Viktoriia Babicheva, ITMO University
Alexandra Boltasseva, Purdue University
Joshua Caldwell, Vanderbilt University
Isabelle Staude, Friedrich Schiller University Jena
U.S. Department of Energy–Office of Basic Energy Sciences
NM09.01: Plasmonics and Metamaterials
Monday PM, April 02, 2018
PCC North, 200 Level, Room 231 B
8:00 AM - NM09.01.01
Optical Metasurfaces Design—A Genetic Algorithm Optimization Approach
Samad Jafar-Zanjani1,Ali Forouzmand1,Sandeep Inumpudi1,Hossein Mosallaei1
Northeastern University1Show Abstract
In this paper, inverse design methodology, along with genetic algorithm (GA) optimization is employed for synthesis of optical metasurfaces. Despite many advantages of the ultimate forms of ultra-thin layers, the performance of metasurfaces is not yet satisfactory due to the limited range of either amplitude or phase gradients achievable with ‘units of canonical shapes’. In addition, the existing design methodologies by nature do not explicitly support incorporation of multi-functionality, multi-band and broadband applications within one unit cell. Utilizing inverse design methodologies, here we design ‘binary-digitized unit cells’ to overcome the above limitations by pushing the achievable phase and amplitude gradients to fundamental limits. We propose an optimization scheme based on our in-house developed Finite-Difference Time-Domain (FDTD) algorithm, genetic algorithm and GPU computing to optimize the ‘binary-digitized unit cells’ to achieve all possible combinations and independently controllable phase and amplitude gradients at a broad range of frequency spectrum. We present a library of such optimized unit cell patterns that can be utilized and transformed into ultra-thin metasurfaces with a variety of novel applications. In this fashion, conventional applications of metasurfaces, including beam-steering, lensing, and holography can be extended systematically by overcoming the inevitable limitations dictated by regular canonical unit cells. For example, by arranging the optimized unit cells in a unique fashion we present highly-efficient digitized plasmonic metasurface for directive radiation and beam-steering in space. Or by independently controlling the phase gradients at a finite set of frequencies one can obtain challenging functionalities such as achromatic bending and focusing of light irrespective of the frequency range. This will be of extreme benefit to the fields of photonics and metasurfaces, and the future of functional metasurfaces.
8:15 AM - NM09.01.02
Chiral Metamaterial Platform with Tunable Near and Far Field Chiroptical Response
Pavlos Pachidis1,Vivian Ferry1
University of Minnesota1Show Abstract
Chiral plasmonic metamaterials have been proposed as a promising platform for optoelectronic devices with exotic applications, such as negative index of refraction materials and superlenses that can break the diffraction limit. Chiral metamaterials have unit cells that lack mirror symmetry and inversion centers, and exhibit asymmetries in response to circularly polarized light that are orders of magnitude greater than the ones observed from naturally occurring chiral molecules. In the far-field, these asymmetries are manifested in the circular dichroism (CD) signal that quantifies the difference in absorption of left and right handed light. Despite the extensive literature on the far-field optical response of chiral metamaterials, our understanding of the chiral electromagnetic light-matter interactions remains limited. To develop design principles for chiral metamaterials, it is important to characterize and tune the optical chirality C of the electromagnetic fields in the vicinity of the nanostructures in the chiral unit cell, and elucidate the complex relationship between the near- and far-field optical response of chiral systems.
Here, we used Finite-Difference Time-Domain calculations to simulate the optical response of a stacked gold L-shape resonator system. In this system, the sign of the CD spectrum can be controllably changed through lateral shifts in the relative position of two gold L- resonators, which tunes the interaction strength. These small structural reconfigurations change the energetic ordering of the hybridized modes and alter the CD response of the system without changing its handedness. We examined the sensitivity of the system to small structural perturbations, and demonstrated that the CD spectrum can abruptly reverse under small (less than 1 nm) reconfigurations of the L-resonators. The abrupt change in the far-field response reflects the interesting evolution of the near-fields under small reconfigurations of the system.
Calculations revealed that superchiral fields around the plasmonic structures change their magnitude and localization based on the position of the upper L-resonator relative to the bottom one. The optical chirality, and volume of the superchiral fields are much larger for the mode with enhanced extinction cross section. In this way, we demonstrated that small structural modification abruptly change not only the cross section but also the chiral near field response of the L-shape system under illumination with circularly polarized light.
The ability of our chiral L-shape assembly to abruptly change the far-field chiroptical response makes it ideal for experimentally fabricating a fast switchable chiral platform with dynamically tunable CD response. Meanwhile, the tunable chiral local electromagnetic fields could be utilized to examine how the change in the chiral interactions between the nanostructures translates into a change in the far field response of chiral systems.
8:30 AM - NM09.01.03
Nanophotonic Designs for Efficient Propulsion and Radiative Cooling of the Starshot Lightsail
Ognjen Ilic1,Cora Went1,Artur Davoyan1,Deep Jariwala1,Michelle Sherrott1,William Whitney1,Joeson Wong1,Harry Atwater1
California Institute of Technology1Show Abstract
Breakthrough Starshot is an ambitious project with the goal to design and build a laser-propelled spacecraft that can reach Proxima Centauri b, an exoplanet 4.2 lightyears away from Earth, in just 20 years. In order to propel the spacecraft to relativistic speeds (~0.2c), an ultrathin, gram-sized, lightsail must be stably accelerated under MW/cm2 laser intensities operating in the near-IR spectral range. Because radiative cooling in space is the only mechanism for nanocraft thermal management, the Starshot Lightsail requires multiband functionality: it must simultaneously exhibit very low absorptivity in the (Doppler-broadened) laser beam spectrum in the near-IR, and high emissivity in the mid-IR for efficient cooling. These engineering challenges present an opportunity for nanophotonic design. In this work, we show that optimized nanoscale optical structures could play an important role in the lightsail design due to their ability to achieve desired optical response while maintaining low absorption in the NIR, significant emissivity in the MIR, and a very low mass.
To address the issues of efficient propulsion and thermal management, we combine material properties of very weak sub-band absorption in semiconductors with phonon-polariton driven emission in the MIR in materials such as silica. By way of nonlinear optimization, we survey a range of canonical nanophotonic structures (including thin-film slabs, multi-layer stacks, and 2D photonic crystal slabs) to reveal a tradeoff between reflectivity, mass, absorptivity, and emissivity. Our analysis compares several relevant figures of merit for the interaction between the laser and the lightsail and points to optimal designs for propulsion and thermal management.
8:45 AM - NM09.01.04
Highly Efficient Circularly Polarized Light Detection Based on Chip-Integrated Metasurface
Ali Basiri1,Xiahui Chen1,Pouya Amrollahi1,Jing Bai1,Joe Carpenter1,Zachary Holman1,Chao Wang1,Yu Yao1
Arizona State University1Show Abstract
Detection and generation of circularly polarized (CP) light is an essential operation in optical communication, quantum computing, molecular spectroscopy, magnetic recording and imaging applications. Increasing demand for subwavelength and high efficiency detectors has intrigued numerous research groups to approach this problem by designing twisted optical metamaterial and helical structures, spiral plasmonic lens and chiral organic transistors. More recently, owing to the notion of chirality, the detection of handedness of light has been made possible in the context of hot electron injection in plasmonic metamaterials, as well as in all-dielectric metasurface [1-2]. However, complicated fabrication procedure, bandwidth limitation and low values of transmission and circular polarization dichroism are still deemed as impeding factors for most practical applications.
Here we have experimentally demonstrated a hybrid metal-dielectric metasurface for CP light detection in transmission mode. First, based on the birefringence effect we design a quarter waveplate (QWP) by patterning a PECVD-grown silicon layer in the form of a periodic array with rectangular unit cells. This can be achieved by electron beam lithography, followed by inductively coupled plasma etching. The degree of anisotropy in phase accumulation between the fast and slow axes of QWP can be tailored by engineering the aspect ratio, occupation factor and silicon thickness. These degrees of freedom, on the other hand, provide a significant versatility to tune the operation wavelength (visible to NIR) and bandwidth broadening (few hundreds of nanometers). Moreover, due to low loss in dielectric materials in contrast to plasmonic structures, the measured transmission is as high as 95%, associated with a remarkable degree of circular polarization (DOCP>98%). Depending on the handedness of incident light, the QWP output will be linearly polarized +45 or -45 degrees with respect to the major or minor axes of QWP. Therefore, integration of a metallic grating separated by a fused silica spacer layer can almost completely block or pass the output, hence forming a binary detector. The extinction ratios measured in experiment are up to 13, while the overall transmission is close to 90%. Further design optimization can lead to even higher extinction up to 400.
The proposed structure exhibits various advantages including scalability and CMOS compatibility, compact footprint (few tens of micron) and superior DOCP, high extinction ratio and transmission. Moreover, it shows robustness against imperfections in fabrication process which is deemed desirable in comparison with other chiral metamaterial designs in literature. Therefore, it can be a great candidate for imaging, sensing applications and communication systems.
 Wei Li, et al., Nature Communications 6, 8379 (2015)
 Jingpei Hu, et al., Sci Rep. 7: 41893 (2017)
9:00 AM - NM09.01.05
Chip Integrated Plasmonic Flat Optics for Mid-infrared Polarization Detection
Jing Bai1,Chu Wang1,Xiahui Chen1,Ali Basiri1,Chao Wang1,Yu Yao1
Arizona State University1Show Abstract
Polarization detection is an essential topic due to enriching applications including safe optical communication, remote sensing, polarization imaging and biomedical applications1. Polarization, unlike intensity of the light, cannot be directly detected by conventional photodetectors. Currently, the widely used polarization detection methods require bulky optical components such as polarizers and waveplates, which make it challenging for device integration and minimization. Flat optics based on plasmonic structure open a new path for polarization detection with ultra-compact size 2-5. Polarization detection in MIR range is especially attractive due to wide applications in biomedical fields like cancer detection and molecule chirality detection. Yet, MIR polarization detection is even more challenging than that in visible and NIR due to the material absorption limitations. Here we present the theoretical modeling and experimental demonstration of MIR polarization detection based on integrated plasmoinc flat optics composed of optical antenna and nanogratings. Our technique provides complete measurement of full stokes parameters and thus enables the detection of light with any polarization state, including partially polarized light. Moreover, it has the advantages of being ultracompact, capable to work in MIR range with high extinction ratio and easy to integrate with photodetectors. The MIR polarization detector consists of 6 detection units, including 4 nanograting units and 2 circularly polarized light detection units. According to our theoretical modeling, the nanograting units and the CP detection units show high extinction ratio for linearly and circular polarized input light in MIR, respectively. We have also demonstrated experimentally circularly polarized light detection with extinction ratio of 6.2 and linearly polarized light detection with extinction ratio of 45.5. With all 6 elements, we have performed full-stokes polarization measurement of arbitrary polarization states. The measured Stokes Parameters are reasonably well consistent to the input polarization of the light. The average error of S1, S2, S3 is 0.035, 0.025, and 0.104, respectively. And the average error of DOLP and DOCP is 0.036 and 0.103, respectively. The device performance can be further improved by increasing the extinction ratio of the linearly and circular polarization detection units through optimization of design parameters as well as fabrication processes. The detector we proposed can be easily redesigned to any wavelength from NIR to MIR by changing the design parameters of the optical antennas, which is promising for multi-wavelength or broadband polarization detection.
1. Snik, F., et.al. SPIE Proceedings 2014, 90990B.
2. Afshinmanesh, F., et.al. Nanophotonics 2012, 1, (2).
3. Li, W., et.al. Nat Commun 2015, 6, 8379.
4. Pors, A., et.al. Optica 2015, 2, (8), 716.
5. Chen, W. T. et.al. Nanotechnology 2016, 27, (22), 224002.
9:15 AM - NM09.01.06
Gigahertz All-Optical Modulation Using Reconfigurable Plasmonic Metamaterials
Xiangfan Chen1,Biqin Dong1,Chen Wang1,Fan Zhou1,Cheng Sun1
Northwestern University1Show Abstract
We report the design of reconfigurable metamaterial consisting a large array of nanowire featuring U-shaped cross section. These nanowires, also named as nano-scale metamolecules, support co-localized electromagnetic resonance at optical frequencies and mechanical resonance at GHz frequencies with a deep-sub-diffraction-limit spatial confinement (~λ2/100). The coherent coupling of those two distinct resonances manifests a strong optical force, which is fundamentally different from the commonly studied forms of radiation forces, gradient forces, or photo-thermal induced deformation. The strong optical force acting upon the built-in compliance further sets the stage for allowing the metamolecules to dynamically change their optical properties upon the incident light. The all-optical modulation at the frequency at 1.8 GHz has thus been demonstrated experimentally using a monolayer of metamolecules. The metamolecules were conveniently fabricated using CMOS-compatible metal deposition and nano-imprinting processes and thus, offer promising potential in developing integrated all-optical modulator.
