Session E12: Nanostructures and Metamaterials 4

Modeling non-equilibrium thermal radiation phenomena using a direct simulation method

Thermal radiation is a field of physics that finds rich applications in energy efficiency, energy conversion, and related topics. It is well-understood for bulk materials in thermal equilibrium through the application of Kirchoff’s law. Nonetheless, the recent emergence of nanoscale materials with exotic physical properties (such as violation of detailed balance) reveals that these systems can in general depart from thermal equilibrium, particularly over short time scales. These findings give rise in turn to two challenging questions: (1) How can we describe non-equilibrium radiative transport far from equilibrium (i.e., in a non-perturbative regime)? and (2) what are the fundamental limits, if any, on cooling in this regime? To begin to address both of these questions, we develop a first principles-based model of thermal radiation, based on the fluctuation-dissipation theorem. After verifying that it reproduces Kirchoff’s law in thermal equilibrium in the far field, then apply it to radiative transfer in the near field. This provides limits on radiative transport which are distinct from the equilibrium analysis. The potential effects of the presence of nonlinear media (e.g., Kerr media) are also investigated using this framework. Furthermore, we find that the presence of non-equilibrium states also significantly impacts the physics of related problems, such as the Casimir force.   

Observation of weak value amplification in polarization-tailored Fano interference in waveguided plasmonic crystals

Resonance arising from the interference of a continuum and a discrete mode is known to give rise to a typical asymmetric spectral line shape and is called Fano resonance. We present a systematic study of Fano resonance in a variety of precisely designed wave guided plasmonic crystals that consists of metal nano-patterns deposited on ITO waveguide. Using a custom-designed dark field microscopy setup we record the Muller matrix spectrum of scattered light, that contains the entire polarization response of the device under study. We demonstrate large variation of the asymmetric line shapes of scattered light, tuned via pre- and post-selection of polarization states. We present a phenomenological model to quantify the dramatic control of spectral asymmetry in scattered intensity from Fano resonant systems in terms of physically accessible parameters of interference.The approach based on pre and post selection of optimized polarization states of light is further used to develop a novel concept of 'weak measurement' using the asymmetric spectral line shape of Fano resonance as a natural pointer generated within the system. The details of the results will be presented and their implications towards development of novel polarization controlled photonic devices will be discussed.   

Metamaterials with index ellipsoids at arbitrary k-points

Ordinary materials have their index ellipsoids centered at k = 0. We propose a new type of metamaterial possessing multiple index ellipsoids centered at arbitrary nonzero k-points. Their locations in momentum space are determined by the connectivity of a set of interpenetrating metallic scaffolds that constitute the metamaterial, independent of the detailed geometry of the scaffold, which determines the group velocity of the modes. The existence of the quasi-static modes at non-zero k-points can be understood by solving Poisson’s equation and can be viewed as a real-space structure induced synthetic gauge potential that shifts the light cone in momentum space. Such metamaterials have broadband functionality as the long wavelength properties arise from global network connectivity rather than local resonance. For example, when interfaced with ordinary materials, they have an orientation-dependent coupling effect that can be used to design broadband directional antennae. We performed microwave experiments to confirm our findings.   

Local phase method for designing highly efficient metasurface devices

Recently, metasurfaces have emerged as a powerful and versatile paradigm for making many optical devices especially optical lenses. However, the approach usually used to engineer metasurface devices assumes that neighboring elements are identical, by extracting the phase information from simulations with periodic boundaries, or that near-field coupling between particles is negligible, by extracting the phase from single particle simulations. This is not the case in general and the approach thus prevents the optimization of devices that operate away from their optimum. Here, we propose a versatile numerical method named local phase method (LPM) to obtain the phase of each element within the metasurface while accounting for near-field coupling with different neighbors. Quantifying the phase error of each element of the metasurfaces with the proposed local phase method paves the way to the design of highly efficient metasurface devices including, but not limited to, deflectors, high numerical aperture metasurface concentrators, lenses, cloaks, and modulators.   

