The research field of nanophotonics has made tremendous progress in the past 10 years. Many of these achievements result from the increasing degree of control over nanofabrication and nanocharacterization that has evolved in the past years. Taking advantage of the extreme control over nanophotonic light fields that we have today, we are now in an ideal position to bring the photonics field a next step further, and to investigate the coupling of light with other degrees of freedom such as nanoscale mechanical motion, acoustic phonons, electron spins and excitons.
We will illustrate this emerging new trend of hybrid nanophotonics with several characteristic examples:
- plasmonic optomechanical oscillators: we demonstrate the integration of optically resonant plasmonic nanocavities with silicon-nitride-membrane mechanical oscillators. In these hybrid structures mechanical motion is transduced to scattered optical fields, enabling measurements of displacement sensitivity down to the picometer level.
- plasmonic transparent conductors: we shows how resonant plasmonic nanowire networks serve both as efficient light coupling/trapping geometries and low-resistivity electrical current collectors. We show the integration of nano-imprinted silver nanowire networks with high-efficiency silicon heterojunction solar cells, polymer solar cells, and present their possible use in touch-screen displays.
- cathodoluminescence microscopy: the high-energy electron beam in an electron microscope is coupled to resonant modes in photonic nanostructures, enabling spatial mapping of optical modes with deep-subwavelength resolution, as we will show. This technique provides the “purest” form of optical excitation spectroscopy, as a single electron drives individual polarizable nanophotonic building blocks.
Hybrid nanophotonic concepts will lead to new devices and functionality such as mechanical sensors, RF signal processing, bio-sensors, quantum information processing, ultra-high efficiency solar cells, light-emitting diodes, opto-electronic integrated circuits, imaging systems, and much more.

Classical polarized light carries spin and reveals spin-orbit coupling when propagating along a curved trajectory, however this interaction typically is weak. Utilizing a large phase-gradient, a metasurface can greatly enhance this interaction because it can bend light abruptly within an extremely small thickness due to the large induced momentum. Conceivably, this strong photonic spin-orbit coupling on a metasurface could drive the electrons collectively and lead to direct electric Hall currents flowing transversely to the light-bending direction, even at normal incidence and without external magnetic fields. Yet such a photon-spin-induced electric Hall effect has never until now been demonstrated. Here we report the first observation of this direct coupling between photon spin and electron orbital momenta on a metallic metasurface consisting of complementary nanoantennas. By inputting opposite photonic spins, we directly detect the changes inversion of the transverse current direction, with the longitudinal current (induced by the photon-drag effect) being constant. This effect enables an electrical way of detecting photonic spin-orbit interactions and provides a viable route to directly integrate conventional electronic chips with the additional degrees of freedom from the spin and orbital angular momenta of light for future information processing and communication applications

Optical matter is a unique class of materials. Typical optical matter is consisting of dielectric microparticles that are self-organized and stabilized by optical binding forces in an optical field. Noble metal nanoparticles usually exhibit strong scattering in the visible light spectrum, resulting in strong optical binding forces that offer the opportunity to construct optical matter from nanoparticles. Here we report our recent work on controlling and tailoring the self-organization of plasmonic nanoparticles in structured optical fields. Structurally stable 2D and 3D clusters with various geometries have been photonically synthesized from Ag nanoparticles with nanometer scale precision in their inter-particle separations. In particular, we show how Ag nanoparticles organize themselves in response to the optical trap shape, intensity gradient, phase gradient, polarization of light, and even the spatial assembly pathways. Our work paves the way for rational design of photonic clusters in optical fields, and we envision that these photonic clusters will find applications in various areas, such as sensing and quantum optics.

Enantiomers are pairs of chiral molecules that are non-superimposable mirror images of each other. The handedness of enantiomers is highly related to their biological function, and effective separation of enantiomers is crucial for the development of functional drugs. Since enantiomers are identical in their chemical composition and mass, existing methods of enantiomer separation largely rely on the unique ways in which each molecule in an enantiomeric pair interacts with other chiral reagents. Here we introduce a novel method of separating enantiomers using optical forces. To generate forces of sufficient strength to trap single molecules, we use a plasmonic coaxial optical trap composed of a 120-nm-diameter silver core and a 25-nm silicon dioxide channel. The coaxial resonator thickness is 150 nm, which provides two strong transmission peaks at 487 nm and 702 nm, corresponding to the Fabry-Perot resonances of the resonator. When light is scattered from the aperture, a large field gradient is generated close to it; linearly-polarized light can provide up to 1.8 pN of lateral force per 100 mW of transmitted power, sufficient to capture dielectric nanoparticles as small as 10 nm in diameter at 20 nm above the coaxial aperture. Interestingly, the near-fields of our coaxial aperture also generate forces that can differentiate between different chiral structures. The two distinct lateral forces arising from chiral molecules interacting with the coaxial aperture are related to the optical rotation dispersion and the circular dichroism of the molecule, and can be derived from electromagnetic chiral density and the energy flow vortex. To calculate the optical forces exerted selectively on R and S enantiomers, we use a combination of finite difference time domain simulations and analytic methods. We consider a 10-nm-diameter dielectric chiral particle (similar in size to a typical large protein) placed 20 nm away from the aperture. Our calculations indicate that, for left-handed circularly polarized illumination, the total maximum transverse force can reach 0.9pN/100mW for the S enantiomer and 0.3pN/100mW for the R entantiomer. Importantly, these forces are in opposite directions: the S enantiomer is attracted toward the trap, while the R enantiomer is repelled. Such significant differences in both magnitude and sign of the transverse forces for the enantiomer pairs result in large differences in trapping potentials: we observe a maximum trapping potential depth of -7.6kT per 100mW transmitted power for the S-enantiomer and a positive trapping potential for the R enantiomer. On-going experiments are employing atomic force microscopy to directly measure these forces and to maneuver single chiral molecules with a specific handedness by switching polarization states. Our presentation will describe this theoretical and experimental work and discuss optimized coaxial arrays for all-optical enantiomer-pure chemical synthesis and chiral drug purification.

Fundamentally plasmon oscillations are an opto-mechanical phenomenon. Incident optical fields induce the coherent motion of electrons that produce large fluctuations of charge density at ‘hot spots&’ in resonant structures. Despite these large fluctuations, during linearly polarized illumination a resonant structure, e.g. a metallic nanorod, will necessarily assume a uniform charge distribution twice during each optical cycle. This behavior mimics the electrical field vector of the linearly polarized excitation, which must also have zero magnitude in the plane perpendicular to the direction of propagation twice during an optical cycle. In contrast, the electrical field vector of circularly polarized light has constant magnitude. During an optical cycle, the electric field vector rotates in the plane normal to the wave propagation. Consequently, if plasmonic structures are resonant with circularly polarized excitation, they can exhibit regions of strongly modified carrier density for the duration of the optical cycle.
Here, we study a class of achiral toroid and ‘sun burst&’ patterned resonant plasmonic nanostructures that show persistent, circulating charge density waves during circularly polarized illumination. The direction of the continuously circulating wave (clockwise or counterclockwise) depends on the handedness of the incident beam. Our interest stems from whether these charge density waves can support circular electric currents (DC) manifest experimentally as static magnetic fields during illumination. Using full-wave optical modeling (FDTD method), and mechanistic calculations of the circulating potential acting on the electronic density of states in the toroid resonators, we outline the conditions that maximize optical excitation of circulating electric currents for the case of Au and Ag resonators. We show that in the limit of very weak coupling to the solenoid-like electron transport, or when < 1 x 10^-6 % of the electron population that contributes to the charge density wave enters the circular transport modes, relatively strong magnetic fields, > 1 G, can be expected. We discuss scanning probe measurements for monitoring the induced magnetic field, as well as the relationship between this phenomenon and the inverse faraday effect observed in continuous media.

The recent demonstration of resistless nano-imprint lithography (RNIL) as a scalable technique for the fabrication of nanoscale structures in metallic films provides great promise for the mass production of plasmonic devices. Computational and experimental studies indicate that nanocavities in a metallic film exhibit distinct localized resonances. These cavity resonances are accompanied by strongly enhanced and localized electric and magnetic fields and can modify the lifetime and other emission properties of quantum sources such as quantum dots, NV centers in nanodiamond and fluorescent molecules. Furthermore, cavities produced by RNIL support a higher energy, lowest-order, localized surface plasmon mode than a similarly sized dielectric-backed aperture. Applications for these nanocavities include plasmonic color filters, enhanced solar energy harvesting and the production of novel polarization states. Here we demonstrate enhanced fluorescence of CdSe/CdS/ZnS core-shell quantum dots (QDs) via coupling to plasmonic nanocavities imprinted using RNIL into a silver film.
A silicon master template was patterned using electron beam lithography and reactive ion etching. The pattern consisted of rectangular nanorods in a periodic array, with a period of 300 nm. The pattern was transferred to a 360 nm thick silver film on a silicon substrate using RNIL with a force of 19 kN at room temperature. The resulting cavities were 200 nm long by 60 nm wide by 40 nm deep. The QDs were mixed with SU8 2005 and then drop-cast over the sample and cured.
Fluorescence microscopy was performed on the sample by pumping the QDs with a 532 nm pulsed laser and collecting the 650 nm emission. Time correlated single photon counting (TCSPC) lifetime measurements indicate that excited state lifetimes of the QDs in the vicinity of the nano-imprinted plasmonic nanocavities are reduced. Preliminary results indicate around a 4-fold brightness enhancement and a lifetime reduction of around 70%, when compared to the QD-SU8 matrix on an unpatterned Ag film.
This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). This research was supported under the Australian Research Council's Discovery Projects funding scheme (project number DP110100221) and a Melbourne Centre for Nanofabrication Technical Fellowship.

We study the optical properties of molecules deposited metallic nanostructures with respect to the free molecules. We show theoretically and experimentally that molecular excited states can be strongly coupled to plasmonic modes. Upon coupling new hybrid states are form, the lower and the higher polariton. These modes have the characteristic of both molecular and plasmonic states. As the coupling strength grows, a new mode emerges, which is attributed to long-range molecular interactions mediated by the plasmonic field. The new, molecular-like mode repels the polariton states, and leads to an opening of energy gaps. By tuning the plasmonic modes to be on/off resonance with respect to molecular system excited state, one can shift these hybrid modes and by that modify the photophysical and even the chemical properties of these molecules, and form a new kind of tunable hybrid materials.
In the same aspect, we study and demonstrate coupling between plasmonic modes of metallic nanocavities (holes). The metallic nanocavities display long range coupling at distances of hundreds of nanometers for selected hole distance/wavelength combinations. Such strong coupling drastically changes the symmetry of the charge distribution around the nanocavities as is evidenced by the nonlinear optical response of the medium. Upon coupling, the emission intensity is strongly dependent on the fundamental beam polarization, and is either suppressed or enhanced.
References
middot; From Individual to coupled metallic nanocavities. Salomon, A.; Prior, Y. ; Kolkowski, R.; Zyss,J.; Journal of optics, accepted 2014
middot; Role of mode degeneracy in molecule-surface plasmon strong coupling. Salomon A. Wang, S. Hutchison, J.A., Genet, C. Ebbesen, T.W. accepted ChemPhysChem 2013
J. Phys. Chem. C, 2013, 117 (43), pp 22377-22382
middot; Collective Plasmonic-Molecular Modes in the Strong Coupling Regime Salomon, A.; Gordon R.J.; Prior, Y.; Seideman, T.; Sukharev, M.;
Phys. Rev. Lett 109, 073002, 2012
middot; Molecule - light complex: dynamics of hybrid molecule - surface plasmon states. Salomon, A. Genet, C. and Ebbesen, TW. Angewandte Chemie, 48, Pages: 8749-8751, 2009.

Quantum optics involves the coupling of quantum emitters to their electromagnetic environment. Because this coupling is related to the concentration of the optical field, it is typically constrained by the diffraction limit of light. One way to circumvent this limitation is by moving to quantum plasmonics, which uses surface plasmon polaritons (SPPs) instead of photons. However, despite the advantages of this approach, quantum plasmonics has not yet been fully explored, largely due to the difficulty of creating the necessary structures. We address this problem by combining state-of-the-art quantum emitters with plasmonic structures (waveguides and reflectors) that approach theoretical performance limits. We synthesize highly luminescent colloidal quantum dots (photoluminescence quantum yields >90%) and precisely place them on template-stripped silver wedge waveguides. We demonstrate efficient coupling of the quantum-dot emission into guided plasmonic modes in the waveguide. In addition, by adding efficient reflectors, high-Q plasmonic resonators are obtained. More generally, the flexibility and fidelity of the resulting quantum plasmonic systems indicates that they will enable a broad set of nanoscale quantum optical measurements and devices.

Light-matter interaction probed at the nanoscale attracts much attention nowadays, both for its fundamental interest and potential applications, ranging from all-integrated optics to single photon sources [1].
Hybrid plasmo-photonic 2D crystals exhibit a complex resonance pattern, which has been characterised using both numerical simulations and measurements of the electric near-field patterns obtained with a scattering scanning near-field optical microscope (s-SNOM) [2]. We show that the resonance modes exhibited by such a hybrid nanostructure can be recovered in the far-field by using narrow-band fluorescence nano-reporters. To this end, different types of semiconductor quantum dots (QDs), acting as nanoreporters of the local interactions, were dispersed on top of these hybrid structures. We show that the emission rates are affected, in a unique way, by the complex interaction occurring between the plasmo-photonic modes and the excitons.
We also report on the experimental observation of an asymmetric wavelength-dependence of the emission rate enhancement in such 2D crystals [3]. This feature strongly contrasts with the traditional Lorentzian line shape exhibited at a resonance by the Purcell factor. This unusual dispersive behavior is shown to be reproducible for different combinations of emitters and structures. It is further retrieved from FDTD simulations. It is mainly due to the fact that a hybridized mode, resulting from the coupling of a Bragg and a SPP modes, is coupled to the emitters. Further studies of the emission detuning by hybrid plasmo-photonic structures are expected to benefit many related fields and notably sensing and bio-sensing technologies.
Furthermore, we show experimentally that frequency-bin entangled photons can propagate through these plasmo-photonic nanostructures [4]. Our measurements clearly show the robustness of frequency-bin entanglement, which survives after interactions with the plasmo-photonic structures, offering possibilities for a variety of applications in which quantum states can be encoded into the collective motion of a many-body electronic system without demolishing their quantum nature.
Finally, we show some recent investigations of the strong coupling regime between excitons and the resonances modes of such plasmo-photonics structures, the so-called "excimons".
[1] W.L. Barnes, A. Dereux and T.W. Ebbesen, Nature 424, 824 (2003)
[2] S. Ungureanu, B. Kolaric, J. Chen, R. Hillenbrand and R.A.L. Vallée, Nanophotonics, 2, 173, (2013)
[3] P. Fauché, S. Ungureanu, B. Kolaric, R.A.L. Vallée, Journal of Materials Chemistry C, 2014, DOI: 10.1039/C4TC01787K
[4] L. Olislager, B. Kolaric, W. Kubo, T. Tanaka, S. Ungureanu, R.A.L. Vallée, P. Emplit, S. Massar, in preparation.

Semiconductor nanowires (NWs) are filamentary crystals with a diameter of few tens of nanometers. Thanks to their dimensions and high refractive index, have the ability to collect and trap the light into a sub-wavelength volume ^[1]. Metal nanostructures have shown in the past the ability of modifying and enhancing light response of nanoscale objects^[2]. The realization of this hybrid system opens a way to novel applications and the use optical fields to manipulate and enhance the light interaction with semiconducting materials^[3,4].
We demonstrate for the first time how, by hybridizing a single nanowire photo#8208;detector with a bow#8208;tie antenna structure, it is possible to control the polarization response of a nanowire. Bow#8208; tie antenna concentrate radiation inside the body of the nanowire, thereby increasing the absorption of light polarized across the nanowire axis. The amount of light absorbed is strictly dependent on the antenna design and density. We demonstrate enhancement in light absorption, as well as for second order phenomena such as the generation of second harmonics and Raman scattering ^[3]. We perform photoconductivity measurements demonstrating that a hybrid structure formed by GaAs NWs and an array of bow-tie antennas is able to modify the polarization response of a NW^[5]. The large increase in light absorption for transverse polarized light changes the NW polarization response, including the inversion.
This study constitutes an important step for the understanding of light coupling in engineered nanodevices. It opens a path to applications aiming to combine the semiconducting properties of nanostructures with the plasmonic properties of metal. Hybrid nanoantenna-NW systems can certainly bring progress to the use of nanowires for next generation photo#8208;detectors and solar cells.
References
^[1]P. Krogstrup, et al., Nature Photonics, vol 7, 306-310 (2013)
^[2]C. Forestiere, et al., Nano Letters, vol 12, 2037-2044 (2012)
^[3]A. Casadei, et al., Nano Letters, vol 14, 2271-2278 (2014)
^[4]S. Heeg, et al., Nano Letters, vol 14, 1762-1768 (2014)
^[5]A. Casadei, et al, Scientific Report, (under review)

The “plasmon hybridization”concept,[1] shows that the plasmon resonances in complex metallic nanostructures interact and hybridize in an analogous manner as atomic wavefunctions in molecules. The insight gained from this concept provides an important conceptual foundation for the development of new plasmonic structures that can serve as substrates for surface enhanced spectroscopies, chemical and biosensing, and subwavelength plasmonic waveguiding and other applications. The talk is comprised of basic overview material interspersed with a few more specialized “hot topics” such quantum plasmonics,[2] molecular plasmonics,[3] active plasmonic nanoantennas for enhanced light harvesting,[4] plasmon induced hot carrier generation,[5] and chemical reactions.[6]
[1] N.J. Halas et al., Adv. Mat. 24(2012)4842
[2] T.V. Teperik et al., PRL 110(2013)263901; Opt. Express 21(2013)27306
[3] A. Manjavacas et al., ACS Nano 7(2013)3635
[4] M. W. Knight et al., NL 13(2013)1687; A. Sobhani et al., Nature Comm. 4(2013)1643
[5] A. Manjavacas et al., ACS Nano 8(2014)7630
[6] S. Mukherjee et al., JACS 136(2014)64; Y. Kang et al., Adv. Mat. 26(2014)6467

We report the large-area assembly of gold anisotropic nanoparticles into lithographically defined templates with control over their angular position using a capillary-based approach. We elucidate the role of the geometry of the templates in the assembly of anisotropic nanoparticles consisting of different shapes and sizes. These insights allow us to design templates that immobilize individual triangular nanoprisms and concave nanocubes in a shape-selective manner, and filter undesired impurity particles from a mixture of triangular prisms and other polyhedra. Furthermore, by studying the assembly of two particles in the same trench, we elucidate the importance of interparticle forces with this novel method. These advances allow for the construction of face-to-face and edge-to-edge nanocube dimers as well as triangular nanoprism bowties. As an example of the fundamental studies enabled by this assembly method, we investigate the surface enhanced Raman scattering (SERS) of face-to-face concave cube dimers both experimentally and theoretically, and reveal a strong polarization dependence of the local field enhancement.

We fabricated coatings with a silver nanocube aggregation gradient by applying different surface pressures during slow Langmuir-Blodgett deposition. The randomly distributed nanocube aggregates with gradually changing surface coverage open up new ways to continuously control the absorption and phase of the incoming light over a broad optical spectrum, while being polarization sensitive. Optical characterization under total internal reflection conditions (TIR) reveals that the broadband light absorption depends on the relative orientation of the particle aggregations to the polarization of the incoming light, which has not been investigated yet. By using electromagnetic simulations, we found that the electric field vector of the s-polarized light matches with the different types of silver nanocube aggregations to excite variable plasmonic resonances. Based on this matching conditions, the s-polarization shows an outstanding tunablility of the plasmonic resonance by the angle of incidence (64nm per 10° angle of incidence). Further observation of these modes with spectral ellipsometry reveals a steep phase change at the plasmonic resonances. With a very low surface coverage below 25%, we observed a polarization-selective absorption of 80% (theoretical 98% at peak value) of the incoming light over a broad optical range in the visible region from 400 to 700 nm. This low cost, large area, gradiental variable and easily scalable assembled plasmonic material designs is of particular interest for broadband light absorption, phase-sensitive sensors, and imaging.

Optical metasurfaces are ultrathin, two-dimensional arrays of subwavelength resonators that have been demonstrated to control the flow of light in ways that are otherwise unattainable with natural materials. However, metasurfaces pose major challenges in their nanofabrication, prominently in terms of minimum reliable feature size, high throughput and large area fabrication, which can be prohibitively expensive and time-consuming using conventional lithography techniques. Optical metasurfaces that operate in the visible are inherently challenging to fabricate due to their multi-scale architecture, and requirement for deeply sub-wavelength features, often requiring dielectric spacing of a few nanometers.
Self-assembly provides an alternative fabrication approach that overcomes these limitations in scalability and design. Here we show that colloidal, single-crystalline anisotropic nanocrystals can be organized into metasurface architectures with nanometer-precision using robust, scalable assembly methods. These metasurfaces exhibit extreme in- and out-of-plane electromagnetic coupling that is strongly dependent on nanocrystal size, shape, and spacing. We show that this fabrication approach enables plasmonic metamaterials to be tuned for diverse applications ranging from metasurfaces that display near-ideal electromagnetic absorption--up to 99.8%--tunable from the visible into mid- IR wavelengths, to quasi-2D metasurfaces that demonstrate photoluminescent enhancement of over 120x, for an active region less than 150 nm thick.

Laser induced mechanical shockwave was developed as an effective tool to tailor multi-crystalline Au nanostructures and tune nanogaps in plasmonic nanoantennas. The shock wave generation during laser shock irradiation of graphite was explained using Fabbro's model. Atomic movement of plasmonic gold structure under shockwave compression was investigated using molecular dynamic simulations. Force - displacement relationship was monitored to analyze mechanical property change of nanostructures during laser shock compression. Atoms were found to move in radial direction and nanodisks were found to keep their symmetry after severe plastic deformation. Geometrical changes at different laser energies were quantitatively measured using SEM and AFM images. Surface roughness was found to be decreased after laser shock compression. Different nanogaps down to sub-10 nm could be effectively controlled by varying laser fluences. Optical response of the processed nanostructures were compared with initial structures by numerical simulations and experiments. It was found that after laser shock, nanodisks were enlarged, brought closer, and field enhancement at the center of two adjunct nanodisks was hugely increased. The processed nanostructures had higher sensitivity in low- concentration molecular sensing experiment. Optical dark field spectroscopy experiment showed that the resonance of these processed structures had a red shift compared to the initial structures. Laser induced mechanical shockwave is an effective tool to improve optical properties of multi-crystalline plasmonic nanoantennas.

The origin of unique optical properties of gold nanorods such as distinctive extinction bands in the upper visible or near-infrared region is surface plasmon (SP) oscillation of free electrons. Surface electrostatics of gold nanorod is tremendously important to understand how electrons interact with optical waves. Electrostatic property of gold nanorods has been known to be centrosymetric, however, we have shown that gold nanorods have anisotropic charge accumulation on their surface, which can be defined as a polarized surface charge density. By using off-axis electron holography, we have observed anisotropic electrostatic potential of gold nanorods experimentally. We believe that their anisotropic potential is from different distribution of cetyltrimethylammonium bromide (CTAB), capping ligand having positive charge, on the gold nanorod surface. This anisotropic distribution of CTAB is identified by high resolution scanning electron microscopy (SEM), implementing secondary electron detector in high resolution transmission electron microscopy (TEM) system. We believe that this anisotropic potential formed by CTAB layer results in anisotropic charge accumulation on the gold surface as compensate the potential gradient. As the result, the inner potential of gold nanorod itself has no gradient as well known the fact that the electric field anywhere beneath the surface of a charged conductor is zero. Computation of electrostatic potential of gold nanorod considering this charge distribution from CTAB is well matched with experimental hologram that we observed.

