Photonics offers effective solutions to the problem of routing, controlling and processing electromagnetic signals in integrated optical and microwave communication systems based on waveguides, optical fibers and photonic circuit. However, development of another class of photonic components - nonreciprocal devices - is critical for successful implementation of all-optical communications and signal processing.
As photonics research moves into increasingly subwavelength regimes it is vital to develop techniques enabling functionality of nonreciprocal devices at this scale. Scaling down optical devices to several-wavelengths and subwavelength dimensions and their integration in photonic circuitry requires novel approaches to light manipulation.
One of such approaches, which holds a great promise for the entire field of photonics, is based on a concept of metamaterials and metasurfaces. In these artificial photonic structures advances of micro- and nanofabrication techniques allow judicious manipulation of material features at the subwavelength scale which makes it possible to envision and implement the most unprecedented and exotic electromagnetic properties in the entire electromagnetic spectrum.
Here I show that the presence of magneto-optical substrates can have a dramatic effect on the optical response of metasurfaces producing an analog of electromagnetically induced transparency in Fano-resonant meta-molecules. The simplest implementation of such magnetically induced Fano resonances in a meta-surface comprised of an array of resonant antenna pairs placed on a ferrite substrate is analyzed by simple symmetry considerations and by rigorous numerical approach. Time reversal and spatial inversion symmetry breaking introduced by the DC magnetization are shown to make the metasurface bi-anisotropic and optically active. This causes Fano interference between the otherwise uncoupled symmetric and anti-symmetric resonances of the meta-molecules giving rise to a sharp asymmetric transmission peak through the otherwise reflective meta-surface.
For the case of oblique wave incidence, one-way Fano resonances can be achieved due to the combination of spatial dispersion and gyromagnetic effect. It is shown that spectral positions, radiative lifetimes and quality factors of Fano resonances can be controlled by the magnitude of the external magnetic field. While similar tunability may be achieved with other methods, the approach based on gyromagnetically induced coupling to dark sub-radiant resonances is unique because of its nonreciprocal nature. This novel class of nonreciprocal effects may lead to a new generation of tunable and nonreciprocal Fano resonant systems for various applications where strong field enhancement, tunability and nonreciprocity are simultaneously required. One-way absorbers, one-way sensors, and one-way cloaking elements are just a few examples of such applications.

Reflection of light is an unwanted feature for many optoelectronic devices. For example, an air-glass interface reflects 4% of the incident light, which reduces the performance of glass-encapsulated solar panels, and creates unwanted glare on tablet computers and smart phones with glass-based touch screens.
Conventional anti-reflection (AR) coatings based on interference have vanishing reflection if the refractive index of the coating equals radic;(n_substraten_air), and the coating thickness equals a quarter wavelength inside the material. To create an AR coating on glass (index n=1.52) an AR coating with a refractive index n=1.23 is required. However, there exist no materials in nature with such a low refractive index that can be readily applied as a thin film.
Here, we demonstrate a large-scale metasurface composed of a square array of sub-wavelength silica cylinders (280 nm diameter, 110 nm high, 325 nm pitch) that forms a highly-efficient AR coating for glass substrates. The silica metasurface is fabricated in a single-step process using soft-imprint lithography in a liquid silica solgel using a 6-inch-diameter PDMS stamp. The stamp is molded from a silicon master wafer that is patterned using interference lithography. Using this soft-imprint technique we demonstrate very high control over pattern features, thickness, and large-scale uniformity.
We apply the metasurface AR coating on both sides of a borosilicate glass slide and perform reflection and transmission measurements using an integrating sphere in the lambda; = 400-965 nm spectral band. We find that the reflectivity due to both front and back glass interfaces, averaged over the AM1.5 solar spectral density, is strongly decreased from 7.87% (without coating) to only 0.38% (with coating).
We find that the effective refractive index of the sub-wavelength-patterned metasurface is close to the geometrical average determined by the silica/air filling fraction. The silica exhibits no measurable absorption losses. Diffraction losses are avoided by keeping the array pitch below 325 nm. By varying the silica cylinder diameter and pitch, the effective index of the metasurface can be tuned in the range n=1.01-1.31. The coating can then be applied on substrates with an index up to n= 1.74, making it applicable to high-index specialty glass, sapphire, etc. By tuning the cylinder height, the AR performance can be optimized for any wavelength in the visible/near-infrared spectral range. Finally, we demonstrate the printing of the silica metasurface AR coating on mobile phone screens and solar modules.

Thin metamaterial layer, or metafilm, is of interests for many years due to the possible applications to planar optoelectronics, such as solar cell, photodetector and solar water splitting. In this work, we theoretically and experimentally demonstrate that semiconductor metafilm less than the thickness of 50nm can achieve critical coupling (perfect absorption) with designer absorption spectra and furthermore, can be operated for broadband near-unity absorption.
Specifically, judiciously designed germanium (Ge) nanobeams are packed in space well below diffraction limit on metal reflector. Near unity absorption can be attained at the target wavelength, which is identical to the resonant wavelength of individual Ge nanobeam. Typically, such a phenomena is not expected in deep subwavelength array of dielectric nanobeam whose modal field penetrates into the nearby building blocks and optical mode coupling occurs with ease.
Using this interesting property, we continue to design multi-resonant metafilm by placing distinct resonant building blocks in deep subwavelength limit. Whereas single type of nanobeam array critically couples particular wavelength of resonance with large absorption crosssection, multi-resonant beam array can simultaneously couple separate wavelengths of interests by encoding resonant property of individual nanobeam into the net-absorption property of metafilm. In this way, very high mode density can be created across broad visible spectrum. This approach suggests new intriguing designing strategy for broadband near-unity light absorption, which is fundamentally different from general thin film light trapping based on the over-coupling to the system&’s resonance.

We study the geometry dependence of the second harmonic and third harmonic emission from gold nanostructures by designing arrays of nanostructures whose geometry varies from bars to U shapes (split ring resonators). We find a near field overlap integral and nonlinear scattering theory accurately predict the optimal geometry for nonlinear emission.
Nanostructures and metamaterials have attracted interest in the nonlinear optics community due to the possibility of engineering their nonlinear responses. Much debate surrounds whether the nonlinear susceptibility can be represented by an anharmonic oscillator model which implies a direct relation between the nonlinearity and a product of its linear susceptibilities.
We show that the optimum geometry for nonlinear generation can be predicted from an overlap integral between the local nonlinear susceptibility and the mode of the nanostructure at the emission wavelength. This conclusion is independent of the specific structure studied and can be applied in order to predict the nonlinear generation from any plasmonic nanostructure or metamaterial.

Optical metamaterials, with unconventional optical properties, have been utilized to demonstrate various optical nonlinearities as a new type of nonlinear media. The reported studies indicate that the corresponding nonlinear responses are closely correlated to the linear behavior, which is quite beneficial for the customized nonlinearity within the framework of metamaterials. Recently, we experimentally demonstrated voltage tunable optical nonlinearities in a metamaterial absorber in the visible and near-IR wavelength range. On the other hand, chiral metamaterials, exhibiting giant optical chirality compared with those in natural materials, provide one additional degree of freedom, the handedness, in manipulating electromagnetic waves. In addition, the chiral resonance and field enhancement of chiral metamaterials promise handedness-sensitive nonlinearity, as we recently demonstrated with chiral-selective nonlinear imaging in an array of twisted-arc structures.
The question remains, is it possible to achieve electric-field-induced second-harmonic (EFISH) generation along with the handedness-sensitivity? In the work presented here, we numerically investigate the electric field tunable second harmonic generation and its handedness-sensitivity produced by an optical chiral metamaterial. By performing full-wave simulations, we first design a fishnet structure inspired optical chiral metamaterial with two silver layers perforated by twisted elliptical holes and separated by a nanometer thick dielectric spacer without chi;⁽²⁾. A ~100 fold light intensity enhancement in the dielectric spacer and a circular dichroism (CD) as large as ~0.5 are observed at wavelengths corresponding to the resonance dips of both LCP and RCP. The continuous dual layered metallic structure, the strong linear chiral response and the corresponding field enhancement make the proposed metamaterial a good platform for tunable EFISH generation with handedness-sensitivity. By modifying the equation system of a commercial finite-element package (COMSOL), we perform nonlinear simulations to solve two coupled electromagnetic models, one for the fundamental and the other for the SHG. The simulated results reveal a correlation between EFISH generation from the chiral metamaterial and the chiral resonances associated with the fundamental wave. For both LCP and RCP excitation, the EFISH output signal peaks at the excitation wavelength corresponding to the linear chiral transmittance dips and could be purposely modulated by varying the applied electric field. The electrically tunable and handedness-sensitive nonlinear generation reported in this work provide a path forward in utilizing optical chiral metamaterials as a multi-functional electro-optic information processing devices.

Symmetries play a fundamental role in physics. In this talk, I will discuss the fundamental role of symmetries at the nanoscale resonant level in constructing optical metamaterials and plasmonic systems. I will propose the first strategy for the - long awaited isotropic negative index at optical frequencies by proposing a classification of ring-resonators with striking similarities with the Kramer degeneracy of odd electrons and its demonstation in a self-assembled plasmonic system. I will also discuss the possibility to construct new optical modes, i.e. bound staes in the continuum in metamaterials via their resonances. These controls open the way to novel optical devices.

