Realizing quasi-monochromatic switchable thermal emission from electro-optically induced topological phase transitions.

Explorations into the photonic analogs of topological materials have garnered significant research interest due to their application potential. Particularly in planar systems, the prospects of engendering extinguishable topological states can have wide-ranging implications. With an objective of employing these concepts for thermal emission engineering, here, we design and numerically investigate a quasi-monochromatic highly directional mid-infrared source elicited from inversion symmetry-protected topological interface states. Notably, by relying on the architecture of electro-optic effect-induced topological phase transitions, we introduce the possibility of ultrafast switching of thermal radiation. These reversible phase transitions, being free from carrier transport are inherently fast and evoke thermal emission modulation with a modulation depth upto 0.99. Specifically, our platform exhibits a near-perfect extinguishable spectral emission peak at [Formula: see text]m with a quality factor of over 18500, displaying negligible parasitic emissions. Furthermore, the optimized interface state manifests itself for only one of the polarization modes, resulting in polarized emission under resonance conditions. To establish a methodical approach to parameter optimization, we also model our platform as a leaky mode resonator using the framework of temporal coupled-mode theory. We believe, our findings can provide a way forward in establishing complete control over the optical characteristics of the infrared thermal emitters.

Enhanced Visualization of Latent Fingermarks on Rough Aluminum Surfaces Using Sequential Au and Zn/ZnS/ZnO Depositions.

Detection and visualization of fingermarks on rough and diffuse surfaces is a relatively challenging task. We succeeded in developing latent fingermarks on scratched and rough aluminum surfaces by sequential deposition of a thin layer of gold followed by one of zinc or zinc-based compounds on the fingermarks. The best image enhancement was achieved with sequential Au and ZnS depositions. Using this combination, we could enhance the visualization of latent fingermarks aged over 65 days in normal conditions. The optical reflectance from the fingermarks with the deposited layers of metal/dielectric is analyzed as a stratified medium. Significant contrast in the reflectance from the regions of the ridges and the valleys of the fingermark would enhance the visualization. Our results show that the Au and ZnS bi-layer combination can have a large reflection contrast and improved fingermark visualization at wavelengths corresponding to the green light for specific thickness of ZnS.

Optical limiting by nonlinear tuning of resonance in metamaterial absorbers.

Metamaterial resonant absorbers (MRA) have intense local field enhancements that can be used to elicit large nonlinear responses. An MRA, composed of gold disks separated by a ZnS thin film from an underlying gold layer, shows optical limiting for the reflectivity of 8 ps pulses at 1064 nm due to the Kerr nonlinearity of gold and ZnS. Fluorescence from multiphoton absorption due to large fields localized in the ZnS layer is measured, and the effective χ(3) of the layer is enhanced by 3 orders of magnitude compared to bare ZnS thin films. Self-consistent nonlinear electromagnetic simulations confirm that the nonlinear absorption is caused by a shift of the resonance with increasing intensity.

Reconfiguring gratings of slanted plasmonic nanocolumns by ion beam irradiation.

Gratings with slanted plasmonic nanocolumns of silver (Ag) on top fabricated by physical vapor deposition at large oblique angles on predefined gratings show unique plasmonic properties. These aligned nanocolumns with high-aspect ratios can be uniformly re-oriented to any desired angle of slant by ion beam irradiation. A focused ion beam (FIB) has been used as the ion source here. The plastic deformation of the nanocolumns arises due to defect formation caused by the energetic ions and can enable the complete tuning of the photonic and plasmonic properties of the grating through the slant angle. The reorientation can be uniformly carried over large areas of 0.2 mm(2) with the FIB and the diffraction patterns from the reoriented grating show large changes in the diffraction efficiencies. Electromagnetic simulations of the grating structures reveal large changes in the photonic properties with the slant angle such as diffraction efficiencies and the electromagnetic near fields.

Origami with negative refractive index to generate super-lenses.