As our society become progress, the possibility of people to be exposing to various hazardous environments. The needs to develop high sensitive and selective bio/chemical sensor have been increased. The π-conjugate polymers are a good candidate for the developing sensor applications because of their specific detection capability through a large change in the electrical and optical signals. In this study, we present a high performance optical sensing using core-shell hybrid nanostructure consist of silver (Ag) nanoparticles (NPs) and polydiacetylene (PDA). Our ultimate aims are improvement and control in the detection ability of selectivity and sensitivity by means of surface plasmon resonance (SPR) coupling effect. We fabricate a core-shell NP in deionized (DI) water solution through sequentially processes of reprecipitation and hydrothermal thermal method. The formation of the Ag/PDA core-shell hybrid NPs was visualized through scanning electron microscope (SEM) and high resolution-transmission electron microscope (HR-TEM) images. The nanometer-scale photoluminescence (PL) and Raman characteristics of core-shell NP were studied by using a laser confocal microscope (LCM) with a help of color charged-coupled device (CCD) images. The variation of light emission color and intensities of the core-shell hybrid nanostructure were observed depending on the amounts and kinds of target materials. Our results show that the PL intensity of Ag/PDA core-shell hybrid NP considerably increased compare to pristine PDA materials when the attachment of target bio/chemical materials because of the strong SPR coupling effect.

Recently, plasmonic coupled optical cavity has gained much attention due to its attractive properties in light manipulation, e.g. high Q optical resonance,[1] local field enhancements[2,3] and extraordinary transmission.[3] The strongly enhanced local filed originated from the plasmonic resonance hybridizing with the optical cavity mode has great potential applications. Therefore, it has been widely proposed to be used in enhancing semiconductor emission,[4] plasmonic laser,[1] WGM resonator based bio-sensor,[2] SERS detecting[5] and other related fields requiring superior optical performances.
In this work, the coupling effect between plasmonic mode and optical cavity mode has been demonstrated in self-assembled metal/dielectric hollow-nanosphere (HNS) arrays. The further employment of the strongly enhanced local filed originated from the inter-coupling of the plasmonic cavities for high sensitive SERS sensing was proposed. Large scale, two-dimensional (2D) metal/dielectric HNS arrays were successfully fabricated using the self-assembled polystyrene (PS) nanospheres as the template. Series orders of whispering gallery mode (WGM) resonances in the HNSs and plasmonic coupled WGM resonances in the double-shell HNSs have been experimentally and theoretically demonstrated by the transmission spectroscopy and simulated resonance spectra and near field patterns in FDTD method. Highly localized near field around the HNS cavities originated from the plasmonic WGM resonance and its strong multiple coupling effect between the HNS arrays ensure this plasmonic cavity arrays to be a suitable SERS substrate. Extraordinary SERS performances were then achieved on the HNS arrays with optimized size and gaps. Additionally, more significant photocatalytic self-cleaning process can be effectively accomplished on this recyclable SERS chips under visible light irradiation rather than conventional UV illumination.

The study of catalytic processes and redox reactions on metal nanocrystal surfaces is extremely challenging. Rates of reaction are highly sensitive to a variety of parameters including the particle size and shape, the degree of crystal faceting, the occurrence of underpotential deposition (UPD) and the role of substrate interactions. These factors are all convoluted in ensemble electrochemical measurements and consequently, one approach for obviating these problems is to study redox processes on single metal nanoparticles. We report the electrodeposition of metallic silver onto single gold nanostars adsorbed to indium tin oxide (ITO) electrodes. The electrochemical process was studied in-situ by correlated dark field spectroscopy and scanning electron microscopy (SEM). We demonstrate that bulk nucleation and growth on the ITO can be circumvented by careful application of chronoamperometry enabling controlled, selective growth of small amounts of silver onto single gold nanocrystals. This is possible because the binding energy for silver to gold is large enough to enable selective deposition at very low overpotentials. In order to increase the probability of UPD and hence control the deposition process, we have used gold nanostars which present a wide range of crystal facets and possess a high double-layer capacitance, due to their high surface-to-volume ratios. We carry out chronoamperometry at very low silver ion concentrations (0.5 micromolar) in 0.1 M NaNO₃ as the supporting electrolyte. SEM proves that deposition occurs preferentially on less atomically coordinated gold sites at the tips. The surface plasmon resonance blue-shifts by several tens of nanometers following the formation of spherical deposits of silver on the star tips, moving the scattered colour from the near infrared to red. The spectral shifts can be accurately modelled using finite element simulations. These results demonstrate that the morphological properties of individual bimetallic particles can be engineered electrochemically by monitoring optical changes.

Chiral structures are ubiquitous in nature, such as DNA, peptides and amino acids, which exhibit very small chiroptical effects. However, chiral nanoplasmonic structures exhibit strong chiroptical effects, which are not only due to the chiral geometry of nanoparticles, but also to nanoplasma used to manipulate the local electric and magnetic fields. In our study, silver nano-spirals (AgNSs) with adjustable geometric parameters have been fabricated in wafer scale by glancing angle deposition (GLAD) under controlled low temperature. When AgNSs are dispersed in ethanol, it has been demonstrated that circular dichroism of individual AgNSs can be tuned by adjusting a set of spiral dimensional parameters. In addition, a gigantic chiroptical response is obtained from the array of randomly distributed AgNSs deposited on sapphires in the UV-visible range, which has not been reported before to our best knowledge.

Materials with narrow-band high absorption ef#64257;ciency are particularly desirable for various applications including monochromatic photodetectors, sensors and thermal emitters. Previous theoretical work suggests that ideal narrow-band absorbers can be fabricated based on ordered two-dimensional arrays of metallodielectric core-shell microspheres on a gold substrate.^[1] In contrast to absorbers prepared by lithography, a low-cost fabrication method based on colloidal crystals is proposed to produce large area structures with almost perfect light absorption.
Here, we present a facile approach for the preparation and assembly of sub-micron metallodielectric particles on metallic substrates. The particles consist of a silica core surrounded by a platinum shell and a second shell made of silica. In this work, we developed a fabrication method for the production of almost perfectly smooth metallic shells of different thicknesses (3-30 nm) by a seeded growth method. Furthermore, we optimized the coating of the metallic surface with SiO₂, so that very homogeneous and smooth insulating shells in a range of 30 nm down to very thin shells of 2 nm were accessible. The metallic character of the core-shell SiO₂@Pt particles as well as the insulating character of the SiO₂-shells of the core-shell-shell SiO₂@Pt@SiO₂ particles could successfully be demonstrated by low-temperature conductivity measurements. Finally, we established a drop-casting method for self-assembly of high density particles with diameters of 350-400 nm, which enabled the formation of at least partially ordered monolayers of the core-shell-shell SiO₂@Pt@SiO₂ particles on metallic substrates. Optical measurements of these monolayers show a structurally related absorption in the IR region.

Metamaterial structures composed of ordered arrays of metallic nanoparticles and nanocavities are able to support strong plasmon and Fano resonances in the optical frequencies, where the appeared Fano dips can be utilized in bio/chemical sensing and spectroscopic purposes with a significant sensitivity. Herein, we utilize two concentric compositional nanoshells (Al/Al₂O₃) to design nanomatryushka structures in periodic arrays, where each one of Aluminum nanoparticles is covered by a certain thickness of the oxide layer. Depositing studied Aluminum nanomatryushka arrays on metasurfaces, we determined the optical response of the structure by calculating and drawing the extinction cross-sectional profiles. It is shown that the proposed structure is able to support multiple strong Fano resonances in the visible spectrum. Evaluating the plasmon response of the metamaterial configuration for the presence of various semiconductor metasurfaces (Silicon and GaN), the quality of Fano dips is analyzed for different regimes. Ultimately, we investigated the behavior of the appeared Fano dips for perturbations in the refractive index of the surrounding medium. In this method, we measured the accuracy and sensitivity of the metamaterial structure by plotting the linear figure of merit (FoM) and quantifying this parameter. This understanding paves a method to utilize entirely Aluminum nanoparticles in designing metamaterials structures that can be employed in designing ultra-sensitive bio/chemical sensors with low-cost and high efficiency.

Anisotropic linear assemblies of plasmonic NPs are highly interesting for their potential applications in the fields of sensing, metamaterials, wave guiding and light harvesting, due to their anisotropic optical properties. In this work, we present protein capped gold nanoparticles, which exhibit extremely high colloidal stability, even at super high NP concentrations and remarkable physicochemical and wettability properties[1-3]. In this work, we present highly ordered macroscopic (cm²) linear arrays of protein coated gold NPs via wrinkle assisted assembly.[4] The resulting anisotropic and nanostructured assemblies exhibit sophisticated polarization dependent optical properties.
[1] M. Chanana, M.A. Correa-Duarte, and L.M. Liz-Marzán, Small, 2011, 7, 2650
[2] M. S. Strozyk, M. Chanana, I. Pastoriza-Santos, J. Pérez-Juste and L. M. Liz-Marzán, Adv. Funct. Mater., 2012, 22, 1436.
[3] M. Chanana, P. Rivera_Gil, M. A. Correa-Duarte, L. M. Liz-Marzán and W. J. Parak, Angew. Chem. Int. Ed., 2013, 52, 4179
[4] Alexandra Schweikart, Nicolás Pazos-Pérez, Ramoacute;n A. Alvarez-Puebla and Andreas Fery, Soft Matter, 2011,7, 4093-4100

It is well known that the interface between a smooth silver surface and a dielectric enables propagation of surface plasmon polaritons (SPPs). Unfortunately, the direct excitation of SPPs from incident light is not possible at this smooth interface. Instead a surface structure providing a lateral impulse is needed, e.g. nanoparticles or gratings. The resulting roughened silver surfaces allow photons to be absorbed into SPPs which reduces reflection and enables improved photovoltaics and thermo-photovoltaics.
In our study we investigate the interface between a silver surface and air. Specifically, we introduce a facile approach that enables the large-area preparation of sophisticated metal nanostructures which allow for an efficient excitation of SPPs. Polydimethylsiloxane (PDMS) stamps containing nano-relief surface structures are prepared by cast and cure replication of wafer-based nano-templates.
The stamp surface is then coated with an ultra-thin silver island film. We have found that with exposure to humidity or liquid water the coalescence of the silver islands can be tuned from initially separated islands to smooth continuous films. After this tuning of the nanoparticle morphology the ultrathin silver film is transfer-printed on a smooth silver surface. Hence, the local roughness at the silver surface created after printing can be well controlled. Moreover, the ultrathin silver film is transferred only from the top regions of the nano-patterned stamp surface providing well controlled superimposed nanopatterns, e.g. gratings.
The resulting silver surfaces are analyzed by optical spectroscopy. We demonstrate blackened silver surfaces providing a large absorption of around 80% in the entire visible spectrum and for any incident angle. Moreover, the studies enable a new experimental access to the interaction of localized plasmons and delocalized SPPs.

Since the discovery of the effect named "Surface Enhanced Raman Spectroscopy" [1], a lot of work has been made to understand and control this effect [2], and making a complete list is now impossible. This work led to the construction of new scanning probe microscopes, microscope TERS (Tip Enhanced Raman Spectroscopy) [3] which have supplanted systems coupled with near-field optical spectrometers with low imaging capability [4]. Beautiful studies of nanoresonant objects could then be conducted with this TERS technique as well as molecular monolayers [5].
However, the TERS effect is not simply used for all of our samples, depending on its physical properties, leading us to propose a fundamental study coupling the ability to manipulate silver or gold nano-particles under a confocal Raman microscope, with multi-wavelength excitation laser. Our goal is to produce nano-assemblies of nano-particles in the vicinity of the single object or molecular materials [6] to be studied: this allows obtaining a plasmon spectrum with tunable resonance, according to the assembly structure [7]. Chemical contact with the molecule is avoided as well.
In this work, we are developing different nanostructures of metal nanoparticles (gold or silver) by AFM, coupled with a confocal micro-Raman spectrometer, allowing us to observe step by step the evolution of the vibrational spectrum of a sensor system (typically the advanced characterization of a single carbon nanotube, CNT).
These experimental data are associated with finite element models of far and local electromagnetic fields. Thus we achieve to a better understanding about the tunning of plasmon resonance modes of our nanostructures, including sensitive breaks symmetry modes. On the other hand, the consequences of their interactions with a substrate or molecule dipole moment, depending on the excitation wavelength (especially inelastic scattering), are studied. All this allows us to understand and to predict the phenomena observed experimentally.
[1] D.L. Jeanmaire, R.P. Van Duyne, J. Electroanal. Chem., 66 (1975), p. 235
[2]A.Bouvrée, A. D&’Orlando, S. Martin, G. Louarn, J.Y. Mévellec and B. Humbert, Nanostructured Gold surfaces: applications to the SERS and to the lightening rod effect, Gold Bulletin (2013), Vol 46, Issue 4, pp 283-290
[3] A. Hartschuh, N. Anderson, L.Novotny, Near-field Raman spectroscopy using a sharp metal tip, Journal of Microscopy, vol.210, Issue3, pages 234-240, June 2003
[4] Grausem, J., Humbert, B., Spajer, M., Courjon, D., Burneau, A., & Oswalt, J. (1999), Journal of Raman spectroscopy, 30(9), 833-840.
[5] G Picardi, M Chaigneau, R Ossikovski, C Licitra, G Delapierre, Journal of Raman Spectroscopy, 40 (10), 1407-1412, 2009
[6]Tong, L., Li, Z., Zhu, T., Xu, H., & Liu, Z. (2008), JPC C, 112(18), 7119-7123.
[7]Chuntonov, L. & Haran, G., Effect of Symmetry Breaking on the Mode Structure of Trimeric Plasmonic Molecules, JPC C 115(40):19488-19495, (2011)

Aluminum is the most abundant metal on the earth and recently has attracted much interest in nanophotonics because of its optical properties in the UV and visible regions. Single nanoparticles of aluminum, however, have broad, highly-damped plasmonic resonances at wavelengths longer than ~500 nm because of high optical losses. Here we show that the resonant lineshape narrows by a factor of three throughout the entire visible region by placing the nanocrystals on a thin film of aluminum. The nanocrystals and the film are separated by a self-terminating oxide, which is only a few nanometers thick. This results in a strong interaction between the two systems, counteracting the intrinsic optical losses of aluminum and increasing the radiative and non-radiative lifetimes of the nanocrystal plasmon. This capability expands the practicality of aluminum for nanophotonics in the visible range, resulting in vividly colored samples that enable applications ranging from full-color printing to sensing and surface enhanced microscopies.

We present the correlation between structural and optical properties of truly three-dimensionally gold/air percolated nanoparticles, nanosponges. The nanosponges provide a variation of the local dielectric function on the nanoscale, due to the size of the interior filaments, which is well below 20 nm. The inner morphology, i.e. the three-dimensionally gold/air percolation, can be adjusted changing the fabrication parameters.
Here, we report that the scattering spectra of single nanosponges are mainly determined by the three-dimensional percolation of each individual nanosponge. In addition, our experiments, in agreement with numerical calculations, indicate that the nanosponges are less lossy than solid gold nanoparticles, supporting our original aim to find a novel low loss plasmonic material.
Consequently, this, together with their large surface-to-volume ratio and the very small size of the interior filaments, renders single nanosponges as promising candidates for metamaterials in the visible range, for sensing applications and for photo-catalysis.
Financial support was provided by the ERC grant 257158 “ActiveNP” and Deutsche Forschungsgemeinschaft (DFG, grant SCHA 632/20-1).

In the present investigation SERS sensitivity and plasmonic-photonic interference coupling (P-PIC) in amorphous TiO₂ nanotube (A[TNT]) and crystalline silver (Ag) nanoparticles was studied. TNT were synthesized by two step anodization on titanium (Ti) substrate. Using thermal evaporation, 20 nm Ag film overcoating on TNT structures was synthesized, which coalescence to form Ag nanoparticles, acting as a three dimensional (3D) surface enhanced raman spectroscopy (SERS) substrate for rhodamine 6G (R6G) molecule detection. The current decay time studied under 1 sun condition in potassium hydroxide (KOH) electrolyte using two electrode system for A[TNT] samples is larger as compared to C[TNT] samples. The nanoarchitecture phase formation is confirmed by X-ray diffraction (XRD) analysis, Raman spectroscopy and Energy Dispersive X ray (EDAX) analysis. Surface morphology and the salient features are studied by scanning electron microscopy (SEM). Current decay time is also studied and compared for the samples respectively which show large current decay time for A[TNT]-Ag as compared to C[TNT-Ag]. UV- visible absorption studies shows larger red shift in the absorption band gap edge of A[TNT]-Ag samples as compared to C[TNT-Ag] samples. Moreover the TNT length variation in A[TNT]-Ag samples results into constructive or destructive interference, which in turn affect the P-PIC and R6G molecule detection as well as SERS intensity which was approximately three time higher than C[TNT-Ag].

Recent advances in plasmonic metamaterials have defied common sense in optics, where the optical properits of an artificial material can be tailored at will. Metamaterial absorbers have recently been demonstrated to exhibit a large or even perfect absorption within a certain frequency range. Since the metamaterial absorbers offer a unique surface condition with tailored absorption properties as well as strong plasmonic enhancement, a wide variety of potential applications have been proposed, such as high-efficiency thermal emitter and high-sensitive bio-chemical sensing. Here, we demonstrate a high-sensitive surface-enhanced molecular detection technique by utilizing the resonant coupling of plasmonic modes of a metamaterial absorber and infrared (IR) vibrational modes of a molecular self-assembled monolayer (SAM). IR absorption spectroscopy of molecular vibrations is of importance in material/medical science and security detection, since it provides essential information of the molecular structure, composition, and orientation. For direct (far-field) detection of extremely small amounts of molecules, surface enhanced IR absorption (SEIRA) using a tailored plasmonic nanostructures has been extensively studied to dramatically improve the sensitivity by several orders of magnitude. Our metamaterial approach offers not only significant plasmonic enhancement but also a low-background detection scheme, thus further lowering the detection limit of direct IR absorption spectroscopy. The fabricated metamaterial consisted of 1D array of Au micro-ribbons on a thick Au film separated by an MgF₂ gap layer. The surface structures were designed to exhibit an anomalous IR absorption at ~ 3000 cm^-1, which spectrally overlapped with C-H stretching vibrational modes. 16-Mercaptohexadecanoic acid (16-MHDA) was used as a test molecule, which formed a 2-nm thick SAM with their thiol head-group chemisorbed on the Au surface. In the FTIR measurements, the symmetric and asymmetric C-H stretching modes were clearly observed as reflection peaks within a broad plasmonic absorption of the metamaterial. The low-background detection scheme with tailored plasmonic enhancement by the metamaterial absorber dramatically improved the sensitivity down to ~ 1.8 attomoles within the diffraction-limited IR beam spot. Our metamaterial approach thus may open up new avenues for realizing ultrasensitive IR inspection technologies.

Over the last few decades, researches in the field of surface enhanced Raman scattering (SERS) have been dramatically developed due to its powerful analytic performance to biological and chemical analytes. Especially, the microfluidic systems coupled with SERS have been recently demonstrated for in-situ monitoring of chemical reactions and detect analytes. However, in-situ SERS measurement in microfluidic systems have limitations due to the destruction or contamination of SERS-active metallic nanostructure in the fluid channel by fluidic force.
Droplet-based open-channel microfluidic systems have several advantages compared with conventional microfluidics systems in terms of low cost, and simple manufacture. In particular, open-channel microfluidic systems on superhydrophobic surfaces with extremely low adhesion have a key advantage that the droplet can be easily roll-off without surface contamination.
Here, we demonstrate a novel droplet-based open-channel SERS measurement system on superhydrophobic guiding track. SERS-active structures were fabricated on a copper plate with a shallow groove via galvanic displacement reaction, and extreme water repellent properties were achieved through self-assembled monolayer coating. Droplet can be successfully guided along the pre-defined groove on the superhydrophobic surface, and transported to the laser spot for Raman measurement. By using this novel system, successive manipulation and SERS measurement on the same surface can be achieved using Rhodamine 6G droplet with a low concentration of 10^-7M without any contamination.

Metallic nanomaterials exhibiting distinct surface-plasmon-resonance effects have gained increasing attention over the past few years due to their broad applications in photonics, biological imaging, drug delivery, sensors and surface enhanced Raman scattering (SERS) ¹. One of the main drivers for the development of plasmonic materials is the desire to improve the sensitivity of SERS for exploring structure and reaction pathways at surfaces as well as for sensing ². The development of nanofabrication methodologies for tailoring both particle shape and size has been intensified recently, giving special attention to the preparation of noble metal nanoparticles (Cu, Ag, Pt, Pd, and Au). ³ Over a wide range of studies, the challenge is to develop metallic substrates combining high sensitivity, reproducibility, stability and easy of preparation. ⁴ The sensing feature of MNPs results from their collective charge density oscillations and is known as localized surface plasmon resonance (LSPR). ² In recent studies, we reported a new method of fabrication of MNPS based on the self assembly of metallic precursor-loaded homopolymer dispersion. Then, the deposition of this dispersion on a given substrate (glass, silicon, etc.) allows the formation of highly sensitive nano-objects particularly relevant for sensing and SERS (figure 1). Herein we report a facile way to detect few biomolecules (picomol) the LSPR and SERS features of a highly sensitive biosensor. In the present projet, we aim to investigate the physico-chemical mechanism of formation of MNPs in order tune their physical and optical properties, which is essential for applications. We highlight the possibility to measure few molecules of bacteria, which is an un-preceded result using a plasmonic sensor.
1. Akil S., Jradi S., Plain J, Adam P.-M., Bijeon, J.-L., Sanchez C., Bachelot R. and Royer P. , Chemical communications, 2011, 47, 2444.
2. Akil S., Jradi S., Plain J, Adam P.-M., Bijeon, J.-L., Bachelot R. and Royer P. (2012), RSC Advances, 2, 7837.
3. Jradi S., Balan L., Zeng X., Plain, J., Lougnot D., Royer P., Bachelot R., Akil S., Soppera O., Vidal L. (2010), Nanotechnology, Volume 21, 095605.
4. Seung Joon Lee, Brian D. Piorek, Carl D. Meinhart and Martin Moskovits. Nano Lett., 2010, 10, 1329.

Plasmonic optical biosensors have become an important area of research because they are extremely sensitive to surface changes within subwavelength dimensions and show significant potential for on-chip integration, low cost processing, and massive parallelization. Pushing this trend, the discovery of plasmon-enhanced optical transmission through an array of nano-structured holes has led to the development of an entirely new generation of optical biosensors. While nanoholes have been typically used in a liquid environment, plasmonic sensing of gas-phase analytes is also an important area of research and development. In this paper, we employ a template stripping method to produce plasmonic nanohole arrays on analyte-sensitive substrates for real-time vapor sensing. Template stripping involves cleaning a silicon master template, depositing a thin metal film, and then stripping the film from the template with an adhesive backing layer. The resultant surfaces are very smooth, and dozens of identical chips can be created by reusing the same template. While many fabrication methods are available, template stripping is ideal in our current case by offering the ability to chemically pattern the substrate before stripping from the template. Indeed, our device is designed to operate by explicitly exploiting simultaneous plasmonic resonances within the substrate as well as within the vapor being tested. Because the substrate is in contact with vapor due to the open-hole geometry, red-shifts (air-side plasmon resonances) and blue-shifts (substrate-side plasmon resonances) are seen at the same time during exposure to <10 ppm ethanol vapor in nitrogen. We also show negative control experiments with similar concentrations of m-xylene. Since we are able to chemically pattern the substrate prior to template stripping, our devices have potential for multiplex sensing. Initially, we demonstrated these sensors utilizing an inverted microscope and an imaging spectrometer system. While useful for sensor characterization, this setup requires bulky optics, microscope objectives, and large spectrometers and cannot lead to a compact, on-chip solution. To resolve this issue, in this paper we also show that by placing our nanohole chips directly onto the front window of a commercially available, low-cost CMOS imager, all of these optical components can be circumvented. By properly positioning the nanoholes, an illumination source (e.g., an LED), and a suitable spacer layer, transmitted light will diffract from the nanohole array, spread into a spectrum over the space of a few millimeters, and land on the imager as a full spectrum. This spectrum, with plasmon-enhanced transmission peaks, is monitored in real-time and used for vapor sensing as before. This on-chip solution circumvents the bulky components (e.g. microscopes, coupling optics, and spectrometers) needed for traditional plasmonic biosensing setups while maintaining high sensitivity and multiplexing capability.