Metamaterials have been of significant interest for nearly two decades. Through plasmonics and spoof plasmonics various single frequency metamaterials have been formed. While there is interest in multi-resonant metamaterials, little work has been done beyond arrays containing unit cells with multiple components of spatially separated geometric structures. This approach limits the ultimate size of these metamaterials. In this work we propose periodic metamaterials using a single element that has geometries embedded within each other. These metamaterials have Ag or Au 2D gratings and various geometric openings in the metals. We show that when these 2D gratings are placed on thin low index insulators such as sapphire which is in turn grown on a plasmonic metal, high Q multi-resonances are possible. these resonances are due to the modes of surface plasmon polaritons (SPP's) that result from light coupling into the grating. The use of embedded geometries in the grating metals allws us to modify the existing resonances and add new ones that were previously not possible. Experimenatlly measured reflectance agrees very well with simulated spectra, allowing us to uniquely identify the resonances and the impoact of the embedded geometries. We find that the SPP's generated by the grating-insulator metal structures can be significantly modified in the visible through the shape and symmetry of the embedded geometry, leading to combined localized and traveling plasmon modes. These effects are also verified experimentally by using a metalized AFM tip placed at various positions above the grating unit cell. This perturbs the local electric fields and resutls in a change in the resonances. Finally, we also consider the role of the metal used in the metamaterials and show that coinage metals provide the best SPP confinement of plasmons in thin dielectric layers. These results have significant implications to increasing quantum efficiencies of thin film detectors and energy harvesting devices, whose thicknesses are less than the penetration depth of light.

Versatile control of electromagnetic waves in nano-scale structured materials is the cornerstone of today&’s photonic technology. The degree of light manipulation demonstrated so far was especially astounding for hyperbolic metamaterials (HMMs), where elliptical iso-frequency surfaces are morphed into surfaces with exotic hyperbolic topologies. Metamaterials allow achieving properties normally limited or not found in naturally occurring materials. One such property is optical nonreciprocity, a rare and generally weak characteristic of materials to differentiate between opposite propagation directions of light. This property, which manifests in magnetic and nonlinear materials, is of immense importance for devices such as optical isolators and circulators, widely used to stabilize laser operations and to route signals in optical telecommunication networks. In this context HMMs can be exceptional candidates offering both enhancements of nonreciprocal effects and broadband operation unavailable in other classes of metamaterials. As demonstrated here, introduction of magneto-plasmonic response into HMMs results in unprecedented nonreciprocal characteristics such as one-way topological phase transitions and broadband one-way hyperbolic regimes.
Optical nonreciprocity is a subtle phenomenon which only occurs when the optical system lacks both time-reversal and inversion symmetries. In a particular case of layered HMMs magnetized in the Voigt geometry, the nonreciprocity is achieved for p-polarization by periodically stacking asymmetric unit cells made of a plasmonic layer sandwiched in between two different dielectrics. By applying both an exact transfer matrix technique and a novel nonreciprocal homogenization theory we identified three distinct nonreciprocal hyperbolic regimes in such metamaterials. The first regime - the nonreciprocal two-way hyperbolic regime - is characterized by the iso-frequency contours consisting of two asymmetric branches of hyperbola corresponding to forward and backward propagation directions. The second regime - the forward elliptical/backward hyperbolic regime - is characterized by the hyperbolic contour for the backward and the elliptical contour for the forward propagation directions. The third regime - theone-way hyperbolic regime - appears with the further increase of the DC magnetization for the Type-I HMMs when the ellipse for the forward propagation collapses to a point.
To summarize, the theory of nonreciprocal HMMs was developed and a variety of nonreciprocal hyperbolic regimes and one-way topological transitions between hyperbolic and elliptical dispersion were demonstrated. Due to the uniform broadband character of the one-way hyperbolic regimes, the effects discovered have potential for practical application in optics and photonics. In addition to the visible domain studied here, the results presented remain valid in the infrared domain for nonreciprocal HMMs composed of doped semiconductors in magnetic field.

Hyperbolic metamaterials (HMM), a class of artificially engineered materials with a highly anisotropic permittivity response originating from opposite signs of the principal components of the electric tensor, have attracted significant interest in recent years due to their ability to manipulate the propagation light in exotic ways. Such materials enable distinctive optical phenomena such as negative refraction, super-resolution imaging, and enhanced spontaneous emission. Here we exploit the hyperbolic iso-frequency characteristic of a planar type-II HMM (composed of alternating, 15-nm-thick, sputtered films of Ag and SiO₂) to achieve high-sensitivity proximity detection of metallic and dielectric nanoparticles in transmission. The iso-frequency surface is unique in that propagation of light inside the HMM over the entire visible-range is allowed only for electromagnetic modes having tangential spatial frequencies k_x exceeding the free-space wavevector k₀ by over a factor of two (k_x >2k₀). The nanoparticle detector consists of a 480-nm thick slab of HMM having an input side consisting of a template-stripped ultra-smooth Ag surface and an exit side consisting of a SiO₂ film coated with asymp; 6-nm-thick Ag nano-islands. As a result of the optical bandgap of HMM, light illuminating the input surface at any angle (intensity I₀) is effectively blocked from transmitting through the slab; only a vanishingly small evanescent-amount (intensity I₁) leaks through, corresponding to an experimental optical density OD = log(I₁/I₀) asymp; 5 at lambda;₀ = 633 nm. Bringing a nanoparticle into deep-subwavelength proximity or contact with the pristine input-surface of the detector opens up efficient transmission channels (intensity I'₁ for a given nanoparticle density), enabled by in-coupling of high-spatial frequency evanescent modes to propagating modes inside the HMM, followed by out-coupling on the exit surface through scattering from Ag nano-islands into free-space. The corresponding increase in transmission per-unit area is represented by an optical contrast ratio defined as γ = I'₁/I₁. Experimental detection of a layer of Au-nanoparticles (asymp; 50 nm diameter, average surface density asymp; 1 µm^-2), dispersed from solution on the device, yielded γ =1500 µm^-2 (a contrast-ratio equivalent to induced transparency of the fractional surface area occupied by the nanoparticles). Two-dimensional finite-difference-time-domain (FDTD) simulations predict high-contrast detection of particles of diameter down to asymp; 10 nm whether composed of metals (Ag, Cr) or dielectric (SiO₂). In addition, γ is found to exponentially decrease as a function of particle-surface gap, with a deep-subwavelength decay length of asymp; 10 nm. Due to its high sensitivity to ultra-small particle sizes in deep-subwavelength proximity, this HMM-based device hints at promising applications in bio-chemical sensing, particle tracking and contamination analysis.

Invisibility cloaks - devices that reroute electromagnetic waves around an object so that the existence of the object does not perturb light propagation. We demonstrate an ultrathin invisibility carpet cloak device, which is capable of making an object undetectable at visible frequency. The cloak is designed using a metasurface that can locally correct the light phase in a subwavelength scale. It could be a general scheme for flexible transformation optical devices.

The fabrication of high-performance optical nanostructures based on metals is challenging. Extreme squeezing of optical energy in metallic nanostructures, such as gaps and tips, is possible by utilizing surface plasmon waves, yet they are highly susceptible to defects and nanometric surface roughness. Also, many applications in sensing, spectroscopy, and quantum plasmonics utilize gap plasmons in ultra-thin nanogaps, but the fabrication of single-digit nanometer gaps in metals is extremely difficult. This presentation will focus on top-down nanofabrication techniques to overcome these challenges. First, template stripping utilizes precisely engineered silicon master templates to replicate ultra-smooth patterned metal films. Second, the challenge of producing single-digit nanometer-scale or even sub-nanometer-scale plasmonic gaps can be addressed by a new technique known as atomic layer lithography. This simple yet highly effective patterning method is capable of wafer-scale production of sub-nanometer-wide gaps that extend along centimeter-scale loops. Applications of ultra-flat and ultra-small metallic structures produced via these methods will be presented, including sensing and surface-enhanced spectroscopy of molecules and thin films in the visible, infrared, and terahertz frequencies.

Chiral molecules occur as pairs of enantiomers that are non-superimposable mirror images of each other. Separation of enantiomers is nontrivial, as enantiomers are identical in all scalar physical properties such as density, molecular weight, enthalpy of formation, and electronic and vibrational frequencies. However, since many applications in chemistry and biology rely on enantiomerically pure samples, chiral resolution, the separation of opposite enantiomers, is of critical importance.
Here, we present a novel method for chiral resolution based on the preferential optical absorption of left/right-handed molecules near resonant dielectric materials and metamaterials. It is well known that enantiomers show differential absorption when illuminated by circularly polarized light with different handedness. If the difference is large enough, this asymmetric absorption can result in the photodestruction of one enantiomer while preserving the other, producing an enantiomerically pure product. In most molecules, the rate of asymmetric absorption of circularly polarized light is too small for efficient photodestructive chiral resolution. Our approach is based on enhancing local electric and magnetic fields to enhance both the optical chirality, C, of the molecule and the rate of asymmetric absorption, g, which is proportional to C divided by the electric energy density. To enhance both C and g, we consider nanophotonic materials that exhibit a simultaneous enhancement in electric and magnetic fields while minimizing the electric energy density.
First, using analytic Mie calculations, we calculate the spatially-dependent g in silver and silicon nanospheres that support electric and magnetic Mie resonances. In 10 nm radius Ag spheres that support only strong electric dipole resonances, we find a local 2x enhancement of g that is spatially restricted to a 2nm radius spot size in the forward scattering direction at 389 nm. 75 nm radius Si nanospheres, in contrast, show a local 2x enhancement of g that covers a 75 nm radius spot size in the backscattering direction at 565 nm. Finally, we use full-field finite-difference time domain simulations to investigate the rate of asymmetric absorption near silicon Dolmen structures. These structures are composed of two parallel 2 mu;m x 500 nm bars and one orthogonal 500nm x 500nm bar, all of height 1.2 mu;m, and exhibit both Fano-type resonances and polarization rotation properties in the 4 mu;m range, which is comparable with the vibrational resonance of limonene, our target molecule. These structures enable a rotation of the displacement field from the electric field, thus minimizing the electric energy density, U_e α E#8901; D, while still enhancing the electric field. Interestingly, our calculations indicate that optimal enhancement in asymmetric absorption does not require chiral nanostructures, and point toward resonant dielectric metasurfaces as promising platform for creating a light driven method of chiral resolution.

