Periodic structures with minimum feature sizes in the scale of the mean radiation wavelength or less attract considerable interest due to the peculiarity of their electromagnetic (EM) response. When interference and diffraction effects become sufficiently strong, novel and interesting phenomena emerge in reflectivity, transmissivity, absorbance and even infrared thermal emission. The nanotechnology processing enables the production of high-efficiency diffraction gratings with quite small periods, down to the nanometer range, with aspect ratios higher than in spectroscopic gratings. In this paper we present the spectral measurements (transmission and thermal emission) of GaAs and silicon samples with lamella 1D gratings and mesa 2D structures. We also present the theoretical and simulation tools developed for the design and analysis of multilayer lamellar grating structures.
In this paper we present a study of infrared spectral thermal emission from varius grating structures. The structures include various lamellar grating layers of metals, silicon or GaAs on the same semiconductor substrate. The gratings have different periods, groove widths and groove depths, with feature sizes comparable to the radiated measurement wavelengths (2.5 - 25 μm). The measurement temperatures for all samples were in the range 27 to 740°K. Lateral and vertical optical confinement in the grating layers can occur. In the semiconductor grating layer in the case where the material is partially transparent lateral optical coupling exist which affect the spectral emission. In addition vertical confinement of the electromagnetic field exists which corresponds to "organ-pipe" like modes. The vertical confinement is enhanced in the case where the grating scructure is coated with metal or degenerate semiconductor. These phenomena resulted in thermal emission spectral oscillation for the wavelength range larger than the grating period.
Infrared spectral transmission, reflection and thermal emission from diffraction gratings with differing periods, groove widths and groove depths were experimentally and theoretically studied. The structural dimensions are comparable to the measured spectral wavelengths in the range 2.5 to 25 microns. For calculating the optical properties (transmission and reflection spectra), we have used an in-house S-Matrix Propagation Algorithm (SMPA) technique which is unconditionally stable versus changes in structural dimensions, optical constants and truncation order. We have experimentally studied the planar angular transmission and reflection spectrum of Si and GaAs grating samples, using FTIR spectrometry over the spectral range from 2.5 μm to 25 μm. At λ < Λ, the transmitted intensity is quasi-periodic with respect to wave number. A similar property also appears in the reflection spectra. The theoretical results for spectral transmission are in good agreement with the experimental results for the wavelength range 2.5 to 25 μm.
Theoretical work of our group is placed in the general frame of efforts to improve numerical performance and efficiency of rigorous coupled-wave analysis of grating diffraction. Mathematical transformation of Maxwell equations for a multi-layered structure to evolution equations in functional space is presented. By-construction numerically stable symbolic algorithm to solve these equations using the notion of in-layer scattering operator is proposed. On the base of this algorithm a toolbox for simulation of diffraction from multi-layered grating structures, implemented by a graphical user interface is developed. An example of simulation using this in-house software is exposed.
The infrared normal spectral emissions from degenerate (metallic-like) silicon and metallic (nickel) lamellar grating structures were investigated. The gratings were micromachined on (110) silicon wafer was with differing periods, groove widths and groove depths, where the dimensions of all samples were with feature sizes comparable to the measurement wavelengths (2.5 - 25 μm). The measurement temperatures for all samples were in the range 27 to 740 °C. Infrared normal transmission through diffraction was also measured. In general, it was found that the spectral emission of the metallic gratings was different from the degenerate silicon grating. This because the bulk absorption in the silicon samples was affecting the emission.
The effects of vacuum and (gamma) -irradiation as well as their joint effects on electrical and optical properties of phototransistors and imaging charge coupled devices are discussed. These effects will be related not only to bulk effects, but also to surface effects which are enhanced due to vacuum environments.
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