Simultaneous LSPR and electrical detection using ultrathin gold films with incorporated nanoholes is presented. The sensor is electrochemically accessible and enables label-free sensing in a controlled fashion.
Films with dimensions smaller than the electron mean free path are essential to measure large resistance changes due to small alterations in thickness or close proximity of charged species [1]. Additionally, nanoholes within thin films enable optical sensing based on localized surface plasmon resonance. Our device is fabricated on a glass wafer coated with 12 nm of niobium pentoxide (Nb₂O₅). Colloidal lithography was used to generate nanoholes. The evaporated gold films are 20 nm thick, contacted by 100 nm thick gold traces, and (apart of the central sensing region of the film) covered by SU8. This chip is fully integrated into a custom-made flow cell. The design of both the flow cell and the chip allows for reproducible and accurate measurements with ideal fluid exchange behavior.
Iodide sensing is one of the possible applications for this device. Iodide is an essential element for humans and animals, but insufficient intake is still a major problem. Most current detection methods are costly and require both a sample pretreatment and a long analysis time [2,3]. However, for determining iodide concentrations of human samples in developing countries or for comprehensive and long-term studies in environmental waters, low cost and simplicity are becoming increasingly important.
Our device takes up on these issues with sensing based on iodide induced electrochemical etching of ultrathin gold films. For example potassium iodide (KI) dissociates in aqueous buffer and the iodide anions can be attracted using voltage. The resulting etching can be measured by a change in the impedance of the thin film. The underlying mechanism is demonstrated by simultaneous cyclic voltammetry experiments and by measuring resistance change in buffer solution. As sensing methodology, an amperometric multistep method is presented in buffer as well as in an environmentally relevant fluid (lake water) with limits of detection in the range of 1 µM (127 µg L^-1) and 2 µM (254 µg L^-1), respectively.
The versatility of this sensing platform is also shown with binding studies. Attachment of thiolated single-stranded DNA demonstrates this concept. Iodide etching can thereby be used as a controlled cleaning step, enabling the reuse of the device.
We believe this sensor not only enables low cost measurements of iodide concentrations and binding events, but also opens up opportunities for a variety of biosensing applications.
References
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2. Shelor CP, Dasgupta PK. Anal Chim Acta2011, 702, 16-36.
3. Khazan M, Azizi F, Hedayati M. Scimetr2013, 1, 1-9.

Plasmonic materials and devices aim to exploit the unique optical properties of metallic nanostructures to enable routing and manipulation of light at the nanoscale. A critical parameter in delivering the plasmonic devices is the materials&’ preparation methods, which should allow for the production of nanostructures with tunable plasmonic properties. Two issues, however, emerge: Firstly, as far as specific applications are concerned, such as plasmonic writing, the protection of these structures from environmental poisoning is of paramount importance; any production process should be able to form plasmonic nanoparticles (PNPs) below the dielectric surface. In that context, laser annealing (LA) of Ceramic:Ag nanocomposites is proposed as a promising route for the production of buried PNPs for optical encoding, lens coloration and decorative purposes. LA is a simple patterning tool providing freedom of design, fast processing, compatibility with large-scale manufacturing and allows for the use of inexpensive flexible and organic substrates such PET and polycarbonate. Secondly, the scale of the production process would dictate the maximum size of the envisaged devices and their application range in everyday life. Large-scale and simultaneously cold processes are still required for the effective production of buried PNPs. The sputter deposition of ceramic/metal nanocomposites and multilayers would address both the aforementioned limitations, as they are capable of producing metals buried into dielectric ceramics with fascinating optical response, while they are compatible with a variety of large-scale deposition configurations such as in-line, roll-to-roll, or close-field sputtering, which are becoming nowadays the manufacturing routes of choice. In the present work, we present a new entirely-cold route by combining the deposition of ceramic/metal nanocomposites and multilayers (in particular, AlN:Ag, AlN/Ag, Y2O3/Ag and CeO2/Ag) multilayers with one ultra-short UV LA step. We demonstrate that this LA step is capable of driving the subsurface modification of plasmonic metal/dielectric films and multilayers, and delivers tunable LSPR behavior from Ag nanoparticles that are formed and dispersed in a depth of several nm away from the free surface. This large-scale entirely-cold process is very unlike previous attempts dealing with the treatment of thin monolayered films on top of a rigid substrate. We perform an extensive, thorough and in depth investigation of the laser-matter interactions considering the morphological and microstructural features of the films. The experimental results are complemented by detailed photo-thermal calculations, which are used to identify the fundamental light-matter interactions and heat diffusion mechanisms in the films and multilayers, and obtain insights on the basic mechanisms of morphology changes upon LA.

Colorimetric sensor provides a way to sense and analyze various chemicals and biomaterial targets based on local refractive index changes. However, pre-existing colorimetric sensors, which incorporates one of the optical phenomena such as photonic crystal, whispering gallery mode and localized surface plasmon, shows limited resonance peak shifts which makes it hard to analyze and thus requires extra instruments. Furthermore often the structural color comes from superposition of several different colors rather than a single color, which makes it harder to talk about the sensitivity. Many approaches to increase the sensitivity of colorimetric sensors are usually focused on the structure that interacts with the light and produces plasmonic colors such as extra ordinary transmission from nano holes. However, another important aspect of colorimetric sensor is signal amplification since the density of target molecule in the environment is often far below the concentration that can induce the detectable color shift.
Here we demonstrate sandwich type plasmonic ELISA which incorporates glucose-glucose oxidase reaction for the signal amplification based on the aluminum anodized oxide structure and provide computational analysis of effective 3D colorimetric plasmon sensor structure on a novel aluminum anodized oxide (AAO) by FEM optical simulation. As the target molecules are anchored to capture antibody on substrate, the detection antibody with glucose oxidase attaches to target molecule thus inducing reduction of Au ions in solutions of ELISA. The reduced Au is deposited on the thin Au deposited AAO nanostructure resulting shift of plasmonic color. Furthermore color shift is calculated through FEM method and the nanostructure is optimized in terms of pore depth, pore size, etc. When the incident white beam is reflected from the Al mirror at the bottom of AAO structure, the interference between incoming wave and reflected wave forms and the wave is absorbed on the top Au layer due to the localized surface plasmon effect. As more Au is deposited on the nanostructure stronger color change occurs.

Plasmon excitation in metal nanostructures decays into short-lived hot-electrons and hot-holes. It is possible to harness the energy of these hot-carriers before they thermalize and are dissipated as heat. Here, we describe a scheme to extract the energy of hot-carriers into an optical form. Our novel scheme allows photon upconversion, i.e. emission of higher energy photons by absorbing low energy photons. While current state-of-the-art solid-state upconverters suffer from low efficiency, narrow-band operation, and lack of tunability, our hot-carrier upconversion overcomes these limitations. Specifically, we consider a metal/semiconductor quantum well heterostructure where the electrical current generated by hot-carriers in the metal nanostructure drives the adjacent quantum well emitter. When metal injects both hot-electrons and hot-holes into the quantum well, charge neutrality is maintained and a current loop is established to continuously drive the quantum well. This balanced injection allows the semiconductor quantum well to emit photons corresponding to its energy bandgap, which, importantly, can be higher than the energy of the photons exciting plasmons in the metal. First, we develop a theoretical model to determine the upconversion efficiency. We consider a small metallic cube placed adjacent to an ideal quantum well emitter. Upon plasmon excitation, the hot-carrier distribution in the metal cube is computed using Fermi&’s golden rule. Small metal cubes (less than 10 nm on a side) generate hot-carriers with efficiency greater than 80%. Injection of these carriers into the quantum well limits total upconversion quantum efficiency, but 5 nm silver cubes can still exhibit upconversion efficiencies exceeding 20%. Gold nanoparticles are less efficient, but can still generate upconverted photons with an efficiency of 10%. We then verify this scheme experimentally using blue emitting InGaN/GaN multiquantum wells adjacent to gold nanodisks. Electron beam lithography is used to fabricate 10 thick gold nanodisks of various diameters, ranging from 10 to 50 nm, on the InGaN/GaN quantum wells. These disks absorb at a wavelength of 530 nm, and following hot-carrier injection, enable emission from a three stack multi-quantum well (5 nm GaN and 3 nm In_0.13Ga_0.87N) at 423 nm. Our presentation will describe the efficiency of upconversion, its dependence on illumination intensity and its dependence on shape and size of gold nanostructures. Interestingly, the proposed hot-electron upconversion scheme does not require coherent illumination, can be quite broadband, is tunable-by-design, and more efficient than the existing lanthanide and molecular upconversion schemes.

The decay of surface plasmon resonances is usually a detriment in the field of plasmonics, but the possibility to capture the energy normally lost to heat would open new opportunities in photon sensors and energy conversion devices. In the context of hot-electron devices, the large extinction cross-section at a surface plasmon resonance enables nanostructures to absorb a significant fraction of the solar spectrum in very thin films. Despite the significant experimental work in this direction, a complete theoretical understanding of plasmon-driven hot carrier generation with electronic structure details has been evasive. Theoretical studies of plasmonic systems have traditionally focused on their optical response, including quantum jellium models of nanostructured systems. While extremely valuable, these models do not capture the material dependence of this process and miss interband transitions in noble metals.
Recently we analyzed the quantum decay of surface plasmon polaritons and found that the prompt distribution of generated carriers is extremely sensitive to the energy band structure of the plasmonic material. In this context, we have developed a general theoretical and computational framework using a multi-scale model that combines long-range electromagentic solvers for the plasmons with density functional theory in a maximally-localized Wannier basis for the electrons. With a Feynman diagram approach to processes involving plasmons, electrons and phonons built upon this model, we present realistic first principles calculations for nanostructured plasmonic systems including transport of plasmonic hot carriers to the surface and injection over a barrier. Additionally, using this diagrammatic approach we present results on higher order processes such as multi-plasmon decays in metals which are critical for plasmon-driven upconversion.

Cooling is a significant end-use of energy globally and a major driver of peak electricity demand. Air conditioning of buildings, for example, accounts for 15% of the primary energy used to generate electricity in the United States. Moreover, global energy use for cooling buildings is predicted to grow to be ten times what it is today by 2050, with most of the growth coming from the developing world. A passive cooling strategy that cools without any electricity input could therefore have a significant impact on global energy consumption. To achieve cooling one needs to be able to reach and maintain a temperature below the ambient air. At night, passive cooling below ambient air temperature has been demonstrated using a technique known as radiative cooling or night-sky cooling, where one uses a device exposed to the sky to radiatively emit heat to outer space through a transparency window in the atmosphere between 8-13 µm. Peak cooling demand however occurs during the daytime. Daytime radiative cooling below ambient under direct sunlight has never before been achieved because sky access during the day results in heating of the radiative cooler by the sun.
We show how thermal nanophotonic approaches and designs enable one, for the first time, to achieve passive radiative cooling below ambient air temperature during peak daylight hours. We first highlight the theoretical requirements necessary for daytime radiative cooling and discuss the need for a photonic approach. We then present a nanophotonic design that has the required spectral characteristics to achieve daytime radiative cooling: it is strongly reflective over visible and near-IR wavelengths but strongly emissive between 8 and 13 µm. We then present results of the first experimental demonstration of daytime radiative cooling, where we achieve a temperature of nearly 5°C below the ambient air temperature under direct sunlight. Using a thermal nanophotonic approach, we design and fabricate an integrated photonic solar reflector and thermal emitter consisting of 7 layers of HfO₂ and SiO₂ that reflects 97% of incident sunlight while emitting strongly and selectively in the atmospheric transparency window. Even when exposed to direct solar irradiance of greater than 850 W/m² on a rooftop, the photonic radiative cooler achieves an average of 4.9°C below ambient air temperature over one hour, and has a cooling power of 40.1 W/m² at ambient. Furthermore, we develop a theoretical model capable of accurately predicting the photonic radiative cooler's performance as a function of time of day, based on ambient conditions and the spectral absorption/emission characteristics of the radiative cooler.
These results demonstrate that a tailored, nanophotonic approach can fundamentally enable new technological possibilities for energy efficiency, and further indicate that the cold darkness of the universe can be used as a renewable thermodynamic resource, even during the hottest hours of the day.

Thermophotovoltaic (TPV) devices are highly attractive in harvesting and converting thermal energies, including solar energy and wasted energies from internal combustion engines, fuel sources, etc, into electricity at a very high efficiency. Solar thermophotovoltaics (sTPV) seek to overperform the Shockley-Quiesser limit for single junction solar cells by converting the broadband solar spectrum of sun light into much narrower emission wavelengths so that the photon energies can be more efficiently absorbed by the photovoltaic cells. 3D metallic photonic crystals (MPhCs) are promising candidates for the choice of sTPV emitters as they exhibit strong selectivity in absorption due to the tailoring of the photonic density of states by the periodic structures. However, reasons preventing the 3D MPhCs emitters being widely used in the sTPV devices are that the thermal stability of the materials being currently studied (i.e. W and Ta) and their fabrication process are still far from satisfactory. Therefore, it is necessary to find better material systems that can both potentially meet the requirements for having superior thermal stability and simultaneously being easily processed. Rhenium (Re) and its alloys, as another well-known refractory metal system, are highly reputed for the outstanding thermal and other physical properties in terms of non-structured architectures and have been broadly used in extreme conditions as bulk materials. However, the reported work in studying the engineered microstructures of Re and their optical responses are rare. In this work, we have been pursuing the development of the first rhenium nickel alloy based MPhC via electrodeposition from low temperature nontoxic aqueous electrolyte. The resulting 3D mesostructures are showing outstanding thermal stabilities for at least 1000 C annealing for extended hours, much better compared to the tungsten MPhCs fabricated via similar electrochemical approaches. The flexibility in adjusting the Re and Ni composition ratio also allows comprehensive studies of the metallurgy aspects of the heavily structured metallic microstructures, which could bring significant insights on the optimization of the 3D architectures under high temperature applications. Analysis on the selection of surface protective coatings from transparent dielectrics and the optical response of the MPhCs are also performed.

We will show that composite photo-catalysts combing plasmonic metallic nanoparticles of noble metals and semiconductor nanostructures exhibit improved photo-chemical activity compared to conventional photo-catalytic materials.^1,2 We will also show that plasmonic silver nanoparticles, optically excited with low intensity visible light, exhibit direct photo-catalytic activity. We will discuss underlying mechanisms associated with these phenomena.^2,3,4 We propose that this new family of photo-catalysts could prove useful for many heterogeneous catalytic processes that cannot be activated using conventional thermal processes on metals or photo-catalytic processes on semiconductors. I will show an example of such a process.⁵
D. B. Ingram, S. Linic, JACS, 133, 5202, 2011
Suljo Linic, Phillip Christopher and David B., Nature Materials,10, 911, 2011.
Ingram P. Christopher, H. Xin, S. Linic, Nature Chemistry, 3, 467, 2011.
P. Christopher, H. Xin, M. Andiappan, S. Linic, Nature Materials, 11, 1044, 2012.

We present new spectroscopic techniques that enable visualization of nanoparticle phase transitions in reactive environments and light-matter interactions with nanometer-scale resolution. First, we directly monitor hydrogen absorption and desorption in individual palladium nanocrystals. Our approach is based on in-situ electron energy-loss spectroscopy (EELS) in an environmental transmission electron microscope. By probing hydrogen-induced shifts of the palladium plasmon resonance, we find that hydrogen loading and unloading isotherms are characterized by abrupt phase transitions and macroscopic hysteresis gaps. These results suggest that alpha and beta phases do not coexist in single-crystalline nanoparticles, in striking contrast with conventional phase transitions and ensemble measurements of Pd nanoparticles. Then, we then extend these techniques to monitor nanoparticle reactions in a liquid environment. By constructing a flow chamber, we directly monitor growth and assembly of colloidal plasmonic metamaterial constituents induced by chemical catalysts. Lastly, we introduce a novel tomographic technique, cathodoluminescence spectroscopic tomography, to probe optical properties in three dimensions with nanometer-scale spatial and spectral resolution. Particular attention is given to reconstructing a 3D metamaterial resonator supporting broadband electric and magnetic resonances at optical frequencies. Our tomograms allow us to locate regions of efficient cathodoluminescence across visible and near-infrared wavelengths, with contributions from material luminescence and radiative decay of electromagnetic eigenmodes. The experimental signal can further be correlated with the radiative local density of optical states in particular regions of the reconstruction. Our results provide a general framework for visualizing chemical reactions and light-matter interactions in plasmonic materials and metamaterials, with sub-nanometer-scale resolution, and in three-dimensions.

Due to their unique optical properties, plasmonic nanostructures find a broad range of applications including sensing, subwavelength optical components and light management structures in photovoltaics. For such applications, structures with well-defined plasmon resonances are required, i.e. the plasmon resonance needs to occur in a certain energy range. At the same time substrate-supported systems are typically needed, which is a particularly important challenge with respect to tailoring optical properties since plasmon resonances are strongly sensitive to the refractive index environment. Consequently the introduction of an interface will alter the optical properties of the plasmonic nanostructures. In addition structural control such as adjustable inter-particle distances and surface coverage are of utmost importance for the preparation of optically functional superstructures since these can strongly affect the optical performance. The steady developments in wet-chemical synthesis of plasmonic nanoparticles with control over particle size, shape and their distribution as well as the particle functionalization provide a nearly infinite toolbox of optical building blocks. However, typically a single distribution of particles sizes is realized which omits the effective screening of a range of plasmon resonances.
In this presentation we demonstrate a versatile approach to prepare substrate-supported, macroscopic arrays of plasmonic gold nanoparticles with locally varying particle size and hence plasmonic properties[1]. The arrays are based on polymer-encapsulated gold nanoparticles[2], which were deposited on glass substrates by simple spin-coating. A controlled growing protocol was used to overgrow the gold cores in-situ, i.e. directly using the substrate-supported particle array. Analysis of the kinetics of overgrowth revealed diffusion-limited growth. This allowed us to control the size of the gold cores by adjusting the duration of growth, yielding a continuous gradient in particle size. The size of the particles was investigated by electron microscopy and atomic force microscopy, while absorbance spectroscopy was used to analyze the plasmonic properties of the array at different positions. Simulations based on Mie theory were used to support the experimental findings and to investigate the size and refractive index dependent plasmonic features of our particle superstructure.
[1] M.B. Müller, C. Kuttner, T.A.F. König, V.V. Tsukruk, S. Förster, M. Karg, A. Fery, ACS Nano2014, 8 (9), 9410-9421.
[2] M. Karg, S. Jaber, T. Hellweg, P. Mulvaney, Langmuir2011, 27, 820-827

It is well known that the plasmon resonance frequency of metal nanostructures depends on the free carrier density in the metal. Earlier work has shown that the plasmon resonance can be tuned by electrostatically controlling the carrier density: increasing the carrier density leads to a blueshift of the resonance, while decreasing it leads to a redshift. Interestingly, the reverse effect, the optical generation of an electrostatic potential, has so far not been observed. Thermodynamically, however, such a plasmoelectric effect is expected to occur. Here, we demonstrate direct experimental evidence of plasmoelectric potentials in the range 10-100 mV on colloidal assemblies and plasmonic hole-array device geometries.
When positioned on a grounded conductive substrate, metal nanoparticles can exchange electrons with the substrate, causing small shifts in the plasmon resonance absorption spectrum. Under monochromatic illumination detuned from the resonance peak wavelength, such electron transfer is thermodynamically favorable when it increases the particle absorption as that leads to a minute temperature increase corresponding to an increase in entropy. As a result, in steady state, an electrochemical potential builds up that is negative when the particle illuminated on the blue side of the plasmon resonance, and positive when it is illuminated on the red side.
To test this experimentally, 60 nm Au colloids were dropcasted on an ITO/glass substrate. Next, a dark-field scattering spectrum was measured, showing a clear plasmon resonance peak around lambda; = 550 nm. Kelvin probe force microscopy (KPFM) was used in non-contact mode to measure the potential difference between tip and substrate. The tip is positioned close to the colloids, and the sample is illuminated from the bottom with monochromatic radiation while the wavelength is scanned from 480 - 640 nm (1 W cm^-2). We observe a clear negative light-induced surface potential up to -12 mV for illumination below the resonance wavelength; a positive potential up to 14 mV is observed above the resonance, in agreement with the thermodynamic model.
Next, we studied optically-induced plasmoelectric potentials on sub-wavelength hole arrays in a 20 nm thin Au film that were fabricated on a glass substrate using e-beam lithography. The hole arrays with different collective absorption resonances are made by varying the array pitch from 175 - 300 nm, resulting into spectrally controlled absorption resonances between 580 - 710 nm. Surface potentials as high as 100 mV are observed (100 mW cm^-2 illumination intensity), changing from negative to positive at the plasmon resonance. By varying the array geometry the plasmo-electric potential spectrum is tuned over a broad spectral range. The hole arrays are a first step towards plasmo-electric circuitry in which power can be harvested.
M.T. Sheldon, J. van de Groep, A.M. Brown, A. Polman, H.A. Atwater, Science 346, in press (2014), published October 30, 2014

Nanomaterials exhibiting plasmonic resonances have been attracting intense attention for their tunable optical properties. Via control of the interaction of the nanoparticles (and surrounding media) with designed shapes such as nanospheres and nanorods, plasmonic resonances can be tuned to achieve unique optical properties such as Fano resonances and transparency windows. Such complex nanostructures have been traditionally fabricated using the electron-beam lithographic methods. In this work, we demonstrate the self-assembly of plasmonic nanoparticles via simple aerosol processing. Two kinds of typical plasmonic nanoparticles are used as model materials. The first type of nanoparticles is boron-doped silicon (Si) nanocrystals (NC), which are synthesized using a nonthermal plasma technique and show B-doping-induced plasmonic absorption in the infrared (IR) region. By aerosolizing and drying droplets of a mixture of small B-doped Si NCs and larger polystyrene latex (PSL) spheres, assembly of mesoscopic particles consisting of a hexagonally stacked PSL sphere scaffold with B-doped Si NCs infilling the voids in between is achieved. In-flight annealing can be used to remove the PSL sphere scaffold, leaving behind mesoparticles of B-doped Si NCs forming an inverse opal void structure.
The second type of nanoparticles is noble metal gold (Au) nanorods with plasmonic resonances in the UV-visible region. Mesoscale assemblies are obtained in which Au nanorods are embedded in a matrix of silica nanospheres and therefore well separated from each other. Such assemblies may provide a route to eliminate aggregation effects of the Au nanorods but efficiently maintain sharp plasmonic resonance of Au nanorods to enable spectral engineering of the plasmonic absorption.
This presentation will describe the aerosol assembly routes and preliminary data on optical properties of these plasmonic meso-structures.
This work was supported primarily by the Army Office of Research under MURI Grant W911NF-12-1-0407.