Hyperbolic metamaterials represent a class of synthetic electromagnetic media in which electromagnetic waves exhibit unprecedented hyperbolic dispersion extending toward very large wavenumbers. The latter property was shown to be crucial for multiple applications from superlensing to enhancing emission of light. Metamaterials based on different materials, including noble metals, doped semiconductors, and even ferrites, have been proposed to achieve the hyperbolic dispersion over entire electromagnetic spectrum. As opposed to the plasmonic origin in optical and infrared metamaterials, in the microwave domain the hyperbolic regime can be achieved with the excitation of magnons (spin-waves), representing the collective excitations of magnetic subsystem of ferrites in which the magnetic permeability is negative near the ferromagnetic resonance.
#8203;With the aim to endow hyperbolic metamaterials with another unique property of nonreciprocal response, we designed a microwave hyperbolic metamaterial made by stacking two ferrites and one dielectric subwavelength-thick layers. Such three-layer stacking allows breaking of the inversion symmetry of the structure which, together with the gyromagnetic activity of ferrites, enables the nonreciprocal response. To explore properties of such low-symmetry hyperbolic metamaterial, we develop an exact transfer matrix technique and derived an analytic expression for the nonreciprocal hyperbolic dispersion. In addition to the exact treatment, the structure was homogenized by applying the subwavelength approximation and a compact analytical expression for the nonreciprocal hyperbolic dispersion of electromagnetic waves was derived.
The developed theory was applied to design a metamaterial with extremely nonreciprocal response characterized by a one-way hyperbolic dispersion (either of the Type-I or Type-II) when, within a certain range of frequencies, the light can only propagate in a particular direction but not in the opposite direction. By examining the electromagnetic field in the metamaterial we found that the hyperbolic regime originates from the excitation of surface spin-waves of the individual ferrite layers which evanescently couple to each other to form a hyperbolic continuum, with the nonreciprocity stemming from the nonreciprocal dispersion of the spin-waves. Nonreciprocal regimes characterized by solutions which are elliptical in one and hyperbolic in the opposite direction have also been found. The possibility of dynamic control of the nonreciprocal regimes and switching between them by changing the external DC magnetic field was established.
In summary, nonreciprocal hyperbolic metamaterials proposed here can be of great promise for practical applications and envision a new generation of nonreciprocal and tunable microwave devices possessing a variety of unique characteristics, including broadband nonreciprocal response and ability to transfer evanescent fields nonreciprocally or even one-way.

With a strong acceleration since the year 2000, the number of papers dedicated to a ‘nano&’ subject, nowadays, Raman spectrometry is a method of choice to characterize and to understand nano-materials and colloids [1]. Moreover Raman offers a ‘bottom-up&’ approach of nanostructured materials, which comes as a good complement to methods like electron microscopy or X-ray diffraction.
Surface- and tip-enhanced Raman and LSPR spectroscopies have developed over the past 15 years as unique tools for uncovering the properties of single particles and single molecules that are unobservable in ensemble measurements [2]. Measurements of individual events provide insight into the distribution of nanoparticle properties that are averaged over in ensemble experiments. Localized optical spectroscopy can provide detailed information on the identity of molecular species and changes in the local environment, respectively.
Our group develops a new approach combining an AFM and a confocal-Raman microscope, where AFM microscope is used to image and to manipulate nano-particles under the confocal optical microscope coupled at the Raman spectrometer [3]. Our optical device allows us to scan the resonance effects by tunning the wavelengths of excitation.
In this presentation, we will show the results obtained with some symmetric structures of assembling of gold nano-particles, in the vicinity of a single and isolated carbon nanotube (CNT). We will investigate the consequence on the super-resolved Raman spectra and as a function of the different sub-wavelength-scale geometries of AuNPs aggregates. We will discuss the different interactions between AuNPs aggregates and the carbon nanotube, including their impact on resonance effects. In particular, we will focus on the enhancement of the local electrical field by metallic nano-structures to probe single objects.
These experimental data are interpreted according to finite element models of far and local electromagnetic fields. Thus, on one hand, we achieve to a better understanding about the tunability of plasmon resonance modes of home-tailored nanostructures, including sensitive breaks symmetry modes [4]. On the other hand, the consequences of their interactions with a substrate or molecule dipole moment, depending on the excitation wavelength (especially in the case of the inelastic scattering), are studied. All this allows us to understand and predict the experimental observation.
[1]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, 283-290
[2] 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
[3]Tong, L., Li, Z., Zhu, T., Xu, H., & Liu, Z., JPC C, 112(18), 7119-7123 (2008)
[4]Chuntonov, L. & Haran, Journal of Physical Chemistry C 115(40),19488-19495, (2011)

We use spectroscopic imaging to assess the spatial variations in upconversion luminescence from NaYF₄:Er³⁺,Yb³⁺ nanoparticles embedded in PMMA on Au nano-cavity arrays over a wide range of excitation intensities. The nano-cavity arrays support a surface plasmon (SP) resonance at 980nm, coincident with the peak absorption of the Yb³⁺ sensitizer. Spatially-resolved upconversion spectra show a 30X to 3X luminescence intensity enhancement on the nano-cavity array compared to the nearby smooth Au surface, corresponding to varying excitation intensities from 1 W/cm² to 300kW/cm², spanning the non-linear and saturation power dependence regimes. Our analysis shows the power dependent enhancement in upconversion luminescence can be almost entirely accounted for by a constant shift in the effective excitation intensity, which is maintained over five orders of magnitude variation in excitation intensity. The variations in upconversion luminescence enhancement with power can be modeled by a 3-level-system near the saturation limit, which agrees well with the experimental observations. This amplification of the excitation field is independent of the emission wavelength, indicating the enhancement in upconversion emission is due to entirely to inceased absorption by the Yb³⁺ sensitizer. Analysis of the statistical distribution of emission intensities in the spectroscopic images on and off the nano-cavity arrays provides an estimate of the average enhancement factor independent of fluctuations in nano-particle density.

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. These arrays are typically composed of metallic Ag or Au nanostructures shaped like split-rings, nanowire pairs, or nanorods, commonly referred to as meta-atoms. The meta-atoms, rationally arranged with dielectric or semi-conducting components, give rise to localized or propagating surface plasmon resonances that then induce capacitive or inductive coupling between individual meta-atoms. This gives rise to a collective electromagnetic response, which enables the composite metasurface to be treated as an effective medium, capable of an impressive range of properties, such as narrow- and broad-band absorption, graded birefringence for light steering, and total phase control.
An unexplored approach to tailor the wavelength range, bandwidth, and mode volume of a colloidal metasurface is to exploit in-plane coupling between individual meta-atoms--in addition to the established out-of-plane coupling between meta-atoms and a backplane--providing an additional design variable for colloidal metasurfaces. We demonstrate a robust, scalable assembly method for a perfect absorbing metasurface that uses colloidal single-crystalline nanocrystals that self-assemble to produce architectures with nanometer-precision. These metasurfaces exhibit extreme in- and out-of-plane electromagnetic coupling that is strongly dependent on nanocrystal size, shape, and spacing, and display near-ideal optical absorption tunable from the visible to mid-infrared wavelengths.

The ability to create graded index (GRIN) materials with arbitrary index distributions and with high resolution in three dimensions would prove transformative for optics design and applications. Where conventional optics restricts designs to surfaces and thicknesses, GRIN materials would open the design space to include the entire volume of optical elements—providing dramatically reduced size, weight, and element count important for next generation optical platforms. More excitingly, realizing well-defined GRIN profiles in actual materials will lead to transformative technologies for cloaking, imaging, transformation optics, solar collection, and advanced optical systems where multiple optical functions can be combined into single elements. However, current fabrication techniques applied toward common optical materials are fundamentally limited to diffusion profiles and are unable to realize arbitrary distribution GRIN design.
Here, we describe the writing of graded index structures using multiphoton lithography (MPL), a 3D direct-write technique where a pulsed laser beam is focused within precursors solutions or photoresists to drive photochemical reactions (e.g., photopolymerization) that are restricted in 3D to the beam focus. Using MPL, we fabricated grating structures in hydrogel materials comprised bovine serum albumin (BSA) protein and then quantified the index change of the written structures. We were able to successfully demonstrate index changes of 10^-2, which is similar to laser densified glass and polymer systems, with a Δn/Δx of 1.5 x 10^-3/µm. Finally, we show that these direct-write density variations in BSA structures can be converted to SiO₂, opening up the possibility of transforming MPL written structures to other optical material systems. These initial results provide a foundation to realize functionally graded materials with highly complex and user-defined index profiles, that can potentially be translated into or incorporate other dielectric or metallic material systems ideal for creating submicron resonant structures.

