Coherent thermal radiation in thin films and its application in the emissivity design of multilayer films

Abstract

The infrared transmission spectra of a 0.54-μm-thick Ge film and a 20-μm-thick Si film were experimentally measured. As the incident radiation was in the wavelength range from 1.5 μm to 10 μm, the Ge film demonstrated a strongly spectral coherence. However, thermal radiation of the Ge film was found to be spatially incoherent due to its extreme thinness. The Si film exhibited significantly spectral and spatial coherence. The results confirmed that thermal radiation of a monolayer film could be coherent spectrally and spatially if the film thickness was comparable with the wavelength. The optical characteristic matrix method was applied to calculate the transmission spectra of the Si and Ge film, and the results agreed well with the measurements. This method was further used to analyze two multilayer films composed of five low emissive layers. Their emissivities were found to be highly emissive at a certain zenith angle, and the emissive peak could be controlled by careful selection of film thickness.thermal radiation, thin film, cvoherence, emissivity, multilayer film

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“…Recent experiment on the normal transmittivity of a 0.54-micron-thick Ge film on a CaF 2 substrate by the PE Spectrum GX Fourier Transformed Infrared (FTIR) spectrometer showed a distinct coherence effect [84] . The wavelength range was from 1.5 to 10 microns, which is a typical range in the research of thermal radiation.…”