Fabrication techniques based on self-assembly have the opportunity to construct interesting plasmonic materials that would be difficult, if not impossible, to build using conventional lithographic tools. In such cases, there exists the potential to investigate unique plasmonic interactions which have not been observed previously. Recently, DNA has emerged as a particularly powerful tool for directing the assembly of plasmonic nanostructures into one-, two-, and three-dimensional architectures as a result of its versatility in programming both the symmetry and spacing of the resulting superlattices. In this work, we utilize circular disk nanoparticles as building blocks that can be considered “plasmonically two-dimensional”, i.e. they are sufficiently thin and anisotropic so as to confine the localized dipolar oscillation to occur only in a 2D plane while supporting extremely weak or non-existent transverse modes. These particles are then assembled into extremely well-ordered one-dimensional superlattices via DNA hybridization interactions occurring between their flat faces. Because of the two-dimensional nature of the plasmon modes in these particles, coupled resonances display exclusively pi-bonding-type modes, even in orientationally-averaged measurements. Therefore, in contrast to the majority of examples of nanoparticle association, strongly blue-shifted plasmon resonances are observed in particles transitioning from a discrete to a (1D) assembled state. We explore these observations in the context of the plasmon hybridization model and conclude that this system represents the unusual case where antibonding modes are bright while bonding modes are dark, as the latter exhibit no net dipole for coupling to far-field light. Systematic modulation of the particle size and spacing in one-dimensional superlattices yields results that are consistent with this interpretation.
Unlike many other particle assembly tools, DNA offers the capability of programming the specificity of nanoparticle binding by tailoring the sequences that govern particle association. As a result, more complex one-dimensional superlattices can be constructed in which particles of two different sizes are forced to assemble into a biperiodic (i.e. alternating) arrangement. In this case, more complex plasmon coupling is observed as modes that were previously forbidden by symmetry, are now allowed. Specifically, both blue-shifted and red-shifted resonances are observed upon nanoparticle assembly, as both antibonding and bonding modes are capable of dipolar coupling to the electromagnetic field. Finally, one-dimensional arrays which are biperiodic in both size and shape (e.g. alternating triangular prisms and circular disks or alternating circular disks and nanorods) have been constructed in order to examine more exotic forms of plasmon coupling.

We report an integrated narrowband light source based on thin MoS₂ emissive material coupled to the high quality factor whispering gallery mode resonances of a microdisk cavity with a special coupler enabling easy free space coupling of emission while yielding high spatial coherence. The active light emitting material consists of MoS₂ bilayer flakes with a thin atomic layer deposited SiO₂ protective coating yielding 20 times brighter chemically enhanced photoluminescence compared to as-exfoliated monolayers on the microdisk. Quality factors as high as 900 are observed with correspondingly high temporal coherence. We also experimentally demonstrate effective index tuning of cavity coupled emission over a full free spectral range. The thermal response of this system is also studied. This work provides new insights for achieving laser action with an atomically thin active medium.

Two-dimensional (2D) transition metal chalcogenide (TMDC) materials have been emerging as one central topic of the entire physical science and engineering. These materials show remarkable excitonic properties and offer a tantalizing prospect of scaling all kinds of semiconductor photonic devices down to a truly atomic scale!
In this talk, I will show our recent results that demonstrate unique optical functions in 2D TMDC materials, which cannot be obtained with all other material systems. I will demonstrate that the refractive index of 2D MoS₂ is completely dominated by excitonic effects, rather bandstructures as all other materials. Additionally, I will present our recent results on the excitonic dynamics in 2D heterstroctures that consists of multiple dissimilar monolayers epitaxially or non-epitaxially stacked in the vertical direction. Our results indicate extremely efficient interlayer relaxation and transition of exictons in the 2D heterostructures. This suggests that 2D heterostructures may provide unprecedented capabilities to engineer excitons for the development of exotic photonic devices.

Localized surface plasmon resonances (LSPRs) supported in doped semiconductors are rapidly emerging as a route to the deep sub-wavelength confinement of infrared light. Near-field coupling of closely spaced resonators is important for achieving extreme local fields, but requires precise control of dopant atom placement. This task remains difficult for bottom-up nanoscale syntheses, including those for semiconductor nanowires. Here, we identify and show the impact of axially graded carrier density profiles on mid-infrared LSPRs supported by Si nanowires synthesized via the vapor-liquid-solid technique. This behavior is studied by examining the near-field coupling of multiple resonators along the nanowire length via in situ infrared spectral response measurements and simulations within the discrete dipole approximation. We find residual carrier densities as high as 10²⁰ cm^-3 in the spacer region between each intentionally fabricated resonator (i.e., doped segment), an observation attributed to the so-called “reservoir effect.” Lowering substrate temperature during the spacer segment growth dramatically reduces this residual carrier density and results in an optical response that is indistinguishable from nanowires with ideal, atomically abrupt carrier density profiles. Our experiments demonstrate methods for determining and controlling axial dopant profile in semiconductor nanowires, and have important implications for the manipulation of near-field plasmonic phenomena in Si.

A localized surface plasmonic resonance (LSPR) is the coupled oscillation of free carriers dielectrically confined to a nanoparticle. LSPRs have previously been explored in metal nanoparticles where the resonance can be tuned by the size, shape and composition of the nanoparticle. Recently, LSPRs have been observed in semiconductor nanocrystals including phosphorus-doped silicon and aluminum-doped zinc oxide. Plasmonic properties of semiconductor nanocrystals are unique in that they are able to be tuned not only by size, shape and composition but also by free carrier concentration. Doped nanomaterials are of great interest due to their exciting optical and electronic properties but the actual behavior of dopants in nanocrystals is still poorly understood. In this work we look at the plasmonic properties of phosphorus-doped and boron-doped silicon nanocrystals (Si NCs) as a way of characterizing dopant behavior. The doped silicon nanocrystals were synthesized in the gas phase using a RF powered low temperature plasma.
Phosphorus-doped and boron-doped Si NCs both exhibit plasmonic behavior dependent on their active dopant concentrations however the conditions under which they demonstrate plasmonic behavior are very different between the two dopant types. Phosphorus-doped Si NCs have a plasmonic resonance immediately after synthesis with the resonance disappearing after oxidation while the boron-doped SiNCs must be oxidized or annealed before a plasmonic resonance can be observed. Both boron-doped and phosphorus-doped SiNCs exhibit a blue shift in plasmon peak position with low temperature annealing. It was observed that the majority of dopant atoms are inactive and located at the nanocrystals surface. Results also suggest phosphorus dopants are more readily incorporated into the core of the particle during synthesis and boron dopants are activated post-synthesis due to the completion of a fourth bond on trivalently bonded boron dopant.
This work was supported by the Army Office of Research under MURI Grant W911NF-12-1-0407. Part of this work was carried out in the College of Science and Engineering Characterization Facility, University of Minnesota, which has received capital equipment funding from the NSF through the UMN MRSEC program and the College of Science and Engineering Minnesota Nanocenter, University of Minnesota, which receives partial support from NSF through the NNIN program.

The mid-infrared (mid-IR) spectral range (3-30µm) has become a burgeoning and dynamic field of research both for fundamental exploration as well as for more applied research in health and the environment, security and defense, communication, and sensing. At the same time, the areas of plasmonics and metamaterials have experienced explosive growth over the past decade, fueled in part by rapid developments in fabrication, characterization, computational science, and theory. Yet, the integration of plasmonic structures into mid-IR optical systems has been slower to evolve. While scaling metamaterial and plasmonic geometries to mid-IR wavelengths is actually fairly straightforward, replicating the near-IR and visible optical properties of constituent materials in plasmonic and metamaterial systems is less trivial, leading to very different behavior of scaled systems in these two wavelength ranges.
In this talk, I will discuss our group&’s recent work developing novel optoelectronic and plasmonic devices and structures for mid-IR applications. I will demonstrate the advantages and disadvantages of utilizing traditional plasmonic metals in mid-IR structures, and use this discussion to motivate our recent work with highly doped semiconductors as designer mid-IR metals for plasmonic, metamaterial, and epsilon-near-zero applications. In particular, I will focus on the promise of these new plasmonic materials for nano-scale confinement of micron-scale wavelengths, and for potential applications in sensing, thermal emissivity control, and integration with new types of mid-IR optoelectronic devices. Results demonstrating all-semiconductor perfect absorbers and nano-antennas, as well as hybrid plasmonic/optoelectronic devices will be presented. Finally, I will discuss how our advances in mid-IR photonic and optical materials can be leveraged to explore the forbidding world of far-IR optics.

The widespread interest in plasmonic technologies is motivated by a wealth of emergent optoelectronic applications. For the plasmonics community, the mid-infrared range remains a challenge: The necessary combination of carrier concentration and mobility (>10²⁰/ cm³, mu;>300) cannot be accessed with traditional semiconductors or metals. Besides 2D materials such as graphene, and patterned metal surfaces, conductive metal oxides (CMOs) are investigated as suitable hosts for mid-IR optoelectronic applications. In contrast to metals, degeneratively doped semiconductors such as CMOs do not suffer from loss caused by intra-band transitions or electron-electron interactions due to high charge carrier concentration. Here, we will discuss the outstanding optical properties of Dysprosium doped Cadmium Oxide (CdO:Dy) in the context of mid-IR plasmonics. CdO is a prototypical transparent conducting oxide with excellent n-type conductivity and high charge carrier mobility. Thin heteroepitaxial CdO:Dy films routinely exceed mobilities of 450 cm²/(V s), making them an ideal basis for the next generation of plasmonic applications. Additionally, doping techniques allow for precisely controlled tuning of the plasma frequency and the resulting energy range of plasmonic effects. The combination of tunability and optical properties of CdO:Dy make it an ideal host material to study emerging plasmonic applications in the mid-IR.
One research area of interest is to study the coupling of vibrational bands of organic molecules to mid-IR surface plasmons supported by CdO:Dy. Since mid-IR vibrational bands carry chemical information, the mid-IR frequency band is of particular interest. For a model experiment, gas phase molecules offer a clean approach to study this effect since there is no substantial physical or chemical interaction with the plasmonic host material. We will present experimental data describing mid-IR Rabi splitting due to coupling of mid-IR SPPs to the vibrational modes of N₂O gas molecules. A concentration dependent splitting is observed and a remarkable sensitivity towards the presence of N₂O will be demonstrated. Experimental data shows excellent agreement to theoretical modeling of the gas/SPP interaction and we will discuss potential applications for chemical sensing and spectroscopy. Furthermore, we will provide an outlook to further optoelectronic applications that will be enabled by high µ metal oxides such as CdO:Dy.

We report a series of experiments on light interaction with indium tin oxide nanorod arrays (ITO-NRA) in the near infrared spectrum. In the IR, the imaginary part of the ITO dielectric function is about a factor of four smaller than silver due to the absence of intra-band transitions. The plasma frequency, its mobility, and conductivity can also be tuned via materials processing. We show how these unique properties, in addition to the use of periodic 3D aligned ITO-NRA, can help research where comparison between theory, simulation, and experiment is critically needed. We illustrate this with a few examples: 1. Through theory-based architectural design of the ITO-NRA, we observed strong Fano-type coupling of photonic and the longitudinal plasmonic modes of the ITO-NRA. The experiments compared well with theory, in particular, we saw the presence of very sharp resonances and high Q factors. 2. We also demonstrate the observation of ultrafast dynamic modulation of the plasma frequency, omega;_p in ITO-NRAs upon photoinjection of electrons into the conduction band of ITO, and mapped the field distribution of the plasmon resonance using pump-probe spectroscopies. In these experiments, with a UV pump and NIR probe, where we monitored the extinction of the localized surface plasmon resonance (LSPR), we observed a photoinduced absorption that increases in oscillator strength and energy with increasing pump fluence, which was consistent with the changes in LSPR expected for an increase of omega;_p. 3. Via transient spectroscopy measurements, we report how energy is transferred to excite high energy electrons, and their subsequent transfer of energies to different channels of losses in the system. We suggest how these results can be applied to communication, biosensing and imaging in the near infrared.

The field of plasmonics is based on the capability of metallic nanoantennas to generate significantly enhanced and highly confined electromagnetic fields. Especially in the mid-infrared spectral range, where chemical sensing, thermal emission and detection open new applications for plasmonics, the losses associated with noble metals can be avoided: certain Phase Change Materials (PCM) and polar dielectrics are promising alternative materials with low losses for nano-antenna applications. Due to a small imaginary part of the dielectric function of both material classes, resonance damping is reduced.
In the first part of this talk, I will present our latest results on active mid-infrared plasmonics, i.e. the tuning of nanoantennas resonances via variation of the refractive index n of an embedding medium based on phase-change materials (PCMs). PCMs offer a huge contrast in the refractive index n due to a phase transition from amorphous to crystalline state, which can be thermally, optically or electrically triggered. Specifically we use the two PCMs InSb and GST-326, which provide a huge contrast in ε₁ and a negligibly small ε₂ in the mid-IR spectral range [1]. We present resonance tuning with a maximum shift of about 31% and a tuning figure of merit (FOM) of more than 3.6. Furthermore we will show reversible resonance tuning by applying single ultrafast optical pulses [2].
In the second part, we will use Phonon-polariton-based IR antennas made from polar dielectrics which exhibit lower losses and larger Q-values compared to metallic nanoantennas. Due to the low intrinsic loss, surface phonon polaritons on planar surfaces of polar crystals (e.g. SiC) provide sharp resonances that enable enhanced near-field coupling [3] and refractive index sensing [4]. More recently, resonant cavities for surface phonon polariton such as SiC nanopillar arrays [5] and single circular microcavities [6] are proving to become a good alternative for realizing widespread nano-antenna applications in the infrared spectral range. The possibility to switch or tune resonant properties on the surface phonon polariton resonant structures using phase-change materials will be also addressed.
[1] A. U. Michel, T. Taubner, et al. Nano Letters, 13, 3470 (2013).
[2] A. U. Michel, P. Zalden, T. Taubner et al. ACS Photonics, 1, 833minus;839 (2014).
[3] R. Hillenbrand, T. Taubner & F. Keilmann Nature, 418, 159-162 (2002).
[4] J.D. Caldwell, et al., Nano Letters 13, 3690-3697 (2013).
[5] T. Wang, P. Li, et al. Nano Letters, 13, 5051-5055 (2013).

In a conventional resonant cavity, the higher the quality factor of the cavity, the more sensitive the structure is with respect to changes in the temporal (i.e., frequency) and the spatial (i.e., physical construct) characteristics. Specifically, in a standard high-Q cavity, small changes in the body of the cavity, e.g., a dent in the sidewall, may cause the shift in the cavity&’s resonance frequency. In our ongoing research on the extreme-parameter metamaterials we have found that it is possible to alter these properties by employing materials or structures with effective relative permittivity and permeability near zero. Such materials with epsilon-and-mu-near-zero (EMNZ) have unique properties in the light-matter interaction (A. M. Mahmoud and N. Engheta, “Wave-Matter Interactions in Epsilon-and-Mu-Near-Zero Structures”, Nature Communications, in press.) Within such EMNZ media, curl(E)=0 and curl(H)=0 simultaneously, and thus the electric and magnetic phenomena are decoupled while still temporally dynamic. This leads to the “static optics” paradigm in which we may have optical phenomena while the field distributions are spatially static-like. In this talk, we will discuss some of the exotic features of cavities utilizing such EMNZ structures. We have explored how such EMNZ media can be designed using the proper combination of materials such that to achieve the effective permittivity and effective permeability near zero, and then we employ such structures as the filling materials within a cavity with perfectly electric conducting walls. We have explored how for such cavities, while they may have high quality factors and thus sensitive to temporal variation of a signal, their spatial characteristics are highly flexible. Using the analytical methods and numerical simulations, we have shown that such cavities can be significantly deformed spatially (while keeping their cross-sectional areas fixed), but at the same time they are highly sensitive to the temporal variation. We will discuss the feeding of such cavities, and how the wave interacts with the structures as it goes through such unusual resonant cavities. Such cavities may play important roles in light-matter interaction, both in the wave optics and quantum optics arenas. We will present our results and discuss future directions and possibilities.

Solar energy is a promising solution for future energy demands. Amplifying light absorption has been regarded as one of key challenges in achieving maximal efficiency for solar energy harvesting. Recently, plasmonic nanostructures have attracted many researchers for their potential ability to greatly enhance the light harvesting process. The primary issue in achieving plasmon-enhanced light harvesting lies in the precise architecture of metal nanostructures and photosensitizers because different kinds of energy transfer interactions can predominate depending on their geometries. While there have been significant advances in fabrication routes for metallic nanostructures, obstacles such as low throughput and high cost in case of top-down approaches and the need for extra schemes in the arrangement and adhesion of colloidal nanoparticles on substrates limit their applicability to artificial light harvesting systems. Furthermore, synthetic strategies for precise incorporation of photosensitizers to such fine metal nanostructures, which often occurs in a random manner, have yet to be established for the development of complex plasmon-enhanced light harvesting assemblies. Therefore, inexpensive, scalable, and accurate synthetic strategies for assembling plasmonic metal nanostructures, as well as strategies for hybridizing them with targeted photocatalytic systems, are in high demand to realize practical applications of plasmonics in solar-to-energy conversion.
Here we propose and demonstrate an innovative scheme to fabricate elaborate core-shell nanohybrid architecture, in which the coupling between plasmonic resonator and photosensitizer is controlled in a sub-nanometer scale through a simple, solution-based process. The simplicity of our scheme owes largely to the multi-purpose polydopamine (PDA) nanolayers inspired by mussel adhesion. PDA not only facilitates the formation of metal nanoparticles that support surface plasmons, but also serves as a scaffold to incorporate photosensitizers around metal cores as well as an adhesive between nanohybrids and the substrates. According to our simulation and experimental data, the core-shell configuration greatly enhances light absorption in photocatalytic systems to augment artificial photosynthesis. Furthermore, material-independent surface chemistry of PDA makes our approach to be widely applicable to various substrates independent of their chemical composition and shape. The design flexibility allows the synthesis of assorted sets of plasmonic light harvesting assemblies with desired optical properties, providing an effective platform for plasmon-enhanced solar energy conversion applications.
Our Recent Publications Related to This Presentation:
M. Lee, J. U. Kim, J. S. Lee, B. I. Lee, J. Shin, C. B. Park, Advanced Materials2014, 26, 4463-4468.

Plasmonics can increase photoconversion at energies below the band edge by hot electron transfer and non-radiative coupling through the near field. Non-radiative energy transfer occurs during the initial stages of the plasmon and can happen through an insulating barrier, whereas hot electron transfer occurs after the plasmon dephases and requires direct contact. Alternatively, the plasmonic metal can increase the lifetime of photo-excited or transferred carriers due to the formation of a Schottky barrier at the interface. In this presentation, the metal-semiconductor interfacial barrier in a plasmonic nanostructure is varied from insulating to Ohmic to Schottky, exploring the effect on both plasmon-assisted carrier creation and excited carrier lifetimes. The role of the coherence of the plasmon will be discussed, showing that the plasmon dephasing rate is modulated by the interfacial barrier height, and is directly linked to the balance and efficiency of hot electron injection and resonant energy transfer. The effect of the Schottky barrier height on the initial enhancement in photoexcited carrier density versus subsequent back-transfer or prolonged charge separation will also be discussed. Interestingly, the results presented indicate that direct metal-semiconductor contact is not always optimal for enhancing below-band edge photoconversion. The presentation will explore and outline the importance of engineering the interface for efficient plasmonic enhancements.

Electrolysis of water is the decomposition of water into oxygen and hydrogen gas due to electric current being passed through the water. Production of hydrogen from water requires large amounts of energy so it is uneconomic compared to the production from coal or natural gas. For this reason, H₂ gas produced using renewable energy sources rather than fossil fuels, such as H₂ generated from water utilizing solar radiation, has thus attracted much interest as a clean energy carrier. Ever since Fujishima and Honda reported photoelectrochemical (PEC) water splitting using a titanium oxide electrode in 1972,[1] many researchers has intensively studied water splitting using semiconductor photoelectrodes or photocatalysts. Since photocatalytic or PEC water splitting resembles photosynthesis in green plants it is regarded as a form of artificial photosynthesis. Most attention photocatalyst material is Fe₂O₃ and titanium oxide. Also, efficiency, reliability, and cost of photocatalyst are important, and efficiency is biggest issue. Titanium oxide has reliability with low cost, but has low efficiency of light absorption that does not absorb visible light by 3.2 eV band gap. Recently reported, surface plasmon resonance to boost visible light absorption of titanium oxide photocatalyst, and enhanced hydrogen production of photochemical cell[2]. The resonance of plasmon and meta-structure same effect in electromagnetic. Meta-structure has light absorption rate higher than the surface plasmon[3].
In our study, it is fabrication of TiO2 based high-efficiency visible light operating artificial photosynthesis electrochemical cell by focusing enhanced plasmon generation, using the 3D meta-structure of titanium oxide photocatalyst with Au nano-structure. Our propose photoelectrode of combined photocatalyst with meta-structure has Au layer thickness of 100 nm on glass, and titanium oxide layer deposited thickness of 10 ~ 20 nm, and Au dot is formed on titanium oxide layer by thermal dewetting. Briefly, titanium oxide photocatalyst is located between Au dot and the Au layer, and hydrogen production efficiency is improved by the plasmon resonance energy of meta-structure transfer in the titanium oxide photocatalyst. We report improvement of photocatalytic efficiency and dependent of light absorption by the titanium oxide thickness and the Au dot size.
[1] A. Fujishima, K. Honda, Nature 238, 37 (1972).
[2] Syed Mubeen, Joun Lee, Martin Moskovits, Nature Nano 8, 247 (2013)
[3] Jing Wang, Yiting Che,n Min Qiu, J. A. Physics 109, 074510 (2011)

During the past decade plasmonic nanostructures have been widely studied and implemented for enhanced light emission. While the majority of research groups succeeded to enhance the radiative decay rate of localized emitters, practical improvement of large scale electroluminescent devices is not straightforward. Further development is particularly important for solution-processable semiconductors that emit very inefficiently in the near-infrared region. Single-walled carbon nanotubes and certain low bandgap conjugated polymers exhibit high hole and electron mobilities and are thus promising for optoelectronic devices such as light-emitting field-effect transistors (LEFET). With the goal to enhance overall electroluminescence we simulated and engineered plasmonic nanostructures suitable for LEFETs based on high-mobility, near-infrared light-emitting materials. Three-dimensional finite-difference time-domain calculations correlate very well with the observed increase of electroluminescence and additional spectroscopic studies confirm the role of localized surface plasmons and Purcell effect. We believe that this approach will lead to more efficient near-infrared emitting devices.

Au-TiO₂-Ti metal-insulator-metal (MIM) nanodiodes were used to probe hot electron flows generated upon photon absorption. When light is absorbed in a gold thin film, which is the top electrode of the MIM nanodiode, hot electrons are generated. These hot electrons can travel ballistically across the potential barrier, formed by a thin film of TiO₂, leading to a photocurrent. Here, we demonstrate amplification of the hot electron flow by localized surface plasmon resonance on a connected gold island nanostructure fabricated by annealing the Au-TiO₂-Ti nanodiodes at elevated temperatures. We show changes in the morphology of the gold thin films that lead to changes in the conductive properties of the MIM junction and amplification of the photocurrent. The photocurrent exhibited an exponential dependence on the thickness of the insulator, which is consistent with the tunneling mechanism in hot electron transport. Analysis of these data suggests an intrinsic correlation between the photocurrent due to hot electron flow and localized surface plasmon resonance in the Au-TiO₂-Ti nanodiodes.

Polymer solar cells (PSCs) have many advantages over their inorganic counterparts, including low fabrication costs and application versatility. However, the power conversion efficiency (PCE) at present is significantly low, which is mostly limited by poor carrier mobility of organic materials. It&’s quite important to increase the absorption of active layer without increasing the thickness. An effective method for achieving light trapping inside PSCs to increase the absorbance is the use of metallic NPs that produce localized surface plasmon resonance (LSPR), which is excitated through the resonant interaction between the local electromagnetic fields of incident light and the surface electron density surrounding metallic NPs causes local enhancement in the electromagnetic field. The LSPR is known to hold tremendous potential for transformative applications in optics-based technologies, and it relies mostly on nanostructures made of noble metals, essentially gold and silver. However, these metals have inherent limitations hindering the development of plasmonic devices toward the blue and ultraviolet parts of the spectrum. Overcoming those issues, aluminum plasmonics is now emerging. In striking contrast with noble metals, aluminum is an inexpensive and abundant material, a key advantage for commercial and industry-related applications. A rapidly increasing number of paper underlines its potential and demonstrates that aluminum nanostructures exhibit surface plasmon resonances down to the deep (below 200 nm), with moderate losses.
Herein we report the results of a study that amphipathic core-shell aluminum nanoparticles (Al NPs) are incorporated to the anode buffer layer of the PTB7:PC₇₁BM blend PSCs. Al NPs are synthesized by a facile wet chemical process. The PSCs achieved the PCE of 6.29% that is 20% improvement comparing with the pristine device. This improvement is contributed to the enhancement of the LSPR effects of Al NPs being incorporated in the buffer layer and the light absorption of devices.