We present a method to organize cyanine dye molecules into aggregates in thin films and describe the thin film optical properties.
Aggregates precipitate in droplets of the evaporating precursor solution during spin coating. The formation of droplets is forced by phase separation or dewetting, depending on the coating conditions either J- or H-aggregates can be formed.
Cyanine dye aggregates are best known for intense and narrow absorption bands, going along with an exciton delocalized over the coherence length of the aggregate. Our films on the other hand stick out for prominent scattering in the absorption band (resonance light scattering). Similar to localized surface plasmon resonances (LSPR), also scattering can be explained with the coherently oscillating electron cloud of the aggregate in response to incident light. Because dye aggregate extinction has narrowest bandwidths, the wavelength selectivity exceeds the selectivity of localized surface plasmon resonances. Inherent to the fabrication process, aggregate domains can be easily organized into periodic structures, making the method appealing for photonic applications. As an example, we fabricated a 2D grating where the narrow absorption range of the aggregates leads to wavelength specific scattering.
Reference: J.-N. Tisserant, R. Brönnimann, R. Hany, S. Jenatsch, F. Nüesch, R. Mezzenga, G.-L. Bona, J. Heier, Resonance light scattering in dye aggregates forming in dewetting droplets, ACS Nano 2014, DOI: 10.1021/nn5040839.

Nanoparticles from colloidal solution have controlled nanomaterial interfaces allowing for tuning of the plasmon resonances as well as mitigating losses and affecting extinction spectra; nanoparticles effectively serve as meta-molecule building blocks to tune optical properties. In addition, colloidal assembly is beneficial as a high-throughput, wafer scale deposition method. We will present experimental data coupled with theoretical simulations showing arrangements of nanoparticles deposited from colloids serve as plasmonic and metamaterial surfaces. We have achieved robust surface enhanced Raman scattering (SERS) sensors approaching single molecule detection limits reproducibly over large areas using colloidal assembly. We will show by varying driving forces for assembly, diffusion versus electrophoresis, nanoparticle clusters with gaps between nanoparticles of 4 nm down to 1 nm, respectively, are obtained.
Arrays of tightly coupled metal and metal- dielectric nanoparticles also support narrow band resonances, Fano resonances, based on “dark” electric and/or magnetic resonances. We will discuss how material interfaces can be used to mitigate losses that eliminate Fano resonant features. For example, the extinction and absorption efficiencies resulting from an array of linear trimers of Au nanoshells in homogeneous environment show that efficiency is affected by changing dye concentration in nanoshells. The use of dyes as gain media induces sharpened Fano resonance features (attributed to the meta-molecule nature of the linear trimers) and increased maximum absorption efficiency at 422 THz. Using similar methods, circular nanoclusters (CNC) of metal nanoparticles can support a magnetic Fano resonance at 472 THz via dipole moments forming a current loop under oblique TE-polarized plane wave incidence. In particular, array-induced resonances are narrower than single-CNC-induced ones and also provide even larger field enhancements, in particular generating a magnetic field enhancement of about 10-folds and an electric field enhancement of about 40-folds for a representative metasurface. Since natural magnetism fades away at infrared and optical frequencies and artificial magnetism is cumbersome to achieve in these regimes, as conventional split ring resonators are difficult to scale down to optical wavelengths, nanoparticles assembled from colloids are a scalable approach to engineer materials&’ electromagnetic properties.

The properties of individual semiconductor and metallic nanoparticles have been extensively investigated. When these nanoparticles are controllably arranged in a particular geometry, new and fascinating properties emerge. I will present a few recent experiments in which we assemble semiconductor/metallic nanoparticles into a particular geometry using AFM nanomanipulation method.

Surface Plasmon Polaritons (SPPs) are fluctuations of the free electron density in metals coupled to electromagnetic waves. SPPs at optical frequencies show a significant momentum mismatch with respect to the light incident on a flat metal/dielectric interface, therefore coupling strategies generally rely on prisms (e.g. Otto-Kretschmann configuration) or metal gratings to excite them.
This talk will focus on alternative methods to generate SPPs at optical frequencies using light diffraction by individual nanocorrugations etched in metal films followed by in-plane interference to generate high, spatially localized fields at the metal/active layer interface.
Sub-wavelength localization of the SPPs excitation source (using for example nano-slits, grooves and holes in metal films) allows for control of the SPP propagative phase, thus enabling plasmonic interferometry at the nano- and micro-scale.
By properly varying the separation distance and in-plane distribution of several nanoscatterers, the optical interference of SPPs can be spatially modulated and spectrally tuned. This property, together with the highly confined nature of SPPs, can be employed to enhance the optical absorption in thin film solar cells, and improve the sensitivity and selectivity of high-throughput, real-time biochemical sensors.
Plasmonic Interferometry can also be employed to measure the coherence length of light sources, as well as in-situ determination of the optical constants of dielectric materials.

In recent years, all-dielectric nanoscale resonators, such as nanowires, nanopillars and nanodisks, have emerged as viable, low-loss alternatives to plasmonics. Several groups including ours have demonstrated that such dielectric resonators possess characteristic Mie-type resonance modes enabling strong concentration of light intensity in sub-wavelength volumes based on the index of refraction of the dielectric. Most spectral features due to dielectric resonances observed to date are broad and attributed predominantly to the generalized size and shape of the individual resonators. In the present work, we investigate periodic square arrays of silicon nanopillar resonators on silicon and observe an additional, narrow resonance using angle-dependent optical reflectivity measurements. This mode appears superimposed on the broad resonance associated with individual, non-interacting resonators, but spectrally tunes with array period and incident angle in a manner consistent with the first-order diffraction condition. On the basis of these results, we assign the new resonance to a collective lattice mode involving a coupling between a local resonator and an in-plane diffractively-excited propagating mode, in a close analogy to the collective lattice modes observed in plasmonics. The measurements are accompanied by FDTD simulations in which similar features are observed reinforcing this interpretation. The new collective mode exhibits a number of desirable characteristics - narrow spectral width, easy tunability via simple parameters, strong sensitivity to the refractive index of surrounding medium, and long-range coupling - making it attractive for sensing and energy harvesting applications.

Dielectric optical antenna resonators have recently emerged as a viable alternative to plasmonic resonators for metamaterials and nanophotonic devices, due to their ability to support multipolar Mie resonances with low losses. Interestingly, in contrast to plasmonic resonators which naturally possess only electric resonances, dielectric particles exhibit a series of both magnetic and electric resonances. The majority of previous works on dielectric resonators have focused on silicon as the material of choice where several studies have demonstrated that silicon nanospheres support both electric and magnetic resonances in the visible and near-infrared spectral ranges. However, there have been no similar experimental demonstrations in the mid-infrared range, where high index semiconductor materials such as Si and Ge are practically lossless. In this work, we experimentally investigate the mid-infrared Mie resonances in Si and Ge subwavelength spherical particles. In particular, we leverage the electronic and optical properties of these semiconductors in the mid-infrared range to design and tune Mie resonators through free-carrier refraction.
Si and Ge semiconductor spheres of varying sizes of 0.5-4 mu;m were fabricated using femtosecond laser ablation. The sizes of the spheres was controlled by the irradiance of the laser beam. Using single particle infrared spectroscopy, we first demonstrate size-dependent Si and Ge Mie resonances spanning the entire mid-infrared (2-16 mu;m) spectral range. Subsequently we show that the Mie resonances can be tuned by varying material properties rather than size or geometry. We experimentally demonstrate doping-dependent resonance frequency shifts that follow simple Drude models of free-carrier refraction. We show that Ge particles exhibit a stronger doping dependence than Si due to the smaller effective mass of the free carriers. We also demonstrate the emergence of plasmonic resonances for high doping levels and long wavelengths. These findings demonstrate the potential for tuning infrared semiconductor Mie resonances by optically or electrically modulating charge carrier densities, thus providing an excellent platform for tunable electromagnetic metamaterials.

Showing original optical properties, high refractive index dielectric nanostructures are very promising candidates for new photonic and photovoltaic devices. Particularly interesting is the possibility of tailoring optical resonances like the ability to guide, scatter and absorb light within specific spectral ranges. It is thus possible to enhance or suppress light-matter interaction within certain wavelength ranges by changing the size, shape or the material of a high index nanostructure.
Applications of nanoobjects such as silicon nanowires can be found for instance in photovoltaics, where resonances are engineered in order to ideally match the solar spectrum. Furthermore, they offer an attractive alternative to plasmonic nanostructures for field enhanced spectroscopy applications as, despite of lower local field enhancement, they exhibit far lower absorption losses than metals in the visible and infrared.
For the design of application-dedicated nanostructures, knowledge about the optical near-field distribution is crucial. As near-field optical microscopy (SNOM) is usually difficult to perform and interpret, far-field techniques may represent a more practicable approach. We report on the experimental observation and characterization of second-harmonic generation (SHG) from individual silicon nanowires deposited on glass substrates and its use to perform nonlinear optical microscopy. The Second Harmonic, which is forbidden in the centrosymmetric silicon crystal, is generated at the Si-NW surface and is found to be highly sensitive to the presence of resonant modes. SHG 2D mappings of single Si-NWs of different sizes show a distinctive dependence on the fundamental light polarization and provide insight in the distribution of the local electric field intensity. The observed near-field distribution is similar to formerly performed experiments using a Si nanocrystal plane placed in the vicinity of the Si-NW as a probe for the local electric field and could be verified in numerical simulations.
In addition to the SHG, we evidenced a spectrally broad nonlinear emission of third order, which we attribute to photoluminescence from the native silicon oxide layer, induced either by three photon absorption processes and/or indirectly by Third Harmonic Generation from the silicon NW core.
Finally we observed that the SHG in silicon is intense compared to SHG from bulk silicon. This fact, together with its CMOS compatibility, renders silicon nanostructures very promising for photonic applications e.g. in low cost optical networking.