9:30 AM - NM09.01.07
Passive PT-Symmetry in Semiconductor-Metal Hybrid Nanoantenna Dimers
Alexander Hwang1,Gururaj Naik1
Rice University1Show Abstract
Non-Hermitian systems can possess real eigenvalues if their Hamiltonians have parity- and time-symmetries (PT-symmetry). Such systems have been actively studied because they demonstrate extensions of conventional Hermitian quantum mechanics into a more generalized framework. Recently, PT-symmetry has gained much attention, especially in optics because of ease of implementation. PT-symmetry in optics translates to a conjugate-symmetric refractive index distribution, i.e. a balanced loss-gain system. Implementing such systems is easier for larger-scale photonic systems than for nanophotonic systems. Thus, so far, experimental studies have primarily focused on larger-scale photonic systems, though there are proposals for nanoscale optical devices with PT-symmetry.
Nano-optical devices often have high losses, which require equally high gain to implement PT-symmetric potentials. Such high gains are impractical and alternative methods to circumvent this problem have been investigated. One such alternative is a passive PT-symmetric system, where the characteristics of PT-symmetry can be observed using lossless and lossy components. These passive systems are simpler to implement using various fabrication techniques and materials available to nanophotonics. Here, we demonstrate a PT-phase transition in a passive dimer system consisting of silicon and silver nanoparticle pairs fabricated using electron-beam lithography. We characterize the system’s PT-symmetric behavior by measuring the scattering spectrum and far-field radiation pattern as a function of coupling, or distance between the particles. From the scattering spectrum, we can deduce the real and imaginary eigenvalues by identifying resonant peaks and linewidths. At the same time, the far-field radiation pattern, observed from the back Fourier plane image, represents the eigenmodes of the system.
In the PT-symmetric phase, where coupling is strong, far-field radiation is dipolar symmetric and scattering spectrum shows two resonant peaks. As coupling is weakened by increasing separation distance, the resonances move closer together in frequency, with little change in linewidth. At the exceptional point, these resonances coincide, resulting in a degenerate mode. Decreasing coupling past the exceptional point leads to a PT-broken phase, where the system exhibits a single resonant peak, with smaller linewidths. In the PT-broken phase, the far-field radiation pattern becomes increasingly asymmetric as coupling lowers, with more scattering towards the lossless particle. This behavior in the transition from the symmetric to symmetry-broken phase demonstrates passive PT-symmetry breaking at the nanoscale. Our understanding of PT-symmetry in this nanoantenna dimer opens opportunities to explore the rich physics underlying PT-symmetric nanosystems.
10:15 AM - NM09.01.08
Colloidal Doped Plasmonic Metal Oxide Nanocrystals—Precise Control Over Shape, Size, Dopant Type and Their Radial Distribution
Ajay Singh1,2,Delia Milliron2
Los Alamos National Laboratory1,The University of Texas at Austin2Show Abstract
Colloidal synthesis of doped metal oxide nanocrystals provides a great opportunity and easy route to generate materials that has unique optoelectronic properties with promising applications such as smart windows, displays, sensing and photo-catalysis etc. By introducing the free carriers with different type of dopants (n- or p-type) in the metal oxide nanocrystals, their surface plasmon resonance can be tuned precisely from near IR to mid-IR range. Similarly, like metals, the optical response of plasmonic metal oxide nanocrystals can be manipulated by controlling the shape, size of the nanocrystal and free electron concentration. The effect of nanocrystal shape, size on the enhancement of their local electrical field strength and surface plasmon resonance have paved the way for new technologies and better sensing opportunities. The sharp faceted nanocrystals exhibit enhanced electric fields at corners and edges, which give us an opportunity to explore different morphologies of the NC for sensing application. Here, we will be presenting a solution route to synthesize plasmonic metal oxide nanocrystal (doped Indium Oxide) with defined shape, size, dopant type and radial distribution of dopant in the nanocrystals. Also, with co-doping (cation, anion or both) in these nanocrystals, we can shift the surface plasmon resonance to higher energies and can also influence the shape of the nanocrystals. Further, we will present near field enhancement property of single nanocrystals via EELS mapping and quantify both near field and far field plasmon property via COMSOL electromagnetic simulations.
10:30 AM - NM09.01.09
Plasmonic Metal Nanostructures for Use in Solar-Thermal Thermionic Optical Power Converters
Nicki Hogan1,Matthew Sheldon1
Texas A&M University1Show Abstract
Through systematic tailoring of the optical properties of lithographically patterned plasmonic nanostructures it is possible to optimize solar absorption and thermal reemission for photo-thermal heating to temperatures well above ambient. We outline a method to take advantage of such resonant photothermal heating in addition to photo-excited hot electrons to promote electron emission from the metal with high efficiency. Due to the close relation to purely thermionic emission this process is termed Hot-Electron Enhanced Thermionic Emission (HEETE). This dual mechanism of electron emission may provide a technique to more efficiently utilize optical power and can theoretically out-perform traditional semiconductor based solar cells.
To address design of such nanostructures, we have developed a simple model of the photo-thermal response of a plasmonic absorber with allows us to explore features such as spectral width of absorbance and emittance as well as angular dependence of emission. Additionally, it allows us to examine the roll of non-radiative thermal loss pathways such as conduction and convection. While these pathways normally dominate, placing the structure in vacuum is a simple way to minimize this loss. In such conditions temperature increases of well over 900 K are achievable without additional optical concentration. The nanostructures that reach these temperatures have high absorption, greater than 90%, in the visible up to 1100 nm and emissivity of approximately 2% through the infrared as well as minimized emission at oblique angles.
Using full wave optical simulations (FDTD method) and particle swarm optimization algorithms, where we were able to use temperature as calculated by our model as the figure of merit to identify possible nanostructures. We found a range of structures that will have the desired absorbance and emittance properties which are made of a variety of noble metals such as gold, silver, and copper. When coated with a dielectric material such as aluminum oxide to increase the thermal tolerance of the nanostructures while minimally impacting the emission characteristics, our designs takes advantage of highly absorbing plasmon resonances in the visible as well as the naturally low emissivity in the infrared which are both characteristic to metals without losing the thermal stability of higher melting point refractory materials. The dielectric coating also allows for accurate temperature measurements of the structure via in-situ anti-stokes Raman thermometry. Test HEETE devices have been nanofabricated on thermally isolated Si3N4 membranes to minimize thermal conduction to the surrounding substrate. Initial temperature measurements demonstrate that these plasmonic arrays greatly exceed the temperatures of ideal blackbodies under solar fluence.
10:45 AM - NM09.01.10
Active Control of the Photoluminescence Emitted by Quantum Dots Using Metallic Nanoparticles and Photochromic Molecules
Gwénaëlle Lamri1,Jana Nieder2,Edite Figueiras2,Jean Aubard3,Pierre-Michel Adam1,Christophe Couteau1,Nordin Felidj3,Anne-Laure Baudrion1
University of Technology at Troyes, France1,Iberian Nanotechnology Laboratory2,ITODYS3Show Abstract
Quantum dots (QDs) can lead either to the enhancement or to the quenching of their photoluminescence , provided that they are coupled with metallic nanoparticles (MNPs). Such MNPs, well known to sustain Localized Surface Plasmon (LSP) resonances, may indeed affect the QDs photoluminescence. The distance between QDs and MNPs is one of the switch parameters between both regimes. The goal of this study is to control the coupling distance (different from the physical distance) between QDs and MNPs by changing the refractive index of the surrounding medium using photochromic molecules. These molecules are optical switches, which move from a transparent state to a colored one by absorbing UV light. The spectral overlap and the lifetime of each optical phenomenon are the key parameters, since the photochromic molecules can couple to LSP to induce strong coupling  or couple to QDs to quench the photoluminescence .
In this study, the Fluorescence Lifetime Imaging Microscopy (FLIM)  has been performed to record QDs photoluminescence lifetime and intensity. We fabricated silver nanoparticles arrays covered with a protective SiO2 layer and we spin-coated different mixtures on top of it. Firstly, we studied this sample spin-coated with QDs in a PMMA matrix and then, we studied the same sample spin-coated with QDs and photochromic molecules diluted in a PMMA matrix. The QDs photoluminescence lifetime and intensity have then been explored before and after the photochromic transition, above and nearby the MNPs arrays.
The analysis of the results shows a Förster Resonant Energy Transfer between the QDs (donors) and the colored form of the photochromic molecules (acceptors). In addition, it is observed an optical activation of the resonant coupling between QDs and MNPs due to the photochromic transition.
 Enhancement and quenching regimes in metal-semiconductor hybrid optical nanosources, P. Viste et al., ACS Nano, vol.4, n° 2, p. 759-764 (2010).
 Reversible strong coupling in silver nanoparticle arrays using photochromic molecules, A.-L. Baudrion et al., Nano Lett. 13, p. 282−286 (2013).
 Reversible Modulation of Quantum Dot Photoluminescence Using a Protein-Bound Photochromic Fluorescence Resonance Energy Transfer Acceptor, I. L. Medintz et al., J. Am. Chem. Soc., 126, p. 30-31 (2004).
 Munster, Erik B. van, and Theodorus W. J. Gadella. « Fluorescence Lifetime Imaging Microscopy (FLIM) ». In Microscopy Techniques, edited by Jens Rietdorf, 143 75. Berlin, Heidelberg: Springer Berlin Heidelberg, 2005. https://doi.org/10.1007/b102213.
11:00 AM - NM09.01.11
Spray-on Vanadium Oxide Films for Plasmonics and Bolometers
Seth Calhoun1,Rachel Evans1,Rikki Leyva1,Robert Peale1,Isaiah Oladeji2
University of Central Florida1,SIsom Thin Films LLC2Show Abstract
Vanadium Oxide is well established for use in infrared bolometers because of its high temperature coefficient of resistivity (TCR). The metal-to-insulator transition of VO2 has attracted recent interest for switchable infrared plasmonic devices. We demonstrate VOx nanocrystalline thin films grown by aqueous spray deposition, which allows perfectly conformal coatings for convenient fabrication of (e.g.) plasmonic slot waveguides and metasurfaces. X-ray diffraction analysis shows that samples annealed post-growth in nitrogen have composition VOx with x close to 2, while X-ray photoelectron spectroscopy on this film gives an x value of about 1.5. Its measured TCR is as high as -2.7 %/degC, which compares favorably with traditional sputtered films. Samples annealed in air have higher crystallinity in the more-oxidized insulating phases V4O9 and V2O5. Thus, for phase change applications, the degree of crystallinity can be increased, and value of x tuned, by post-growth annealing in oxidizing or reducing atmospheres.
11:15 AM - NM09.01.12
Nonreciprocal Nanophotonics with Dielectric and Plasmonic Metasurfaces
Stanford University1Show Abstract
The propagation of free-space electromagnetic signals is generally governed by time-reversal symmetry, meaning that forward- and backward-travelling waves will trace identical paths when being reflected, refracted or diffracted at an interface. Breaking time-reversal symmetry promises significantly improved photo-voltaic efficiencies and optical diodes, but is challenging to achieve in compact optical devices. Here, we introduce two nanophotonic designs that enable nonreciprocal transmission of visible and near infrared light within subwavelength optical paths. First, we design an all-dielectric, 100-nm-thick Si metasurface for non-reciprocal signal propagation. Owing to the high-quality-factor resonances of the metasurface and the inherent Kerr nonlinearities of Si, this structure acts as an optical diode for free-space optical signals. This structure also exhibits nonreciprocal beam steering with appropriate patterning to form a phase gradient metasurface. Secondly, we design a plasmonic metamaterial that exhibits broadband and wide angle nonreciprocity. A parity-time symmetric distribution of saturable loss and gain leads to nonreciprocal transmission over a 50 nm wavelength range and 60 degree angular range at visible frequencies. Compared to existing schemes, these platforms enable time-reversal-symmetry breaking for arbitrary free-space and modal optical inputs in a simple, robust materials platform.