Negative refractive index materials (NRIM) enable unique effects including superlenses with a high degree of sub-wavelength image resolution, a capability that stems from the ability of NRIM to support a host of surface plasmon states. Using a generalized lens theorem and the powerful tools of transformational optics, a variety of focusing configurations involving complementary positive and negative refractive index media can be generated. A paradigm of such complementary media are checkerboards that consist of alternating cells of positive and negative refractive index, and are associated with very singular electromagnetics. We present here a variety of multi-scale checkerboard lenses that we call origami lenses and investigate their electromagnetic properties both theoretically and computationally. Some of these meta-structures in the plane display thin bridges of complementary media, and this highly enhances their plasmonic response. We demonstrate the design of three-dimensional checkerboard meta-structures of complementary media using transformational optics to map the checkerboard onto three-dimensional corner lenses, the only restriction being that the corresponding unfolded structures in the plane are constrained by the four color-map theorem.

Broadband infrared metamaterial absorber with visible transparency using ITO as ground plane.

Metamaterials that have broadband absorption at MIR frequencies, and yet, are transmitive at visible frequencies are fabricated using a semi-conducting Indium Tin Oxide (ITO) film as ground plane. The metamaterial absorber consists of an array of uniform aluminum disks separated by a Zinc Sulphide (ZnS) dielectric spacer layer from the ITO ground plane. The metamaterial was fabricated by a simple, cost-effective vapor deposition through a shadow mask. Compared with the usual metal/dielectric/metal tri-layer absorbers, the metal/dielectric/ITO absorber shows a peak absorbance of 98% and >70% over the mid-infrared regime from 4 to 7 µm. The broadband nature arises due to smaller dispersion in the dielectric permittivity of ITO compared to that of plasmonic metals at mid-infrared frequencies.

Focusing light in a bianisotropic slab with negatively refracting materials.

We investigate the electromagnetic response of a pair of complementary bianisotropic media, which consist of a medium with positive refractive index (+ε, +µ, +ξ) and a medium with negative refractive index(-ε, -µ, -ξ). We show that this idealized system has peculiar imaging properties in that it reproduces images of a source, in principle, with unlimited resolution. We then consider an infinite array of line sources regularly spaced in a 1D photonic crystal (PC) consisting of 2n layers of bianisotropic complementary media. Using coordinate transformations, we map this system into 2D corner chiral lenses of 2n heterogeneous anisotropic complementary media sharing a vertex, within which light circles around closed trajectories. Alternatively, one can consider corner lenses with homogeneous isotropic media and map them into 1D PCs with heterogeneous bianisotropic layers. Interestingly, such complementary media are described by scalar, or matrix valued, sign-shifting parameters, which satisfy a new version of the generalized lens theorem of Pendry and Ramakrishna. This theorem can be derived using Fourier series solutions of the Maxwell-Tellegen equations, or from space-time symmetry arguments. Also of interest are 2D periodic checkerboards consisting of alternating rectangular cells of complementary media which are such that one point source in one cell gives rise to an infinite set of images with an image in every other cell. Such checkerboards can themselves be mapped into a class of 3D corner lenses of complementary bianisotropic media. These theoretical results are illustrated by finite element computations.

Metamaterial saturable absorber mirror.

We propose a metamaterial saturable absorber mirror at midinfrared wavelengths that can show a saturation of absorption with intensity of incident light and switch to a reflecting state. The design consists of an array of circular metallic disks separated by a thin film of vanadium dioxide (VO(2)) from a continuous metallic film. The heating due to the absorption in the absorptive state causes the VO(2) to transit to a metallic phase from the low temperature insulating phase. The metamaterial switches from an absorptive state (R≃0.1%) to a reflective state (R>95%) for a specific threshold intensity of the incident radiation corresponding to the phase transition of VO(2), resulting in the saturation of absorption in the metamaterial. The computer simulations show over 99.9% peak absorbance, a resonant bandwidth of about 0.8 µm at 10.22 µm wavelengths, and saturation intensity of 140 mW cm(-2) for undoped VO(2) at room temperature. We also carried out numerical simulations to investigate the effects of localized heating and temperature distribution by solving the heat diffusion problem.