Weyl points are degenerate energy states resulting from band crossing of linear dispersions in three dimensional momentum space. Unlike Dirac points in the two dimensional systems, Weyl points have been shown to be stable and the associated surface states are predicted to be topological surface states with non-trivial Chern number [1]. These topologically protected surface states may potentially lead to various interesting phenomena such as backscattering immune transport.
We fabricate and characterize photonic crystals in the infrared regime with Weyl points present in their band structures. Full wave FDTD simulations were utilized to optimize the unit cell size and material index of the gyroid structures. Three dimensional two-photon lithography method was used to fabricate optimized geometry from simulations in to polymers. We used sputtering process to coat the polymer structure with high index materials such as amorphous silicon at low temperature conformally. Optical properties of these gyroid geometries with high effective refractive index are characterized with angled resolved Fourier transform infrared spectroscopy (FTIR) in order to map out the bulk and surface band structures in the momentum space. Initial FTIR measurement at normal incidence has shown strong absorption related to both structured polymer and a-Si structures.
L. Lu, L. Fu, J.D. Joannopoulos, M. Solja#269;icacute;, “Weyl points and line nodes in gyroid photonic crystals”, Nature Photonics 7, 294-299 (2013)

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.

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
[1] Z. Fang et al., Nano Lett. 14, 299 (2014).
[2] M. Gullans et al., Phys. Rev. Lett. 111, 247401 (2013).
[3] F.H.L. Koppens, D.E. Chang, and F.J. García de Abajo, Nano Lett. 11, 3370 (2011).
[4] Z. Fei et al., Nature 487, 82 (2012).
[5] J. Chen et al., Nature 487, 77 (2012).
[6] A. Principi et al., Phys. Rev. B 88, 121405(R) (2013).
[7] A. K. Geim and I. V. Grigorieva, Nature 499, 419 (2013).
[8] L. Wang et al., Science 342, 614 (2013).
[9] A. Woessner, M.B. Lundeberg, Y. Gao et al., arXiv:1409.5674 (2014).
[10] A. Principi et al., Phys. Rev. B 90, 165408 (2014).
[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.

Plasmonic electrodes, consisting of thin films of nanostructured metal, can exhibit a number of optical, electrical and morphological effects that can be exploited to improve performance parameters of organic thin-film optoelectronic devices. Here we investigate the use of two different types of disordered plasmonic back electrodes - nanoporous metal (NPM) films and continuous metal nanoparticle array (MNPA) films - for organic light-emitting diodes (OLEDs) and organic photovoltaics. NPM with periodic arrays of sub-wavelength holes have attracted a lot of attention due to the discovery of extraordinary optical transmission (EOT) arising from surface plasmons supported by the NPM. Recently it has been found that NPM films can also exhibit a phenomenon called absorption-included transparency (AIT) in addition to EOT when thin-film absorber layers are applied to the nanoporous metal. Interestingly, while EOT depends sensitively on periodicity of the nanopores, AIT occurs even for single pores and, therefore, does not require a periodic arrangement of the nanopores. In this work we employ large-area disordered NPM back electrodes for light extraction and emission enhancement from light-emitting polymer layers for OLED applications. We find that while light-scattering by the nanoporous metal is highly dependent on pore diameter, enhancements in light emission from the polymer layer coatings do not correlate well with pore diameter. Photoluminescence emission intensity enhancements of up to 12 and 30 are found for the disordered NPM/polymer composites with pore depths of 50 and 100 nm, respectively (with pore diameter approximately constant), which we attribute to a combination of far-field scattering and AIT effects. In addition to NPM, we investigate the morphological and plasmonic mode contributions of MNPA to organic photovoltaic active layer performance. In this work, we find evidence for both surface plasmon polariton and in-plane AIT-type effects in MNPA electrodes coated with a range of organic semiconductor absorber layers.

Erbium is a well-studied optical material in part because the near-infrared, ⁴I_13/2 → ⁴I_15/2 (lambda; = 1550 nm) transition coincides with the transmission window of silica fibers used in optical telecommunications. In this study, nanometer scale, Au plasmonic structures are patterned onto erbium-doped yttrium aluminum garnet (YAG) crystals. The YAG crystals are intrinsically doped with erbium at a 0.5% dopant level in order to admit lasing at the lambda;=1550 nm erbium transition. We have previously designed dual-band optical antennas with resonances at lambda; = 1.8 µm and 2lambda; = 3.6 µm that match the fundamental and half-harmonic transitions of PbSe quantum dots. In this new study, we similarly design dual-resonant optical antennas to match the ⁴I_15/2 → ⁴I_11/2 (pump, lambda; = 980 nm) and ⁴I_13/2 → ⁴I_15/2 (luminescent, lambda; = 1550 nm) erbium transitions. The two resonance frequencies of the Au nano-optical structures are designed to feature spatial overlap of the field-enhanced regions. Further, this compound system of Au-nanostructures and local erbium emitters breaks the centro-symmetry conditions necessary to admit a chi-two optical nonlinearity in addition to the strong chi-three nonlinearity known to be present in bulk Au samples. We provide direct detection of second harmonic generation and simultaneous pump sensitization and luminescence enhancement accomplished using these dual-band optical resonators. Also, time resolved photoluminescence measurements show that simultaneous pump and luminescent enhancement of erbium are qualitatively different than previously demonstrated singly-resonant experiments. The results have important implications for the use of erbium as an optical or plasmonic gain medium, as well as the design of optical antennas for the enhancement of local emitters.

In optoelectronics, optical loss is usually undesired, as it is responsible for power dissipation and light attenuation. The concept of parity-time (PT) symmetry, however, exploits the interplay between the material loss and gain to attain novel optical phenomena such as exceptional point and unidirectional light propagation. Here we experimentally demonstrate a PT symmetry breaking laser that allows unique control of the resonant modes. In contrast to conventional ring cavity lasers with multiple competing modes, our on-chip InGaAsP/InP based PT microring laser exhibits intrinsic single-mode lasing regardless of the gain spectral bandwidth. Thresholdless parity-time symmetry breaking due to the rotationally symmetric structure leads to stable single-mode operation with the selective whispering gallery mode order. Our chip-scale semiconductor platform provides a unique route towards fundamental exploration of PT physics and next-generation active optoelectronic devices for optical communication and computing.

Parity-time (PT) symmetric structures have brought new importance to the imaginary component of the refractive index, k”, in photonic and plasmonic systems. Coupled waveguides with asymmetric gain/loss profiles and single waveguides with Bragg-like gain/loss striations have formed the basis of a new class of metamaterials with unique electromagnetic properties. These properties are contingent on k”, in that if additional gain and loss are added to the system an ‘exceptional point&’ or ‘phase change&’ is reached, beyond which the electromagnetic properties of a system significantly shift. Such properties can be utilized to realize asymmetric and even non-reciprocal light propagation. Here, we present coaxial waveguides as new PT-symmetric plasmonic architectures. We consider a plasmonic coax consisting of a 60-nm-diameter silver core surrounded by a 25-nm-thick silica ring, embedded in a semi-infinite silver cladding. The dielectric ring is sectioned into 4N sections, with loss (positive k”) and gain (negative k”) included periodically each as one of the 4N sections. Using a perturbative analytic model with results confirmed with a numerical eigenvalue and eigenmode solver, we explore the evolution of the field distributions and the dispersion relations as k” is increased from 0 to 0.2 for the 13 modes (m=0, 1, 2, 3, 4, 6 of which 1-6 are degenerate pairs of modes) supported by the structure for energies less than the surface plasmon resonance for N=0.5, 1, 1.5, and 2. We find the modes have distinct exceptional points, and that they are highly altered by N, even though the gain and loss remain balanced and k” is the same. When N is an integer, the periodic distribution of gain and loss in the structure overlaps with a propagating mode, leading to an exceptional point for the matching mode for any k” greater than zero. These matched modes become thresholdless, while unmatched modes experience exceptional points at higher values of k”. For example, for N=1, the original passive degenerate azimuthal m=1 modes split into an amplifying mode and a loss mode for arbitrarily small values of k”, as the field intensities overlap only with the gain or loss sections of the coax respectively. Different integer values of N match different modes, so one can fabricate waveguides that possess strong preferences for particular modes, such as N=2 with m=2. This selectivity can be extended to the azimuthal direction by introducing a second periodic modulation in for the real part of the index. Above the exceptional point, this property leads to a preferential circulation of the fields in the waveguide, creating circular polarization independent of the excitation condition. Therefore, this geometry may provide a means of generating chiral circularly polarized light from linearly polarized light via a non-chiral structure.

Control of the radiative properties of emitters such as molecules, quantum dots, and color centers is central to nanophotonic and quantum optical devices, including lasers and single photon sources. Plasmonic cavities and nanoantennas can strongly modify the excitation and decay rates of nearby emitters by altering the local density of states. Here, we demonstrate large enhancements of fluorescence and spontaneous emission rates of molecules embedded in plasmonic nanoantennas with sub-10-nm gap sizes. The nanoantennas consist of colloidally synthesized silver nanocubes coupled to a metallic film which is separated by a ~5 nm self-assembled polyelectrolyte spacer layer with embedded molecules. Each nanocube resembles a nanoscale patch antenna whose plasmon resonance can be changed independent of its local field enhancement. By varying the size of the nanopatch, we tune the plasmonic resonance by ~200 nm throughout the excitation, absorption, and emission spectra of the embedded molecules demonstrating giant fluorescence enhancement for antennas resonant with the excitation wavelength. Next, we directly probe and control the nanoscale photonic environment of the embedded emitters including the local field enhancement, dipole orientation and spatial distribution of emitters. This enables the design and experimental demonstration of Purcell factors ~1,000 while maintaining high quantum efficiency and directional emission. Full-wave simulations incorporating the nanoscale environment accurately predict the experimentally observed emission dynamics and reveal design rules for future devices. Finally, progress on coupling colloidal CdSe/ZnS core-shell quantum dots to the plasmonic nanopatch antennas will be discussed.