11:45 AM - NM09.01.13
Optical Properties and Plasmonic Performance of Crystalline and Amorphous Titanium Nitride Nanoshells as Selective Solar Absorbers
Zak Blumer1,Martin Kordesch1
Ohio University1Show Abstract
Metal nanostructures are capable of plasmonic selective absorption in specific regimes of the electromagnetic spectrum, consequently leading to a higher ratio of absorption to re-radiation than that of a blackbody absorber. Selective absorbers are candidates for solar thermal energy generation, especially those materials with a large imaginary part of their dielectric function, such as gold. Titanium nitride (TiN) is a ceramic metal that absorbs in the visible and near-infrared range, with a large imaginary part of its dielectric constant—similar to gold and an ideal characteristic for absorption. It is also significantly less expensive, more chemically stable, and more thermally resistant than both gold and silver. TiN thin film shells (100-300 nm) have been grown on glass microbeads (3-10 μm diameter) on a heated boro-aluminosilicate glass substrate (300-600 K) by radio frequency (RF) sputtering physical vapor deposition (PVD) at 7 mTorr (ρAr = 6, ρN = 0.5 mTorr) for 3-24 hours. Raman spectra for the films show consistent peaks at 210, 310, and 550 rel. cm-1. X-ray diffraction (XRD) spectra confirmed crystallization of TiN at higher temperatures, with two peaks representing the (200) and (220) orientations. An analysis of the optical properties of the TiN nanoshells, obtained by spectroscopic ellipsometry (SE), will be presented.
NM09.02: Late-Breaking News
Monday PM, April 02, 2018
PCC North, 200 Level, Room 231 B
1:30 PM - NM09.02.01
“Crypto-Display” in Dual-Mode Metasurfaces by Simultaneous Control of Coherent and Incoherent Optical Responses
Gwanho Yoon1,Dasol Lee1,Ki-Tae Nam2,Junsuk Rho1
Pohang University of Science and Technology1,Seoul National University2Show Abstract
Recently, multi-functional metasurfaces have been demonstrated based on polarization dependency, nonlinear optical effect and superposition of metasurfaces; nevertheless, conventional multi-functional metasurfaces have a severe limitation. We call them “coherent metasurfaces”. When incoherent light such as sunlight is shone on coherent metasurfaces, desired phase distribution cannot be developed; i.e., no information is obtained. In contrast, “incoherent metasurfaces” work only under incoherent light. They show colors or pseudo-holograms by controlling transmission or reflection spectra of incoherent optical waves. This incoherent response of the metasurfaces can be exploited for practical applications because incoherent light is more common than coherent light. However, no metasurface has achieved both coherent and incoherent functionality simultaneously.
Here we propose the first dual-mode metasurface that operates under both coherent and incoherent light simultaneously. The term “dual-mode” represents independent control of coherent response to transmitted light and of incoherent response to reflected light. Our dual-mode metasurface deploys parallel dielectric nanoantennas based on Pancharatnam-Berry phase to control spatial phase distribution of coherent light, and the reflection spectrum of incoherent light. Conventional metasurfaces based on Pancharatnam-Berry phase only control the orientation of each nanoantenna to manipulate phase distribution, but the reflection spectrum is also controllable by changing the sizes of nanoantennas. The nanoantenna sizes affect cross-polarization transmittance, so we find a pair of nanoantenna designs that have equal cross-polarization transmittance near the target wavelength of 635 nm. Based on the pair of designs, we design and experimentally demonstrate a crypto-display that contains encrypted information as an example of the dual-mode metasurface. Under incoherent white light the crypto-display works as a typical reflective display, whereas under coherent light, the encrypted information is revealed in the form of a hologram. Furthermore, the encrypted information does not affect the reflected image, so the information encoded in the crypto-display is not revealed unless coherent light is shone on it. Our device and design approach provide a way to develop novel security technologies such as steganography, anti-counterfeiting measures, and ghost imaging applications.
1:45 PM - NM09.02.02
Novel Chalcogenide as a Material Platform for Tunable Nanoantenna Arrays in the Visible and Near Infrared Spectrum
Li Lu1,Ramon Paniagua-Dominguez2,Vytautas Valuckas2,Robert Simpson1,Arseniy Kuznetsov2
Singapore University of Technology and Design1,Data Storage Institute, A*STAR (Agency for Science, Technology and Research)2Show Abstract
Nanostructures made of dielectric materials can have analogous properties to plasmonic structures for manipulation of light, with the advantage of having lower dissipative losses . Wide bandgap phase change chalcogenides may be tailored to have a large refractive index with a low absorption in the visible and near infrared spectrum, and thus they are a promising platform for tunable metasurfaces in the visible and near infrared spectrum. For one of the most commonly used chalcogenides, Ge2Sb2Te5 (GST), the refractive index at 840nm is rather large, approximately 4.5 for the amorphous state and 5.5 for the crystalline state. However, the extinction coefficient of GST at 840nm is approximately 1.5 and 3.6 for the amorphous and crystalline states respectively, which renders it very lossy and, therefore, impractical for realistic applications. In comparison, the properly designed wide bandgap phase change chalcogenide, which is used herein, has a refractive index of approximately 3.0 and 3.5 for the amorphous and crystalline states at 840nm, with an extinction coefficient near 0 at 840nm for both states, thus meeting the high index and low loss requirements for high efficiency devices.
We designed nanoantenna arrays metasurfaces based on a wide bandgap chalcogenide for the visible and NIR spectrum. The device operates in transmission mode and allows manipulation of the phase of the transmitted wave, and is tunable through structural phase transitions in the chalcogenide material. The proposed device exploits both the refractive index change in chalcogenide and the concept of Huygens’ metasurface  to exhibit a very high transmission (>80%) with full angular 2π phase control. Based on the structural phase change property of the chalcogenide, we designed a gradient metasurface using two different mechanisms: geometrical tuning and partial crystallization. Both designs allow dynamic control of the transmitted light at a wavelength of 840nm. The transmitted beam deflection could be tuned by adjusting the individual crystallization levels of the wide bandgap phase change material, with overall efficiencies exceeding 40% with respect to the incident power.
In conclusion, both simulations and experimental results will be presented that demonstrate nanoantenna array metasurfaces, which are based on a wide bandgap phase change materials, can achieve tunable control of light beams in the visible and NIR spectrum. These results suggest that phase change materials have a further application beyond data storage in high-speed spatial light modulator and phase arrays, with potential applications in dynamic holography.
Li Lu acknowledges his scholarship from Singapore Ministry of Education.
 Kuznetsov, Arseniy I., et al. "Optically resonant dielectric nanostructures." Science 354.6314 (2016): aag2472.
 Yu, Ye Feng, et al. ''High-transmission dielectric metasurface with 2π phase control at visible wavelengths." Laser & Photonics Reviews 9.4 (2015): 412-418.
2:00 PM - NM09.02.03
Tunable Moiré Chiral Metamaterials and Their Applications in Ultrasensitive Sensing
Mingsong Wang1,Zilong Wu1,Yuebing Zheng1
University of Texas at Austin1Show Abstract
Plasmonic chiral metamaterials with strong optical chirality and high tunability in visible and near-infrared light regimes have emerged as promising candidates for photonic sensors and devices. Here, we demonstrate a new type of chiral metamaterials, known as moiré chiral metamaterials (MCMs), to overcome limits in current chiral metamaterials that rely on local structural chirality or site-specific symmetry breaking. Consisting of two layers of identical achiral Au nanohole arrays stacked into moiré patterns, the ultrathin (~70 nm, which is only ≈1/10 of the operation wavelength) MCMs exhibit strong chiroptical effects. The optical chirality can be precisely tuned by the relative rotation between the lattice directions of the two Au nanohole arrays. We have further demonstrated that the MCMs can distinguish a therapeutic chiral drug, R-thalidomide, from its medically toxic enantiomer (S-thalidomide) at picogram level in a label-free manner.
Moreover, we have exploit Fano coupling as a new mechanism to achieve ultrathin active chiral metamaterials of highly tunable chiroptical responses by adding a dielectric spacer layer in MCMs. Our simulations and experiments reveal that spacer-dependent Fano coupling exists in the MCMs, which significantly enhances the spectral shift and line shape change of the circular dichroism (CD) spectra of the MCMs. We further use a silk fibroin thin film as an active spacer layer in the MCMs. With the solvent-controllable swelling of the silk fibroin thin films, we demonstrate tunable Fano coupling and chiroptical responses of the silk-MCMs using different solvents and their mixtures. Impressively, we have achieved the spectral shift over a wavelength range that is more than one full width at half maximum and the sign inversion of the CD spectra in a single ultrathin (1/5 of wavelength in thickness) MCM. Finally, we apply the silk-MCMs as ultrasensitive sensors to detect trace amount of solvent impurities down to 200 ppm, corresponding to an ultrahigh sensitivity of >105 nm/refractive index unit (RIU) and a figure of merit of 105 /RIU. With their strong and tunable optical chirality, in combination with robust cost-effective fabrication, the MCMs will become critical components for chiroptical devices. Our results also pave a way towards active chiral metamaterials of high tunability, ultrathin thickness and large-scale fabrication for a wide range of applications.
2:15 PM - NM09.02.04
Robust Extraction of Hyperbolic Metamaterial Permittivity Using Total Internal Reflection Ellipsometry
Cheng Zhang1,Nina Hong2,Chengang Ji3,Wenqi Zhu1,Xi Chen3,Amit Agrawal1,Zhong Zhang3,Tom E. Tiwald2,Stefan Schoeche2,James N. Hilfiker2,L. Jay Guo3,Henri J. Lezec1
National Institute of Standards and Technology1,J. A. Woollam Co.2,University of Michigan-Ann Arbor3Show Abstract
Hyperbolic metamaterials (HMMs) are highly anisotropic structures that exhibit metallic (i.e., Re (ε) < 0) and dielectric (i.e., Re (ε) > 0) response along orthogonal directions. They have been utilized to demonstrate various phenomena, including broadband light absorption, enhanced spontaneous emission, asymmetric light transmission, engineered thermal radiation, and sub-diffraction imaging. The key to the array of rich phenomena enabled by HMMs is their highly anisotropic permittivity. HMMs reported to date are often described by numerically calculated permittivity tensors based on effective medium theory (EMT), which utilizes constituent metal and dielectric permittivities reported in the literature or measured by spectroscopic ellipsometry. However, the accuracy of calculation is limited by the known precision of experimental layer thicknessness and local permittivities, as well as non-modelled effects such as layer roughness, strain, and inter-layer diffusion.
In this work, we demonstrate how both the in-plane and out-of-plane effective permittivities of an HMM operating at ultraviolet, visible, and near-infrared frequencies can be accurately extracted using a coupling-prism-enabled spectroscopic ellipsometry technique based on total internal reflection (TIR). For reference, this technique is compared to two other spectroscopic ellipsometry methods commonly used to date for HMM characterization, namely (1) interference enhancement (IE), in which reflection-mode ellipsometry exploits a substrate decorated with a silicon oxide layer to enhance light-HMM interaction, and (2) reflection plus transmission (R+T), which adds normal-incidence transmittance spectroscopy to standard reflection-mode ellipsometry. Although both IE and R+T techniques have been successfully used for characterizing isotropic thin absorbing films, we show here that neither method is able to robustly extract HMM out-of-plane effective permittivity. In contrast, the TIR method is demonstrated to provide robust permittivity extraction having well-converged fitting parameters. In particular, measurement sensitivity is improved compared to both the IE and R+T cases via prism-mediated enhancement of the out-of-plane electric field inside the HMM. The TIR technique requires neither modification of the HMM sample itself nor substantial re-configuration of a standard ellipsometer, and can therefore serve as a reliable and easy-to-adopt technique for the characterization of both HMMs and a variety of other anisotropic metamaterials.
2:30 PM - NM09.02.05
Ultrahigh Density Plasmonic Nanopillar Arrays on Plastic Substrates: Material Platforms for Ultrasensitive Raman Sensors
Dong-Ho Kim1,Ho Sang Jung1,Sung-Gyu Park1
With increasing concerns about environmental pollution, opiate abuse and terrorism, people have become more sensitive to hazardous substances that threaten public health and safety. Raman spectroscopy is a very useful analytical tool, giving molecular fingerprint information even with portable readers. However, since inelastic (Raman) scattering of light is inherently weak, the Raman-based sensors cannot detect the substances in trace amounts. The extraordinary enhancement of Raman signals from molecules adjacent to metallic nanostructures, which is called as surface-enhanced Raman scattering (SERS), has been discovered by Professor Van Duyne in 1976. Over the past 40 years, scientists have continuously expanded the theoretical understanding of the plasmonic phenomena and developed various nanomaterials to be SERS-active.
