Browsing by Subject "Absorption per unit mass"

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    High Absorption per unit Thermal Mass Subwavelength Perforated Membrane as Uncooled Thermal Infrared Detector
    (2023-01) Das, Avijit
    In the first project, a theoretical and experimental investigation of photon diffusion is discussed in highly absorbing microscale graphite. A Nd:YAG continuous wave laser is used to heat the graphite samples with thicknesses of 40 µm and 100 µm. Optical intensities of 10 kW/cm^2 and 20 kW/cm^2 are used in laser heating. The graphite samples are heated to temperatures of thousands of kelvins within milliseconds, which are recorded by a 2-color, high-speed pyrometer. To compare the observed temperatures, the differential equation of heat conduction is solved across the samples with proper initial and boundary conditions. In addition to lattice vibrations, photon diffusion is incorporated into the analytical model of thermal conductivity for solving the heat equation. The numerical simulations showed close matching between experiment and theory only when including the photon diffusion equations and existing material properties data found in the previously published works with no fitting constants. The results indicate that the commonly overlooked mechanism of photon diffusion dominates the heat transfer of many microscale structures near their evaporation temperatures. In addition, the treatment explains the discrepancies between thermal conductivity measurements and theory that were previously described in the scientifc literature. In the second project, a subwavelength perforated metamaterial absorber is developed for a maximum absorption-to-thermal mass ratio to construct an uncooled thermal infrared (λ∼8−12 µm) detector operating at a time constant of ∼7.7 ms, faster than the video frame rates, with a noise equivalent temperature difference (NETD) of 4.5 mKand a detectivity of 3.8×10^9 cm√Hz/W. The designed metamaterial absorber consists of Ti, SiNx, and Ni nanoscale films with an overall fill factor of ∼28%, where subwavelength interference and Fabry Perot resonance induce an absorption per unit mass of approximately 1.3−27.6 times higher than the previously reported infrared absorbers. We read out the fabricated detector optically via Mach Zehnder interferometer.