Concentrating optical energy at deep sub-wavelength scale is a great challenge because of the diffractive nature of light and it has numerous applications such as imaging, nanolithography, high-density data storage, etc. The near-field scanning optical microscopy (NSOM) has been widely used in sub-diffraction limit optical imaging. By working in the near-field of a sub-wavelength aperture in an opaque film, people can utilize the high wave vector components in the evanescent waves to generate and probe the features far smaller than the optical diffraction limit. In general, the achievable resolution of NSOM is mainly limited by the skin depth of the opaque film to a few tens of nanometers. And their major technical obstacle has been the trade-off between spatial confinement and coupling efficiency. The typical efficiency for optical power transmission through a NSOM probe is at the order of 10^-5 or lower for a deep sub-wavelength size opening, limited by the wave vector mismatch between the aperture and propagating light. Efficient light focusing has been realized by utilizing the large wave vectors of surface plasmon polaritons (SPPs). Various plasmonic structures have been proposed to enhance NSOM efficiency. Despite the successes in achieving highly enhanced and confined light spots, their performance is usually impaired in the presence of a sample at its close proximity due to the change of resonance.
We report a new plasmonic coupling scheme, named hybrid quasi-3D plasmonic focusing, which is capable to obtain efficient optical confinement reaching the sub-10 nm region for the purposes of lithography and imaging. In this new scheme, we focus two sets of phase mismatched SPPs into standing-wave-like SPPs with preferable off-plane components. And the obtained SPPs are then converted into localized modes and excite a nanoscale pin structure. This scheme is optimized for efficient nanoscale energy coupling to the sample surfaces, which makes it an ideal device for many demanding applications, such as imaging, lithography, sensing, and data storage. Comparing with other schemes, the hybrid quasi-3D plasmonic focusing can provide several orders of magnitude larger optical energy efficiencies reaching 10% or higher. A novel type of plasmonic nanofocusing structure, called the push-pin plasmonic lens (PPL), will be shown as an example. And we experimentally demonstrated the PPL device in the aNSOM experiment. Our numerical study also shows that the PPL can simultaneously collect the near-field waves scattered by the sample surface. The PPL directly converts the collected near-field waves into farfield propagating light without the needs of high NA optics. The achieved SNR is several orders of magnitude higher than the conventional aNSOM scheme. In addition, the quasi-3D devices feature their planar thin-film structures and can be easily fabricated into a large array. It has the potentials to enable the development of parallel aNSOM systems.

A spaser is a nano-optical light generator utilizing plasmonic modes of metallic nano-particles. Here, we discuss its similarities with the conventional laser within an electrodynamic framework. Strategies to decrease the threshold for optical pumping by varying the nanoparticle shape, size, as well as the spasing modes are considered. Universal dependences, figures of merit, and the role of saturation are illustrated with analytical and numerical examples. We show that, the minimization of the spasing threshold in realistic spasers requires the use of multipolar modes and more diverse structures, such as spheroids and multi-layer core-shells.
In the electrostatic limit (small particles), the threshold gain depends only on the dielectric constants of the metal and the gain material. A variation of nanoparticle shape, composition, or spasing mode may shift the plasmonic resonance to a wavelength where the dielectric constants define a minimal threshold, but it cannot improve this material-imposed optimum. This general argument is illustrated by detailed numerical studies of the thresholds for two experimentally relevant geometries: silver spheroids and spherical shells embedded into a gain material. For both cases, we include retardation for finite-sized particles. This allows us to quantify how an increasing particle size increases the spasing threshold.
The operation of a continuous spaser above the threshold can be described within a purely electrodynamic framework with an intensity-dependent dielectric function. This allows one to obtain local fields and cross-sections in both self-oscillating and driven regimes.
The behavior of a driven spaser depends on three main dimensionless control parameters: driving frequency normalized to the spaser frequency; pumping, characterized by the strength of the Lorentzian gain, normalized to its threshold value, and the magnitude of the incident E-field, normalized to its saturation level. Material parameters strongly influence the normalization values, but different spaser types undergo similar qualitative changes when the normalized parameters are modified.
Financial support was provided by the ERC grant 257158 “ActiveNP” and ONR MURI Grant N00014-13-064.

Glasses are recognized as the ideal hosts to incorporate plasmonic metal nanoparticles (NPs), semiconductor NPs, and luminescent rare-earth (RE³⁺) ions and exploit them for nanophotonic applications. This is due to their unique optical properties, stability, absence of high energy bond vibrations and inertness towards the incorporated NPs. However, conventional methods of metal-glass nanocomposite fabrication involve ion-implantation or sputtering and subsequent heat-treatment under H2, UV-light/X-ray/ γ- or laser irradiation. They are (i) multi-step, (ii) require expensive set-up, (iii) risk of sample damage (iv) formation of NPs only in surface layers. [1]
Here we develop two novel glass-systems K₂O-B₂O₃-Sb₂O₃ and K₂O-B₂O₃-Sb₂O₃-ZnO. Using these hosts, we demonstrate for the first time the strategy for single-step in-situ fabrication of metal NPs within bulk glasses. The selective reducing property of the main component Sb₂O₃ has been used to synthesize a series of novel composites co-embedding metal NPs (elliptical Au, elongated Ag NPs and Au_core-AuAg_shell NPs) and RE³⁺ ions for enhanced upconversion and solar panel applications. [2-4]
RE³⁺ doped glasses are critically important to develop visible lasers, color displays, remote sensing, optical communication, bar-code reading, etc. Neighboring plasmonic metal NPs can modify the radiative decay properties of RE³⁺ ions. The plasmon polaritons propagate along the metal/glass interface. Surface plasmons resonance results in concentration and enhancement of the local electromagnetic field (LFE) or hot-spots around coupled metallic NPs.[5] The luminescent RE³⁺ ion in the vicinity experiences drastic enhancement its excitation due to LFE and the emission rate also increases. [2-4]
We observe that the LFE effect is stronger on electric dipole transitions of the RE³⁺ than the magnetic dipole ones. LFE induced by nano Au enhance the (i) electric dipole 4G5/2 → 6H9/2 636 nm red upconversion of Sm3+ by about 7 fold, (ii) 4S3/2 → 4I15/2 536 nm green and 4F9/2 → 4I15/2 645 nm red emissions of Er3+ by 2 and 5 fold and (iii) 4G7/2 → 4I9/2 540 nm green and 4G7/2 → 4I15/2 650 nm red upconversion emissions of Nd3+ by 9 and 11 fold respectively. LFE induced by nano Ag enhance both the green and red upconversion emission of Er3+ by 8 fold. The Aucore-AuAgshell NPs enhance the red upconversion of Sm3+ only by 2 fold due to smaller LFE effect of bimetallic NPs. [2] These novel hybrid nanocomposites may find applications in advanced displays, light emitting diodes and solar cells. All the Au-doped antimony glasses are dichroic. They transmit the blue light and reflect the brown light, which make them very interesting material comparable to the historic Lycurgus Cup. [3]
References
[1] F. Gonella, Rev.Adv.Mater.Sci. 14 (2007) 134
[2] T. Som, B. Karmakar, Nano Research 2 (2009) 607
[3] T. Som, B. Karmakar, Plasmonics 5 (2010) 149
[4] T. Som, B. Karmakar, JAP 105 (2009) 013102
[5] A. Polman, Science 322 (2008) 868

Converting light to electricity is important for many applications from detectors and sensors to solar cells. Depending upon the application, both resonant and non-resonant optical structures are important. In this talk I will present our recent work on a variety of architectures used to confine and control light on the nanoscale for applications in solar energy. Structures can be designed to act as broadband antireflection coatings, localized couplers to guide modes, and optical concentrators. To surpass the efficiency limit of traditional photovoltaic devices, I will discuss novel new methods using nanoscale optical concentration, photonic crystals to effectively modify the semiconductor bandgap, and the generation and collection of hot electrons in plasmonic structures.

Nanophotonic light-trapping concepts such as photonic crystals, nanowires, grating couplers or plasmonic gratings are widely investigated for achieving high efficiency of state-of-the-art solar cells. Commonly, the applied nanopatterns are optimized by 3D electromagnetic simulations providing a high spatial resolution. In contrast, the experimental evaluation of the performance is in most cases limited to macroscopic quantities like the external quantum efficiency and angular resolved absorptance which do not provide information about the nanoscopic physics of the waveguide mode. The understanding of the impact of local variations of the size, the shape and dielectric properties of the nanostructures however, is inevitable for optimization.
In this contribution, we present Scanning Near-Field Optical Microscopy (SNOM) as an experimental approach to gain access to the electric field intensity of propagating waveguide modes on a nanoscale. We demonstrate a direct visualization of waveguide modes coupled into a periodically nanopatterned test device made of hydrogenated amorphous silicon.
The spectral resonance condition for light, coupled by the nanopattern to an individual waveguide mode, is investigated. Therefore, the intensity of the propagating waveguide mode is measured at various wavelengths. Light coupling is maximized when the periodicity of the electric field intensity of the investigated waveguide mode matches the periodicity of the nanopattern. In case of optimal light coupling, a pronounced resonance in the external quantum efficiency of the solar cell is observed. This way the observed nanophotonic effects are directly associated to the efficiency of the solar cell.