From a practical point-of-view, cost-effective high-throughput methods of fabricating SERS substrates are in great demand. In this regard, we introduce a novel approach for fabricating SERS substrates on plastic films. Maskless plasma etching of a plastic film produces nano-protrusions on the surface, which serving as selective growth sites for nanostructure development during the subsequent metal deposition step. These simple two steps result in ultrahigh density (>100/μm2) plasmonic nanopillar arrays. Besides of SERS performance (i.e., enhancement factor > 107), the reproducibility and uniformity are thoroughly examined in 4 inch wafer scale. The detections of forensic drugs (heroin, Fentanyl, methamphetamine) and explosives (TNT, RDX, PETN) in low concentrations have been demonstrated on our SERS substrates (KIMStrates) using a portable Raman reader (Metrohm Raman Ltd.). We strongly believe that this economical and reliable SERS substrate can be a material platform for ultrasensitive Raman sensors in various applications of SERS technology.
2:45 PM - NM09.02.06
Influences of Geometric Inversion of Nanostructures on Antireflection for High Angle of Incidence Considering Mie Scattering and Guided Mode Resonance
Seungmuk Ji1,Jihye Lee1,Young-Shik Yun1,Jong-Souk Yeo1
Yonsei University1Show Abstract
Antireflective (AR) nanostructures observed in nature, such as the corneal surface of Moth eye, the wing scales of butterflies etc., exhibit an excellent AR performance compared to the quarter wave thin films over a wide range of incident angles and wavelengths, thus attracting great interests in the field of optical and optoelectronic devices. Especially, anti-reflectivity for wide angles of incidence is significant in display and photovoltaic applications to improve visibility and photo-conversion efficiency, respectively. Since the AR nanostructures with 3D geometry are fabricated on a substrate, it is necessary to consider the scattering and coupling of the light due to the geometry of the nanostructures on the substrate, in dealing with the anti-reflectivity by the incident angles.
In this work, we investigate effects on the AR properties of two geometries, nanocones (NCs) and inverted nanocones (INCs) which can be generated by geometric inversion in the nano-imprinting process. We fabricate the two geometries by repetitive polymer replication processes by using photo-curable polymers and nanostructured quartz molds and evaluate the specular reflectance for visible range with various incident angles from 6° to 75°. The measured spectra are analyzed in the view of Mie scattering and guided mode resonance.
We find that, unlike the INCs, the NCs enable to maintain Mie scattering efficiency against changes in the incident angles because the scattering fields are concentrated at the apex of the NCs. This phenomenon is verified by computational simulations based on finite-difference time domain methods. The concentrated scatterings on NCs allow the more propagation of incident fields and for this reason, the NCs provide better AR performance than the INCs. We observe the presence of guided mode resonance from the measured spectra and analyze it by considering the phase matching in 2D hexagonal nano-grating structures. Additionally, we find that INCs can exhibit stronger guided mode resonance and internal reflections, which can be another reason why AR performance is degraded in the INCs. By utilizing these findings on both-sided antireflective nanocones, we achieve extremely low average reflectance (5.4 %) at very high incidence angle of 75° for entire visible range.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1D1A1B04033182)
3:30 PM - NM09.02.07
Collective Optical Effects in Exciton-Plasmon Nanomaterials
Maxim Sukharev1,Renaud Vallee2,Abraham Nitzan3
Arizona State University1,CNRS researcher Centre de Recherche Paul-Pascal2,University of Pennsylvania3Show Abstract
Nanomaterials comprised of molecular excitons optically coupled to surface plasmon-polaritons at metal interfaces are considered. Using semiclassical theory we investigate optical properties of such systems under strong coupling conditions and high molecular concentrations. It is shown that exciton-plasmon materials in addition to conventional Rabi splittings exhibit collective optical exciton resonances. The results of theory are compared with experimental measurements for 3D opal plasmonic arrays. We also examine the radiative decay rates at high molecular concentrations. It is shown that the decay rates are significantly reduced due to strong exciton-exciton coupling.
3:45 PM - NM09.02.08
Tunable Plasmon-Exciton Interactions in Hybrid Systems of Single Plasmonic Nanoparticle and Two-Dimensional Transition Metal Dichalcogenides
Mingsong Wang1,Yuebing Zheng1
The University of Texas at Austin1Show Abstract
In recent years, the development of two-dimensional transition metal dichalcogenides (2D TMDs) has aroused great interests in a variety of optoelectronic applications including photodetectors, optical chemical sensors, light-emitting diodes, lasers, and opto-valleytronic devises because of their high ON/OFF current ratios, low sub-threshold switching, strong photoluminescence, controllable valley polarization and high thermal stability. Despite their excellent optoelectronic properties, the light-matter interaction in 2D TMDs is weak due to their atomic thickness, thereby limiting their optoelectronic applications. Benefiting from the capability of surface plasmons (SPs) in concentrating light beyond the diffraction limit, plasmonic metal NPs have been applied to enhance light-matter interactions in quantum emitters including dye molecules and quantum dots through mechanisms such as Fano interference, strong coupling, plasmon-induced resonance energy transfer, and plasmon-enhanced emission. Therefore, there is an emerging trend of exploiting light-matter interactions in hybrid systems consisting of 2D TMDs and plasmonic metal NPs for boosting the performance of 2D TMD-based optoelectronic devices. Herein, we report two tunable plasmon-exciton interactions that are novel in 2D TMD-plasmonic NP hybrids: (1) tunable plasmon-induced resonance energy transfer from a single Au nanotriangle (AuNT) to monolayer MoS2; (2) tunable Fano resonance and plasmon-exciton coupling in a single AuNT on monolayer WS2 at room temperature. In the first case, we report the first observation and tuning of plasmon-trion and plasmon-exciton resonance energy transfer (RET) from a single AuNT to monolayer MoS2. We achieved these phenomena by the combination of rational design of hybrid 2D TMD-plasmonic NP systems and single-nanoparticle measurements. By combining experimental measurements with theoretical calculations, we conclude that the efficient RET between SPs of metal NPs and excitons or trions in monolayer MoS2 is enabled by the large quantum confinement and reduced dielectric screening in monolayer MoS2. In the second case, we report tunable Fano resonances and plasmon-exciton coupling in 2D WS2-AuNT hybrids at room temperature. The tuning of Fano resonances and plasmon-exciton coupling was achieved by active control of the WS2 exciton binding energy and dipole-dipole interaction through controlling the dielectric constant of the surround medium. Specially, Fano resonances are controlled by the exciton binding energy or the localized surface plasmon resonance (LSPR) strength through tuning the dielectric constant of surrounding solvents or the dimension of AuNTs. Additionally, we observe a transition from weak to strong plasmon-exciton coupling when increasing the dielectric constant of surrounding solvents. Our results provide guidance on systematic tuning of the Fano line-shape and Rabi splitting energies at room temperature for 2D TMD-plasmonic NP hybrids.
4:00 PM - NM09.02.09
3D-Printed Infrared Metamaterials
Joseph Tischler1,Nicholas Sharac1,Michael Meeker1,Swathi Lyer1,Chase Ellis1,Keith Perkins1,Sharka Prokes1,Chul Soo Kim1,Erin Cleveland1,Diogenes Placencia1
U.S. Naval Research Lab1Show Abstract
Intense research on two-photon polymerization (2PP) processes has led to the development of sophisticated commercial apparatus capable of producing arbitrary 3D polymer scaffolds with spatial resolutions as high as 170 nm. Generally speaking, these polymer-based constructs do not interact with photons due to their low conductivity and low dielectric constants. Therefore, they do not make good optical metamaterials by themselves; however, metals and materials with high dielectrics constants such as polar dielectrics (e.g., Si, hBN and SiC) do. In this work we produced novel optical metamaterials by combining 2PP (or 3D-printed) structures with e-beam evaporation, atomic layer deposition and/or reactive-ion etching. Furthermore we compare optical measurements performed on these structures with full-wave electromagnetic simulations, demonstrating the strength of these fabrication methods of chiral/non-chiral structures suitable for applications such as SERS, SEIRA, light steering, and sub wavelength light focusing.
4:15 PM - NM09.02.10
Ultra Low Loss Polaritons in Hexagonal Boron Nitride
Thomas Folland6,Alexander Giles1,Siyuan Dai2,Igor Vurgaftman1,Timothy Hoffman3,Song Liu3,Lucas Lindsay4,Chase Ellis1,Ioannis Chatzakis1,Thomas Reinecke1,Joseph Tischler1,Michael Fogler2,James Edgar3,Dimitri Basov5,Joshua Caldwell6
U.S. Naval Research Laboratory1,University of California, San Diego2,Kansas State University3,Oak Ridge National Laboratory4,Columbia University5,Vanderbilt6Show Abstract
Conventional optical components are limited to size-scales much larger than the wavelength of light, as changes to the amplitude, phase and polarization of the electromagnetic fields are accrued gradually along an optical path. However, advances in nanophotonics have produced ultrathin, so-called “flat” optical components that beget abrupt changes in these properties over distances significantly shorter than the free space wavelength. While high optical losses still plague many approaches, phonon polariton materials have demonstrated long lifetimes for localized modes in comparison to plasmon-polariton based nanophotonics. Our work predicts a further 14-fold increase in the optic phonon lifetime and we experimentally report a ~3-fold improvement through isotopic enrichment of hexagonal boron nitride (hBN). We establish commensurate increases in the phonon polariton propagation length via direct imaging of polaritonic standing waves by means of infrared nano optics. Our results provide the foundation for a materials-growth-directed approach towards realizing the loss control necessary for the development of phonon polariton based nanophotonic devices.
4:30 PM - NM09.02.11
Electrochromic Tuning of Transparent Gold Nanorods with Poly[(3,4-propylenedioxy)pyrrole] Shells in the Near-Infrared Region
Jing Zhou1,Ju Won Jeon2,James Ponder1,Jeffrey Geldmeier1,Mahmoud Mahmoud3,Mostafa El-Sayed1,John Reynolds1,Vladimir Tsukruk1
Georgia Institute of Technology1,The University of Alabama2,The University of Texas at San Antonio3Show Abstract
Active control of the plasmonic properties in a dynamic and reversible manner enables applications such as plasmonic sensing and photovoltaic devices. Herein, we present electrochemically tunable hybrid nanostructures composed of gold nanorods encapsulated with directly polymerized poly[(3,4-propylenedioxy)pyrrole] (PProDOP) nanoshells with controlled thicknesses. This system displays narrow visible-near infrared absorption bands upon applying a variable electric potential due to the remarkable transmissivity of PProDOP at various oxidation states. The PProDOP, synthesized by in-situ chemical oxidative polymerization using a mild oxidizing agent, has demonstrated outstanding electrochemical performance such as reversible electroactivity, high transmissivity in visible-range at various oxidation states, as well as a low oxidation potential (-1.06 V vs. Fc/Fc+). It was revealed that the stable reversible modulation of the observed plasmonic response of the gold nanorods was caused by the variation of the refractive index of PProDOP shells at different oxidation states as confirmed by finite-difference time-domain (FDTD) simulations. A surface plasmon resonance (LSPR) band of gold nanorods at 800 nm was shifted reversibly by 24 nm upon multiple cycling of electric potential. Overall, these core-shell structures with electrochemical plasmonic tunability in the near-infrared region allow for tailoring of the optical and electrochemical properties of pre-programmed plasmon responses for active control of colorimetric appearance across the visible range and toward the near-infrared.
4:45 PM - NM09.02.12
Plexciton in Fullerene-gold Nanostructures
Fu-Cheng Tsai1,Cheng-His Weng1,Yu Lim Chen2,Wen-Pin Shih1,Pei-Zen Chang1
National Taiwan University1,National Taiwan Normal University2Show Abstract
This research develops a plasmon-exciton system, which is composed of the fullerene film and the gold nanostructure, applicated in the optics and optoelectronics. Fullerene exciton can be excited and interacted with the surface plasmons produced from the gold nanostructure, and this interaction results in the plasmon energy transporting out of the near-field range. We demonstrate this effect by the cavity structure that sandwiches the fullerene films between a monolayer gold nano-islands and a gold film. The gold film act as a plasmonic mirror producing the image charges and its electromagnetic field couples with the extended plasmonic field from the nano-islands. The coupling phenomenon makes the reflection spectra exhibiting the asymmetric curve-lines, and it brings our cavity structure displaying a bright, saturated, and nearly omnidirectional visible colors. Furthermore, the plasmon-exciton interaction has a significant advantage in the plasmoelectric effect, which is an energy converting rout from plasmon to electronic. The strength of plasmoelectric potential is dominated by the effective temperature. Because of the extended plasmon energy by the plasmon-exciton interaction and the ultralow emissivity of the fullerene, the temperature of the hot spots may reach thousands of kelvins. The high temperature makes the output voltage is up to 277 mV under the UV illumination with the intensity of 10 mW/cm2. The efficiency is hundreds of times as large as the voltage produced from the single layer of gold nanoparticles. With this advantage of high plasmoelectric voltage, fullerene films have broad use in many optoelectronic applications, such as solar cell, catalysis, and photovoltaic devices.
