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.
The diffractive plenoptic camera (DPC) was developed as a system that would capture the spectral and spatial information of a scene in one snapshot. While the DPC couples a diffractive optic with plenoptic camera designs to provide snapshot spectral imaging capabilities, it produces rendered images with low pixel count and low spatial resolution. A modified setup of the DPC, the intermediate image (II)DPC, was built and tested for the first time and compared to both the DPC and a diffractive-optic camera as a system that could improve the cutoff spatial frequency of the rendered images. This paper reports on the spatial resolution achieved for different configurations of the IIDPC and looks at the factors limiting performance. The IIDPC improved on the cutoff spatial resolution over the DPC over a wavelength range of 750 to 790 nm for a design wavelength of 770 nm and improved resolution over a diffractive-optic camera at wavelengths below 750 nm or above 790 nm, with the best results achieved for IIDPC configurations with the largest magnification. Frequency analysis of each system determined that the optic limiting performance was the microlens array. Models showed that decreasing the microlens size improved resolution but reduced the spectral range for the DPC, while decreasing the f / # of the microlenses improved the resolution for the IIDPC. These results will help optimize the designs of future systems.
Two different configurations of the Diffractive Plenoptic Camera (DPC), the DPC and the Intermediate Image (II) DPC, had previously been built and their performances compared. The DPC couples a diffractive optic with plenoptic camera designs that provide snapshot spectral imaging capabilities but produce rendered images with low pixel count and low spatial resolution. The IIDPC, a modified setup of the DPC, was introduced as a system that could improve the spatial resolution. The IIDPC improved resolution over a narrow centralized spectral range, while the DPC had sustained resolution over a larger spectral range. Further study of both systems was desired to understand what the limiting factor in their performance was. Frequency analysis of both systems was carried out to determine the limiting component of each system. The limiting optic in both systems was determined to be the Microlens Array (MLA).
Recently a novel Fresnel zone light field spectral imager was developed that provides snapshot spectral imaging using no moving parts or scanning. This system combines a Fresnel zone plate as the primary optic to conduct both imaging and dispersion with a microlens array configured like a plenoptic camera. This encodes spectral information onto the detector array like a traditional plenoptic camera encodes range information, and both systems require post-processing to produce final images. While algorithms will significantly affect final performance, the ability to judge the optical performance of a particular hardware design before post processing is important to set a base line for algorithm comparisons as well as establish how components in the imaging chain impact performance. For remote sensing scenarios we propose a figure of merit based on shifts of the images formed by each microlens of a point source and derive design equations linking this to system parameters. This talk examines how the Fresnel zone plate and microlenses affect the blur of the point source images and how the sampling of the images by the detector array impacts the figure of merit. For remote sensing scenarios an image shift corresponded to a particular source wavelength. The sampling by the detector array added uncertainty to image shift measurements providing a measure of the spectral resolution due to the hardware. The image shift figure of merit describes how spectral information is encoded in the raw data by the hardware and can be used to estimate performance prior to post-processing.
Recent interest in building an imaging system using diffractive optics that can fit on a CubeSat (10 cm x 10 cm x 30 cm) and can correct severe chromatic aberrations inherent to diffractive optics has led to the development of the Fresnel zone light field spectral imaging system (FZLFSI). The FZLFSI is a system that integrates an axial dispersion binary diffractive optic with a light field (plenoptic) camera design that enables snapshot spectral imaging capability. This system suffered from poor resolution and a modified FZLFSI based on full resolution light field rendering has been built and tested. The modified FZLFSI shifts the optical elements to different positions which change the way the light field is encoded on the focal plane array. The new encoding increases the available spatial information at the expense of some spectral information. The system was tested for different internal system parameters, at a range of wavelengths, and the resulting tradeoffs between spatial and spectral performance were studied. The performance of the modified FZLFSI was compared to that of the conventional FZLFSI and optimal internal system parameters identified for different imaging scenarios.
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