Revisiting the parking problem under the light of random plasmonic arrays and metasurfaces

For a long time, random and complex media have been solely considered as a nuisance for controlling waves. Development of techniques such as time reversal, wave shaping with SLM and SMM have allowed to harness the very high numbers of degrees of freedom that those media intrinsically support. Hence random media were shown to improve focusing compared to free space or to increase the bit rate in telecommunications, to create media with very large bandgaps or to increase the absorption. Lastly metasurfaces, 2D devices designed at the subwavelength scale have brought considerable attention in photonics. Their capabilities to control the phase and the amplitude of waves lead to the design of flat lenses, holograms... Metasurfaces are designed in a periodic framework. To break this paradigm and harness the disorder for metasurfaces, processes like the near field coupling between elements as well as the packing of random elements must be thoroughly investigated. Here, we present our study of rectangular plasmonic particles of the order of 150 to 500 nm long, dispersed on a 2D plane. We investigate the maximum density achievable as a function of their aspect-ratio, as well as the local order and their optical consequences.   

Unconventional Dirac Polaritons in Cavity-Embedded Honeycomb Metasurfaces

Pseudorelativistic Dirac quasiparticles have emerged in a plethora of artificial graphene systems that mimic the underlying honeycomb symmetry of graphene. However, it is notoriously difficult to manipulate their properties without modifying the lattice structure. Here we theoretically investigate polaritons supported by crystalline honeycomb metasurfaces. Despite the trivial dipolar nature of individual resonant elements, we unveil rich Dirac physics stemming from a non-trivial winding in the light-matter interaction. As a result, a new kind of type-II Dirac point emerges which exists simultaneously with its conventional type-I counterpart. By modifying only the photonic environment via an enclosing cavity, one can manipulate the location of the type-II Dirac points, giving rise to distinct polariton phases. This striking tunability enables one to alter the fundamental properties of the emergent Dirac polaritons while preserving the lattice structure - a unique scenario which has no analog in real or artificial graphene systems. This new paradigm of exploiting the photonic environment will markedly expand the realm of Dirac physics at the subwavelength scale.   

Thermally Stable Emitters Selected by Pareto Optimization for Solar Thermophotovoltaics

A central idea of solar thermophotovoltaics (STPV) is to spectrally tune the solar spectrum to better match the bandgap of a target PV cell in order to exceed the Shockley-Queisser limit of ~33%. We propose a new design of the selective emitters to tailor thermal emission by exploiting critical coupling between reflection in a Bragg reflector and absorption in optically tunable layer. After Pareto optimization was applied to computationally identify a small number of record-setting emitter structures, we fabricate several multilayer stacks in which the Bragg reflector is composed of SiO₂ and TiO₂ layers and the tunable absorber is a W-Al₂O₃ alloy with variable volume fraction. Plasma-enhanced atomic layer deposition (PE-ALD) allows precise control over each layer’s thickness and density as well as the alloy composition. We evaluate the spectral efficiency and spectral density of each structure and compare it to computational models. We further investigate the stabilizing effect of ALD oxide overlayers on underlying layers that are otherwise prone to oxidation. The multilayer stacks exhibit excellent thermal stability upon repetitive annealing at 1000 °C under inert atmosphere.   

Actively Controlled Purcell Enhancement of Colloidal Quantum Dots in Gated Plasmonic Heterostructures

Controlling light emission of quantum emitters, such as semiconductor quantum dots, is a central theme of nanotechnology. Typically, the emitted power from an array of quantum emitters is modulated by changing the optical or electrical pump intensity, within a given nanostructured environment. We propose and demonstrate a conceptually different approach to dynamically control the power radiated by the emitter. We experimentally demonstrate the modulation of the radiative emission rate, the emitted power, and the quantum efficiency, at constant optical pump intensity, via dynamical changes to the local density states. The local density of states is modulated by changing the carrier density, under field effect gate control, in a plasmonic titanium nitride layer nearby the emitters. Our proof-of-principle experiment, which uses a TiN/SiO₂/Ag plasmonic heterostructure, demonstrates a new active plasmonic mechanism for modulating visible light emission that is extensible to other type of quantum emitters. Moreover, it also may pave a path towards ultrathin LCD-free displays with long durability, reduced pixel size and large viewing angle.   