Viktoriia Babicheva, ITMO University
Alexandra Boltasseva, Purdue University
Joshua Caldwell, Vanderbilt University
Isabelle Staude, Friedrich Schiller University Jena
U.S. Department of Energy–Office of Basic Energy Sciences
NM09.03: Nonreciprocal and Nonlinear Metasurfaces
Tuesday AM, April 03, 2018
PCC North, 200 Level, Room 231 BC
10:30 AM - NM09.03.01
Multiphysics Simulation of Reconfigurable Phase-Change Material Based Meta-Surfaces
Dmitry ChigrinShow Abstract
Today it is possible to engineer the building blocks of artificial materials (meta-materials) with feature sizes smaller than the wavelength of light. The ability to design meta-atoms in a largely arbitrary fashion adds a new degree of freedom in material engineering, allowing to create artificial materials with unusual electromagnetic properties rare or absent in nature. Achieving tunable, switchable and non-linear functionalities of meta-materials at individual meta-atom level could potentially lead to additional flexibility in designing active photonic devices. These include among others, meta-materials based on phase-change materials, whose properties could be altered by thermal or photo-thermal means. In this presentation, our recent results on developing appropriate numerical methods to study hybrid meta-material structures containing phase-change materials will be discussed. Meta-atoms based on plasmon polaritonic or phonon polaritonic materials are considered depending on the application spectral range. We develop appropriate phenomenological models of phase transition and self-consistently couple them with the full wave electromagnetic and heat transfer solvers. Developed methods are used to design meta-surface based tunable components.
11:00 AM - NM09.03.02
(Invited) Challenges, Trends and Prospects for Photonic Materials, Metamaterials and Metasurfaces
National Science Foundation1Show Abstract
Fundamental research in photonic materials has been one of the core drivers of progress in optical and photonic technologies, including photonic nanotechnologies, as well as in classical and quantum optical information processing. In particular, optical metamaterials - artificial composites with an engineered electromagnetic response - have recently emerged as a nascent class of novel materials that may offer the control and robustness required for a vast array of applications, ranging from precision wavefront sculpting through cloaking and to electromagnetic control at the single-quantum level. Using highlights of recent accomplishments and awards made through the Electronic and Photonic Materials Program at the NSF, I will address the current trends and recent breakthroughs in studies of optical metamaterials and metasurfaces, as well as their main materials challenges. I will additionally discuss recently identified gaps, both epistemic and technological, and the best approaches for addressing these challenges.
11:15 AM - NM09.03.03
High-Frequency Reststrahlen Bands in Molecular Crystals for Surface-Phonon Polariton Applications
Adam Dunkelberger1,Kenan Fears1,Daniel Ratchford1,Roderick Davidson1,2,Andrea Grafton1,2,Jeff Owrutsky1
U.S. Naval Research Laboratory1,NRC Research Associate Program2Show Abstract
Surface-phonon polaritons (SPhPs) have emerged as attractive alternatives to plasmon polaritons because of the extremely high quality factors of their localized and propagating resonances (SPhPRs). The high quality arises from their dependence on concerted nuclear motion rather than electronic motion. These resonances can only be supported when the resonance frequency lies in the Reststrahlen band of a material, the region between the longitudinal and transverse optical phonons and characterized by metal-like optical constants. To date, most SPhPs have been observed in polar semiconductors, where the Reststrahlen band tends to occur at wavelengths longer than 6 micrometers. Shorter wavelength, higher frequency SPhPRs could potentially be useful for a variety of chemical applications like sensing or energy-transfer modulation. Here, we present infrared reflection spectroscopy of a number of molecular crystals that possess Reststrahlen bands in chemically relevant frequency ranges. We identify and assign resonances that appear within the Reststrahlen bands of some of these materials, specifically W(CO)6, and comment on the range of resonances that can be supported on this class of SPhP materials.
11:30 AM - NM09.03.04
Ultrafast and Highly Nonlinear Metasurfaces
Igal BrenerShow Abstract
The new design paradigm that metamaterials and metasurfaces provide, such as the ability to tailor the local near fields as well as the far field radiation patterns, are enabling new schemes for ultrafast control of light and nonlinear optical sources. In this talk I will describe some of these new developments when metasurfaces made from conventional and new oxide semiconductors are used combined with short pulse excitation using single and multiple pulses. I will describe i) recent results of multiple harmonic mixing spanning a wavelength range from the near infrared to the ultraviolet using highly nonlinear GaAs based metasurfaces, ii) ultrafast subpicosecond polarization switching of light with high contrast using plasmonic cavities containing high mobility CdO films.
NM09.04: Nanoparticles and Metamaterials
Tuesday PM, April 03, 2018
PCC North, 200 Level, Room 231 BC
1:30 PM - NM09.04.01
Designing Active Plasmonic Metasurfaces from Colloidal Nanocrystal Building Blocks
University of Pennsylvania1Show Abstract
Colloidal plasmonic nanocrystals (NCs) are known for their size- and shape-dependent localized surface plasmon resonances. Here we show these plasmonic NCs can be used as building blocks of mesoscale materials.1 Chemical exchange of the long ligands used in NC synthesis with more compact ligand chemistries brings neighboring NCs into proximity and increases interparticle coupling. This ligand-controlled coupling allows us to tailor a dielectric-to-metal phase transition seen by a 1010 range in DC conductivity and a dielectric permittivity ranging from everywhere positive to everywhere negative across the whole range of optical frequencies. We realize a "diluted metal" with optical properties not found in the bulk metal analog, presenting a new axis in plasmonic materials design and the realization of optical properties akin to next-generation metamaterials. We harness the solution-processability and physical properties of colloidal plasmonic NCs to print NC superstructures for large-area, active metamaterials. We demonstrate quarter-wave plates with extreme bandwidths and high polarization conversion efficiencies in the near- to-mid infrared.2 By fabricating colloidal NC superstructures on the surface of hydrogels, we fabricate optically-responsive sensors suitable for large-area monitoring of soil moisture. Finally, by combining superparamagnetic Zn0.2Fe2.8O4 NCs and plasmonic Au NCs, we fabricate multifunctional, smart superparticles, that in suspensions, switch their polarization-dependent transmission in the infrared in response to an external magnetic field.3
(1) Fafarman, A. T.; Hong, S.-H.; Caglayan, H.; Ye, X.; Diroll, B. T.; Paik, T.; Engheta, N.; Murray, C. B.; Kagan, C. R. Nano Lett. 2013, 13 (2), 350–357.
(2) Chen, W.; Tymchenko, M.; Gopalan, P.; Ye, X.; Wu, Y.; Zhang, M.; Murray, C. B.; Alu, A.; Kagan, C. R. Nano Lett. 2015, 15 (8), 5254–5260.
(3) Zhang, M.; Magagnosc, D. J.; Liberal, I.; Yu, Y.; Yun, H.; Yang, H.; Wu, Y.; Guo, J.; Chen, W.; Shin, Y. J.; Stein, A.; Kikkawa, J. M.; Engheta, N.; Gianola, D. S.; Murray, C. B.; Kagan, C. R. Nat. Nanotechnol. 2016, 12 (3), 228–232.
2:00 PM - NM09.04.02
From Macro- to Nano- Materials as New Types of Light Sources
Centre de Recherche Paul Pascal1Show Abstract
The numerical design, synthesis and characterization of advanced colloidal structures and foams based on High Internal Phase Emulsions for application in plasmonics, nano- and macro- photonics has proven to be very attractive, especially in the fields of lasing and new single photon sources.
In this talk, we will report a review of our work in these domains and explain the salient features of the involved effects.
As such, i) we provide experimental evidence of plasmonic super-radiance of organic emitters grafted to Au@SiO2 nanospheres at room temperature. This observation of plasmonic super-radiance at room temperature opens questions about the robustness of these collective states against decoherence mechanisms which are of major interest for potential applications.
ii) We demonstrate both experimentally and theoretically how to manipulate strong coupling between the Bragg-plasmon mode supported by an organo-metallic array and molecular excitons in the form of J-aggregates dispersed on the hybrid structure . We observe experimentally the transition from a conventional strong coupling regime exhibiting the usual upper and lower polaritonic branches to a more complex regime, where a third nondispersive mode is seen, as the concentration of J-aggregates is increased. Owing to numerical simulations, we could confirm the presence of the third resonance and attribute its physical nature.
iii) We demonstrate lasing oscillation in Colloidal Photonic Crystals (CPCs) based on a defect mode passband effect . The spectroscopic measurements and theoretical simulations match well and reveal that the relatively low-threshold lasing exhibited by the structure can uniquely be attributed to the efficient coupling of the spontaneous emission of the dye to the defect mode of the CPC.
Finally, iV), we have numerically predicted and experimentally shown the coexistence and competition of random lasing (RL) and stimulated Raman scattering (SRS) in active disordered random media: foams based on silica HIPEs [3, 4]. We developed a simple model which includes both mechanisms coupled through diffusion equations. We found that the prevalence of a nonlinear mechanism over the other is determined by the degree of scattering. The competition was explained in terms of disorder-dependent pump depletion and fluorescence saturation.
 P. Fauché, C. Gebhardt, M. Sukharev, M., R. A. L. Vallée, Scientific Reports 7, 4107 (2017).
 K. Zhong, L. Liu, X. Xu, M. Hillen, A. Yamada, X. Zhou, N. Verellen, K. Song, S. Van Cleuvenbergen, R. Vallée and K. Clays, ACS Photonics 3, 2330-2337, (2016).
 N. Bachelard, P. Gaikwad, R. Backov, P. Sebbah and R. A. L. Vallée, ACS Photonics, 1(11), 1206–1211 (2014)
 P. Gaikwad, N. Bachelard, P. Sebbah, R. Backov and RAL Vallée, Advanced Optical Materials, 3(11), 1640–1651 (2015).
2:15 PM - NM09.04.03
(Invited) Navy Perspective on Metamaterials
Office of Naval Research1Show Abstract
Metamaterials provide the ability to design material properties beyond those possible with conventional materials. Research programs at ONR and other agencies are pursuing fundamental and applied goals with the aim of mapping out the potential of metamaterials for a variety of applications. These programs have pursued optical, RF and acoustic materials for applications across the electromagnetic spectrum, from novel RF antennas to devices utilizing optical magnetism. Acoustic metamaterials are of unique interest to the Navy for underwater applications. Common challenges associated with loss, bandwidth, and scalability will be discussed along with the needs for innovative designs, materials, and fabrication approaches.
NM09.05: Wave Propagation and Disorder
Tuesday PM, April 03, 2018
PCC North, 200 Level, Room 231 BC
3:30 PM - NM09.05.01
The Delay-Bandwidth Limit in Nanophotonics and Electrostatic Wave Propagation in Time-Modulated Lattices
Andrea Alu1,Sander Mann1,Mykhailo Tymchenko1,Dimitrios Sounas1
The University of Texas-Austin1Show Abstract
The delay-bandwidth limit refers to the trade-off between the time delay that can be applied to a signal traveling through a device and its bandwidth. Recently, there have been several studies showing that this bound can be broken in nanophotnics, including time-modulated photonic crystals, nonreciprocal cavities and terminated unidirectional waveguides. In our talk, we will discuss various approaches to overcoming the delay-bandwidth limit, their relation to Lorentz reciprocity, and a novel approach to slow down waves using temporally modulated network, which enables a quasi-electrostatic signal transport with large group velocities over broad bandwidths of operation.