The incorporation of noble metal plasmonic nanoparticles (NPs) into the structure of the photovoltaics promises devices with increased efficiency and enhanced stability. Especially, in the case of the solution processed organic and printed photovoltaics (OPVs), the use of novel, large scale compatible, alternative routes for the efficient fabrication of plasmonic OPVs is considered of major importance.
In this work plasmonic solutions comprised by Ag nanoparticles immersed in organic solvents (e.g. chloroform) compatible with the OPV processes are used for the fabrication of plasmonic OPVs by: i) mixing of the Ag NPs plasmonic solution with the photoactive solution (P3HT:PCBM) towards the development of hybrid-plasmonic photoactive blends and ii) by electrospaying the Ag NPs plasmonic solution on top of the PEDOT:PSS layer and introducing in this way a plasmonic layer at the interface between the hole transport layer (PEDOT:PSS) and the active (P3HT:PCBM) layer.
The plasmonic nature of the hybrid-plasmonic photoactive blend and the electrosprayed plasmonic layer was confirmed by Vis-UV Spectroscopic Ellipsometry, while Scanning Probe Microscopy was used for the surface morphology characterization.
The OPVs devices were tested under AM1.5G at 1000 W/m² of illumination and showed that both alternative routes succeeded to the fabrication of efficient plasmonic OPVs with power conversion efficiency > 3.5% and comparable to the non-plasmonic / reference OPV. Interestingly, the OPV devices based on with electrospraying deposited plasmonic layer showed exhibited improved electrical properties as well as enhanced stability and reproducibility.

We present ensembles of surface-ordered nanoparticle arrangements, which are formed by template-assisted self-assembly of monodisperse, protein-coated gold nanoparticles in wrinkle-templates. Centimeter square areas of highly regular, linear assemblies with tunable line width are fabricated and their extinction cross-sections can be characterized by conventional UV/Vis/NIR spectroscopy. Modeling based on electrodynamic simulations shows a clear signature of strong plasmonic coupling with an interparticle spacing of 1-2 nm. We find evidence for well-defined plasmonic modes of quasi-infinite chains, such as resonance splitting and multiple radiant modes. Beyond elementary simulations on the individual chain level we introduce an advanced model, which considers the chain length distribution as well as disorder. The step towards macroscopic sample areas does not only open perspectives for a range of applications in sensing, plasmonic light harvesting, surface enhanced spectroscopy, and information technology, but also eases the investigation of hybridization and metamaterial effects fundamentally.

The theory of spatial transformations (ST) allows a practical approach to control and direct electromagnetic waves in unprecedented ways by carefully adjusting the propagating material electromagnetic parameters in space, providing theoretical support for metamaterial design, and thus opening the door to novel devices. However, ST implies challenging materials requirements in terms of spatial variations of permittivity and permeability, which are difficult to achieve by conventional manufacturing technologies. Fused deposition modeling - a form of 3D printing - is capable of placing different materials, typically polymers, in three dimensions with precision, and has previously been demonstrated for the fabrication of dielectric metamaterial-type devices (variations in permittivity ε) that operate at microwave frequencies. However so far, the printed devices are dielectric materials with little or no magnetic response (i.e. permeability mu; = 1), restricting the freedom of design and limiting the performance of devices e.g. severe reflection losses can arise due to an impedance mismatch Z = (mu;/ε)^1/2 with any adjacent medium.
In this study, we present 3D printing of a polymer-based composite filament containing nickel zinc ferrite particles. Strategies to disperse the ferrite in the filaments and the printed part are described. A range of ferrite loadings have been investigated, with a maximum printable fraction of 23 vol. %. The printed magnetic response has been investigated as a function of frequency, and extends to beyond 2 GHz, which is rare amongst magnetic materials. To further extend the magnetic response to even higher frequency, a hybrid composite can be introduced. The microstructures of the printed materials were characterized showing an adequate homogeneity of particle distribution. The permittivity and permeability of the developed feeding materials measured by a stripline technique have been shown to be well-controlled and reproducible. This approach brings design freedom to manipulate both permittivity and permeability spatially, along with capability to do this in relatively complex geometries and structures direct from a CAD file, enabling a new range of novel metamaterial structures in practice.

Magnetic nanostructures composed of 3-d transition metals and dielectric materials are essential in magneto-plasmonics. New functionality can be achieved from the combined roles of generating magnetoplasmonic oscillations when excited by an incident optical radiation that are further controlled by external magnetic fields. The excitation condition strongly depends on the magnetic spin orientation in the ferromagnetic layers, metal-dielectric and metal-metal interface states, and the refractive indices and permeabilities of the layers involved. In this work, we designed, optimized and fabricated Ti/Au/Co/Au nanostructures and investigated magnetic and microstructure, optical and magneto-optical properties. This is the continuation of our previous efforts [1-3] but with the focus on magnetoplasmonics and how the magnetic, microstructure of the nanostructure would have effect on the magneto-optic properties. The magnetic characterization of the Ti/Au (t_Au)/Co (t_Co)/Au (t_Au) trilayers measured with the in-plane magnetic field to the multilayer surface for both the as-deposited and annealed multilayers show isotropic easy-axis magnetization along the multilayer plane. The chemical composition and layered information of the Au (t_Au)/Co (t_Co)/Au (t_Au) trilayer, verified using X-ray reflectivity and diffraction analyses show distinct fcc-Au and fcc-Co phases. The optical characteristics and sensing performance of the single Au and Co layers, Co/Au multilayers, and Au/Co/Au trilayers in standard surface plasmon resonance and magneto-optical surface plasmon configurations suggests a one fold increase in sensitivity of the magneto-optic configuration over the surface plasmon configuration. Further work on the effect of interface and magnetic orientation on the magnetoplasmonic properties and experimental evidence of increasing sensitivity of the magneto-optical surface plasmon resonance configuration over the surface plasmon resonance configuration is currently ongoing.
References
[1] W. Robertson and E. Fullerton, Re-examination of the surface-plasma-wave technique for determining the dielectric constant and thickness of metal films, Journal of Optical Society of America, vol. 6, no. 8, pp. 1584-1589, 1989.
[2] C. Rizal, B. Moa, J. Wingert, and O. Shpyrko, Magnetic anisotropy and magnetoresistance properties of Co/Au multilayers, IEEE Transactions on Magnetics, no. 99, a 6-pages regular article, Published online in IEEE Explore on Aug 28, 2014.
[3] C. Rizal, B. Moa, and A. Brolo, Recent advances in ferromagnetic multilayer based magnetoplasmonics sensors for potentail biomedical
applications, Conference Proceedings of the 37th Canadian Medical and Biomedical Enginering Society Conference (CMBES), Vancouver, Canada, 4 pages, May 19-23, 2014.

Plasmonic devices on optoelectronic integrated circuits including waveguides [1], detectors [2], and modulators [3] have been proposed for high-speed and -capacity information processing. Dielectric-loaded surface plasmon polariton (DLSPP) waveguides, composed of dielectric stripes on metal films, enable strong confinement of surface plasmon polariton (SPP) fields in the cross section perpendicular to the SPP propagation direction while keeping relatively low propagation losses [4]. In this paper, we propose multimode interference (MMI) waveguide crossings for DLSPPs and demonstrate their low-loss transmission and low crosstalk. Although DLSPP waveguide crossings consisting of two simple stripes have been proposed [5], the transmission loss and crosstalk of the crossings have not been quantitatively evaluated. Here, we numerically simulate SPP fields at tilted mirror-imaged crossings with and without a MMI structure using a finite difference time domain method and calculate the transmission loss and crosstalk. We also compare the transmission loss and crosstalk when different dielectric materials, SiO₂, Al₂O₃, and TiO₂, are used. The crossing structures are operated by light with a free-space wavelength of 1310 nm. In the simple stripe crossing, the transmission loss and crosstalk become larger with decreasing crossing angle from 90° to 10°. For example, in the case of a 30°-angled SiO₂ stripe crossing on an Au film, the transmission loss is minus;2.65 dB, and crosstalk is minus;6.93 dB. To suppress this high loss and crosstalk, we propose tilted mirror-imaged MMI crossings, and optimize the widths and lengths of MMI for each dielectric material. For a 30°-angled SiO₂ mirror-imaged MMI crossing, the transmission loss is minus;0.93 dB, and crosstalk is minus;18.6 dB. We found that scattering losses can be suppressed at the edge of crossing structures by optimizing mirror-image locations. Additionally, scattering losses, which increase transmission losses and crosstalk, are larger when materials with higher refractive index are used. In conclusion, we confirmed by numerical simulations and experiments that transmission loss and crosstalk characteristics are improved by using mirror-imaged MMI crossings and appropriate materials. The resulting simple structure with low and crosstalk will be beneficial for miniaturizing plasmonic devices and flexible patterning of optical interconnections.
[1] M. Fukuhara et al., Appl. Phys. Lett. 104, 081111 (2014).
[2] T. Aihara et al., IEEE Photon. J. 5, 6800609 (2013).
[3] D. Pacifici et al., Nature Photon. 1, 402-406 (2007).
[4] T. Holmgaard et al., Phys. Rev. B 75, 245405 (2007).
[5] B. Steinberger et al., Appl. Phys. Lett. 88, 094104 (2006).

In the past two decades, electromagnetically induced transparency (EIT) has been considered as one of the promising ways to control the speed of light and nonlinear effects by forming a sharp transmission peak within a wide absorption or reflection spectrum range. Among researches creating EIT phenomena, metamaterials have realized the exotic way of controlling the light within thin-films, which has opened a new way for various attractive applications such as slow light devices, optical communication systems, and nonlinear optical devices. The EIT-like properties in metamaterials have been generally achieved by the destructive interaction between the split half rings with different lengths or between bright meta-atom and dark meta-atom structure for the specific polarization. From these interactions, the incident field can be transmitted through the sharp spectral window by the metamaterial array without any disturbance from the meta-atom resonators. However, due to several damping factors in the EIT-like metamaterials such as the finite conductivity of the metallic structure and the intrinsic loss of the supporting substrates, the conventional EIT analogues have suffered from the inevitable power attenuation so far.
In this study, we propose a novel EIT analogue by using so-called Babinet crosslink meta-atoms (BCM) at terahertz frequencies. As a BCM unit cell, the two cut-wire (CW) structures and single complementary cut-wire (CCW) aperture are integrated into one meta-atom structure. According to Babinet principle, since the complementary structure has orthogonal polarization compared with its original structure, two CW patterns in the BCMs are oriented for the parallel polarization direction so that they exhibit a broad reflection band, whereas the CCW aperture in the BCMs is oriented for the perpendicular polarization direction so that it exhibits a sharp transmission band to produce EIT properties. By utilizing this EIT analogue, the proposed BCM structures can achieve extraordinary focus of incident wave and subsequently transmit it through the structural window in each meta-atom, whereas the conventional EIT analogues merely transmit the incident wave at their transmission peak. Measurement results verify that the maximum field amplitude and the quality factor of the BCMs are enhanced by 20 times and 1.3 times in comparison to those of the incident field and the conventional CCW apertures, respectively. Therefore, by using the proposed BCMs, we can expect to overcome the power loss problem of incident waves in the conventional EIT analogues. Moreover, based on this exceptional field enhancement property, the proposed BCMs are expected to be utilized for the exotic applications such as low-power bio-chemical sensing and imaging devices, slow light applications with higher spatial resolution, and phase hologram systems.

Several new plasmonic materials have recently been introduced in order to achieve better temperature stability than conventional plasmonic metals and control field localization with a choice of plasma frequencies in a wide spectral range. Here, low surface roughness (001) SrRuO₃ thin films suitable for plasmonic applications are fabricated and studied. This films, exhibiting different optical parameters are obtained by pulsed laser deposition. The influence of different oxygen pressures (20-300 mTorr) during deposition on the structural, electrical and optical properties of thin films was studied, and the relation between charge carrier dynamics and optical constants in the near-infrared spectral range was elucidated. The obtained results indicate the suitability of SrRuO₃ thin films for plasmonic and metamaterial design applications with controlled plasma frequency and epsilon-near-zero wavelength in the 3.16-3.82 eV and 1.11-1.47 mm spectral range, respectively. The applications may range from the heat-generating nanostructures in the near-infrared spectral range to metamaterial-based ideal absorbers as well as epsilon-near-zero components where the interplay between real and imaginary parts of the permittivity in a given spectral range is needed for optimizing a spectral performance.

Optical properties and thermal dissipation of resonantly excited plasmons are important in applications for optoelectronics, biomedicine, energy, and catalysis. Measured thermal emission and dynamics of gold nanoparticle-polydimethylsiloxane (AuNP-PDMS) thin films exceeded that attributable by finite element analysis to Mie absorption, Fourier conduction, Rayleigh convection, and Boltzmann radiation. Refractive index matching experiments and measured temperature profiles indicated AuNP-PDMS films internally reflected light and dissipated power transverse to the film surface. Models have been developed to evaluate and predict both the geometric optics and thermal dissipation rates in a range of 2- and 3D plasmonic nanocomposites. Measured optical responses of multi-component systems were predictable within 0.04 units of estimates based on geometric optics, an approach that allows the summative optical response of a sequence of 2D elements comprising a 3D assembly to be analyzed. Balancing micro- and macro-scale thermal dissipation rates supports estimation of thermal dynamics within 1.6% for colloid AuNP suspensions, 4.4% for AuNP-PDMS dispersions, and 17% for AuNPs deposited on conductive ceramics. The thermal response measured for uniform AuNP-PDMS dispersions was 8-fold higher than AuNPs deposited on ceramics, while a novel diffusive-reduction technique produced thin, scalable AuNP-containing layers with even greater thermal response. A photon-to-heat conversion of up to 3000°C/watt was observed in the asymmetric AuNP-PDMS films, which represents a 3-230-fold increase over previous AuNP-functionalized systems. These approaches could guide design and deployment of optothermal plasmonic energy materials, sensors, and therapeutics.

Gold thin films possess unique plasmonic properties, and are thus widely investigated in applications for optical devices. Coupling them with the selective light reflection and transmission of 3D photonic crystals would result in novel optical materials which have potential use for light modulation, surface-enhanced spectroscopy, optoelectronic detection and photovoltaics. However, deposition of gold films on the inner walls of porous structures has been difficult due to high aspect ratios of the features and mass transport limitations.
Here we demonstrate the deposition of gold thin films onto the structure of titania log pile 3D photonic crystals using a supercritical fluid deposition technique. Dissolved in supercritical carbon dioxide, dimethyl(acetylacntonate)gold(III) were reduced to noble gold by hydrogen, and formed thin films on all the surfaces of the titania substrate.
The deposited films were highly pure, with uniform step coverage on both the outer surface and the inner walls of the log pile structure. The film thickness were uniform and could be tuned to tens of nanometers, thus maintaining the open log pile stucture. A series of depositions were made with different thicknesses, and their optical properties were investigated.

Very recently, a new class of two-dimensional (2D) metamaterials, called metasurfaces, has become an emerging frontier in the metamaterials research. The central idea of metasurfaces is to introduce the desired phase profile by patterning planar subwavelength structures, which offers an additional, yet important degree of freedom to mold light flow. Taking advantage of the exceptional plasmonic response of graphene at infrared frequencies, we demonstrate that judiciously designed graphene structures, such as arrays of 1D ribbons or 2D patches, can strongly interact with infrared light, so that we are able to control the reflected infrared light in a prescribed manner. The graphene metasurfaces are designed on a dielectric/metal substrate. Utilizing the Fabry-Perot resonance of the dielectric slab and the plasmonic resonance of the graphene nanostructure, the phase of the reflection can range almost from -180 degrees to +180 degrees, while the amplitude of the reflection can be kept in a small variation. As a result, we can realize focusing, negative reflection and strong polarization conversion very efficiently even with the atomically thin graphene structures. The optical responses and functionalities of the graphene metasurfaces are expected to be highly tunable by electrical or chemical doping.

Monolayer graphene has been shown to support plasmonic modes that display extremely high confinement factors and active tunability from the THz to the mid-IR. These properties make graphene an intriguing candidate material to use in highly miniaturized and active plasmonic devices. Recently, multiple experiments based on nano-fabricated graphene structures have sought to exploit these properties to create plasmonic metasurfaces that use electrostatic gating to control optical absorption and emission at mid-IR frequencies.
In this presentation, we report a new technique of utilizing graphene plasmonic modes to create strong light modulation. Unlike previous methods where absorption was driven by only the optical resonances in graphene ribbons, our proposed device is based on a double resonance effect, where the graphene plasmonic ribbon resonances are matched to the optical resonances in a subwavelength metallic slit array which is designed to exhibit extraordinary optical transmission (EOT) at a certain resonance frequency. It is well known that the EOT phenomenon stems from surface plasmons (SPs) induced on the top and the bottom surfaces of the metal layer which couple with each other through the metallic slits. In the proposed structure, the graphene ribbons are located in the subwavelength metallic slits to block the coupling channel. As the resonant frequency of the graphene plasmons is tuned to the resonance of the EOT structure, a large amount of optical absorption can be driven into the graphene ribbons, thus blocking the coupling channel between the metallic SP&’s and suppressing the EOT phenomena.
The advantages of the proposed structure is that very strong modulation efficiency in transmission is achievable even in a low hole mobility in the graphene and a Fermi level that accessible via electrostatic gating. Full wave electromagnetic simulations show modulation efficiencies of 77.0% at 7.45mu;m with a hole mobility of 1,500cm²V^-1sec^-1 and the Fermi energy of 0.39eV. For a hole mobility of 10,000cm²V^-1sec^-1, we show that the modulation efficiency can reach 95.8%. If there is no subwavelength metallic slit array, the bare graphene ribbons shows the modulation efficiencies of 32.4% and 68.1% with the hole mobilities of 1,500cm²V^-1sec^-1 and 10,000cm²V^-1sec^-1, respectively. Experimental results for double resonance modulation will also be discussed.

Conventional optical components, such as lenses and prisms, control light propagations based on accumulated phase shift of each light path length. In this way, light is refracted or reflected along phase shift distribution achieved by the component&’s surface structures. On the other hand, metasurface, which consists of surface metallic nanostructures, has been extensively studied as a new way to realize anomalous light bending. Since any phase discontinuities can be induced by plasmon resonance of the metallic nanostructures, light bending through the metasurface can be designed at will. However, the phase discontinuities of metasurface are solely determined by structural parameters, thus dynamic tuning of the light bending by the metasurface are still challenging. In this paper, we propose and numerically investigate a tunable metasurface made of an array of graphene ribbons to dynamically control terahertz (THz) wavefront. The proposed metasurface consists of graphene micro ribbons on a SiO₂ layer with a thick silver substrate. The graphene ribbons are designed to exhibit a localized plasmon resonance at 5 THz to induce additional phase shift of the reflected THz wave. When a gradient of the phase shift is induced continuously and periodically along the metasurface, the reflected wave from a normal incidence is bended against the metasuface. By tuning Fermi level of the graphene ribbon via electrostatic gating or chemical doping, the plasmonic responses of the metasurface can be changed, thereby realizing dynamic control of THz wave propagation. To design the metasurface, we performed a set of numerical simulations and investigated the plasmonic responses of the graphene ribbon by changing Fermi level. From the calculation results, we find that a wide range of the phase-shift control up to 2π can be achieved by properly arranging the Fermi-level distribution (0.2 ~ 0.8 eV) along the metasurface. When the phase shift changes continuously and linearly along the metasurface, a clear wavefront of the reflected THz wave is formed with an anomalous bending angle. By changing the Fermi-level distribution to induce different phase shifts, the reflected angles can be dynamically changed from -53 to +53 degrees, while keeping the reflectance as high as 60%. We also investigated response time of the proposed device and found that the propagation direction of THz wave could be switched within 0.6 ps. Our proposed metasurface paves the way toward the next step of plasmonic metasurface study for an advanced light wave control.

Terahertz Split Ring Resonators (SRRs) have been proven to be good candidates for biological and chemical detection compared to other sensing technologies like mechanical, electrical and optical detection. However, currently developed SRRs have very low quality (Q) factors. This is one of the biggest obstacles for the development of SRR based sensors because a low Q-factor leads to low sensitivity and low selectivity. There are many new approaches being investigated to enhance the Q-factor by adopting advanced resonator designs and by employing new resonator and substrate materials. However, the improved Q-factor is still not high enough to satisfy scientists and engineers working in industry for commercialization. In order to enhance Q-factor, a novel resonator, a nanowire based Terahertz SRR using displacement current between nanowires, has been designed, fabricated, and characterized. Higher quality factors were observed from both simulations and experimental measurements when compared to a conventional SRR. Specifically, the novel THz SRR has been modeled using ANSYS, High Frequency Structure Simulator (HFSS) software and has shown significantly enhanced Q-factor up to a few hundred. The results have demonstrated that our novel THz SRR has great potential for high sensitive sensing and biomedical detection.

Split Ring Resonator (SRR) arrays have aroused huge attentions among scientists and engineers because of their widely usages in microwave biomedical sensing. However, due to its unidirectional property, multi-directional detections and responses to different orientations of SRR arrays are difficult to accomplish. To improve the sensing capability of SRR arrays, we designed three-dimensional (3D) micro-scale isotropic and anisotropic metamaterials, which were realized by both symmetrically and asymmetrically arranging SRR arrays in 3D microscale cubic structures. Resonant behaviors and transmission properties of the 3D metamaterials with different orientations of the SRR arrays were analyzed by ANSYS, High Frequency Structure Simulator (HFSS) software. In addition, our 3D SRR metamaterials showed the coupling behaviors of SRR arrays in the 3D cubic structure. Based on these results, we fabricated 3D cubic metamaterials with patterning SRR arrays on a dielectric layer using origami-inspired self-folding approach, which allows us to define 3D patterning on each face of the 3D structure. This 3D cubic metamaterial is a good candidate for biosensing applications due to its isotropic and anisotropic properties as well as multiple-detection capability. Moreover, by applying different parameters (i.e. geometry and dimensions) of the SRR arrays on each face of the 3D cube, a nano/microscale gyroscope can also be realized by detecting different transmission and/or reflection spectrum, which can be generated by different orientations of the cube.

The field of plasmonics is ripe with potential applications ranging from chemical and biological sensing to enhanced photovoltaics [1, 2]. At the heart of these plasmonic devices are resonances that establish the devices&’ unique function. The ability to control these resonances is essential in advancing various applications of plasmonics. In order to control resonances, we must be able to quantitatively observe them. Typically, the approach to analyzing plasmonic resonances is to locate the peaks and troughs of transmission and reflection spectra. This qualitative approach is insufficient for non-symmetric or Fano resonances and worse for cases with multiple overlapping resonances. In order to ameliorate the present situation, we describe an effective Hamiltonian formalism [3] to study and tailor resonances of plasmonic systems at optical frequencies. By employing this method, we compute the complex poles of the scattering matrix to investigate resonance dynamics of coupled plasmonic bars. The complex poles provide a quantitative estimate of both resonance frequencies and linewidths. As such, we are able to track the evolution of these resonances and study the coupling behavior between plasmonic bars. Strong coupling between these bars can be understood as plasmon hybridization [4]. By using the effective Hamiltonian model, we identify a negative coupling regime for a particular hybridization. We also show that symmetry breaking within our specific system allows for a very large degree of tunability.
References:
[1] H. A. Atwater and A. Polman, “Plasmonics for improved photovoltaic devices,” Nature Mater. 9, 205 (2010).
[2] N. Liu, M. Hentschel, T. Weiss, A. P. Alivisatos, and Harald Giessen, “Three-Dimensional Plasmon Rulers,” Science 332, 1407 (2011).
[3] N. Moiseyev, Non-Hermitian Quantum Mechanics (Cambridge University Press, United Kingdom 2011).
[4] E. Prodan, C. Radloff, N. J. Halas, P. Nordlander, “A Hybridization Model for the Plasmon Response of Complex Nanostructures,” Science 302, 419 (2003).