Design of highly absorbing metamaterials for infrared frequencies.

Simple designs for polarization independent, metamaterial absorbers at mid-infrared wavelengths and over wide angle of incidence are evaluated computationally. One design consists of an array of circular metallic disks separated from a continuous metallic film by a dielectric film, and shows over 99.9% peak absorbance and a resonant bandwidth of about 0.2 µm wavelengths. The effects of various geometric parameters are analyzed for this metamaterial. Another design consisting of an array of stacked metal-dielectric-metal disks is shown to have an absorbance of over 90% in a comparatively large band of over 1 µm bandwidth, although with a lower peak absorbance of 97%.

Compound resonance-induced coupling effects in composite plasmonic metamaterials.

We study a compound plasmonic system composed of a periodic Au grating array placed close to a thin Au film. The study is not limited to normal incidence and dispersion diagrams are computed for a broad variety of parameters. In addition to identifying localized and propagating modes and the coupling/hybridization interactions between them, we go further and identify modes of compound nature, i.e. those exhibiting both localized and propagating characteristics, and discuss which plasmon modes can exhibit such a behavior in the system at hand and how structural parameters play a central part in the spectral response of such modes.

Interface-induced anisotropy and the nematic glass/gel state in jammed aqueous Laponite suspensions.

Aqueous suspensions of Laponite, a system composed of disklike nanoparticles, are found to develop optical birefringence over several days, well after the suspensions solidified because of jamming. The optical anisotropy is particularly enhanced near the air-Laponite suspension interface over length scales of several millimeters, which is beyond 5 orders of magnitude larger than the particle length scale, suggestive of large-scale ordering influenced by the interface. The orientational order increases with time and is always greater for higher concentration of salt, higher concentration of Laponite, and higher temperatures of the suspension. Although weakly birefringent, Laponite suspensions covered by paraffin oil do not show any enhancement in optical anisotropy near the interface compared to that in the bulk. We suggest that the expedited structure formation near the air interface propagating progressively inside the sample is responsible for the observed behavior. We discuss the observed nematic ordering in the context of glass-like and gel-like microstructure associated with aqueous Laponite suspensions.

Dependence of surface enhanced Raman scattering on the plasmonic template periodicity.

Surface enhanced Raman scattering has been investigated from rhodamine 6G molecules embedded in polymethyl methacrylate (R6G+PMMA) and coated on one-dimensional and two-dimensional gold-dielectric gratings fabricated by laser interference lithographically. The Raman signals from these plasmonic templates are 200 to 400 times larger than the signal from R6G+PMMA coated on plain gold films. The enhancement of the Raman signal varies almost periodically with the period of the grating. Finite-difference time-domain simulations show that large electromagnetic near fields occur at the metallic edges due to the resonant excitation of localized surface plasmon of the gold patches by the pump laser. These give rise to large enhancements of the Raman signal. The dependence on period is due to the combined effects of the localized surface plasmon and the periodic grating that couples the pump laser to the surface plasmon polariton.

Plasmonic space folding: focusing surface plasmons via negative refraction in complementary media.

We extend designs of perfect lenses to the focusing of surface plasmon polaritons (SPPs) propagating at the interface between two anisotropic media of opposite permittivity sign. We identify the role played by the components of anisotropic and heterogeneous tensors of permittivity and permeability, deduced from a coordinate transformation, in the dispersion relation governing propagation of SPPs. We illustrate our theory with three-dimensional finite element computations for focusing of SPPs by perfect flat and cylindrical lenses. Finally, we propose a design of a flat SPP lens consisting of dielectric cylinders arranged in a periodic fashion (along a hexagonal array) on a metal plate.