4:00 PM - NM09.05.02
Tailoring the Optical Properties of Metasurfaces by Deterministic Structural Disorder
Dennis Arslan1,Isabelle Staude1,Stefan Fasold1,Aso Rahimzadegan2,Trideep Kawde1,Sebastian Linß1,Najmeh Abbasirad1,Matthias Falkner1,Manuel Decker3,1,Carsten Rockstuhl2,Thomas Pertsch1
Friedrich Schiller University Jena1,Karlsruhe Institute of Technology2,The Australian National University3Show Abstract
Based on their ability to provide control over wavefront, polarization and spectrum of light fields while having just nanoscale thickness, optical metasurfaces are promising candidates for flat optical components. Typically, metasurfaces consist of two-dimensional subwavelength arrays of designed metallic or dielectric scatterers. So far, deviations from a periodic, ordered arrangement were usually associated with a deterioration of the metasurface optical properties. However, more recently researchers started recognizing the introduction of controlled disorder as a new handle to engineer the optical response of metasurfaces. For example, the introduction of disorder can decrease unwanted anisotropy in the optical response  and it can enhance the channel capacity of wavefront shaping metasurfaces .
Here we investigate two different types of disordered metasurfaces. In a first study, we consider a chiral plasmonic metasurface consisting of twisted gold-nanorod dimers. Chiral metasurfaces and metamaterials were intensively studied in the past. Most prominently, they can exhibit huge optical activity  and were suggested for applications as polarizing elements [4,5] or nanophotonic sensors. Using polarization spectroscopy and interferometric white-light spectroscopy, we demonstrate that the introduction of rotational disorder at the unit-cell level enables the realization of chiral plasmonic metasurfaces supporting pure circular dichroism and circular birefringence. Importantly, we show experimentally that the polarization eigenstates of these metasurfaces, which coincide with the fundamental right- and left-handed circular polarizations, do not depend on the wavelength in the spectral range of interest. Thereby, our metasurfaces closely mimic the behaviour of natural chiral media, while providing a much stronger chiral response.
In a second study, we concentrate on disordered silicon metasurfaces exhibiting electric and magnetic dipolar Mie-type resonances . Silicon metasurfaces exhibit very low absorption losses in the near-infrared spectral range, thereby opening the door to long-range in-plane interactions between the individual nanoresonators. We systematically investigate how the introduction of different types of positional disorder influences the complex transmittance spectra of these metasurfaces, showing that disorder provides an independent degree of freedom for engineering their spatial and spectral dispersion.
 S. S. Kruk et al., Phys. Rev. B 88, 201404(R) (2013).
 D. Veksler et al., ACS Photonics 2, 661 (2015).
 M. Decker et al., Opt. Lett. 35, 1593 (2010).
 J. K. Gansel et al., Science 325, 1513 (2009).
 Y. Zhao et al., Nat. Commun. 3, 870 (2012).
 I. Staude & J. Schilling, Nature Photon. 11, 274 (2017).
4:15 PM - NM09.05.03
Towards Random Metasurface Based Devices
Matthieu Dupre1,LiYi Hsu1,Junhee Park1,Boubacar Kante1
University of California, San Diego1Show Abstract
Random metamaterials present several advantages compare to their periodic counterparts. (i) The circular symmetry of the elements is statistically restored by the randomness, which allows to design polarization independent metalenses with very anisotropic elements, providing new opportunities to design a. (ii) The random design process optimizes the area of the metasurface. In a periodic metasurface, small and large elements have the same footprint. On the contrary, in a random metasurface, the random design finds more easily a spot for a small element than a large one. This comes to optimize the local density and the footprint of the elements. (iii) The absence of periodicity eliminates any possible spurious diffraction order that can arise for large periods, due to large footprints of elements as in dielectric metasurfaces, or for large numerical aperture lenses.
We will consider arrays of gold plasmonic nanorods in the infrared domain (1500 nm). Such rectangular element is very anisotropic and only polarizable along its longer dimension. Varying the nanorod length from 150 to 500 nm changes the resonant frequency of the element, which allows us to tune the phase-shift provided to an incident plane wave which electric field is parallel to the long axis. On the contrary, the nanorod is transparent to an incoming plane wave with a polarization perpendicular to its main axis. The question that arises is: can we use this simple but anisotropic element to design an isotropic and all polarization metalens?
Here we propose to discuss metasurfaces made of such nanorods that have random positions and orientations. The density of elements is the only tunable parameter in the case of random metamaterial. It influences the phase shift through the scattering cross section of the resonators as well as the near field coupling between them. Hence, we will discuss the parking problem of rectangular elements in 2 dimensions in the context of metasurfaces.The focusing efficiency strongly depends on the density if nanorod per wavelength but also of the dimensionality and of the symmetry of the metasurface Using full wave simulation with CST, we design ordered and nematic 1D and 2D metalens and compare their characteristics.
Finally, we present a experimental realizations of 2D random metalens. The latter are made with conventional top-down fabrication techniques and e-beam lithography. We will show that the resulting lens focus light on diffraction limited focal spots for any polarization.
As a conclusion, this communication will show that random metasurfaces are can be used to realize devices usually made within a periodic framework, opening new perspectives to design metasurfaces.
4:30 PM - NM09.05.04
Wave Propagation via Eigenchannels of Scattering Medium
Missouri University of Science and Technology1Show Abstract
The concept of diffusion is widely used to study propagation of light through multiple scattering media such as clouds, interstellar gas, colloids, paint, and biological tissue. Such media are often called random. This terminology is, however, misleading. Notwithstanding its complexity, the process of wave (e.g. light, sound, electron wave, etc) propagation is deterministic – i.e. given the exact position of scattering centers and the amplitudes of the impinging waves, one can uniquely determine the precise pattern of wave field throughout the system. This pattern can be represented in the basis of eigenchannels of multiple-scattering medium that are based on singular value decomposition of a suitably defined transmission/reflection/absorption matrix, and coupling into different eigenchannels can lead to such a diverse transport behaviors as perfect transmission/reflection/absorption.
The universal bimodal distribution of singular values of transmission matrix in lossless diffusive systems underpins such celebrated phenomena as universal conductance fluctuations, quantum shot noise in condensed matter physics, and enhanced transmission in optics and acoustics. In contrast to the distribution of singular values, the corresponding eigenchannels, are sensitive to the geometry – a specific choice of boundary conditions, placement of macroscopic inhomogeneities in the system, etc. In lossy systems, absorption and its spatial distribution represent the additional degrees of control. In this talk, we will demonstrate effective approaches to modify the eigenchannels in a deterministic way, opening up new opportunities for controlling energy distribution inside complex media via wave-front shaping.
NM09.06: Poster Session: Plasmonics and Metamaterials I
Tuesday PM, April 03, 2018
PCC North, 300 Level, Exhibit Hall C-E
5:00 PM - NM09.06.03
Metal Core-Dielectric Shell-Metal Nanocap Structure for Surface Plasmon-Enhanced Upconversion
Tatsuki Hinamoto1,Kaoru Yamamoto1,Minoru Fujii1
Kobe University1Show Abstract
Rare-earth doped photon upconversion (UC) nanomaterials have numerous applications in different fields such as wavelength conversion layers in solar cells, security inks, and autofluorescence-free fluorescent labels for bioimaging. However, these materials suffer from inherent limitations; the small excitation cross section and the low quantum efficiency are the obstacles for the practical applications. A promising approach to overcome these problems is the formation of nanocomposites composed of UC nanomaterials and metal nanostructures and utilizing the enhanced electric fields accompanied by the localized surface plasmon (LSP) resonances. A variety of nanocomposite structures has been proposed and tested so far, and more than 100-fold enhancement of the UC intensity has been reported [T. Hinamoto et. al, JPCC, 121 (2017) 8077].
In order to maximize the UC enhancement by the LSP resonance, a metal nanostructure has to satisfy some criteria. First, it should have multiple resonances at largely separated wavelengths, because the upconverted photon energy is usually 1.5-3 times larger than the excitation one. A LSP mode at the excitation wavelength is preferably a dark mode with a large absorption cross-section and a small scattering cross-section, while that at the emission wavelength is vice versa. Furthermore, the electric field distribution has to be optimized to maximize the overlap between the field enhancement region and a volume of an UC material. Apparently, metal nanostructures satisfying all these criteria are not simple.
In this work, we develop a metal nanostructure composed of a metal core and a metal nanocap, and placed an UC material in between. In this structure, the LSP modes split into bonding and antibonding ones due to the plasmon hybridization, and the resonance wavelengths can be controlled in a wide wavelength range by the strength of the hybridization. Furthermore, a strong enhancement of the electric fields is expected in the gap, where an UC material exists.
We fabricated the nanocomposites as follows. First, a shell of an UC material (Er and Yb doped Y2O3) about 10 nm in thickness was formed around a Au nanoparticle core (64 nm in diameter) by a homogeneous precipitation method. The composite nanoparticles were placed on a fused silica substrate, and then Ag about 20 nm in thickness was deposited for the formation of a Ag nanocap. The structure of the nanocomposites was characterized by HAADF STEM observations and EDS element mappings. The scattering and UC of single nanocomposites were studied by a dark-field microscopy coupled with a monochromator and visible (CCD) and near-infrared (InGaAs diode array) detectors. The measured scattering spectra and electromagnetic field simulations based on a boundary element method revealed that the nanostructure has two scattering peaks due to the bonding and anti-bonding modes. In this work, an UC enhancement of 5-fold was achieved in not well-optimized structures.
Viktoriia Babicheva, ITMO University
Alexandra Boltasseva, Purdue University
Joshua Caldwell, Vanderbilt University
Isabelle Staude, Friedrich Schiller University Jena
U.S. Department of Energy–Office of Basic Energy Sciences
NM09.07: Phonon- and Vibration-Based Polaritons
Wednesday AM, April 04, 2018
PCC North, 200 Level, Room 231 BC
8:00 AM - NM09.07.01
Investigation of Raman Active Phonon-Polariton Resonances in Selective Epitaxy GaN Nanowire Arrays
Bryan Spann1,Joshua Nolen2,Matt Brubaker1,Thomas Folland2,Todd Harvey1,Joshua Caldwell2,Kris Bertness1
NIST1,Vanderbilt University2Show Abstract
Recently, it has been established that polar semiconductor materials offer a foundation to build novel long-wavelength photonic devices. For instance, their ability to support phonon-polariton resonances that provide highly sub-diffractional electromagnetic fields with exemplary resonant quality factors suggest these systems would be ideal as nanoscale THz emitters. Polar semiconductors also offer wide spectral tunability, with the constituent atomic basis and crystal structure defining the active frequency region. The vast library of these materials allows for photonic applications from the mid-infrared to THz regions. In this work, we investigate the Raman active nature of phonon-polariton modes in selectively grown epitaxial GaN nanowire arrays. We observe strong Raman peaks within the Reststrahlen band of GaN that are hypothesized to originate from localized monopolar and transverse dipolar phonon-polariton modes. These modes occur around 700 cm-1 (~ 14.3 μm), opening a unique spectral region for device applications; which is currently not accessible by other phonon-polariton enabled materials, e.g., SiC and h-BN. Tuning of the apparent resonances are a function of nanowire pitch and diameter for the expectant phonon-polariton modes. Additional measurements were performed using an FTIR microscope that further establish the physical properties of the resonances observed in the Raman microscopy measurements. Interestingly, since the tuning of the modes is geometry dependent, this scheme lends itself well to engineering of the phonon-polariton resonances using the high-precision selective epitaxy process used here.
8:15 AM - NM09.07.02
Unveiling Surface Phonon Polaritons in Complex Nanostructures with Monochromated EELS
Jordan Hachtel1,Joshua Caldwell2,Juan Carlos Idrobo1
Oak Ridge National Laboratory1,Vanderbilt University2Show Abstract
Surface phonon polaritons (SPhPs) have seen a recent surge in interest due to their ability to bring the optical confinement of plasmonics in to the Mid-IR/THz regime, without any of the associated losses. By creating metamaterials and multicomponent superlattices made out of polar dielectric materials (i.e. SiC) SPhPs can be manipulated to achieve active tunability, second harmonic generation enhancement, and structure-induced hyperbolic permittivities. For nanoscale complex structures such as these localized SPhP measurements are key to understanding the interplay between the different components.
Electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) has been a highly successful method of locally measuring optical properties of nanostructures. However, up until recently the Mid-IR/THz regime was inaccessible due to the energy spread of the field emission electron guns. Recent breakthroughs in STEM monochromation have reduced the background in the Mid-IR by orders of magnitude, allowing for low-efficiency signals to be tracked and measured through EELS. Here we examine nanostructured SiC metamaterials in a monochromated STEM to map the excitation of SPhPs directly at the nanoscale.