Abstract- In 1929, Von Neumann and Wigner showed theoretically that certain quantum potentials can surprisingly have bound states above the continuum threshold [1]. States above threshold usually become resonances, i.e., acquire a finite lifetime, due to their coupling to the outside. However, it is possible for certain resonances under specific conditions to achieve an infinite lifetime. These peculiar resonances are called Bound states In the Continuum (BICs). Eigenmodes of systems above threshold (open systems) are inherently complex due to radiating boundary conditions at infinity. Real parts represent the usual resonance frequencies while imaginary parts describe linewidths (inverse lifetimes). BICs are thus eigenmodes with vanishingly small linewidths or, equivalently, infinitely long lifetimes. Such open systems can be effectively described by Coupled Mode Theory (CMT) via non-Hermitian Hamiltonians that account for their non-conservative nature [2].
We consider high-index dielectric resonators, in a rectangular metallic waveguide, that support several multipolar resonances and demonstrate the striking occurrence of multiple BICs as a result of destructive interferences. We highlight the crucial role of symmetries in canceling radiation in the far-field and achieving infinite radiation quality factors. The architecture investigated here, using only dielectric resonators, constitutes a flexible and readily achievable platform for different spectral ranges and for new applications requiring strong light-matter interaction and light localization.
References
1.J. von Neumann and E. Wigner, Phys. Z. 30, 465 (1929).
2.Y. Yang, C. Peng. Y. Liang, Z. Li, and S. Noda, Phys. Rev. Lett. 113, 037401 (2014).

It is well established that the modes of closely spaced plasmonic structures can hybridize to create different modes, in analogy with hybridized orbitals in molecules. Via electrodynamics calculations we investigate a related kind of interaction between plasmonic structures that enables, by nonlinear coupling, the syncing of their modes to have definite phase relationships. This effect is caused via intensity-dependent changes in the refractive index of the dielectric material between the plasmonic structures. We identify parameter regimes in which two selected modes can be synced even when the structure is incoherently illuminated. The effect could be used for several applications, including light intensity enhancement for improved detection of chemical species by surface-enhanced Raman scattering.

Progress in plasmonic research has demonstrated its capability for enhancing many technologies including photodetectors, photovoltaics, and molecular spectroscopy. However, in order to maximize functionality, alternative materials to plasmonic metals that exhibit high optical losses must be explored. Experiments have demonstrated that plasmonic-like effects can be achieved through phonon-mediated collective charge oscillations, called surface phonon polaritons (SPhPs), in polar dielectric material. Recently it was demonstrated by our group that localized SPhP nanopillar resonators supported extreme sub-diffraction (lambda;_res/200) compression of the free-space wavelength, with very low optical losses, resulting in quality factors up to an order of magnitude higher than the best plasmonic devices. In this study, we investigate the evolution of localized SPhP resonances in fixed-height (950nm), fixed-length (400nm) 4H-SiC, cuboidal nanopillars that vary in width (400nm-6,400nm), and thus aspect ratio. FTIR reflectance measurements using incident light polarized parallel or perpendicular to the long-edge (width) of the cuboidal nanostructure were performed to measure the various localized SPhP resonances. Overall, over 10 localized SPhP resonances are identified, where, in general, each resonance is excited by a particular polarization orientation. We find that many of these resonances are associated with high-order SPhP modes that strongly depend on the aspect ratio (AR=width/length) of the nanopillars. Furthermore, we demonstrate that these localized SPhP resonances can be tuned over a broad spectral range by controlling the AR. All resonances exhibit narrow linewidths (3cm^-1 to 30 cm^-1) that correspond to exceptionally high quality factors in the 30-300 range, which significantly exceeds the theoretical maximum quality factor for silver nanoparticles. Such high quality factors result from the low-losses of optical phonons in SiC The use of polar dielectrics to achieve plasmonic-like effects is only in the beginning stages of exploration. Thus, it is expected that optimization in material quality and fabrication techniques, will yield even lower losses for nanostructure, metamaterial and nanophotonic designs enabling sub-diffraction optics in the mid-IR out to the single digit THz. The elongated cuboid geometry is of particular interest, since each polarization can stimulate SPhP resonances that span different spectral ranges; thus, providing a tunable system without having to change the physical geometry of the nanostructure. Such structures can provide unique substrates for molecular sensing via the SEIRA effect, building blocks for mid-IR metamaterials, and tailored thermal emitters.

The radiation pattern and the resonant wavelength of a nanoantenna are significantly influenced by its local environment, in particular, the substrate on which it lies. Past experiments have focused primarily on dielectric substrates where the index of refraction is nominally constant over the range of operating wavelengths, and the resonance tuning is controlled through changes in the antenna size, shape, geometry and/or periodicity. However, novel substrate-antenna interactions can be anticipated from highly dispersive substrates, in particular those where a transition from dielectric to metallic-like behavior is observed. Within such materials, such phase transition results in a spectral point where the real part of relative permittivity becomes zero. Such epsilon near zero (ENZ) substrates have not been explored as it pertains to substrates and their novel light-matter interactions. Here, we probe the influence of ENZ substrates upon the resonance frequency of Au nanorod antenna, whereby the vanishing index of refraction induces a ‘pinning&’ of the antenna resonance to the ENZ frequency, regardless of the antenna length. Further, this is coupled with a large modification in the radiation pattern of the resonating antenna. By using substrate materials of aluminum- (AZO) and gallium-doped zinc oxide (GZO) and silicon carbide, we have demonstrated the generality of this phenomenon over a broad spectral range (near-infrared and infrared, respectively) and to both plasmonic and phonon polariton materials. We have also shown that the corresponding formalism for the resonance condition of the antenna upon the effective index of the substrate/air ambient and the effective length of the antenna can also be generalized to dielectric, ENZ and metallic phases of the dispersive substrates and result in the understanding that the ‘pinning&’ effect is induced by the compensation of the increasing antenna length by a corresponding decrease in the effective index. These results can be useful for overcoming the geometric dispersion of emitters used in sensors, isolating various plasmonic devices for on-chip nanophotonic devices, creating notch filters for nanoantenna arrays, in flat optics designs and beam steering applications.

Distinguishing enantiomers - i.e., molecules of different chirality - presents a significant challenge to chemical, biological, and pharmaceutical industries. While circular dichroism (CD) spectroscopy and vibrational CD spectroscopy can detect the handedness of molecules, these techniques usually require large sample volumes and very sensitive detection schemes.
In this presentation, we describe a novel all-optical method for enantiomer detection. Our method is based on the radiation of chiral emitters near parity-time (PT) symmetric metamaterials. In PT-symmetric optical systems, judicious, balanced amounts of gain and loss lead to intriguing electromagnetic phenomenon such as unidirectional reflection, mode coalescence, and abrupt optical ‘phase transitions&’ near exceptional points (EP). Here, we consider a PT-symmetric metamaterial composed of alternating planar layers of metal (Ag) and dielectric (TiO2) with balanced gain and loss in the TiO2. Each layer is deeply subwavelength in thickness (10-50nm), and the metamaterial is composed of between 1 and 10 periods. We describe how the metamaterial impacts the radiation of achiral and chiral molecules, modelled as electric and/or magnetic dipoles, placed in its vicinity.
Our results indicate four intriguing properties: First, the magnitude of the dipolar emitted power can be modified based on the loss/gain value. For example, a loss/gain factor of 0.3 can modify the emitted power by 5x compared to a loss/gain factor of 0.03. Secondly, depending on the dipole proximity to the gain or loss side, the direction or sign of the emitter power can be reversed - i.e., the emitter can act as a source or a sink. Third, the dipole radiative rate can increase by two orders of magnitude (~100×) at the metamaterial exceptional point. Finally, and perhaps most significantly, chiral enantiomer decay rate can significantly differ based on the handedness of the molecule. For a metamaterial consisting 5 alternating layers of 30-nm-thick Ag and TiO2 layers, left/right enantiomers exhibit a 6x difference in decay rate compared to free space. The results of this study can be used to design new optical spectroscopies for enantiomer selection and detection, with the potential for single-molecule sensitivity.

Optical phonons in polar semiconductors can interact strongly with the incident radiation in the mid-infrared (mid-IR) to terahertz spectral ranges. The resulting hybrid modes of atomic vibrations and light are termed surface phonon polaritons (SPhPs). While surface plasmons generally decay on a time scale of 10-20 fs due to the strong damping by phonon-, decay- and boundary-assisted transitions, the picosecond phonon-phonon scattering times make the SPhP modes a promising alternative for wavelengths in and beyond the mid-IR. While the SPhP modes have been employed in the literature previously, much remains to be learned about their physical characteristics and potential limitations.
In this talk, I will begin by reviewing the characteristics of localized and propagating modes in various SPhP materials and compare them to the plasmon modes supported by metals and doped semiconductors. I will show that the long phonon lifetime leads directly to high Q&’s for subwavelength resonators, while the confinement/loss figure of merit for propagating modes is comparable to surface plasmons in their most favorable geometry for this wavelength range (metal-insulator-metal). This phenomenon can be traced to the much stronger spectral dispersion of the optical constants for SPhP materials in comparison with plasmonic materials. These results are confirmed by measurements on SiC deeply-subwavelength (down to lambda/100) nanopillars with Q&’s as high as 305 for the monopole mode of the pillars. Measurements of the SiC optical constants indicate that as much as a factor of 2 improvement in the Q may be attainable.
Another system of considerable interest is comprised of hyperbolic SPhP materials such as hexagonal BN, with non-overlapping Reststrahlen bands for optical phonons polarized parallel or perpendicular to the c axis. The advantages of the isotropic SPhP materials discussed above apply to these natural hyperbolic systems, with the additional benefit of volume (rather than surface) confined modes arising from the opposite signs of the in-plane and vertical permittivities at a given wavelength. A hyperbolic SPhP mode exhibits no cutoff, regardless of film thickness, although the confinement/loss figure of merit is still comparable to the SPP and isotropic SPhP cases. Some of the peculiarities of the modal characteristics in the upper and lower Reststrahlen bands will be examined.
Experimentally, the localized hyperbolic SPhP modes exhibit Q&’s as high as 283 for deeply subwavelength nanocone resonators. As expected from the sign of the group velocity, the higher-order modes are observed at higher (lower) frequencies in the lower and upper Reststrahlen bands.

Layered van der Waals (vdW) crystals consist of individual atomic planes weakly coupled by vdW interaction, similar to graphene monolayers in bulk graphite. These materials can harbor superconductivity and ferromagnetism with high transition temperatures, emit light and exhibit topologically protected surface states. An ambitious practical goal is to exploit atomic planes of vdW crystals as building blocks of more complex artificially stacked structures where each such block will deliver layer-specific attributes for the purpose of their combined functionality. Infrared (IR) nano-spectroscopy and nano-imaging experiments on hexagonal boron nitride (hBN) have uncovered rich optical effects associated with phonon polaritons in this prototypical van der Waals crystal. We launched, detected and imaged the polaritonic waves in real space and altered their wavelength by varying the number of crystal layers in our specimens [Dai et al. Science, 343, 1125, (2014)]. Unlike surface plasmons in graphene that we have imaged using a similar nano-IR toolset [Fei et al. Nature 487, 82 (2012)], highly confined phonon polaritons are immune to electronic losses and therefore can travel over distances exceeding 10-s of microns. Peculiar properties of phonon polaritons in hBN enabled sub-diffractional focusing and image formation in infrared frequencies using a slab of this layered single crystal. I will also discuss an ability to control plasmonic response of graphene at femto second time scales that we have demonstrated using a unique pump-probe nano-IR apparatus [Wagner et al. Nano Letters 14, 894 (2014)].

Two-dimensional atomic crystals (TDACs), such as graphene[1,2] and hBN [3], support highly confined plasmon- or phonon-polaritons for concentrating electromagnetic energies into manometer scale, which open ups the possibility for many different subdiffractional nanophotonic applications. Here, we present the experimental studies that use these TDACs to overcome the diffraction limit for achieving high-resolution infrared near-field imaging.
First, we introduce a new concept that layered graphene enables the enhancement of evanescent waves for near-field subdiffractive imaging [4]. Compared to other resonant imaging devices like superlenses, the non-resonant operation of graphene-enhanced imaging provides the advantages of a broad intrinsic bandwidth and a low sensitivity to losses, while still maintaining a good subwavelength resolution of better than lambda;/10. Finally, in order to demonstrate this proposal, we show the latest experimental results that monolayer graphene offers a 7-fold plasmonic enhancement of evanescent information, improving conventional infrared near-field microscopy to resolve buried structures at a 500-nm depth with l/11-resolution [5].
Another demonstration has been investigated on hexagonal boron nitride (hBN). We reveal that a thin natural hBN hyperlens offers both imaging of single objects with down to l/32 resolution (0.4-mm at l=12.8 mm), as well as up to 4.2x magnification. This novel functionality can be explained based on the volume-confined, wavelength dependent propagation angle of Type I hyperbolic polaritons [6].
[1] J. Chen et al., “Optical nano-imaging of gate-tunable graphene plasmons”. Nature 487, 77(2012).
[2] Z. Fei et al., “Gate-tuning of graphene plasmons revealed by infrared nano-imaging”. Nature 487, 82 (2012).
[3] S. Dai et al. “Tunable phonon polaritons in atomically thin van der Waals crystals of boron nitride”. Science 343, 1125 (2014).
[4] P. Li and T. Taubner. “Broadband subwavelength imaging using a tunable graphene-lens”. ACS Nano 6, 10107 (2012).
[5] P. Li, T. Wang, H. Boeckmann, and T. Taubner.“ Graphene-Enhanced Infrared Near-Field Microscopy” Nano Letters 4, 4400 (2014)
[6] P. Li, et al., “Hyperbolic phonon-polaritons in Boron Nitride enable sub-diffraction-limited optical imaging”. Submitted.

Graphene plasmonics provides an excellent new platform for strong optical field confinement with relatively low damping. This enables new device classes for deep subwavelength metamaterials [1], single-photon nonlinearities [2], extraordinarily strong light-matter interactions [3] and nano-optoelectronic switches.
The main problem thus far was that strong damping was observed [4,5]. Different reasons for the unexpected strong damping, such as many-body effects in graphene [5] and impurity scattering [6], were proposed as possible explanations. This strong observed damping hindered the further development of graphene plasmonic devices.
Using van der Waals heterostructures [7] new methods to integrate graphene with other atomically flat materials have become available. Graphene encapsulated between two films of hexagonal boron nitride is an example of such a heterostructure and shows extremely high room temperature transport mobility of charge carriers, only limited by the scattering with acoustic phonons in the graphene [8]. It was expected that this high mobility also decreases the damping of plasmons in the graphene.
We present results where we image propagating plasmons in real space in such high quality graphene devices encapsulated between boron nitride by exploiting scattering-type scanning near-field optical microscopy [9]. Frequency dispersion and particularly plasmon damping in real space is determined and we show that these high quality graphene samples show unprecedented low graphene plasmon damping combined with extremely strong field confinement. We identify the main damping channels to be intrinsic thermal phonons in the graphene [10] as well as dielectric losses in the h-BN [11]. The low obtained damping as well as the theoretical understanding of the damping mechanisms are the key for the development of graphene nano-photonic and nano-optoelectronic devices.
References
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[11] J.D. Caldwell et al., Nature Comm. 5, 5221 (2014).

Surface plasmons in honeycomb lattices of Ag nanoparticles exhibit Dirac-like band structures, similar to the electronic band structure of graphene [1, 2]. Full wave simulations for an infinite honeycomb lattice of silver nano-pillars reveal hybridization of localized plasmonic modes between two neighboring pillars and the consequent formation of bonding and anti-bonding modes that are energetically degenerate at Dirac points with a relative phase of Pi. Calculations also reveal that distortion of the honeycomb lattice breaks the lattice inversion symmetry and opens a photonic bandgap, whose width is proportional to the extent of distortion. Further, electromagnetic simulations reveal the existence of Dirac-like plasmonic edge states in finite width nanoribbons of the honeycomb nanoparticle lattice. Nanoscale architecture of the honeycomb lattice may provide a new way to control directional plasmon propagation by selective excitation of surface plasmon edge states without backscattering.
Experimentally, we have utilized cathodoluminescence (CL) spectroscopy to study angular emission patterns at various wavelengths and eventually construct band structures of the silver pillars in honeycomb lattices. In a CL measurement, electron beams are incident on the sample to excite plasmonic modes in the out of plane direction, which is normally difficult to excite via optical measurement. The scattered light due to the decay of surface plasmon excitations is collected by a parabolic mirror and mapped to the momentum space, yielding a direct construction of band structures in the Brillouin zone. In our initial CL measurement, we compared angular emission patterns from a single silver pillar, silver pillar dimers and silver pillars in honeycomb lattices fabricated on a 15 nm thick free standing silicon nitride membrane. The angular emission patterns from a single silver pillar exhibits strong dipole radiation, while silver pillars in dimer have directional radiation resulting from dipole interactions. For silver pillars in honeycomb lattices, we have observed strong radiation patterns near the Brillouin zone edge, integrated over an interval of wavelength including the wavelength of the Dirac points. Efforts on CL measurements with spectral resolution down to 1nm will also be discussed.

This presentation will give an overview of recent progress for extending the range of nanophotonic materials to dielectrics, with a view of widening the frequency range for sub-wavelength light localization. We will show how surface phonon polaritons excited in nanoforests of SiC can lead to ultrasmall mode-volume and moderate Q resonators in the mid-infrared regime, with good potential for surface enhanced sensing. The second part of the talk will focus on hBN as a natural material for achieving hyberbolic dispersion. Finally, we will briefly discuss the potential of dielectric nanoantennas for nanoscopic light localization while at the same time minimizing heating loss.

We review our recent experimental progress on optical metamaterials. We emphasize work on nonlinear optical metamaterials for second-harmonic generation and THz generation as well as work on macroscopic omnidirectional broadband visible-frequency invisibility cloaking in the diffusive regime of light propagation.

Conventional methods to fabricate three-dimensional structures at the nanoscale either rely on multi-step photo- or e-beam lithography or have the restriction of needing a separate mask for every geometry (e.g. nanoimprint lithography). Recently we have shown that true nanometer-scale (< 100 nm) three-dimensional structures can be directly fabricated from gold nanoparticle-laden inks by an electrohydrodynamic printing, NanoDrip technique [1-4].
The as-printed structures are basically nano-composite plasmonic metamaterials consisting of spherical gold nanoparticles (diameter 5nm), ligands and air trapped between the nanoparticles. In this work we theoretically and experimentally demonstrate the absorption capabilities of arrays of as-printed nano-pillars in a three-layer metal-dielectric-nano-composite configuration and study the dependence of their absorption properties on geometrical parameters (pillar height and width, array pitch) and the nano-composite material properties: We show that while the absorption bandwidth is determined by the nano-composite metal-dielectric ratio, which we control by varying the ligand length, the percentage of the absorbed light depends on the pillar height. We can thus freely control the bandwidth and the total absorption of our absorbers. Choosing an appropriate set of parameters, we achieved near-perfect broadband absorption over the entire visible and near-infrared spectral range.
Therefore, by controlling the absorption we can change the gray scale of the sample from white (no pillar, total reflection) to black (total absorption). Choosing an array pitch of 500nm, we are able to print black-and-white images with a pixel resolution close to the diffraction limit. In contrast to other known techniques for fabricating images at this scale, this is a maskless process that can be varied at will from sample to sample according to need. We have also shown that the flexibility of our technique allows us to print on substrates with structured surfaces such as wedges and pyramids, which to the best of our knowledge is not possible with any other technique [5].
1. Galliker, P., et al., Direct printing of nanostructures by electrostatic autofocussing of ink nanodroplets. Nature Communications, 2012. 3: p. 9.
2. Galliker, P., J. Schneider, and D. Poulikakos, Dielectrophoretic bending of directly printed free-standing ultra-soft nanowires. Applied Physics Letters, 2014. 104(7): p. 5.
3. Galliker, P., et al., Open-atmosphere sustenance of highly volatile attoliter-size droplets on surfaces. Proceedings of the National Academy of Sciences of the United States of America, 2013. 110(33): p. 13255-13260.
4. Schneider, J., et al., A Novel 3D Integrated Platform for the High-Resolution Study of Cell Migration Plasticity. Macromolecular Bioscience, 2013. 13(8): p. 973-983.
5. Kress, S.J.P., et al., Near-Field Light Design with Colloidal Quantum Dots for Photonics and Plasmonics. Nano Letters, 2014.

The interaction of light with metal nanostructures gives rise to the collective oscillation of electrons, which can be engineered to resonate at different wavelengths by tuning the size and geometry of the structures. This phenomenon can be used to create high-resolution color prints, i.e. at 100,000 dots-per-inch (dpi), that are orders of magnitude higher than achievable by standard color printers. Though plasmonic color filters have been previously demonstrated that exhibit transmissive colors, the colors transmitted strongly depend on the periodicity of the grating or nanohole array. We recently showed that with a nanostructure with a backreflector design, individual nanostructures can support a single color in reflection mode, without relying on diffractive effects. This effect enables a cluster of nanostructures to be combined or mixed at sub-diffraction dimensions to generate a broad range of colors. Furthermore modification to the geometry of the structures can introduce polarization effects, which allows a single structure to produce multiple colors. We will present the recent progress in plasmonic color printing in metals such as silver, gold, and aluminum, and discuss potential applications and challenges in achieving high-volume and large-scale color prints.

Optical antennas are nanostructures that exhibit strongly localized field enhancement and are thus suited to enhance optical spectroscopies, for example vibrational spectroscopies such as surface enhanced infrared absorption spectroscopy (SEIRA) [1, 2]. While the first SEIRA results on IR antennas have been performed with chemically grown nanowires [1], these IR antennas are nowadays usually fabricated in arrays with electron-beam lithography on planar substrates and are then subsequently characterized with far-field and near-field methods such as scanning near-field optical microscopy (SNOM) [3].
In this talk, we present a simple method for producing upright standing nanoantennas with a high aspect ratio and a tunable resonance frequency by single pulses of femtosecond laser radiation [4]. Irradiation of a thin gold film on a silicon substrate with a single fs-laser pulse results in the formation of a “nanojet”, a nanoscaled needle or nanoantenna with a diameter in the 100nm-range and a height of up to 3 micrometers The resulting antennas exhibit extinction resonances in the mid infrared spectral rage for p-polarized light under grazing incidence. Due to the free charge carriers in the surrounding gold film of the antenna, the resonance condition of the thin-wire monopole antenna can be explained by introducing image charges yielding an observable resonance wavelength of four times the antenna length.
The antenna length, and therefore the resonance frequency can be coarsely tuned over a broad wavelength range by varying the numerical aperture of the focusing objective used for the focusing of the fs-laser radiation for the structure manufacture. A fine tuning of the resonance wavelength is enabled by a slight variation of the pulse energy used for structuring. An additional ultrafine tuning of the resonance wavelength with a sub-10 nm resolution is realized by an additional coating process subsequent to the laser structuring.
[1] F. Neubrech et al., #8222;Resonant Plasmonic and Vibrational Coupling in a Tailored Nanoantenna for Infrared Detection“, Physical Review Letters, 101, 157403, (2008).
[2] R. Adato, et al.,“ Ultra-sensitive vibrational spectroscopy of protein monolayers with plasmonic nanoantenna arrays“ PNAS,106, 19227-19232, (2009).
[3] J.M. Hoffmann, et al., #8222;Antenna-enhanced infrared near-field nanospectroscopy of a polymer“, Applied Physics Letters, 101, 193105, (2012).
[4] M. Reininghaus, et al., Optics Express, 21, 32176 (2013).

