Photon diffusion in microscale solids

Abstract

This paper presents a theoretical and experimental investigation of photon diffusion 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 the 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, 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 in 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 scientific literature.

Mid‐Infrared Perfect Absorption with Planar and Subwavelength‐Perforated Ultrathin Metal Films

“…Diluting the metal film only changes the effective permittivity of the film, an effect which can be well-captured by an effective medium approximation, opening a path towards highly controllable and predictable metal film sheet conductivity. To capture the dilution effect, we use a simple Maxwell Garnett effective medium approximation [60,61] 𝜀…”

“…As pointed out in Refs. [37,61,65], such a reduction of the absorbing volume reduces the absorber thermal mass substantially, which is advantageous for bolometric and pyroelectric detector architectures. Furthermore, although the film is effectively mimicking a planar film of a reduced thickness, the microscopic field simulations shown in Figure 3d show substantial field enhancement which is absent from the planar case (Supporting Information).…”

“…Thus, our concept is more oriented toward absorbers and their application in detectors, and where larger cavities are tolerable or even desirable, such as in bolometers, where the absorbing layers are floated above micrometer-sized air gaps. [30,61] The versatile control of the phase and amplitude of the reflection coefficient (and thus the spectral control of absorption) opens up the possibility of building a multifunctional, pixelated selective absorber architecture that can operate across a wide range of IR wavelengths. Moreover, the non-resonant nature of the metal films would be beneficial for constructing ultra-small pixels, while maintaining high absorption efficiencies in ultrasmall volumes, which is highly desirable for modern bolometric and pyroelectric sensor elements.…”

“…Moreover, the non-resonant nature of the metal films would be beneficial for constructing ultra-small pixels, while maintaining high absorption efficiencies in ultrasmall volumes, which is highly desirable for modern bolometric and pyroelectric sensor elements. [11,31,32,61] The diluted "meshes" we use could also be used for realizing multifunctional transparent electrodes, [23] since their ultra-small effective thickness should enable optical transparency in the visible range while maintaining high DC conductivity (low resistivity). To investigate the potential parameter space associated with amplitude and phase control in thin metal films, we envision a checker-board type structure, where different combinations of filling factors in both the absorbing and phase layers would be built in the same vertical stack, as shown schematically in Figure 5a.…”