In this paper, we present a design for a narrowband absorber based on metamaterials in the infrared wavelengths for
wavelength-selective uncooled hyperspectral imaging systems. The proposed narrowband absorber integrated with
microbolometer focal plane arrays has the potential to increase the detection sensitivity of the microbolometers. The
design of the metamaterial unit cell consists of a resonant metallic 'cross' structure which has a resonance in the IR
wavelengths and is placed on a dielectric substrate with a metal back plate. In order to achieve a very high absorption of
the electromagnetic radiation, the designed metamaterial needs to have minimal transmission and reflection within its
spectral response window. Minimal reflection is achieved through impedance matching of the metamaterial with the free
space whereas zero transmission is ensured through the metal back plate. Moreover, for the purpose of hyperspectral
imaging, the metamaterial structure is combined with a tunable electro-optic material, namely, liquid crystal. Tunability
can be achieved upon applying a voltage across the combined liquid crystal and metamaterial structure thus bringing
about a shift in the resonant frequency. In our simulated model, where losses of metal and dielectric substrate materials
were taken into account, we noted more than ninety percent of absorption can be achieved in a narrow spectral window
for the designed metamaterial structure.
We discuss the design of a Digital Micromirror Device-based Snapshot Spectral Imaging (DMD-SSI) system for NIR
wavelengths. A pair of low/high dispersion glasses was selected for building an NIR relay-lens and a double-Amici
prism, which are needed for generating Compressive Sensing (CS) measurements in experimental settings. Aberrations
of the system were simulated using the Zemax software, which were considered in a numerical model of the system. CS
measurements were generated using this model accounting for those aberrations. We evaluated the quality of the
spatial/spectral data-cubes reconstructed using those non-ideal CS measurements and discuss possible solutions of
enhancing the reconstruction quality.
We report the development of a Digital-Micromirror-Device (DMD)-based Compressive Sampling Hyperspectral
Imaging (CS-HSI) system. A DMD is used to implement CS measurement patterns, which modulate the intensity of
optical images. The 3-dimensional (3-D) spatial/spectral data-cube of the original optical image is reconstructed from the
CS measurements by solving a minimization problem. Two different solvers for the minimization problem were
examined, including the GPSR (Gradient Projection for Sparse Reconstruction) and the TwIST (Two-step Iterative
Shrinkage/Thresholding) methods. The performances of these two methods were tested and compared in terms of the
image-reconstruction quality and the computer run-time. The image-formation process of the DMD-based spectral
imaging system was analyzed using a Zemax model, based on which, an experimental prototype was built. We also
present experimental results obtained from the prototype system.
In this paper, we present both numerical and experimental results for the waveguiding of light using a low-index-contrast
(LIC) self-collimating photonic crystal (SCPhC) in the RF frequency regime. This waveguiding structure
utilizes the unique interactions of light with the periodic structure of the photonic crystal (PhC) to propagate a beam
of light without divergence. This design also employs materials with a low index contrast (LIC), which reduces the
electromagnetic signature of the PhC. This SCPhC was designed by extracting its dispersion contours and
numerically simulating it using HFSS, a commercial 3-D, full-wave FEM software.
In particular, we addressed the issue of coupling the PhC to a coaxial medium by designing an input/output (I/O)
coupler consisting of a coaxial-to-waveguide transition, a rectangular waveguide and a tapered dielectric transition.
We fabricated the SCPhC with a rigid polyurethane foam slab and Rexolite polystyrene rods using an automated
CNC router to drill the periodic lattice in the slab. We also fabricated the dielectric segments of the I/O couplers
with Rexolite slabs using an automated milling machine. Using these I/O couplers and SCPhC slab, we simulated
and subsequently measured experimentally an insertion loss, for the entire system, of -3.3 dB through a 24" PhC
slab, and a coupling loss of -0.95 dB at each coupler-PhC interface.