Session B12: Nanostructures and Metamaterials 2

Nonlinear behavior in hybrid optical resonators

Whispering gallery mode optical resonators have found numerous applications spanning from basic science through applied technology due to their intrinsic ability to amplify an input optical field. Because these devices confine light at the periphery, the circulating optical power directly interacts with both the cavity material and cavity surface. Therefore, gain can be either directly doped into or coated onto the cavity. Here, three different hybrid devices are fabricated from silica resonators.

We have synthesized a suite of metal-doped silica (Zr, Ti) using the sol-gel process and fabricated devices using UV-lithography. The intercalation of the metal in the silica matrix increases the Raman gain coefficient by changing the polarizability of the silica. As a result of the increase, the Raman lasing threshold and Raman efficiency both decrease. The threshold is sub-microWatt and the efficiency is over 40%.

We have synthesized a suite of organic, highly nonlinear optical small molecules to enable high performance Raman lasers and frequency combs. By grafting the small molecule directly to the device surface, the overall nonlinearity of the device is increased. Using this strategy in combination with high Raman gain small molecules, we have demonstrated ultra-low threshold Raman lasers. When combining this approach with high Kerr coefficient materials, we can achieve high efficiency Kerr frequency combs.

By combining metal nanorods with small molecule coatings, we can push the small molecule enhancement further. By coating an optical cavity with small molecule functionalized metal nanorods, a plasmonic-photonic mode is formed. This hybrid mode focuses the optical field into the organic layer, improving the light-nanomaterial interaction. Using this strategy, wide span frequency combs with low thresholds are demonstrated. Additional nonlinear behaviors, such as Anti-Stokes/Stokes generation, are also observed at low input powers.   

Unique Hot Carrier Distributions from Scattering-Mediated Absorption

Light-initiated generation of energetic carriers has attracted attention as a paradigm for photocatalysis and solar energy conversion, and the use of noble metal nanoparticles that support LSPR has been widely explored as a medium for realizing this paradigm. It was recently shown that composite nanostructures enabling the interplay between dielectric scattering resonances and broadband absorption in small metal nanostructures, a phenomenon termed scattering-mediated absorption (SMA), can be used to mediate energetic carrier transfer and selective photochemistry with low-intensity light. We will present results from a multiscale modeling approach for elucidating the hot carrier dynamics initiated by SMA. Our calculations reveal that unique hot carrier distributions and dynamics arise from SMA compared to plasmon excitation, and suggest that it may lead to more efficient hot carrier generation than plasmon resonance under the same external illumination conditions. These results are instrumental in understanding the phenomena of scattering-mediated hot carrier generation, which has potential for expanding the palette of materials that can be utilized for hot carrier mediated photochemistry beyond plasmonic metals and for enabling unique pathways for photocatalytic transformations.   

THz response of High Tc Layered Superconductors from a Microscopic Approach

We studied theoretically the optical properties of a high-T_c layered superconductor material joined by identical alternating Josephson junctions at the microscopic level. The layered high-T_c superconductor is modeled as a periodic photonic crystal with a Josephson junction in the unit cell. In this inherently anisotropic material, the Josephson effect introduces a characteristic dynamical frequency, typically in the terahertz (THz) range. To describe the electromagnetic response, we solved the Maxwell equations inside the crystal taking into account not only the supercurrents due to the Cooper pair tunneling, but also the quasiparticle currents flowing parallel and perpendicular to the superconducting layers. This approach provides the dispersion relation for electromagnetic waves in the system, as well as the components of its nonlocal effective permittivity tensor. A general homogenization method which is valid beyond the long wavelength limit is applied to calculate the components of such a tensor. The homogenization approach also allows us to calculate and analyze the THz reflectance and transmissivity of a layered superconductor slab without using additional boundary conditions.
  

Strong Coupling of Organic Excitons to Surface Lattice Resonances

We report on the fabrication and spectral characterization of films of dyes and dye-doped polymers deposited on top of 2D periodic arrays of gold nanoparticles. Coupling between the dye excitons and the plasmonic surface lattice resonances (SLRs) of the gold particle arrays is investigated by measurement of angle resolved transmission and reflection spectra, and of photoluminescence spectra and images. These results are analyzed to determine the degree of coupling between the organic excitons and the SLRs of the arrays, and to elucidate the nature of the resulting hybrid states, including their dispersion relations. In addition, the effect of coupling on the rate of energy transfer between spatially separated layers of donor and acceptor dyes is examined to determine if the energy transfer is enhanced, as was observed in donor and acceptor films strongly coupled to an optical microcavity. Finally, coupling between the plasmonic arrays and supramolecular aggregates of dye molecules is also investigated. Based on these results, the potential application of SLR-supporting arrays in organic thin film solar cells is discussed.   

