the pump, typically of high intensity, can damage the nanostructures. Nonetheless, such enhancement was exploited in high-harmonic generation via re-collision radiation in Si [2], where a 10x enhancement was observed for all harmonics emitted up to the 9^th. Alternatively, exploiting the latter is advantageous because the structures are resonant with the nonlinear emission which is typically weak, and non-resonant with the pump so are less susceptible to damage. Such enhancement was exploited in Raman scattering experiments [3-5], where the nanoantennas were spectrally-aligned with the Stokes wavelengths of graphene. Under this scenario, single-antenna scattering cross-section enhancements of over 500× were observed [5].
In optoelectronics, plasmon enhancement can be exploited to improve the performance of, e.g., electro-optic modulators [6], photodetectors [7,8], and beam-steering devices [9]. A key requirement is for the plasmonic enhancement to overlap strongly with the active region of the device, which requires that the nanostructure also operate as a device electrode. Such a structure consists of an array of Au nanoantennas contacted perpendicularly by electrical contact lines. The contact lines may be aligned on the nanoantennas such that they are minimally invasive optically. Structures such as these are very promising for beam steering, as each row can be routed to an individual contact such that a 1D phase gradient is applied over the array [9]. Nanostructures that consist of holes in a metal film are also of significant interest [10], especially arrangements of nanoholes (e.g., heptamers), which can be fabricated via focussed ion beam milling because they exhibit interesting and useful Fano resonances [11].

References
[1] Bharadwaj, P., Deutsch, B., Novotny, L., Adv. Opt. Photonics 1, 438-483, 2009
[2] Vampa, G., Ghamsari, B. G., Siadat Mousavi, S., Hammond, T. J., Olivieri, A., Lisicka-Skrek, E., Naumov, A., Villeneuve, D. M., Staudte, A., Berini, P., Corkum, P. B., Nature Physics 13, 659, 2017
[3] Ghamsari, B. G., Olivieri, A., Variola, F., Berini, P., Nanophotonics 3, 363, 2014
[4] Ghamsari, B. G., Olivieri, A., Variola, F., Berini, P., Phys. Rev. B 91, 201408, 2015
[5] Al-Shehab, M., Ghamsari, B. G., Berini, P., J. Opt. Soc. Am. B, 36, F49, 2019
[6] Olivieri, A., Chen, C., Hassan, S., Lisicka-Skrzek, E., Tait, R. N., Berini, P., Nano Letters 15, 2304-2311, 2015
[7] Alavirad, M., Olivieri, A., Roy, L., Berini, P., Optics Express 24, 22544-22554, 2016
[8] Alavirad, M., Olivieri, A., Roy, L., Berini, P., Chin. Opt. Lett. 16, 050007, 2018
[9] Calà Lesina, A., Goodwill, D., Bernier, E., Ramunno, L., Berini, P., IEEE J. Sel. Top. Quant. Electr., DOI: 10.1109/JSTQE.2020.2991386
[10]Gordon, R., Brolo, A. G., McKinnon, A., Rajora, A., Leathem, B., Kavanagh, K. L., Phys. Rev. Lett. 92, 037401, 2004
[11] Hahn, C., Hajebifard, A., Berini, P., Nanophotonics 9, 393-399, 2020

Rationally assembled nanostructures exhibit distinct physical and chemical properties beyond their individual units. This talk will describe fundamentals and applications of metal nanoparticle lattices that show long-range diffractive interactions when the lattice spacing is close to the wavelength of incident light. These collective excitations are known as surface lattice resonances (SLRs) or lattice plasmons. We will discuss advances in nanofabrication techniques that have enabled the patterning of a wide range nanoparticle materials over macroscale (> in²) areas and with units that are highly uniform in size, shape, and crystallinity. Such patterned substrates support SLRs with wavelengths that approach the theoretical limit. We will also describe how different lattice symmetries can result in band-edge modes at high symmetry points in reciprocal space. The confined electromagnetic fields of SLRs can manipulate processes such as exciton-plasmon coupling, nanolasing, and imaging via tunable, flat optics.