KEYWORDS: Plasmonics, Gold, Finite element methods, Polarization, Data modeling, Phase shifts, Analytical research, Instrument modeling, Diffraction, Chemical elements
Plasmonic metasurfaces with in-plane phase elements have a limit in transmission because they only affect the electric field of incident EM radiation. Recently, a set of out-of-plane plasmonic phase elements was designed using a genetic algorithm to work in the infrared as a Huygens metasurface with significantly improved transmission efficiency. A beam-steering metasurface (i.e., blazed transmissive diffraction grating) was fabricated from this design using membrane projection lithography (MPL) and characterized for its bidirectional transmittance distribution function as a function of scatter angle for normally incident light, and linear incident and transmitted polarizations. Measurements were compared with the designed behavior as predicted by finite element method (FEM) simulations that generated near fields for each phase element and propagated them to the far field as a metasurface using a Stratton–Chu formulation, but measurements showed strong zero-order diffraction not present in the simulation along with the designed +1-diffraction order. We analyze this disagreement between measured and ideal results. Further FEM modeling included the introduction of defects into the phase elements consistent with defects expected from the fabrication process and identified lateral displacement of the plasmonic decoration in the MPL structure as a potential cause for the reduced performance of the fabricated device.
KEYWORDS: Data modeling, Polarization, Instrument modeling, Finite element methods, Plasmonics, Scatter measurement, Phase shifts, Optical components, Near field
To improve upon a fundamental limit on transmission of an in-plane plasmonic metasurface device, Out-of-Plane (OOP, i.e. 3D-thin-film) metasurface plasmonic phase elements were designed using genetic algorithm techniques to work in the infrared, λ = 8 μm, and these were fabricated as a beamsteerer using membrane projection lithog- raphy and characterized using scatterometry to measure the Bidirectional Transmittance Distribution Function (BTDF) of the device. BTDF was measured as a function of scatter angle for four different polarization con- figurations: co-polarization and cross-polarization for two orthogonal linear polarization states and simulated using a finite element method (FEM) solver to generate the near fields of each phase element of the device and a Stratton-Chu formulation to propagate to the far field. The measurements showed the designed beamsteering from the device, but also a strong zero-order diffraction not present in the simulations. This disagreement be- tween models and measurements motivated this study to understand what was causing the differences. To that end, FEM models which reduced the coupling between adjacent elements of the beamsteerer were designed to examine methods that would better simulate measurements. Details of the models are discussed. Future work will focus on finding the root cause for this decrease in coupling.
Three-dimensional (3D) metafilms composed of periodic arrays containing single and multiple micrometer-scale vertical split ring resonators per unit cell were fabricated using membrane projection lithography. In contrast to planar and stacked planar structures such as cut wire pairs and fishnet structures, these 3D metafilms have a thickness t ~λd/4, allowing for classical thin film effects in the long wavelength limit. The infrared specular far-field scattering response was measured for metafilms containing one and two resonators per unit cell, and compared to numerical simulations. Excellent agreement in the frequency region below the onset of diffractive scattering was obtained. The metafilms demonstrate strong bi-anisotropic polarization dependence. Further, we show that for 3D metafilms, just as in solids, complex unit cells with multiple atoms (inclusions) per unit cell possess a richer set of excitation mechanisms. The highlight of these new coupling mechanisms is the excitation of the 3D analog to the 2D cut-wire-pair magnetic response.
KEYWORDS: Plasmonics, Chemical elements, Dielectrics, 3D acquisition, 3D modeling, Light-matter interactions, Wavefronts, Interfaces, Antennas, Magnetism
Metasurface optics have offered a fresh perspective into light-matter interactions, providing an unsurpassed means to engineer the wavefront of light transiting a subwavelength interface. However, strictly planar surface architectures using conventional antenna elements have performed quite inefficiently, as they contain purely electric modes and thus do not possess the magnetic modes necessary to generate the optimal Huygens-like scattering profile. And while multi-layer stacks of these 2D sheet admittances have been consistently demonstrated as the only feasible solution to-date for plasmonic-based Huygens-like metastructures, their experimental performance is often degraded by non-analytical behavior or fabrication limitations, leaving dielectric architectures as the best hope for real-world metasurface optical applications. In this work, we propose a new alternative for highly-efficient plasmonic metasurfaces: a 3D architecture which produces a Huygens-like total field and exhibits transmittances of approximately 80% at any targeted phase retardation across the full 2π phase space. The 3D unit cell consists of a cubic silicon cavity, with the interior walls of the cavity modeled as grids of voxels. The grids are initially represented in a binary fashion as a random assortment of either a metal (gold) or a dielectric (air), then iterated through a genetic algorithm routine, flipping the value of individual voxels until a maximum transmittance was reached at the desired total field phase retardation. Optimized designs for eight phase values were chosen to construct a metasurface lens. Simulation, fabrication and experimental results of both the individual element and the lens are presented.
Plasmonic metasurface lenses based on polarization conversion are inherently limited in efficiency, and as a result, they have been given sparse attention in favor of higher-performing dielectric variants. However, recent proposals for expanding the design beyond a single interface offer hope for efficient plasmonic structures. Before developing these multi-layer structures, we wish to better understand dependencies of 2D plasmonic designs. Here we demonstrate the spectral, polarization and geometrical sensitivities of nine large-scale variants of the original V-antenna lens design at λ0 = 8μm. We show that the spectral response oscillates rapidly within a span as small as λ0/320, and that strong focusing can occur at both the designed polarization state, and its orthogonal state–and with differing focal distances. Additionally, we determine that the lens performance is only weakly tied to the size of the discretization, offering only marginal improvement as the discretization approaches a continuum.
The spectral behaviors of an externally-illuminated thermal infrared metamaterial were characterized through simulation and experimental measurement of the power transmittance and reflectance within the 6 - 20μm range. Finite-difference time domain (FDTD) simulations in both 2-D and 3-D environments were swept over a multitude of bent dipole inclusion configurations at normal incidence angles to produce a model which exhibited a dominant electrical resonance in the long-wave infrared (IR) and increased in magnitude, bandwidth and wavelength as a function of the dipole length. Despite the appearance of fabrication defects in the measured samples, it was found the experimental data was in good agreement with the 3-D FDTD simulations, though not at all with the 2-D simulations. These introductory results indicate the dipole inclusion may behave in many ways similar to an antenna in the IR, enabling spectrally- and spatially-selective control of the emission pattern.
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