Negative Refraction in Bimetallic Layered Nanostructures

We theoretically predicted and experimentally confirmed the presence of a negative dispersion pass band in the ultraviolet (UV) optical spectra of a bi-metallic layered nanostructure for p-polarization electromagnetic waves. The bi-metallic nanostructure is in fact a one-dimensional photonic crystal having two metallic layers in its unit cell, namely, aluminum (Al) and silver (Ag). In the lab, the metallic layers were deposited on a quartz substrate by sputtering and the optical spectra were obtained by using a fluororeflectometer for measuring the reflection and transmission of UV and visible light. In regards to the theoretical development, a homogenization technique to calculate not only the components of the nonlocal effective permittivity tensor for the inherently anisotropic crystal, but also its optical spectra was proposed and applied. A negative-refraction pass band was theoretically found and experimentally observed just above the Ag plasma frequency where the parallel and perpendicular components of the permittivity tensor have opposite signs. In this pass band the dispersion relation of the material is a hyperbolic equation in the wave vector space k and, therefore, such a nanostructure behaves as hyperbolic metamaterial with negative refraction.
  

Self-Hybridization within non-Hermitian Localized Plasmonic Systems

Localized surface plasmons resonances occur when a metallic nano-particle is excited by an external electric field. Within the quasi-static limit, Ouyang and Isaacson have shown that the plasmon modes are the solutions of an eigenvalue problem. Depending on the geometry of the particle, this eigenproblem can be non-Hermitian. Apart from bringing additional mathematical difficulties, this particularity of the plasmonic eigenproblem has always been considered as a mere computation detail.
In the present work, we demonstrate that, actually, plasmonic non-Hermiticity has dramatic physical consequences. We show that the bi-orthogonality can trigger the interaction between different plasmon modes within a single particle. Moreover, we relate the splitting energy of this self-hybridization phenomenon to the overlap matrix of the system. This way, we demonstrate that the measurement of the coupling energy enable us to directly measure the degree of bi-orthogonality of the plasmonic system. We unambiguously determine the role of the particle symmetries in the apparition of non-Hermiticity. Using electron energy loss spectroscopy and e-beam lithography, we experimentally realize a self-hybridization in a dagger-like system with a strong experimental coupling energy.   

Tunable Strong Light-matter Interaction in Plasmonic Nanocavities

Probing the strong coupling between quantum emitters and an optical cavity is crucial to both fundamental studies in cavity-quantum electrodynamics, as well as potential applications such as quantum information processing, low threshold lasing and ultrafast single-photon switching. Compared to conventional dielectric cavities, plasmonic nanocavities have the advantage of ultra-small mode volumes and tunable resonances. In this work, we report the observation of strong coupling between dye molecules and film-coupled plasmonic nanocavities. A large splitting energy up to 160 meV is achieved at room temperature. Additionally, a distinct anti-crossing behavior with two prominent branches has been observed when tuning the plasmon resonance, which is a signature of the presence of strong coupling. Moreover, we show that the coupling strength can be tuned by varying the number of molecules that are coupled to the plasmonic cavity or by changing the dimensions of the cavity. Integrating these hybrid nanostructures with active materials is promising for the design of novel active nanophotonic devices operating at room temperature.   

Manipulating Exciton Superradiance Through Disorder in Supramolecular Dye Aggregates

Photosynthetic antennae and organic electronic materials use topological, structural and molecular control of delocalized excitons to enhance and direct energy transfer. Interactions between the transition dipoles of individual chromophore units allow for coherent delocalization across multiple molecular sites. This delocalization, for specific geometries, greatly enhances the transition dipole moment of the lowest-energy excitonic state relative to that of the chromophore and can greatly increase the radiative rate, a phenomenon known as superradiance. In this talk I discuss ordered, self-assembled Light Harvesting Nanotubes (LHNs) that display excitation-induced photobrightening and photodarkening. These changes in quantum yield arise due to changes in energetic disorder, which in turn increases/decreases excitonic superradiance. Through a combination of experiment and modeling, I show that intense illumination induces different types of chemical change in LHNs that reproducibly alter absorption and fluorescence properties, indicating control over excitonic delocalization. Ialso show that changes in spectral width and shift can be sensitive measures of system dimensionality, illustrating the mixed 1-2D nature of LHN excitons.   