We demonstrate the potential for resonant subwavelength dielectric gratings as highly efficient spectrum-splitting filters for ultrahigh efficiency (>50%) multijunction photovoltaics. Spectrum-splitting multijunction solar cells have the capability to avoid the losses due to lack of absorption of sub-bandgap photons and thermalization of excited carriers to the semiconductor band edge. Such architectures overcome the performance constraints of traditional epitaxial monolithic multijunction architectures by incorporating external optical elements to couple light into individual optimized subcells thereby allowing independently contacted subcells and freer choice of subcell materials. Conventional optical filters consist of aperiodic multilayer dielectric thin film stacks that are fabricated by slow and expensive vacuum deposition processes. Resonant subwavelength dielectric gratings on the other hand are a highly attractive alternative since they exhibit near-unity broadband reflectivity and can be fabricated by inexpensive nanoimprint lithography and etching processes. Additionally, chirping the periodicity gives a varying phase delay across the grating allowing incorporation of spectrum-splitting and concentrating in the same optical element.
The subwavelength feature size of the gratings restricts diffraction to the 0^th reflected and transmitted orders. The high-index grating layer supports modes, each with a cutoff frequency. At frequencies between the first and second cutoffs, the two supported modes can be made to destructively interfere such that no light is transmitted. These gratings require high refractive indices and low absorption, leading us to consider Si, GaP, TiO₂, and ZnS as possible materials.
Rigorous coupled-wave analysis simulations show reflection bands with sharp cut-offs between regions of high and low reflectivity for normally incident unpolarized illumination. A silicon grating in an n=1.5 medium with 500 nm periodicity is shown to demonstrate a long-pass filter behavior exhibiting a 200 nm bandwidth and an average in-band reflectivity >90%. Simulations accounting for realistic dispersion of silicon show it to be a viable grating material down to 600 nm. Decreasing the index-contrast between the grating and medium reduces reflection bandwidth for a comparable GaP reflector to 50 nm. Reflectivity in these gratings smoothly varies with the angle of incidence. At oblique incidence, the two polarizations show offset reflection bands leading to reduced reflectivity for unpolarized light.
In this work, we will present efforts to decrease polarization sensitivity at oblique incidence by optimizing parameters including the tiling, periodicity, fill fraction, and shape of the grating unit. The shape is optimized using an inverse design algorithm while the remaining parameters are addressed using iterative forward design optimization.

Light-matter interaction on a sub-wavelength scale enables unprecedented opportunities to design and optimize optical phenomena. The transition from non-resonant, effective metamaterials to resonant optics requires a deep understanding of field distribution and material structures. However, it can open the possibility for new optical functionalities on a large scale. We investigated linear gratings of high refractive index nanobeams as an effective replacement for traditional antireflection coating layers. First, we explored the Maxwell-Garnett regime. After an optimization of the grating filling fraction, we demonstrated polarization-dependent antireflection properties near the solar peak spectrum. Then, we look at the physics of larger structures, capable to support Mie resonances. We demonstrated that similar impedance matching conditions can be achieved taking advantage of the combination of effective properties along with field confinement effects. In particular, we elucidate the deviations from the Maxwell-Garnett effective medium theory in presence of optical resonances.
Analytical calculations using the transfer matrix method and full-field electromagnetic simulations are performed to explore a wide parameter space. Linear gratings were fabricated on silicon wafers by focused ion beam. Reflectivity measurements were performed using a confocal microscopy. We note that the same concept can be applied on any material platform beyond silicon, including transparent conductive oxides.
Finally, we propose a double layer linear structure as an antireflection coating for unpolarized light in solar cells. We will compare our design with the state-of-the-art random roughness approach, demonstrating that a careful engineered metamaterial outperforms the current benchmarks. Implications on a solar cell device will be presented. Moreover, the impact of the angle of incidence of light will be discussed in view of an omnidirectional broadband antireflection metamaterial.

Silicon solar cells have been dominating the PV industry. Thin silicon represents an exciting direction for future solar cells. Here I will show our recent progress on thin single-crystal Si down to 2 micron: fabrication of thin Si, nanoscale photon management, device physics and remarkable mechanical flexibility and robustness. We demonstrate 10 micron thick Si solar cells with 13.7% power efficiency.

Nanophotonics is opening a myriad of unprecedented opportunities for biomedical applications by localizing light beyond the diffraction limit and dramatically boosting the light-matter interactions at nanoscale dimensions. In this talk, I will introduce a number of novel technologies based on nanoscale control of light and fluidics on a chip. I will show how to overcome some of the fundamental limitations of the state of art techniques used point-of-care diagnostics and cancer treatment.
First, an ultrasensitive detection technology with detection limits surpassing the gold standard surface plasmon resonance (SPR) sensors will be introduced. Practical applications of this technology allowing million fold enhanced multiplexing capabilities with minute sample volumes will be shown. Using a bottom-up approach, I will merge nanofluidic and nanophotonic technologies at sub-wavelength dimensions to break the diffusion barrier. For point-of-care applications, a novel sensing scheme enabling label-free detection of biomarker proteins “with the naked eye” will be introduced. Real world applications of these optofluidic-plasmonic sensors for rapid and reliable detection of whole viruses from biological media will be demonstrated. I will extend this nanofluidic approach to a micro-nano cross flow scheme for isolation of rare circulating tumor cells (CTCs) for cancer diagnostics and prognosis. Potential implications of this technique for the analysis of single tumor cells and their metastatic potential will be discussed.

The emergence of multimaterial fibers that combine a multiplicity of materials with disparate properties into a single fiber presents new opportunities for extending fiber applications beyond optical transmission. Enabled by the ability to co-draw different material, fiber devices with acoustic, electronic and optoelectronic properties of increasing complexity have been demonstrated. Having successfully surmounted the obstacle of incorporating disparate materials into a thermally drawn fiber we have recently set our sights on lifting the axial symmetry which has been a characteristic hallmark of fibers in general. Fluid instabilities generated internal to the fiber were harnessed to produce regular sized semiconductor spheres embedded in the insulating fiber cladding with sizes down to tens of nanometers. In this study we demonstrate how fluid instabilities can be directed to occur selectively in specific domains while leaving others continuous. A fiber that contains three cores: the center one being a chalcogenide flanked by two conducting polymer cylinders none of which touch each other initially. Upon heating, the semiconductor core selectively undergoes capillary break-up while keeping the conducting cores continuous, the semiconductor spheres thus formed expand and contact the two conducting buses forming series of self-assembled spherical photodetectors, connected to continuous electrodes. With sphere separation on the order of ten microns, 100,000 electrically contacted devices per meter are self assembled. We demonstrate that these photodetectors show resonant behavior due to the spherical symmetry of the photoconductive core.

Metallic nanogaps can dramatically enhance infrared absorption of molecules situated inside the nanogaps via near-field coupling with gap plasmons. Plasmonic nanogap structures have been fabricated using various techniques such as electron-beam lithography, focused ion beam milling, and electromigration. One of technical challenges for practical sensing applications is to create single-digit-nanometer metallic nanogap structures with high throughput, low cost, and control over the position and uniformity of hotspots. Here we combine atomic layer lithography and template-stripping method to produce wafer-scale plasmonic nanocavity arrays with precisely defined size, shape, and position. The buried metallic nanocavity is protected by a silicon template, which provides a protection against contamination, and can be template-stripped on-demand just prior to experiments. The exposed plasmonic nanocavities show a series of strong Fabry-Pérot resonance peaks, which can be easily tuned by changing the cavity length and the gap size. The resonances cover the mid-infrared spectral region that is important for molecular fingerprinting. The buried nanocavities tightly confine infrared radiation into gaps that are as small as 3 nm (lambda;/3400), in a dense array of millimeter-long high-intensity hotspots along each cavity. Computational modeling shows that the field intensity (|E|²) enhancement is about 1600 at the end facets of the nanogap cavity. To utilize the strong field inside the nanogap, molecules can be backfilled into nanogaps after partially removing the dielectric layer inside the gap. The vibrational modes of adsorbed molecules strongly couple to gap plasmons and generate Fano resonances, with distinct resonance shapes depending on the relative position of the molecule&’s vibrational bands and the plasmon resonance peaks. After forming a self-assembled monolayer of benzenethiol molecules on the metal surface, we measured an infrared absorption enhancement factor of 100,000. By packing a dense array of single-digit-nanometer gaps with uniform hotspots over an entire wafer, along with the extended storage time against contamination, our technique dramatically reduces the cost of fabrication and will help accomplish reproducible surface-enhanced infrared absorption spectroscopy (SEIRA). In addition, our technique can facilitate broader applications of surface-enhanced spectroscopies, such as combining SEIRA and surface-enhance Raman spectroscopy, and also provide a new platform to study fundamental light-matter interactions inside sub-nanometer gaps.