Virtual Design and Analysis of Pareto Optimal Emitter Structures for Thermophotovoltaic Applications

In this work, we focus on composite planar nanostructures that leverage the interaplay between two resonant phenomena that can be realized in simple planar nanoelements: resonant absorption in weakly-absorbing nanoscale films and reflection resonances in multi-layer dielectric stacks (Bragg Reflectors). The interplay between these resonances enables spectral tunability of the composite nanostructures, and yields structures whose thermal emission properties approach the ideal limit of a step-function emitter.
We combined rigourous electrodynamics calculations with a virtual screening technique based on Pareto optimality to identify a small number of promising structures from a search space of more than 5 million structures, and we have begun experimentally realizing and characterizing structures from this small subset. The results we will report on are an important step towards realizing efficient and robust structures for solar and thermal energy conversion.   

Plasmonic Metamasks for Photopatterning of Arbitrary Molecular Orientations

Spatially variant molecular orientations are central to the functionalities of various liquid crystal applications such as Pancharatnam lens, q-plate, command of active matter and programmable stimulus-responsive morphing of liquid crystal polymers. In this talk, we will present a photopatterning technique which allows for high resolution and high throughput patterning of arbitrary molecular orientations by using plasmonic metamasks. Unlike tradition photomasks, the plasmonic metamasks would generate structured light with spatially variant patterns of both intensity and polarization orientations. By projecting such structured light onto photoactive thin films of azo-dyes, orientation patterns can be imprinted in the azo-dyes and then transfer to the bulk of the liquid crystals. A particular aspect of this talk will be new metamask designs for both i-line and g-line photo exposures with high optical transmissions.   

Plasmonic Heating: Efficient and Controlled Heating at the Nanoscale

Heat dissipation and temperature distribution is an essential aspect of nanosystems which can be exploited in a verity of science and engineering applications. It has been always a challenge to bring heat under control in terms of efficiency and localization at the nanoscale. Here, we present theory and simulation of three different nanosystems in which controlled heat generation can be used for specific purposes. First, optimal geometry of arrangement of gold nanoparticles is introduced to form hot spots using continuous laser illumination. It will be proved that Fano resonance is required to make a nano-heater both energy efficient and localized in temperature. Second, we focus on periodic shuriken-like gold indented nanolayers. Simulation of temperature distribution under pulsed laser illumination will be discussed. This structure can be used for selective chemistry and water heating. At last, we introduce methodological approach for the heat generation of ensemble of nanoparticles injected into porcine skin. The theory can be applied for the simulation of thermal imaging and temperature sensing in biological systems.   

Active Metasurfaces for Dynamic Polarization Conversion

Polarization is an important characteristic of electromagnetic waves that has a significant impact on number of applications such as molecular analysis, and quantum communications. Here, we demonstrate that the polarization state of the reflected light can be actively controlled by using indium tin oxide (ITO)-based tunable metasurfaces. The proposed metasurfaces consist of an aluminum back reflector, a 20-nm-thick gate dielectric layer followed by a 5-nm-thick ITO layer on which we fabricate an aluminum nano-antenna array. When applying an electrical bias between the ITO layer and back reflector, the carrier concentration at the gate-dielectric/ITO interface is modulated, resulting in the change of the effective index of the ITO layer. It alters the interaction between the induced plasmonic modes, leading to the modulation of polarization state of the reflected light at telocom wavelengths. By suitably biasing the metasurface structure, the linearly-polarized incident light can be converted to a cross-polarized, circularly-polarized or elliptically-polarized light. This dynamic control of the amplitude, phase as well as the polarization state of the scattered beam provides prospects for various applications, such as adaptive wavefront control, signal monitoring and detection.   

High Power Density Thermionic Energy Conversion Using Nanostructured Electron Optical Grids

Solid state thermionic generators offer a promising route towards highly efficient, direct heat-to-electricity conversion, but their performance has historically been limited by space charge accumulation within the vacuum gap of the converter. Recent approaches to mitigating space charge using positively biased electrostatic grids have suffered from intrinsic efficiency limits arising from grid loss. Here, we describe the successful design, computational modeling, and experimental realization of a nanostructured electrostatic lensing system which accelerates thermionically emitted electrons across the vacuum gap while minimizing grid loss. Using kinetic particle-in-cell electron dynamics simulations, we computationally designed a panel of collector devices with significant enhancements in predicted power density and electronic efficiency over conventional diode converters. The devices were fabricated on semiconductor substrates and characterized in ultra-high vacuum experiments under a Ba dispenser cathode. We report experimental grid loss measurements for these devices, as well as quantitative agreement between simulation predictions and experimental results over a wide range of device configurations.