This work is supported by the Center for Nanophase Materials Sciences (CNMS), which is sponsored at ORNL by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy
8:30 AM - NM09.07.03
Control of Vibration-Cavity Polaritons in the Frequency and Time Domains
Blake Simpkins1,Adam Dunkelberger1,Kenan Fears1,Wonmi Ahn2,Igor Vurgaftman1,Jeff Owrutsky1
Naval Research Laboratory1,National Research Council2Show Abstract
We will focus on using light-matter interactions in an effort to alter the chemical behavior of a target molecular species. This is done through cavity coupling to a molecular vibration. Coupling vibrational transitions to resonant optical modes creates vibrational polaritons shifted from the uncoupled molecular resonances and provides a convenient way to modify the energetics of molecular vibrations and to explore controlling chemical reactivity[i] and energy relaxation.[ii] Here, we experimentally and numerically describe strong coupling between a Fabry-Pérot cavity and several molecular species (e.g., poly-methylmethacrylate, thiocyanate, hexamethyl diisocyanate)[iii] and investigate the transition from the strong to weak coupling regimes. Furthermore, we map the influence of molecule/cavity mode overlap by systematically altering the position of a molecular slab throughout the first and second order cavity resonances with results agreeing well with analytical and transfer matrix predictions.
In the time domain, we use pump-probe infrared spectroscopy to characterize the dynamics of vibration-cavity polaritons for the CO vibrational band of W(CO)6.2 At very early times, we observe quantum beating between the two polariton states, which may account for a lower degree of vibrational excitation observed. After the quantum beating, we interpret our observations as excited-state absorption from polariton modes and uncoupled reservoir modes. The polariton mode relaxes ten times more quickly than the uncoupled vibrational mode and it exhibits a cavity tuning-dependent lifetime which we believe is a result of modifying the relative fractions of cavity and molecular character comprising the polariton. We show that energy relaxation times depend on cavity-vibration coupling and thereby may be a viable way to control the frequency and lifetime of vibration-cavity polaritons and, therefore, may provide opportunities to influence chemical reactivity. This work points out the possibility of systematic and predictive modification of the excited-state kinetics of vibration-cavity polariton systems. Opening the field of polaritonic coupling to vibrational species promises to be a rich arena amenable to a wide variety of infrared-active bonds that can be studied in steady state and dynamically.
[i] A. Thomas, J. George, A. Shalabney, M. Dryzhakov, S. J. Varma, J. Moran, T. Chervy, X. Zhong, E. Devaux, C. Genet, J. A. Hutchison, T. W. Ebbesen, Angew. Chem. Int. Ed. Engl. (2016)
[ii] A.D. Dunkelberger, B.T. Spann, K.P. Fears, B.S. Simpkins, and J.C. Owrutsky, “Modified Relaxation Dynamics in Coupled Vibration-cavity Polaritons”, Nature Communications 7, 13504 (2016)
[iii] B. S. Simpkins, K. P. Fears, W. J. Dressick, B. T. Spann, A.D. Dunkelberger, and J. C. Owrutsky, , ACS Photonics, 2, 1460 (2015)
9:00 AM - NM09.07.04
Optical Frequency-Mixing in III-V Dielectric Metasurfaces
Polina Vabishchevich1,Sheng Liu1,Aleksandr Vaskin2,John Reno1,3,Gordon Keeler1,Michael Sinclair1,Isabelle Staude2,Igal Brener1,3
Sandia National Laboratories1,Institute of Applied Physics, Abbe Center of Photonics, Friedrich Schiller University Jena2,Center for Integrated Nanotechnologies, Sandia National Laboratories3Show Abstract
The recent approach to utilize metasurfaces made from resonant nanostructures has been revolutionizing our perception of nonlinear optical processes. Due to the relaxed phase-matching requirements, simultaneous generation of various nonlinear optical processes can be expected from them. In this work, we show that seven nonlinear processes, including fourth-harmonic generation, four-wave mixing (FWM), and six-wave mixing (SWM) processes can occur simultaneously in GaAs dielectric metasurfaces.
The dielectric metasurfaces used in this work consist of a square array of GaAs nanocylinder resonators that are spatially separated from a GaAs substrate by AlGaO. The nanocylinders have diameters of ~420 nm and support magnetic and electric dipole resonances at ~1520 nm and ~1250 nm, respectively. To explore frequency mixing processes, the GaAs metasurface sample was pumped by two near-infrared femtosecond beams. The optimization of the frequency-mixing signal was achieved by overlapping the pumps’ wavelengths with the two dipole resonances of the nanoresonators. When the two pump pulses spatially and temporally overlap, eleven spectral peaks are observed spanning from UV to near-infrared wavelengths. We divide the newly generated frequencies into two groups: those relying on only one of the two pump beams such as second-, third- and fourth-harmonic generation and two-photon absorption induced photoluminescence; and those relying on both pump beams such as sum-frequency generation, three types of FWM processes, and SWM process. We identify these mechanisms by measuring the power dependence, as well as by matching the photon energy. For example, the observed fifth-order nonlinear effect, SWM, was verified by tunning the wavelengths of the two pumps. To confirm the resonantly enhanced behavior, we also measured the generated nonlinear spectra on an unpatterned sample and observed at least two orders of magnitude smaller signal intensities for most the spectral peaks.
Our demonstration of frequency-mixing in dielectric metasurfaces combines strong material nonlinearities of GaAs, enhanced electromagnetic fields in resonators, and relaxed phase-matching conditions in nanostructures, to allow simultaneously generate of eleven new frequencies. Use of III-V metasurfaces paves a road for realizing ultra-compact optical frequency-mixer for various applications, such as telecommunication technologies.
This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering and 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 multi mission laboratory managed and operated by National Technology and 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 DE-NA0003525.
9:15 AM - NM09.07.05
Phonon-Polaritonic Bowtie Nanoantennas—Controlling Infrared Thermal Radiation at the Nanoscale
Thomas Taubner1,Tao Wang1,2,Peining Li1,3,Dmitry Chigrin1,4,Alexander Giles5,Francesco Bezares6,Joshua Caldwell5,7
RWTH Aachen Univ1,Institute or Materials Research and Engineering2,CIC Nanogune3,DWI - Leibniz-Institute for interactive Materials4,US Naval Research Laboratory5,Universidad de Puerto Rico6,Vanderbilt University7Show Abstract
A conventional thermal emitter exhibits a broad emission spectrum with a peak wavelength depending upon the operation temperature. Recently, narrowband thermal emission was realized with periodic gratings or single microstructures of polar crystals such as SiC [1, 2]. These polar crystals support Surface Phonon-Polaritons (SPhPs) , which offer lower losses and higher resonance quality factors due to longer lifetime than the commonly used Surface Plasmon Polaritons (SPPs). Due to the strong confinement of SPhPs, subwavelength resonators can host different, spectrally narrow modes depending on geometry and period.
Here, we go one step further and investigate the coupling of adjacent phonon-polaritonic nanostructures, specifically deeply sub-diffractional bowtie-shaped silicon carbide nanoantennas. We experimentally demonstrate that the nanometer-scale-gaps can control the thermal emission frequency while retaining emission linewidths as narrow as 10 cm-1.
To prove that the thermal emission originates from of nanoantenna structures and for an unambiguous assignment of the strongly localized SPhP resonant modes, we employ infrared far-field reflectance spectroscopy and compare it with full-wave electromagnetic simulations and near-field optical nanoimaging. The latter is based on scattering-type scanning near-field optical microscopy (s-SNOM) and enables us to directly visualize the rather complex modes of our 3-dimensional nanostructures. We also observe slight differences between individual bowties in our array, again indicating the strong influence of the nanoscale gaps on some of the narrow emission lines.
We believe that the observed narrow emission linewidths and exceptionally small modal volumes will provide new opportunities for the user-design of near- and far-field radiation patterns for advancements in infrared spectroscopy, sensing, signaling, communications, coherent thermal emission, and infrared photo-detection.
 J. J. Greffet et al, Nature 416, 61 (2002).
 J. A. Schuller et al, Nature Photon. 3, 658 (2009).
 J. D. Caldwell, et al., Nanophotonics 4, 44 (2015).
 T. Wang et al., ACS Photonics 4, 1753 (2017).
NM09.08: 2D Materials
Wednesday PM, April 04, 2018
PCC North, 200 Level, Room 231 BC
10:00 AM - NM09.08.01
Hot Plasmons—Hot Carriers in Graphene Give Rise to Mid-Infrared Plasmon-Coupled Radiation Under Ultrafast Optical Excitation
Laura Kim1,Freddie Page2,Seyoon Kim3,Victor Brar4,Joachim Hamm2,Ortwin Hess2,Harry Atwater1
California Institute of Technology1,Imperial College London2,ICFO–The Institute of Photonic Sciences3,University of Wisconsin-Madison4Show Abstract
The decay dynamics of excited carriers in graphene have attracted wide scientific attention, owing to the much lower relaxation rate of excited ‘hot’ carriers than that seen in many three-dimensional materials, owing to the Dirac electronic dispersion of graphene. Plasmons in graphene can significantly reduce the lifetime of photoexcited charge carriers, and this plasmon effect on excited state decay increases with increasing carrier density, as indicated by theoretical calculations and ARPES experiments [1,2,3,4]. In a recent theoretical study, it was also shown that plasmons can be amplified in an inverted graphene and be spontaneously emitted on ultrafast time scale.
We report experimental demonstration of gate-tunable mid-infrared plasmon-coupled radiation from graphene under ultrafast optical pumping, and our experimental results suggest that graphene plasmons excited by ‘hot’ carriers affect the radiative emission rate. We have measured emission for several sample geometries: planar graphene, and non-resonant and resonant gold nanodisks(NDs) on graphene. In infrared emission spectroscopy measurements taken under optical excitation with a Ti:sapphire laser operating at 850nm with 100fs pulse duration, we observe broad radiative emission with features across an energy range of 150meV to 430meV (2.8-8um). The randomly distributed gold NDs on graphene facilitate out-coupling of plasmon excitations to free space light by accommodating the momentum mismatch. In addition, when NDs are resonant with the incoming laser frequency, ultrafast plasmon emission is enhanced in portion to the field intensity concentrated at the location of the graphene sheet. With a surface coverage of 1% for the resonant NDs and less than 3% for the non-resonant NDs, the collected plasmon-coupled light emission intensity is at least a factor of 8 and 4 larger, respectively, than that collected from planar graphene. In all cases, the emission intensity increases for higher graphene carrier density, which is controlled via the changes in applied gate voltage. For a given, moderate laser power, the emission intensity is approximately 1%, 6%, and 70% larger when the graphene Fermi level is at 0.4eV compared to the charge neutral point for planar graphene, and non-resonant and resonant NDs on graphene, respectively. This work has important implications for achieving ultrafast optical control of mid-infrared light emission. Our results are indicative of a spectral modification of plasmon-mediated emission arising from ‘hot’ plasmons in graphene created by ultrafast optical excitation.
1. A. Bostwick et al., Nature Phys., 2007, 3(1), pp.36-40.
2. A. Bostwick et al., Science, 2010, 328(5981), pp.999-1002.
3. F. Rana et al., Phys. Rev. B, 2011, 84(4), pp.045437
4. J. M. Hamm et al., Phys. Rev. B, 2016, 93(4), pp. 041408
5. A. F. Page et al., Phys. Rev. B, 2015, 91(7), pp. 075404
10:15 AM - NM09.08.02
Plasmon-Functionalized 2D Transition Metal Dichalcogenides—Nonlinear Harmonic Generation and Ultrafast Hot Electron Injection
Gregory Forcherio1,2,Luigi Bonacina3,Jeremy Dunklin4,Jérémy Riporto3,5,Yannick Mugnier5,Ronan Le Dantec5,Claudia Backes6,Yana Vaynzof6,Mourad Benamara2,Donald Roper2
U.S. Army Research Laboratory1,University of Arkansas2,University of Geneva3,National Renewable Energy Laboratory4,University of Savoy Mont Blanc5,Ruprecht-Karls University Heidelberg6Show Abstract
Two-dimensional (2D) transition metal dichalcogenides (TMD) functionalized with plasmonic metal nanoantennas (NA) exhibit rich energy conversion capabilities as a material platform for optoelectronics and sustainable energy. This work examined (i) plasmon-enhanced nonlinear second harmonic generation (SHG) and (ii) injection of plasmonic hot electrons into 2D TMD via coordinated multi-photon microscopy, hyper Rayleigh Scattering (HRS), electron energy-loss spectroscopy (EELS), and discrete dipole computation. Augmented local fields by NA surface plasmon resonance enhanced SHG from monolayer MoS2 at efficiencies of up to 0.025 %/W. Hyper Rayleigh scattering (HRS) assessed the second-order nonlinear susceptibility for WS2 monolayers to be 250±12 pm/V. Quantum yield of plasmonic hot electrons transported to 2D TMD was measured locally for two NA-TMD hybrids by EELS, revealing dependence on bonding characteristics at the metal-TMD junction. Highest measured efficiency was 11±5% for NA physicochemically bonded to WS2 edge disulfides via redox-directed self-assembly.