Optical scatterers such as nano-holes, grooves or slits etched in a metal film are efficient sources of surface plasmon polaritons (SPPs). Interference between SPP waves excited by scatterers can lead to unprecedented control of light at nanoscale, with wide applications, e.g. higher-efficiency solar cells, surface enhanced Raman scattering, and compact biochemical sensors. One example, plasmonic interferometry, has the potential to bring the advantages of conventional optical interferometry to micro- and nano-scale for high-throughput, real-time biochemical monitoring.
Our previous work studied the performance of a specific plasmonic interferometer consisting of a rectangular slit-groove pair etched on a metal film. When a broadband light source is incident on the structure, SPP modes are generated at the groove, propagate along the surface to the slit, then interfere with the original incident beam. Light intensity transmitted through the slit of each interferometer carries information about the metal-dielectric interface, which is open for a wide variety of analysis. Here we propose a new structure for the plasmonic interferometer: a slit-groove pair in the form of concentric rings. This system is significantly more complicated due to the optical resonance properties of the ring geometry, which allows for improved field enhancement and sensitivity while providing polarization-independent optical response and easier on-chip integration.
A systematic study will be reported to understand the underlying physics of this system. Keeping the radius of the inner ring fixed, an array of plasmonic interferometers were fabricated with variable slit-groove distance (i.e., increasing radius difference between the two rings). With two variables - wavelength and the distance between the rings - the output intensity spectra can constitute a 2D color map. At certain wavelengths, full widths at half maxima of the normalized transmission spectra were improved due to the optical resonance effect of the structure. Comparing with a rectangular slit-groove plasmonic interferometer, this improvement makes it a better candidate for sensing applications. In addition, we developed an empirical model for the structure, based on SPP propagation and interference. A comparison between experiments, the model and FDTD simulations will be discussed. With full knowledge of the system, we designed a ring slit-groove plasmonic interferometer with optimized parameters for glucose detection in saliva, as a proof of concept.
In conclusion, the mechanism for a marriage of SPP excitation and resonance is illustrated by studying the optical performance of a ring slit-groove plasmonic interferometer array. The effects of varying different key parameters on the optical property of plasmonic interferometers will be discussed in detail. Our findings show that plasmonic interferometry can provide accurate detection of low-concentration biomedical analytes in a small sample volume.

Surface plasmonic polaritons (SPPs) are electromagnetic modes that exist at the interface of metallic and dielectric materials and are a result of sign differences in the dielectric constant of the two materials. It is well known that nanostructures used as optical scatters on the surface of a metal film are efficient sources of SPP waves. By carefully leveraging the arrangement of these nanostructures, the induced SPPs can be used to guide and concentrate electromagnetic energy at a specific localized position because the propagating SPP modes are able to constructively interfere with one another. Such structure, known as a plasmonic concentrator, has a wide variety of applications, including subwavelength focusing, photovoltaics, optical trapping, and bio-sensing.
Here, we theoretically design and experimentally demonstrate a variety of plasmonic concentrators obtained by changing the positions and types of nanostructures (e.g., nano-holes, -slits and -grooves) on the surface of a thin metal film. An analytical model complementing 3D FDTD simulations is used to guide the arrangement of the nanostructures to maximize the enhancement of localized electromagnetic fields. The designed nanostructures are then fabricated in a thin metal film by using focus ion beam (FIB) milling. Finally, optical experiments are performed to show that the fabricated plasmonic concentrators can be used to trap nanoparticles and it also has a high performance on bio-sensing, such as measuring low concentration of glucose or insulin. The use of such plasmonic concentrators to enhance the sensitivity of high-efficiency Ge QD-based photodetectors will also be reported.

Nanopores enable label-free detection and analysis of single biomolecules. Here, we investigate DNA translocations through a novel type of plasmonic nanopore based on a gold bowtie nanoantenna with a solid-state nanopore at the plasmonic hot spot. Plasmonic excitation of the nanopore is found to influence both the sensor signal (nanopore ionic conductance blockade during DNA translocation) and the process that captures DNA into the nanopore, without affecting the duration time of the translocations. Most striking is a strong plasmon-induced enhancement of the rate of DNA translocation events in lithium chloride (LiCl, already tenfold enhancement at a few mW of laser power). This provides a means to utilize the excellent spatiotemporal resolution of DNA interrogations with nanopores in LiCl buffers, which is known to suffer from low event rates. We propose a mechanism based on plasmon-induced local heating and thermophoresis as explanation of our observations.

Nanoplasmonic structures may exhibit nonlocal response when shrinking their critical dimensions to a regime where quantum phenomena are commonly expected to become important. This talk will address a semi-classical hydrodynamic description as a first natural attempt that goes beyond the common description based on the local-response approximation. Including the hydrodynamic response of the Fermi gas adds an intrinsic length scale to the electrodynamics, thus smearing the possible development of singular response inherent to a local-response approximation. The theory is used to explain size-dependent frequency shift and line broadening in individual metallic nanoparticles and the gap-dependent broadening in dimers. Interestingly, the semi-classical gives a good account of the plasmon dynamics even deep into the anticipated ‘quantum regime&’ where dimers separated by sub-nanometer gaps. Turning to graphene flakes, we compare semiclassical and quantum descriptions of the plasmon response, focusing again on the size-dependence of plasmonic resonances. In particular, we find that nonlocal effects in absorption cross sections scale similarly with size as do effects of quantum mechanical edge states. Interestingly, atomic-scale details and edge-state contributions are important for the optical properties even for graphene flakes as large as 20 nanometers.

We derive tight upper and lower bounds of the ratio between decay rates to two ports from a single resonance exhibiting Fano interference, based on a general temporal coupled-mode theory formalism. The photon transport between these two ports involves both direct and resonance-assisted contributions, and the bounds depend only on the direct process. The bounds imply that, in a lossless system, full reflection is always achievable at Fano resonance even for structures lacking mirror symmetries, while full transmission can only be seen in a symmetric configuration. The analytic predictions are verified against full-field electromagnetic simulations.
We consider the resonant antireflection scheme, where a periodic array of resonant subwavelength objects is placed at an air-material interface in order to cancel reflection that otherwise would have occurred at such an interface. Using both general theoretical arguments and analytical coupled-mode theory formalisms, we show that the resonant antireflection scheme can achieve perfect antireflection, provided that the periodicity of the array is sufficiently small, and the resonances in the subwavelength objects radiate into air and the dielectric material in a balanced fashion. We validate the theory using full-field electromagnetic simulations.
We will highlight the connection between the two results, and in addition, we will comment on the implication of the fundamental bound on optical isolation.

Multipolar resonances in sub-wavelength nanostructures enable a variety of metamaterial phenomena and devices. These multipolar light-matter interactions depend on two critical factors: 1) the physical properties of the nanostructure, and 2) the properties of the illuminating radiation. Conventionally, researchers focus on the former and engineer the resonant properties of nanoparticles (NPs) by manipulating their size, shape, and composition. Here, we concentrate on the latter and demonstrate via electromagnetics calculations how unconventional light sources can be used to to selectively excite and enhance coupling to specific multipolar resonances.
Our studies focus on high refractive-index spherical nanoparticles (NPs). As explained by Mie theory, such NPs exhibit a series of multipolar resonances when illuminated by a plane wave. Building from this framework, we describe an analytical method for determining the optical response of NPs illuminated by any focused beam of interest. The illuminating source is decomposed into an angular spectrum of plane waves, and the NP response is determined by integrating over this angular spectrum. We compare the scattering spectra of a spherical NP under conventional linearly polarized (LP) illumination to the spectra obtained from more exotic types of illumination, namely radially polarized (RP) and azimuthally polarized beams (AP). These studies reveal scattering spectra that differ significantly under each illumination condition. For instance, we demonstrate the complete suppression of magnetic (electric) modes under RP (AP) illumination. Additionally, we demonstrate how the orientation of the excited modes varies under different illumination conditions. In particular, we describe a new type of quadrupole excitation inaccessible by LP illumination. In contrast with conventional transverse quadrupoles (i.e. side-by-side dipoles), we show that RP and AP beams excite longitudinal quadrupole configurations (i.e. end-to-end dipoles). We subsequently describe how the multipolar light-matter interaction can be expressed as the product of illumination-independent multipole moments and specific particle-independent field quantities. We show that the electric dipole and electric quadrupole modes are driven by the electric field amplitude and electric field gradients respectively, with similar relations for the magnetic modes. This perspective enables a simplified approach for calculating light scattering and absorption in more complex illumination conditions. For instance, we show how this approach can be used to easily calculate the scattering response of particles located off-axis from a focused beam. This work enables a new understanding of light-matter interactions in metamaterials, and lays the foundation for researchers to identify, quantify, and manipulate multipolar light-matter interactions through optical beam engineering.

Confinement of surface plasmons in sharply faceted systems, most notably probe tips, can enable electromagnetic field enhancement for applications in sensing and nanophotonics. Although well-understood at macroscopic scales, the surface plasmon response at atomic length scales deviates from purely classical models; effects including the nonlocality of the dielectric response as well as the spill-out of electrons impact the optical response of these systems. Previous efforts to model sub-nanometer length scale nonlocality in semiclassical hydrodynamical models have used empirical corrections to Maxwell's equations, and neglect electron spill-out. We study the nonlocal response of atomically-sharp systems from a first-principles approach void of fitting parameters, in a model which implicitly includes nonlocality and in which electron density spill-out is an emergent feature. By considering the linear frequency response of jellium cones with cylindrical symmetry, we lower our computational cost from comparable three-dimensional models, enabling calculations closer to experimentally relevant length scales. We present these calculations, and discuss their implications for sub-wavelength concentration of light in tips for sensing applications.

The impressive recent advances in nanofabrication has made possible the observation of quantum effects in small metal nanostructures. In this case, plasmon excitations have a fundamental role, and that is still not completely clear. The main issue resides in the quite hard and numerically expensive many-body problem for a large number of atoms contained in a metal nanoparticle. In addition, are almost lacking analytical formulations.
We will show our recent research on the transition between classical and quantum regime in systems such as metal nanoparticles interacting with light when the excitation of localized plasmons plays an important role. Particularly, we will highlight that similar effects are possible in polar crystals at lower energies (infrared) due to the phonon localization.