Switching a plasmalike metamaterial via embedded resonant atoms exhibiting electromagnetically induced transparency.

We theoretically demonstrate control of the plasmalike effective response of a metamaterial composed of aligned metallic nanorods when the electric field of the incident radiation is parallel to the nanorods. By embedding this metamaterial in a coherent atomic/molecular medium, for example, silver nanorod arrays submerged in sodium vapor, we can make the metamaterial transmittive in the forbidden frequency region below its plasma frequency. This phenomenon is enabled by having Lorentz absorbers or other coherent processes in the background medium, which provide a large positive dielectric permittivity in the vicinity of the resonance, thereby rendering the effective permittivity positive. In particular, processes such as electromagnetically induced transparency are shown to provide additional control to switch and tune the new transmission bands.

Coherently controlling metamaterials.

Two independent significant developments have challenged our understanding of light-matter interaction, one, involves the artificially structured materials known as metamaterials, and the other, relates to the coherent control of quantum systems via the quantum interference route. We theoretically demonstrate that one can engineer the electromagnetic response of composite metamaterials using coherent quantum interference effects. In particular, we predict that these composite materials can show a variety of effects ranging from dramatic reduction of losses to switchable ultraslow-to-superluminal pulse propagation. We propose parametric control of the metamaterials by active tuning of the capacitance of the structures, which is most efficiently engineered by embedding the metamaterial structures within a coherent atomic/molecular medium. This leads to dramatic frequency dependent features, such as significantly reduced dissipation accompanied by enhanced filling fraction. For a Split-ring resonator medium with magnetic properties, the associated splitting of the negative permeability band can be exploited for narrow band switching applications at near infrared frequencies involving just a single layer of such composite metamaterials.

Delay times and detector times for optical pulses traversing plasmas and negative refractive media.

We show that arrival times for electromagnetic pulses measured through the rate of absorption in an ideal impedance matched detector are equivalent to the arrival times using the average flow of optical energy as proposed by Peatross [Phys. Rev. Lett. 84, 2370 (2000)]. We then investigate the transport of optical pulses through dispersive media with negative dielectric permittivity and negative refractive index choosing the geometry such that no resonant effects come into play. For evanescent waves, the definitions of the group delay, and the reshaping delay get interchanged in comparison to propagating waves. The total delay times for the evanescent waves can be negative in an infinite plasma medium even for broadband pulses. The total delay time is, however, positive for broadband pulses in the presence of an interface when the radiation is detected outside the plasma. We find evidence of the Hartman effect for pulses when the distance traversed in the plasma is much smaller than the free space pulse length. We also show that for a negative refractive index medium (NRM) with epsilon(omega)=mu(omega) the reshaping delay for propagating waves is identically zero. The total delay time in NRM is otherwise dominated by the reshaping delay time, and for broadband pulses in NRM the total delay time is subluminal.

Finite checkerboards of dissipative negative refractive index.

The electromagnetic properties of finite checkerboards consisting of alternating rectangular cells of positive refractive index (epsilon= +1, micro= +1) and negative refractive index (epsilon= -1, micro= -1) have been investigated numerically. We show that the numerical calculations have to be carried out with very fine discretization to accurately model the highly singular behaviour of these checkerboards. Our solutions show that, within the accuracy of the numerical calculations, the focusing properties of these checkerboards are reasonably robust in the presence of moderate levels of dissipation. We also show that even small systems of checkerboards can display focussing effects to some extent.

Resolving the wave vector in negative refractive index media.

We address the general issue of resolving the wave vector in complex electromagnetic media including negative refractive media. This requires us to make a physical choice of the sign of a square root imposed merely by conditions of causality. By considering the analytic behavior of the wave vector in the complex plane, it is shown that there are a total of eight physically distinct cases in the four quadrants of two Riemann sheets.