10:30 AM - NM09.08.03
From Band Gaps to Bound Excitons—Disentangling Optical Transitions and Localized Emitters in TMDCs Even at Nanoscale Dimensions
Nicholas J Borys1,P James Schuck1
Columbia University1Show Abstract
The emergence of two-dimensional (2D) monolayer transition metal dichalcogenides (ML-TMDC) as direct bandgap semiconductors has rapidly accelerated the advancement of room temperature, 2D optoelectronic devices. Optical excitations on the TMDCs manifest from a hierarchy of electrically tunable, Coulombic free-carrier and excitonic many-body phenomena. Investigating the fundamental interactions underpinning these phenomena presents challenges, however, due to a complex balance of competing optoelectronic effects and interdependent properties. We show how optical detection of bound- and free-carrier photoexcitations is used to directly quantify carrier-induced changes of the quasiparticle band gap and exciton binding energies . Pushing to the nanoscale, we demonstrate that a model hybrid architecture, a nano-optical antenna and a ML-WSe2 nanobubble, activates the optical activity of BX states at room temperature and under ambient conditions. These results show that engineered bound-exciton functionality as, in this case, localized nanoscale light sources, can be enabled by an architectural motif that combines localized strain and a nano-optical antenna, laying out a possible path for realizing room-temperature single-photon sources in high-quality 2D semiconductors.
 Kaiyuan Yao, et al., Phys. Rev. Lett. 119, 087401 (2017)
11:00 AM - NM09.08.04
Scalable and Tunable Graphene-Based Infrared Filter
Michael Goldflam1,Isaac Ruiz1,Stephen Howell1,Joel Wendt1,Michael Sinclair1,David Peters1,Thomas Beechem1
Sandia National Laboratories1Show Abstract
We have developed and experimentally demonstrated an actively tunable infrared filter that enables modification of the amplitude of reflected long-wave-infrared light. Tunability results from plasmons excited in an unpatterned sheet of chemical-vapor-deposition grown graphene. Through conventional gating using a periodic metal grating, the Fermi level of the graphene can be modified to change the plasmonic response, resulting in changes to reflectance. The filter enables simultaneous modification of two distinct spectral regions between 600 and 1600 cm-1, whose positions are controlled by the device geometry and graphene plasmon dispersion. Within these bands, the reflected amplitude varies by over 15% and reflectance minima can be shifted over 90 cm-1. As demonstrated though electromagnetic simulations, tuning arises from graphene plasmons excited within the graphene via coupling through the metallic grating. The tuning range is determined by a combination of graphene properties, device structure, and the surrounding dielectrics. Using these parameters, the device architecture demonstrated here is applicable to a broad range of infrared wavelengths.
Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and 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 DE-NA0003525.
11:15 AM - NM09.08.05
Coupled Dirac Plasmons in Topological Insulators
University of Delaware1Show Abstract
Topological insulators (TIs) are layered materials that ideally exhibit an insulating bulk with conducting surfaces. The electrons in these surface states are two-dimensional, linearly dispersing, and exhibit spin-momentum locking. Light can couple to these surface electrons, exciting two-dimensional Dirac plasmons, similar to those excited in graphene. Unlike graphene, however, TI plasmons are expected to be spin-polarized. This causes them to be protected from non-magnetic backscattering, potentially leading to extremely long propagation distances. There are a variety of potential applications for TI plasmons, including THz sensors, optically-driven spintronics, and THz metamaterials. In this talk, I will discuss our recent results measuring the dispersion relationship of coupled Dirac plasmons in topological insulator films. Because the TI films are extremely thin, plasmons excited on the top and bottom surfaces of the film will couple, resulting in Dirac plasmon acoustic and optical modes. By patterning the films into stripes, localized plasmons can be excited. Since we are exciting the optical mode in our films, by changing both the stripe width and film thickness, the coupled plasmon dispersion relationship can be mapped out. Our results show that we are indeed exciting coupled 2D Dirac plasmons and not massive 2D plasmons from either the bulk or from a band-bending two-dimensional electron gas.
Finally, I will discuss recent results on TI films grown with molecular beam epitaxy on high-quality buffer layers. One of the challenges when studying TI films is that the Fermi energy is usually pinned near the bottom of the conduction band, leading to a large density of trivial carriers in the bulk states. These trivial carriers can open up additional scattering pathways for the topological carriers, reducing plasmon lifetimes. We have found that growth of TI films on high-quality buffer layers brings the Fermi energy closer to the Dirac point, reducing the density of trivial carriers. I will close by showing data for Dirac plasmons excited in these high-quality TI layers. Overall, TIs represent an exciting new material class for studies of Dirac plasmon physics as well as plasmonic applications in the THz.
11:45 AM - NM09.08.06
Hyperbolic Behaviour of 2D Materials Measured Using Attenuated Total Reflection
Thomas Folland1,Joseph Matson1,Tobias Maß2,Thomas Taubner2,Joshua Caldwell1
Vanderbilt University1,RWTH Aachen University2Show Abstract
Polaritons in materials with free charge carriers (surface plasmon polaritons) or polar optic phonons (surface phonon polaritons) offer a route to beating the diffraction limit for compact mid- and far-infrared optoelectronics. The latter of these two quasi-particles has received intense scrutiny in recent years due to inherently low losses from phonon scattering, albeit at the limitation of relatively low spectral tunability. One particularly interesting class of polaritons are hyperbolic modes – which occur in highly anisotropic crystals, such as the 2D materials or artificial layered metamaterials. This phenomenon arises from the layered structure, with significantly different vibrational energies in- and out-of-plane directions. The techniques that have been used to study these modes involve nano-structuring, scattering type scanning nearfield optical microscopy (s-SNOM) and photothermal induced resonance (PTIR) techniques. Whilst highly successful these techniques have drawbacks, such as the complex effect of scattering from a nanoscale particle or tip which makes data analysis complex. As a result, accurately measuring the dielectric response of 2D crystals Is extremely difficult using these techniques, but the small size of samples precludes the use of infrared spectroscopic ellipsometry. Here we discuss how prism coupling techniques can be used to measure hyperbolic polaritons and extract dielectric function data from two dimensional crystals.
Specifically, we discuss how the choice of appropriate substrate is critical for successful measurements on two dimensional crystals such as hexagonal boron nitride. Prism coupling requires attenuated total reflection (ATR) between the boundary and substrate, but many low-index materials are not suitable for 2D material preparation. By using a combination of simulations and experiments, we show that by thickening the Si/SiO2 layers conventionally used for mechanical exfoliation of two dimensional materials, we can measure ATR spectra even on high index Si. The measured response is highly sensitive to the thickness of the flakes, as predicted by the dispersion of hyperbolic modes. Specifically, we are able to observe multiple dips in reflection, corresponding to different modes of identical wavevector. Finally, we comment on the repeatability of this technique under both varied measurement and different exfoliation conditions, including the importance of substrate adhesion. One major advantage of this approach is that by using an appropriate prism it is possible to access frequencies that are difficult to measure using both s-SNOM and PTIR techniques (for example the far-IR, where laser sources are limited). Furthermore, by comparison with numerical simulations it is possible to extract dielectric data, like spectroscopic ellipsometry. This can then inform the design of nanostructures to create efficient far-IR thermal emitters and optical components.
NM09.09: Novel Growth of Nanophotonic Materials
Nicholas J Borys
Wednesday PM, April 04, 2018
PCC North, 200 Level, Room 231 BC
1:30 PM - NM09.09.01
Single Crystal Plasmonic Metal Films and Nanostructures on Silicon
Finlay C Macnab1,Xin Zhang1,Gary Leach1
Simon Fraser University1Show Abstract
The advent of commodity single crystal silicon wafers enabled the growth of the electronics industry through the extreme lithographic fidelity and improved device yeild that these substrates afford. The field of plasmonics would benefit from access to low-cost, single crystal metal surfaces in an analogous fashion. Enhanced nanostructure fidelity and film morphology eliminates many fabrication challenges and improves experimental throughput through better agreement between simulation results and device performance. In this work, the fabrication of smooth single crystal metal films of gold, silver, and platinum, on silicon substrates through a modified PVD process was demonstrated. Nanostructuring of these metal surfaces was explored through the use of several novel, low-cost, techniques to generate smooth epitaxial nano-features with excellent plasmonic properties. We will present and discuss our fabrication techniques as well as examples of plasmonic devices with application to photovoltaics, electrochemistry, and non-linear optics.
1:45 PM - NM09.09.02
Epitaxial Electrochemical Deposition of Noble Metal Thin Films and Nanostructures—A New Bottom-Up Strategy for Plasmonic, Nanophotonic and Metamaterial Applications
Gary Leach1,Sasan Grayli1,Xin Zhang1,Finlay C Macnab1
Simon Fraser University1Show Abstract
The ability to deposit and pattern noble metals to form moncrystalline thin films and high-definition subwavelength nanostructures represents a significant challenge in the development of next generation plasmonic, nanophotonic, and metamaterial technologies. Typical physical vapour deposition-based methods result in the deposition of polycrystalline features characterized by structural inhomogeneity that reduces feature quality and increases losses due to grain boundaries. Alternatively, strategies based on the solution phase synthesis of crystalline nanparticles of controlled size, shape, and composition face difficulties in patterning and registering these structures in well-defined locations onto substrates and are of limited utility for electronic applications due to the presence of capping agents. Here, we describe a new method for the deposition of epitaxial, single crystal, noble metal thin films and nanostructures from solution. We demonstrate that epitaxial electrochemical deposition (EED) enables fabrication of large-area, atomically flat, single crystal metal films of desired thickness that are ideal for nanopatterning through ion beam milling. The resulting structures are smooth, homogeneous, manufacturable in high yield, and display thermal and mechanical stability at least one order of magnitude greater than their polycrystalline counterparts. While this chemistry allows for the subtractive manufacture of nanostructure through ion beam milling, EED also enables additive crystalline nanostructure using standard lithographic techniques such as electron beam lithography to enable novel, large area, metamaterial arrays and high aspect ratio crystalline nanostructure. Epitaxial electrochemical deposition represents a new, easily accessible, and cost effective, bottom-up approach to high fidelity nanostructure that will help to enable new plasmonic research and application.
2:00 PM - NM09.09.03
Materials for Photonics Beyond Noble Metals
University of Maryland-College Park1Show Abstract
To date, there is a keen interest in the photonics community to control the optical properties of metal, at both thin film and nanoscale level. While the size, geometry and spatial distribution are often used to modify the localized surface plasmon resonance (LSPR) of nanostructures; the chemical composition is a powerful knob to tune their response in the Vis-NIR range of the spectrum. Here, we demonstrate how the alloying of metals (Ag, Au, Cu, Al) enables the tenability of the dielectric function for applications ranging from energy harvesting to superabsorbers. First, we build a library of the optical response of alloyed thin films, and show that in some cases a mixture can offer a material with dielectric function not achieved by its pure metal counterparts . We numerically and experimentally demonstrate how Al-Cu, potentially CMOS-compatible, can be implemented in a superabsorber thin film stack, providing dual-band near-unity absorption (> 99%) in the Vis-NIR . At the nanoscale, we map the near- to far-field optical response of alloyed nanoparticles, which present local field enhancements at wavelength not accessed by pure metals . We anticipate the investigation of materials beyond noble metals to enable the design and realization of optical systems with superior performance and on-demand response.
 ACS Photonics 3, 507 (2016) – COVER.  Adv. Optical Materials, in press (2017).  Adv. Optical Materials 5, 1600568 (2017) – COVER.
NM09.10: Semiconductor Metasurfaces
Wednesday PM, April 04, 2018
PCC North, 200 Level, Room 231 BC
3:30 PM - NM09.10.01
Science and Applications of Semiconductor Metasurfaces—From Ultra-Efficient Diffraction Gratings to Photon Acceleration and Broadband Harmonics Generation
Max Shcherbakov1,Gennady Shvets1
Cornell University1Show Abstract
Semiconductor metasurfaces represent a unique platform for linear and nonlinear optics. Because s