Today, there is an increasing interest in using nanoscale plasmonic devices to control the polarization and propagation states of light. In this context, connected L-shaped nanoantennas have particularly received interest for their noncentrosymmetric properties in the generation of second harmonic generation, in polarization conversion and birefringence. In this work, we show that it is possible to tune the linear and non-linear optical response of disconnected antennas through the control of the morphology and gap width.
We demonstrate that polarization conversion in coupled dimer antennas, used in phase discontinuity metasurfaces, can be tuned by careful design. By controlling the gap width, a strong variation of the coupling strength and polarization conversion is found between capacitively and conductively coupled antennas. A theoretical two-oscillator model is proposed, which shows a universal scaling of the degree of polarization conversion with the energy splitting of the symmetric and antisymmetric modes supported by the antennas. This picture is supported by extensive electrodynamical simulations based on the 3D-Green's dyadic method.
Using e-beam lithography we fabricated nano-antennas consisting in orthogonal dimers of gold nanorods. Using Spatial Modulation Spectroscopy on individual antennas, we measured first their “pure” bonding and antibonding modes. We then measured the intensity of the scattered light along different polarizations for an incident optical excitation polarized along one of the antenna arms. These measurements allowed to quantify the polarization conversion achieved by these nanostructures. We find good agreement with theory for the scaling of mode splitting and polarization conversion with gap width over the range from capacitive to conductive coupling. Next to linear polarization conversion, we demonstrate single-antenna linear to circular polarization conversion.
Besides the linear optical properties, we have investigated Second Harmonic Generation (SHG) in these structures. Our experimental results supported by numerical simulations show that both the intensity and polarization of the SHG emission strongly depend on the gap width and antenna arm length. Our results provide strategies for designing the linear and non-linear response of plasmonic metasurfaces.
Acknowledgement: This work was supported by the computing facility center CALMIP of the University Paul Sabatier of Toulouse France. This work was supported by DSTL and DGA through a joint UK-France Ph.D. studentship, and by EPSRC through grant EP/J011797/1. O.L.M. acknowledges support through an EPSRC fellowship EP/J016918/1.
[1] Optimal Polarization Conversion in Coupled Dimer Plasmonic Nanoantennas for Metasurfaces
Leo-Jay Black, Yudong Wang, C. H. de Groot, Arnaud Arbouet, and Otto L. Muskens
ACS Nano, 8 (6), pp 6390-6399, (2014)

The location and identity of molecules on the surface of plasmonic nanostructures is critical for defining the interaction between the two materials, which is important for any spectroscopic or sensing application utilizing hybrid organic-plasmonic systems. Unfortunately, the size of both organic molecules and many plasmonic nanostructures of interest are well below the wavelength of light, which means that the coupled molecule-plasmon system will appear as a diffraction-limited spot when imaged by far-field optical microscopy. As a result, probing where molecules are located with respect to the nanostructure remains a distinct experimental challenge. This talk will describe how super-resolution optical imaging can yield insight into the coupling between molecules and plasmonic nanostructures, with implications in surface-enhanced Raman scattering (SERS), surface-enhanced fluorescence, and beyond.

Atomistic simulations of surface-enhanced spectroscopes remains a challenge due the necessity of bridging the different length scale of the molecule and the metal nanopar- ticle. Here we present our recent progress on developing a reliable and efficient hybrid computational method that bridges classical electrodynamics and electronic structure theory. Focus will be on understanding chemical, resonance, nonlinear effects, and inhomogeneous electric fields in surface-enhanced spectroscopies. We will discuss applications of this approach to describe surface-enhanced Raman scattering (SERS) and its nonlinear analogue surface-enhanced hyper-Raman scattering (SEHRS) as well as surface-enhanced Raman optical activity.

Dyes and fluorescent proteins are very important labels for the study of living cells, and single-molecule imaging allows sub-diffraction-limit localization of these labels. Unfortunately, the resolution of such super-resolution imaging is limited by low quantum yields and fast photobleaching. We recently demonstrated that single fluorescent protein emission can be enhanced by coupling to the localized plasmon resonance mode of noble metal nanoparticles, yielding enhanced brightness and photostability (Donehue et al., J. Phys. Chem.C,2014, 118, 15027). However, because enhancement (in the enhanced nanoparticle local electromagnetic field) competes with quenching (due to energy transfer at close distances) in a fluorophore/nanoparticle system, the aim of the current project is to study the distance dependence of plasmon-enhanced fluorescence one molecule at a time, with particular attention on fluorescent proteins, for which the distance dependence of plasmonic fluorescence enhancement has not been extensively studied before.
In this study, fluorescent molecules are separated from a gold nanoparticle by (a) a spacer layer or (b) direct conjugation. In the first case, the enhanced fluorescence of Cy 3.5 and mCherry adsorbing in the vicinity of an encapsulated gold nanorod is quantified with single-molecule fluorescence experiments. We measure an up to 1.5-fold increase in average fluorescence brightness from Cy 3.5 and an up to 3.5-fold increase in average brightness from mCherry, and determine the ideal spacer thickness for both fluorophores to be 12 nm and 10 nm for Cy 3.5 and mCherry, respectively. Electromagnetic simulations are used to interpret these trends. In the latter case, we obtain more precise distance control by recording the fluorescence of single fluorescent proteins directly attached to individual gold nanoparticle by double-stranded DNA linkers with known lengths. Overall, the quantitative relationships uncovered in these experiments reveal plasmon-enhanced fluorescence as a function of distance between a fluorophore and the metal nanoparticle, and reveal particular insight into the possibility of enhancing fluorescent proteins, the ubiquitous labels in biological imaging. Once the optimal distance is determined, this technique can be used in single-molecule imaging with a much improved resolution, with important consequences for even live-cell bio-imaging.

Diffraction-limited light microscopy cannot achieve the control of optical fields necessary for many demanding applications in nano-scale imaging, sensing, and spectroscopy. To address this, the field of plasmonics aims to manipulate light within dimensions much smaller than the optical wavelength by exploiting surface plasmon resonances in metallic nanostructures. Surface plasmons are oscillations of the conduction electrons at the surface of a metal and can squeeze or focus electromagnetic energy into very small volumes. These locally enhanced fields are often called plasmonic “hotspots.” Among other emerging applications, plasmonic hotspots have been used for chemical imaging via surface-enhanced Raman scattering (SERS). With proper image processing, single SERS hotspots can be localized to within ~10 nm for super-resolution chemical mapping. However, to approach the challenges associated with the precise positioning of focused hotspots for sensing, scanning, imaging, or trapping purposes, dynamic placement and characterization of plasmonic hotspots would provide many additional benefits. While focused plasmon waves have been manipulated in various ways, either by changing illumination conditions or by designing special nano-metallic substrates, dynamically shifting plasmonic hotspots under full computer control in sub-wavelength steps, especially for imaging and spectroscopy applications, has not yet been fully explored. In this paper, we present the use of a spatial light modulator (SLM) and computer-controlled holographic laser illumination to dynamically shift (in 100 nm steps for a wavelength of 660 nm) and localize (to within 10 nm) sub-wavelength hotspots on a silver nanohole array surface. By properly engineering the illuminating light field with the SLM, we show that it is possible to position a plasmonic hotspot within and around a single nanohole on-the-fly. Since the hotpots are, in our case, imaged via SERS, our technique has the potential for high-resolution chemical spectroscopy and imaging. For example, due to the large field enhancements at the surface, blinking behavior of the SERS hotspots was observed and processed using a custom Stochastic Optical Reconstruction Microscopy (STORM) algorithm enabling super-resolution chemical imaging. We use this technique to create images of collagen protein fibrils with sub-diffraction-limited resolution. For such super-resolution SERS imaging, dynamically shifting the hotspots is extremely important since static illumination of a plasmonic surface will produce only static plasmonic hotspots, leaving gaps in the reconstructed SERS image. Interestingly, even illuminating the surface with randomly varying phase profiles from the SLM shifts the hotspots sufficiently to achieve high-resolution SERS imaging of a single collagen protein fibril. Beyond imaging, our technique may be useful for sensing or particle manipulation via plasmon-enhanced optical tweezers.

Field effect gate tuning of the carrier density in metal-dielectric-semimetal heterostructures enables the realization of tunable metasurfaces and metamaterials operating at visible to near-infrared frequencies. We illustrate this approach via design of a frequency-tunable epsilon near zero metamaterial with conducting oxide active layers, and a tunable metasurface consisting of a phased array of subwavelength patch antennas. In transparent conducting oxides (TCOs) we can achieve high carrier concentrations, which opens the possibility of tunable permittivity active metamaterial structures. Transparent conducting oxides typically have carrier concentrations between 10¹⁹-10²¹ cm^-3, thus their plasma frequencies can be shifted to near-infrared and visible frequencies. In addition, the carrier concentrations of TCOs can be modulated by applying an electrical bias, in a manner similar to a semiconductor-based field-effect MOS-device, where an accumulation layer is formed due the applied electric field, subsequently inducing a change in the complex index of refraction in the visible and near-infrared region. We demonstrate the design of a tunable ENZ metamaterial based on metal and transparent conductive oxide (TCO) multilayers, using the field effect to tune the permittivity of the TCO across thin gate dielectric layers. Our design consists of a multilayer geometry of two layers of Au, separated by an active layer of indium tin oxide (ITO). We dicuss the designs to achieve electrically tunable birefringence and dichroism in this structure. We also describe the phase and amplitude response of electrically tunable subwavelength antennas and discuss the properties of phased arrays that form tunable metasurfaces.

Hybrid optical antennas with reconfigurable loads have been the subject of numerous theoretical and experimental investigations. For instance, dynamically tunable optical antennas with ultrafast response times have been constructed from active semiconducting materials placed in the gaps of metallic split-ring-resonators and nano-dimers. These advancements provide opportunities to study hybrid plasmonic/semiconductor optical excitations and open up the possibility of designing devices with unprecedented capabilities for sensing and communications.
In these hybrid devices, an active semiconductor load modulates the flow of energy across a gap between two metallic elements. Such antennas are typically thought of as two-state elements. In the off state, there are no free carriers in the semiconductor and the load capacitively couples the two plasmonic elements. In the on state, there is a high carrier concentration in the semiconductor and the load provides a conductive channel across which current can flow. Although these two distinct regimes have been characterized in several recent studies, little attention has focused on the optical properties of such antennas between these two extremes. Here, we theoretically study the evolving optical response of a hybrid metal-semiconductor-metal antenna as the free-carrier concentration in the semiconductor is continuously varied. In particular, we demonstrate qualitatively new behavior arising from epsilon-near-zero (ENZ) properties in intermediate doped semiconductors.
In agreement with optical nano-circuit theory, we show that an ENZ load acts as an ideal optical resistor with an optimized damping response. As a result, electromagnetic scattering is strongly suppressed at the semiconductor ENZ condition. Therefore, if one seeks to optimally modulate the scattering cross section over a broadband range, the antenna should not be switched between the capacitive and conductive states, but rather to and from the ENZ state. In periodic arrays, or metasurfaces, we show how to use these effects to construct spatial intensity modulators capable of dynamically switching between zero and unity transmission. Unlike existing optical modulators, these devices can be ultra-compact with thicknesses as small as 1% of the operating wavelength, which implies high efficiencies and minimal consumption of energy. We conclude by comparing these results to alternative hybrid antennas comprising semiconductor materials coupled to phonon-polariton resonators. In contrast to metallic resonators with strongly resistive optical responses, these resonators have inductive optical responses that result in ultra-narrow linewidths and large absorption cross sections, which leads to unexpected behavior as the free carrier concentration in the load is changed. These results may lead to the development of novel optoelectronic devices for dynamically shaping beams of light and enabling ultra-fast modulation of thermal and molecular emitters.

For plasmonic applications at visible frequencies, silver provides the lowest intrinsic material losses and highest quality factors for both localized and propagating optical modes. Achieving optimal optical properties in silver, however, is frequently hindered by high surface roughness, grain boundaries, and other defects. Although growth of single-crystal material via molecular beam epitaxy and other techniques has demonstrated promise in alleviating these problems, the resulting films are limited to substrates that are appropriately lattice-matched to silver, constraining the applicability of this technique for most plasmonic applications. Here, we develop a new method for the growth of ultrasmooth, epitaxial, and single-crystal silver thin films that can be readily undercut, suspended, and transferred in a scalable manner. We demonstrate the growth of epitaxial silver films on a sacrificial layer of titanium metal grown epitaxially on a 4H-SiC substrate. Characterization of the films with high-resolution x-ray diffraction, transmission electron microscopy, and atomic force microscopy indicates high-quality epitaxy both in and out of plane and ultrasmooth surfaces with a typical RMS roughness of 500-700 pm. Spectroscopic ellipsometry demonstrates optical losses and surface plasmon-polariton propagation lengths equal to Johnson and Christy values over the visible range. Freestanding single-crystal silver nanostructures fabricated by electron beam lithography and highly directional Ar ion etching can be readily undercut and released by selective etching of the titanium sacrificial layer with hydrofluoric acid,,. eliminating the constraint of a lattice-matched substrate. Completely suspended plasmonic nanostructures can also be readily fabricated by this patterning procedure followed by undercut etching and critical point drying, enabling ultra low-loss plasmonic waveguides, plasmonic optomechanical devices, and many other applications. In addition, these nanostructures can be transferred to a secondary substrate using either PDMS peel-off or a nanomanipulator. This work provides a unique pathway to low-loss, single-crystal silver nanostructures on arbitrary substrates and new types of plasmonic devices based on fully suspended, released structures.

If one had the option to arbitrarily modulate the dielectric function (e) of metals, this would enable unprecedented control of the plasmon resonances in nanostructures and thin-films, allowing for the development of a new class of metallic materials with tunable optical responses for the fabrication of optoelectronic devices with unique characteristics, such as thin-film solar cells with enhanced open-circuit voltage, high-performance detectors, and metasurfaces for tunable absorbers and optical filters, among others. Here we present a new class of plasmonic nanostructures based on alloyed metallic nanoparticles (NPs) formed by the combination of Ag, Al, Au and Cu. We fabricate arrays of NPs with controlled sizes (down to 100 nm) and extremely high yield (> 95%) by template deposition. We modulate the optical properties of the nanostructures by varying the alloy composition, achieved by sequential/alternating e-beam evaporation of very thin layers of different metals and by co-sputtering, followed by annealing treatment. Moreover, transmission electron microscopy measurements reveal how intra-diffusion takes place between the different metals at the nanoscale. We incorporate the alloyed NPs into GaAs solar cells and locally measure the spectrally dependent photocurrent enhancements by scanning photocurrent microscopy [1]. We develope and implement a variant of illuminated-Kelvin probe force microscopy to map local variations in the open-circuit voltage of the solar cells with truly nanoscale resolution [2]. Our results show increased optical activity surrounding the nanostructures and strong spectral dependence, only revealed by the high spatial resolution measurements performed here. Further, our Mie scattering calculations suggest that some alloyed metals, such as Al_xAg_1-x, show low optical loss over a broad range of the spectrum.
[1] M. S. Leite et al., ACS Nano, in press. DOI: 10.1021/nn5052585
[2] E. M. Tennyson et al., Nature Nanotech, in review.

Thin film organic solar cells have demonstrated rapidly increasing efficiencies, but typically absorb less than half of the incident solar spectrum. Absorber layer thicknesses are typically less than 200 nm, to achieve high carrier collection, resulting in incomplete light absorption particularly at longer red and near-IR wavelengths. To increase broad-band light absorption, we rigorously design experimentally realizable solar cell architectures based on dual photonic crystals using scattering matrix simulations.
We find an optimized architecture consists of a polymer microlens at the air-glass interface, coupled with a photonic-plasmonic crystal at the metal cathode on the back of the cell. The micro-lens focuses light on the periodic nanostructure that in turn generates strong diffraction of light. Wave-guiding modes and surface plasmon modes together enhance long wavelength absorption. The optimal architecture has a period of 500 nm for both arrays, resulting in absorption enhancement of 49% and photocurrent enhancement of 58% relative to the flat cell, for nearly lossless metal cathodes. The enhanced absorption approaches the Lambertian limit. Misalignment between the two photonic crystals leads to about 1% loss of performance. Simulations incorporating experimental dielectric functions for metal cathode and ITO, using a real space methodology find the enhancement of 38% for the photocurrent and 36% for the weighted absorption, due to parasitic losses mainly in the metal cathode.
This solar architecture is particularly amenable for fabrication since it does not require spin coating of organic layers on corrugated surfaces, but instead requires nano-imprinting an organic layer, followed by metal cathode deposition. We will also discuss alternate approaches for achieving patterning organic absorber layers. Our predicted enhancements also compare well with measurements on simpler microlens based structures, where absorber layers were not patterned. This dual photonic crystal architecture has great potential to achieve >12% efficient single junction organic solar cells, and to control photons by focusing light on nanostructures and plasmonic components.

The interaction of nanoparticles with light is often described in terms of their extinction, scattering, and absorption cross sections. On single nanoparticles these cross sections are very difficult to measure quantitatively, while they can be of great importance. For instance, in single nanowire solar cells efficient conversion of absorbed power to current is crucial, but as long as the absorption is not measured quantitatively this internal quantum efficiency (IQE) can&’t be determined. Here we report on a novel method to measure the absorption cross section of any nanostructure in the visible and near IR using the conventional integrating sphere approach, but adapted to a configuration compatible with diffraction-limited resolution. We present absolute absorption measurements on plasmonic gold spheres and tapered semiconductor nanowires, and show excellent agreement with theory.
Finally, by connecting individual nanowires with carrier-selective contacts, we can measure simultaneously the solar cell absorption and photocurrent spectrum and extract the IQE. Since our system incorporates a piezoelectric scanning stage, we can monitor the IQE as a function of position along the nanowire length, providing valuable information about carrier recombination and extraction that so far has not been directly measured but only inferred from nanowire solar cells results. We see these direct measurements as a crucial step towards achieving ultra-high efficiency nanowire solar cells, as well as a confirmation that absolute absorption measurements on single nanostructures have great utility.

Functional metamaterials are composite materials typically comprised
of at least two constituents: a nanostructured plasmonic metal that
generates determines a resonant optical response, and an active
material that determines the primary function of the device. By
tailoring the local electromagnetic fields, optical metamaterials
improve the performance of optical systems, including absorbers,
emitters, and modulators. In this talk, we demonstrate enhanced
performance of optical limiters, whose tranparency decreases as a
function of incident intensity. These elements are important for
protecting imaging systems and other sensors from damage or from being
dazzled by a bright scene.
Vanadium dioxide is a solid state phase change material that undergoes
a phase transition from a semiconducting (sc) to a semi-metallic (m)
state, with a large corresponding change in optical properties. At
1550nm, the permittivity changes from about 6 (sc) to -2 (m). The
phase transition can be induced optically by absorption of photons in
the visible and near infrared, and when optically actuated is very
fast (sub-picosecond). However, the imaginary permittivity is high in
both states, about 10, so VO2 thin films alone induce too much loss at
to use in practice as a window or coating. By patterning the VO2 and
incorporating a frequency-selective gold metamaterial design, we
demonstrate an improved transparency at low incident power from 65% to
93%. Notably, the threshhold for the onset of limiting is
substantially reduced, from 5.8 mJ/cm^2 to 2.0 mJ/cm^2. With
continuing refinement, we expect to achieve a further improvement to
the application target of 1 mJ/cm^2 threshhold.
We will describe transient spectroscopy techniques used to both to
characterize the enhanced performance of the devices as limiters, and
in ongoing work, to validate our hypothesized models of the
light-matter interactions and the thermal and electronic contributions
to our observations.

In phased array metasurfaces, researchers engineer the phase and amplitude of transmission or reflection at sub-wavelength dimensions. Such metasurfaces have enabled a wide range of devices, including planar optical elements, holographic projections, and electromagnetic cloaks. In these devices researchers control the phase of light by engineering the shape and size of individual optical resonators. An unresolved challenge is to achieve such phase control in a reconfigurable manner. Here, using analytical calculations and FDTD simulations we demonstrate reconfigurable phase control between 0 to 2π while maintaining near-unity transmission in optically pumped silicon metasurfaces.
We describe metasurfaces comprising low-loss high refractive index semiconductor resonators. For static metasurfaces, we design individual resonators to support spectrally overlapping electric dipole (ED) and magnetic dipole (MD) resonances. These combined resonances, each contributing π phase shift, constructively interfere with the incident beam enabling a size-dependent 0 to 2π phase shift in the transmitted beam. Hence each resonator element acts as a Huygens source that can induce an arbitrary phase to the incident beam and the static metasurface comprises a non-uniform array of different resonator sizes. To implement a reconfigurable system, we first optimize the size and periodicity of a uniform resonator array. Coupling between individual nanostructure resonances and diffraction resonances of the periodic array modifies the ED and MD line-shapes and quality factors. For optimized parameters, the ED and MD resonances exhibit identical, overlapping line-shapes, ensuring near-unity transmission. Periodicity-dependent quality factor enhancements lead to resonance narrowing, reducing the refractive index shifts needed to modulate the array transmission. Using models based on THz pump-probe studies, we modulate the silicon refractive index through free-carrier refraction, i.e. by modulating the free-carrier density. We show that such a mechanism enables continuous tuning of the transmission phase between 0 to 2π with less than 1dB of loss. Such tunable Huygens Metasurfaces may form the basis for reconfigurable phased array metadevices that enable unprecedented control over mid-infrared radiation.

Conductive plasmonic metasurfaces, where both the electrical and optical properties are structurally derived, have the potential to replace transparent conductive oxides (TCOs) as front-surface contacts in solar cells. Such front-contacted cells typically employ an indium tin oxide (ITO) layer to simultaneously provide lateral conductivity and function as an anti-reflection (AR) coating. Combining these functions in a single layer results in a compromise between parasitic optical losses and Joule heating losses. The use of ITO also increases cell costs, due to the scarcity and expense of indium, and makes the resulting cells mechanically brittle.
Here we experimentally realize the replacement of this ITO layer on silicon heterojunction (HIT) solar cells by nitride-coated silver nanowire (NW) networks, a geometry which decouples the antireflection coating properties from the sheet resistance of the transparent electrode. NW networks were prepared on flat-front, rear-textured HIT cells with front surfaces that were either ITO free or coated with a minimal (20 nm) ITO layer, and compared with an equivalent reference cell prepared using a standard 80 nm ITO electrode. Fabrication of the NWs was accomplished using substrate-conformal imprint lithography (SCIL), a nanopatterning technique that is low cost, robust against surface contamination, and compatible with wafer-scale fabrication of structures with sub-10 nm precision. The NWs were arranged in square arrays with inter-wire pitches of 300 - 1000 nm, and nanowire widths spanning the range of 40 - 110 nm. Each NW electrode had an area of 2x2 mm, with 40 total electrode geometries tested per wafer.
Electrical tests were conducted under both AM1.5 illumination, and correlated with optical reflectance spectra measured under white light illumination. Optical and electrical properties for these cell designs were also calculated from lambda;=300 - 1110 nm using a 3D finite-difference time domain (FDTD) model and experimentally determined dielectric constants for the solar cell layers. This analysis shows that these NW networks can match the performance of existing ITO coatings. We find that the ITO reference cell loses 1.34 mA/cm² primarily from absorption in the UV/blue spectral regime. By comparison, the minimal-ITO cell with a nanowire grid loses 1.28 mA/cm² with a concurrent twofold reduction in sheet resistance. The ITO-free cell, which uses only NW networks and Si₃N₄, loses slightly more current (2.2 mA/cm²) but retains the reduction in sheet resistance relative to ITO.
This demonstration of NW networks within the optical environment of a functioning solar cell shows that metasurface electrodes offer a practical pathway to increased efficiency in solar cells that are free of rare metals.