Resonant Characteristics of Subwavelength Spherical Scatterers with Radial Inhomogeneities

Scattering by a homogeneous small sphere is a classical canonical problem, that can be found in the heart of many physical models [1]. The problem of a subwalength homogenous sphere can be rigorously analyzed through the corresponding electrostatic scattering point, for homogeneous or piecewise homogeneous spheres [1]. In this work the electrostatic scattering problem by a radially inhomogeneous sphere is considered. Specifically, it is shown that the analytical solution obtained for a graded index sphere with power law permittivity profile (rⁿ) [2] can be expanded for a whole new class of exponential profiles exp((ar)ⁿ). The scattering features are extracted and connected with the polarizability of a solid sphere. As a result, a generalization of the polarizability is introduced, encompassing the particular inhomogeneities to a simple model, and the impact of such inhomogeneities to the overall scattering spectrum is exposed.

[1] C. F. Bohren and D. R. Huffman, Absorption and scattering of light by small particles. John Wiley & Sons, 2008.
[3] L. Dong, G. Gu, and K. Yu, “First-principles approach to dielectric response of graded spherical particles,” Phys. Rev. B, vol. 67, no. 22, p. 224205, Jun. 2003.   

Electrically Small While Acoustically Large Meta-Molecules

Inspired by the natural piezoelectric effect, we introduce hybrid-wave electromechanical meta-atoms and meta-molecules that consist of coupled electrical and mechanical oscillators with similar resonance frequencies. We propose an analytical model for the linearized electromechanical scattering process, and explore its properties based on first principles. We demonstrate that by exploiting the linearized hybrid-wave interaction, one may enable functionalities that are forbidden otherwise, going beyond the limits of today's metamaterials. As an example we show an electrically deep sub-wavelength dimer of meta-atoms with extremely sensitive response to the direction-of-arrival of an impinging electromagnetic wave, this extreme sensitivity stems from the fact that the dimer is acoustically very large and thus gives rise to unique electromechanical resonances. This scheme of meta-atoms and molecules may open ways for metamaterials with a plethora of exciting dynamics and phenomena that have not been studied before with potential technological implications in radio-frequencies and acoustics.   

Model for Describing Plasmonic Nanolasers using Maxwell-Liouville Equations

We present a theoretical study of lasing action when plasmonic metallic structures that show lattice plasmon resonances are embedded in a gain medium. Our model combines classical electrodynamics for arrays of gold nanoparticles with a four-level quantum Liouville model of the laser dye photophysics. Numerical solution was implemented using finite-difference time-domain calculations coupled with a finite difference solution to the Liouville equation. A particular focus of this work is the influence of dephasing in the quantum dynamics on the emission intensity at the threshold for lasing. We find that dephasing in the quantum system leads to reduced lasing emission, but with little effect on the long term population inversion. We also examine rate equation models with and without constraints arising from the Pauli Exclusion Principle. There are factor of two differences between the Maxwell-Liouville results (greater emission intensities and narrower widths) compared to the corresponding results of rate-equation models of the dye states, which indicates the importance of using the Maxwell-Liouville approach in modeling these systems.   

Phonons and excitons for omnipolarization surface waves

Tailoring near-field optical phenomena often requires excitation of surface plasmon or surface phonon polaritons, surface waves occurring at the interface between media with opposite dielectric permittivities. Despite the unprecedented mode confinement of these waves, they are limited by polarization; only transverse magnetic (TM) fields enable their excitation. We demonstrate that high-permittivity materials can support transverse electric (TE) surface-confined propagation, for which we investigate polar dielectrics near their phononic resonances (Reststrahlen bands) and semiconductors near their excitonic resonances that lie at infrared and visible frequencies, respectively. By combining those with plasmonic materials in subwavelength bilayers, we identify regimes where simultaneously TE and TM phase-matched surface waves occur, with figures of merit and confinement factors comparable to plasmons. We introduce a general approach for surface waves computations, applicable to any finite or infinite layered system, with arbitrary individual layer thicknesses, and arbitrary (periodic or aperiodic) sequence of layers, by combining the reflection pole method with parameter-retrieval approaches, traditionally employed for determination of metamaterials’ effective parameters.