Metamaterials composed of an Ag/TiO₂ multilayer stack with a double-periodic unit cell can exhibit omnidirectional negative refraction of energy for UV light. For light incident along the waveguides, the incident plane wave couples to a plasmonic waveguide mode described by a negative mode-index. For out of plane incidence, the propagation inside the material is described by a Bloch mode, where the fundamental harmonic is described by anti-parallel phase and energy propagation. By carefully designing the layer thicknesses in the unit cell, the metamaterial exhibits a three-dimensional omnidirectional optical response in the UV spectral range, where the propagation constant of the waveguide mode and of the fundamental harmonic of the Bloch wave are the same.
We calculate the energy density of the different harmonics contributing to the Bloch wave and find that a large number of harmonics contribute significantly. The m=0,-1,-2,hellip; (m=1,2,3,hellip;) harmonics are described by a negative (positive) phase velocity. From the decomposition we find that the m=1 harmonic dominates, which has a positive phase index [1]. These results are corroborated by optical refraction experiments on a double-periodic Ag/TiO₂ multilayer metamaterial prism in the 380-600 nm spectral range, where m=1 harmonic dominates the transmitted signal.
Next, we investigate how energy refracts through such a metamaterial. We use analytical field profile calculations, for both finite structures and a periodic unit cell, to calculate the direction of the Poynting vector, taking into account losses and dispersion. We find that the unit cell and time averaged Poynting vector of the individual harmonics of the Bloch wave are all oriented in the same direction. By correct design of the metamaterial unit cell, the Poynting vector undergoes negative refraction. Based on these results, we design a planar metamaterial geometry which acts as a flat lens in which the image is formed by the coherent superposition of refracted light from multiple harmonics in the metamaterial.
We use analytical Green&’s tensor calculations to calculate the field profile above a metamaterial slab originating from a horizontal point dipole source placed directly beneath the slab. A clear image is formed, with a full width half maximum of around 300 nm in the plane of the slab. The focus is centered 400 nm away from the top slab interface, depending on the source position and slab thickness. We fabricate metamaterial slabs using electron beam physical vapor deposition to form a multilayer stack with a double-periodic unit cell of 54 nm Ag, 31 nm TiO₂, 34 nm Ag and 31 nm TiO₂. In order to directly observe the focal dimensions in free-space, we perform near-field scanning optical microscopy. By scanning the tip (aperture 90 nm) both laterally through the focus and along the optical axis, we determine the focal shape experimentally.
[1] R. Maas, J. Parsons, E. Verhagen and A. Polman, ACS Photon. 1, 670 (2014)

The interaction between plasmonic nanostructures and propagating light leads to enhanced light absorption, which can increase the sensitivity of infrared (IR) spectroscopy. A widely studied plasmonic nanostructure is the split ring resonator (SRR). The optically resonant response of SRR can be tuned by the shape, size and composition in the visible, IR and THz ranges. Because of light diffraction, the spatial resolution in conventional IR microscopy is limited to several micrometers, bigger than the typical size of SRRs, with resonances in the IR range. Consequently, most commonly, the optical response of SRRs is obtained as an average over several resonators. When the resonators are fabricated in arrays, the interaction between individual SRRs plays an important role in determining their optical response (i.e. resonant frequency, line width and extinction cross section). Those effects were explained previously by retarded electric dipole-dipole interaction [1] and two-dimensional magnetoelectric point scattering lattice [2].
The measurement of optical response in isolated SRR or individual SRR within an array is challenging with conventional techniques. However, recently, the surface enhanced IR absorption in asymmetric-SRR (A-SRR) was measured by our group using the photothermal induced resonance (PTIR) technique [3]. In PTIR, imaging capability of atomic force microscope is combined with chemical sensitivity of IR spectroscopy. The pulsed IR laser is shined onto and absorbed by the sample, then the thermal expansion of the sample by the local heating is measured by monitoring the excitation of mechanical resonances in cantilever, which is in contact to the sample by a sharp tip. For this study, “U”-shaped SRR and A-SRR arrays with different lattice constants were fabricated and characterized in the far-field by Fourier transformed infrared (FTIR) spectroscopy. PTIR measurement were obtained after coating the arrays with a 150 nm thick poly-methyl methacrylate (PMMA) film. Results show that the local PTIR signal of the absorption hot-spot intensity in proximity of “U”-shaped SRRs increase as the array pitch increases and it saturates to the case of isolated SRR. Theoretical calculations based on two-dimensional magnetoelectric point scattering lattice are consistent with the experimental data. However, the response of closely packed A-SRR arrays is not well described by such formalism because of the strong perturbation due to the close interaction between adjacent A-SRR and the resulting hot-spot overlap.

I will present our recent work on the synthesis and self-assembly of colloidal nanocrystals for plasmonics and metasurfaces. Previously, we demonstrated that polymer-grafted metal nanocubes can be organized into nanojunctions that possess intense “hot spots” due to electromagnetic field localization. More recently, we have extended this rational assembly to form a variety of structures, ranging from dimers to interconnected nanocrystal networks. We establish some simple nanocomposite design guidelines using colloidal nanocrystals as plasmonic building blocks. We then use these self-assembled structures to create large-scale, planar arrays that exhibit optical resonances in the visible to near-infrared wavelengths. These metasurfaces can then be used as substrates for enhanced Raman spectroscopy and surface plasmon resonance sensing, or backfilled with a gain medium.

The optical properties of metallic nanostructures can be manipulated by tuning nanoparticle size and shape, or the geometry and relative arrangement of the constituent particles. The sensitivity of the localized surface plasmon resonance to changes in its surroundings, and the enhanced localized electromagnetic field of coupled nanostructures have been widely applied in sensing, surface-enhanced spectroscopies, nonlinear optics. It is challenging to fabricate plasmonic structures with optical activity at visible wavelengths with E-beam lithography or focused ion-beam milling. While DNA has been used as a linker to construct complex three-dimensional architectures, yields are typically low. Here, plasmonic gold nanoparticle dimers and trimers with optical activity in the visible are constructed in a high yield by self-assembly between designed organic scaffolds and monovalent gold nanoparticles. Monovalent gold nanoparticles are prepared through surface reactions on rink-resin-modified silicon substrates. Each monovalent gold nanoparticle has only one reaction site at the end of the multidentate dendron bound on the particle, with the rest of the surface completely passivated. Gold nanoparticle dimers and trimmers are obtained by designing specific organic scaffolds, and this method can be generalized to construct complex three-dimensional nanoparticle assemblies. These types of coupled gold nanostructures can serve as colorimetric sensors, SERS platforms, and building blocks for metamaterials.

The bottom-up fabrication of ordered colloidal assemblies of nanoparticles provides a versatile approach to generating metamaterials architectures, where new physical properties arise from the electromagnetic coupling of individual metal nanoparticles. Recent advancement in the field of self-assembly has produced large-area, uniform arrays of plasmonic nanojunctions. One such development has been orentation-dependent assembly of shaped nanoparticles. Previously, our group demonstrated that polymer-grafted silver nanocubes can be self-assembled into arrays of one dimensional string within polymer thin films.¹ These nanocubes are arranged in either edge-edge and face-face orientations by changing surface chemistry and annealing methods, and that the assembly process is strongly governed by van der waals attraction and steric repulsion. This method has also been established to various other isotropic, anisotropic and combination of these nanostructures.²
Furthering this strategy, I will present our current work on the use of bi-functional ligands to tailor interparticle interaction and guide the assembly of metal nanojunctions in a predictable manner. Homobifunctional polymer ligands maximize the number of face-face orientations whereas heterobifunctional polymer ligands drive the orientation in edge-edge as well as face-face orientations depending on polymer chain length. We also probe the controlled formation of nanocube dimers, trimers, and tetramers by controlling nanoparticle incorporation into a polymer matrix. Using these experiments, we have begun to establish a phase diagram for the assembly process and to aid in the design of colloidal plasmonic nanojunctions. Such a bottom-up approach could provide a powerful fabrication tool for scalable manufacturing and processing of metamaterials.
Reference: 1. B. Gao, G. Arya, and A.R.Tao. Nature Nanotechnology, 7, 433-437 (2012)
2. B. Gao, M. Rozin, and A.R.Tao. Nanoscale,5, 5677-5691 (2013)

We present a plasmon-active hybrid nanomaterial design with electrochemical tunability of the local surface plasmon resonances. The plasmonic-active nanostructures are composed of silver nanocube aggregates embedded into an electrochromic polymer coating on an indium tin oxide electrode with the nanocube aggregation controlled by the surface pressure. Such polymer-nanocube hybrid nanomaterials demonstrated unique tunable plasmonic behavior under an applied electrochemical potential. A significant reversible experimental peak shift of 22 nm at electrical potential of 200 mV has been achieved in these measurements. Finite-difference time-domain (FDTD) simulations show that, under full oxidation potential, a maximal spectral shift of ca. 80 nm can be potentially achieved that corresponds to a high sensitivity of 178 nm per refractive index unit. Furthermore, FDTD modeling suggests that the electrochemically-controlled tunability of plasmonic peaks is caused by reversible changes in the refractive index of the electrochromic polymer coating caused by oxidation or reduction reactions under external electrical potential. Consequently, we define the orthogonal plasmonic resonance shift as a shift that is orthogonal to the redox process responsible for the refractive index change. Based upon these results, we suggest that the combination of anisotropic nanostructures with electrochromic matrix, has the potential to reversibly electrically tune plasmonic resonances over the full visible spectrum.

Highly anisotropic metamaterials, having opposite signs of principle permittivity tensor components, or hyperbolic metamaterials, have been shown to perform, separately, both superlensing and hyperlensing, however most of the techniques are top-down, expensive, cumbersome and most importantly, non-tuneable with regards to their optical properties. A hybrid device which is large area, bottom-up fabricated, cheap, tuneable, freely-floating and having the capability to switch between the two lensing mechanisms is highly desired. In that regard, we demonstrated successful fabrication of a hybrid meta-lens, composed of gold nanowire arrays(AuNWAs) embedded in a flexible polymer matrix(eg. polydimethylsiloxane(PDMS)) by utilizing various techniques, including electrodeposition, supercritical drying and cryo-ultra-microtomy. FESEM, EDX, XRD and UV-VIS confirmed the presence of AuNWAs in PDMS, with successful penetration of the polymer into sub-100nm inter-nanowire gaps, while maintaining their orientation perpendicular to the substrate. A thin(200nm to few mu;m) section of the metalens, being highly conformal, could then be deposited over focus ion beam(FIB) milled sub-wavelength objects on Cr films for superlensing, and on curved Cr hemispherical caps for hyperlensing. Full-wave finite difference time domain(FDTD) simulations showed that both far-field and near-field sub-wavelength imaging was performed, with the hybrid metalens having a resolution of at least lambda;/5 at a wavelength of 1000nm. Images corresponding to sub-wavelength trimer-holed, quadrumer-holed and ring-etched objects on Cr films, were successfully projected into the far field. Magnification factors of 10X were obtained at a distance of 2.5mu;m from the metalens surface, with the image formation being invariant under object rotation — there being 1:1 correspondence between the object and the image projected. The work presented shows that nanowire-polymer based metalenses provide a way forward for realizing super-resolution imaging systems both in the near-field and far-field.

Chiral gold nanoparticles have gained a lot of attention owing to their ability to generate unique optical activities that allow for applications as biological probes, for enantioselective separations, and as dopants in liquid crystals (LCs).^{1, 2} Thus far, there have been three mechanisms explored that provided insight into our understanding of the optical activity of chiral nanoparticles.^3,4 By combining chirality at the nanoscale with the optical activity of liquid crystals, we are developing a system that can provide direct insight into these mechanisms. We previously reported that nematic liquid crystals are suitable candidates to study chirality transfer effects of chiral ligand capped gold nanoparticles.⁵ In this work we will show that cholesterol-capped gold nanoparticles are effective chirality transfer dopants in nematic LCs that allow us to sense, image and rank the chirality of chiral ligand-capped gold nanoparticles based on size, number of chiral ligands, and the synthesis approach. Here, we demonstrated that in-situ capped gold nanoparticles are stronger chiral inducers than nanoparticles obtained by chiral ligand conjugation using polarizing optical microscopy, induced circular dichroism spectropolarimetry, and helical twisting power measurements.⁶
References:
1. Guerrero-Martínez, A.; Alonso-Goacute;mez, J. L.; Auguié, B.; Cid, M. M.; Liz-Marzán, L. M. Nano Today 2011, 4, 381-400.
2. Shivakumar, U.; Mirzaei, J.; Feng, X.; Sharma, A.; Moreira, P.; Hegmann, T. Liquid Crystals 2011, 11-12, 1495-1514.
3. Noguez, C.; Garzoacute;n, I. L. Chem. Soc. Rev. 2009, 3, 757-771.
4. Gautier, C.; Bürgi, T. ChemPhysChem 2009, 3, 483-492.
5. Qi, H.; O'Neil, J.; Hegmann, T. J. Mater. Chem. 2008, 4, 374-380.
6. Sharma, A.; Mori,T.;, Lee, H.-C.; Worden, M.; Bidwell, E.; Hegmann, T. Detecting, visualizing, and measuring gold nanoparticle chirality using helical pitch measurements in nematic liquid crystal phases, 2014, submitted.

Efficient focusing of linear and higher order optical fields holds immense potential in nanophotonics, such as enabling novel on-chip optical functionality. Metal nanoparticle assemblies are promising materials due to their intense inter-particle electromagnetic interactions; however large-area fabrication of ordered arrays with request local architectural precision and surface quality is extremely challenging. In this work, we demonstrate the feasibility of surface-directed assembly to produce deterministic arrays with high yield and controlled orientation of complex homo and hetero architectures of gold nanoparticles. The versatility of the process allows array fabrication on a variety of substrates including polymers. Nanofocusing of the linear and second harmonic generation (SHG) signals are observed in both individual cluster and array experimental methods. For example, ~ 50% of the dimer major axes are within +/- 10^o of the maximum of the polarization-dependent SHG of the array. The circular dichroism ratio (CDR) of C_infin;v heterodimers (R₁/R₂ = 3; l = 1±0.5 nm) varied less than 20% across an array, and due to the quality of the assembly are greater than previously reported. Numerical simulations validated the experimental results, demonstrating the ability to optimize nonlinear second harmonic yield and to induce chiro-optical responses for compact sensors, optical modulators and tunable light sources by rational nanostructure design and fabrication.

This talk will introduce plasmon spectroscopy in the TEM. It will be shown that surface plasmon resonances can be characterized with sub-nanometer spatial precision, and that full-modal plasmon mapping is possible using a probe only 1 nm in diameter.
Plasmon resonances on metal surfaces can be excited by light, which is thus transformed from a free-space wave to a surface-bound oscillation. Efficient plasmon resonators are rather small, with one or two of their dimensions restricted to only a few nanometer, or a few tens of nanometers. Plasmon resonators are often closely spaced as well, to enhance their electrostatic coupling or to induce plasmon-enhanced electron tunneling. Fabrication and synthesis techniques are now available that provide remarkable nanometer precision, but characterization techniques typically lack this accuracy. An often overlooked tool for plasmon characterization is transmission electron microscopy (TEM), which was occasionally used for this purpose in the past [1-4], but is now becoming more popular [5-9] thanks to instrumental developments. Plasmon analysis in the TEM is done with monochromated electron energy-loss spectroscopy (EELS), as it provides the high energy resolution [10,11] that is required to measure plasmon responses in the visible and infrared.
Examples will be presented where the high spatial precision of TEM is combined with monochromated EELS, to demonstrate full-modal plasmon mapping [12], plasmon-enhanced electron tunneling [13-15], and quantification of the plasmon damping parameters [16].
Acknowledgements
The National Research Foundation (NRF) is kindly acknowledged for supporting this research under the CRP program (award No. NRF-CRP 8-2011-07). This work result from close collaborations with the groups of Joel K.W. Yang (SUTD, Singapore), Christian A. Nijhuis (NUS, Singapore), and Erik Dujardin (CEMES, France). Theoretical support from Antonio I. Fernández-Domínguez (UAM, Spain), Stefan A. Maier (ICL, UK), Wu Lin and Bai Ping (both at IHPC, Singapore) is kindly acknowledged.
References
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[3] Wang, Z. L. & Cowley, J. M. Ultramicrosopy 21, 347-366 (1987)
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[5] Nelayah, J. et al. Nat. Phys. 3, 348 - 353 (2007)
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[7] Schaffer, B. et al. Phys. Rev. B 041401 (2009)
[8] Duan, H. et al. Nano Lett. 12, 1683-1689 (2012)
[9] Rossouw, D. & Botton, G.A. Phys. Rev. Lett. 110, 066801 (2013)
[10] Tiemeijer, P.C. Ultramicroscopy 78, 53-62 (1999)
[11] Krivanek, O.L. et al. Nature 514, 209-212 (2014)
[12] Teulle, A. et al. Nature Materials DOI: 10.1038/NMAT4114 (2014)
[13] Marinica, D.C. et al. Nano Lett. 12: 1333-1339 (2012)
[14] Scholl, J.A. et al. Nano Lett. 13: 564-569 (2013)
[15] Tan, S.F. et al. Science 343, 1496-1499 (2014)
[16] Bosman, M. et al. Sci. Rep. 3, 1312 (2013)

Plasmonic metamaterials and metasurfaces have the ability to influence the propagation, confinement, and emission of light on a deep-subwavelength scale. Many of the optical properties of such materials are encoded in the spectrum, the angular intensity distribution, and the polarization of the far-field emission. Angle-resolved cathodoluminescence (CL) imaging spectroscopy (ARCIS) is a powerful platform for studying these properties, as it combines nanoscale excitation resolution, with the capability to measure both spectra and the angular emission intensity distribution. In particular, we use a 30 keV electron beam as a well-defined broad-band excitation source which is sensitive to the optical density of states. This method has been used to study the spectral and angular optical properties of a large variety of dielectric and plasmonic nanostructures. However, thus far we were only able to measure emission intensities and had to disregard the vectorial polarization nature of the light emission. The emission polarization contains valuable information, which can be used to identify multipoles, separate TM and TE modes in waveguides, characterize the coherence of an emission source etc.
Here, we demonstrate a novel CL polarimetry technique in which we retrieve the Stokes vector, i.e. the full polarization state of the far-field emission, as function of angle. To that end, we extend our setup to include a quarter-wave plate (QWP) and a linear polarizer in the beam path. By taking six measurements with the appropriate combinations of QWP and polarizer angle we retrieve the polarization distribution in the detection plane. By applying a correction for the aluminum parabolic mirror optic we then find the emission polarization distribution.
This approach is applied to gold plasmonic bull&’s eye gratings which can coherently couple out the Surface Plasmon Polaritons (SPPs) that are excited by the electron beam. Because the electron beam can be positioned at will, we can study the effect of exciting the bull&’s eye at different positions. For central excitation, the grating is driven in phase leading to an azimuthally symmetric pattern and a radial polarization distribution, as expected from symmetry. However, when we excite off-center the patterns become significantly more complex, showing multiple lobes and alternating regions in angular space in which the polarization goes from circular to linear. To demonstrate the applicability to chiral structures, we move to spiral bull&’s eyes with different handedness, and show that their chirality is reflected in the field distributions. The validity of the polarimetry technique is verified by measuring transition radiation which has a characteristic radial polarization distribution, similar to a vertical dipole source. This work paves the way for polarimetry measurements on a myriad of nanophotonic metamaterial geometries for characterization and better understanding of their optical properties.

Visualizing light-matter interactions in three-dimensions with nanometer-scale spatial and spectral resolution is challenging, as most experimental techniques are limited to gathering two-dimensional (2D) information. Here, we introduce a new technique for nanoscale interrogation of optical properties in three dimensions: cathodoluminescence (CL) tomography. The technique relies on electron-beam excitation of a sample and detection of photons emitted by the sample. It therefore enables direct visualization of material luminescence and radiative decay of electromagnetic modes with very high spatial and spectral resolution.
To demonstrate this new tomographic technique, we consider a 3D metal-dielectric crescent composed of a subwavelength, 200nm diameter spherical polystyrene core coated with a tapered gold shell. This structure is chosen because of its inherently 3D nature and because it supports a variety of radiative optical transitions, including a dipolar plasmonic mode across its tips, which peaks at a wavelength of 850nm, as well as strong material luminescence from interband transitions in the gold, spanning wavelengths from 500-700nm. We first experimentally obtain a series of CL maps of the nanostructure at various orientations in a scanning electron microscope. Next, we use the method of filtered backprojection to reconstruct the CL intensity from the tilt series. The result is the first 3D map of radiative optical properties with nanometer-scale spatial and spectral resolution. By plotting spectra of individual volumetric pixels, we are able to locate regions of efficient CL excitation in three-dimensions spanning the entire visible spectrum. We correlate the experimental signal with calculations of the local density of optical states in particular regions of the reconstruction, as well as measurements of photoluminescence from the gold.
While we have demonstrated CL tomography by reconstructing a metal-dielectric nanostructure, the technique can be applied to a wide variety of inorganic and organic materials systems to achieve high-resolution, three-dimensional visualization of light-matter interactions. For example, this tomographic technique could be used to precisely locate radiative recombination centers in light emitting diodes, probe the nanoscale distribution of defect states in organic photovoltaics, and provide new label-free avenues for biological imaging.

Surface plasmon resonances (SPR) on metallic nanostructures have attracted great interest because they are a promising alternative to confine electromagnetic energy down to nanoscale dimensions, potentially allowing the manipulation of electromagnetic waves below the diffraction limit. Characterization of these structures is imperative for the study, analysis and understanding the optical properties of single structures as well as assemblies of nanostructures. Although optical techniques, such as optical spectroscopy and near field optical microscopy, can characterize single and assemblies of nanostructures, they have limited spatial resolution (<50nm). Energy loss spectroscopy in a scanning transmission electron microscope combines very high spatial resolution, limited only by inelastic delocalization, with spectroscopy resulting in one of the most powerful techniques for characterization of the local optical properties of nanomaterials and in particular for imaging SPR.
Using an ultrastable TEM-STEM equipped with an electron monochromator, and a high-resolution electron energy loss spectrometer, we acquired spectrum images of surface plasmon resonances of lithographically patterned silver rectangular nanostructures. And applying the Richardson-Lucy algorithm in the acquired data we improve the energy resolution (approaching 10meV)[1] of our system and reduce the contribution of the zero loss peak tails allowing the identification of peaks at energies corresponding to the low range of the mid-infrared region of the spectrum. We examine the properties of individual silver rectangles and squares with different aspect ratios and identify multipolar bright and dark modes with a spatial resolution of less than 5 nm. The plasmonic resonances of these rectangular nanostructures are highly tunable from the visible to the mid-infrared region of the spectrum and are strongly dependent on the aspect ratio, result that agrees with calculations of their local density of states. Multiple plasmon modes confined to the center of the rectangular nanostructures are also imaged exhibiting vertical plasmonic resonant modes found in nanocavities. We suggest that these modes arise from the confinement created by the external walls of the nano-rectangles that act as nano-reflectors.
The SPR coupling of rectangular dimers is also studied using the same methodology. Our results suggest a strong coupling and the formation of up to 14 hybrid resonant modes[2] with peak separations as low as 70meV for a dimer with a gap of 100nm. In summary, we characterize dimer as well as single rectangular silver nanostructures using a method that yields high energy and spatial resolution and demonstrate that these nanostructures are highly promising building blocks for the design of plasmonic devices in the visible and infrared frequency.
[1] Bellido EP, Rossouw D, and Botton GA, Microsc. Microanal 20, 767-778 (2014)
[2] Prodan E, Radloff C, Halas NJ, and Nordlander P, Science 302,