Mode-selective thermal radiation from a microsphere as a probe of optical properties of high-temperature materials

R. Morino, H. Tajima, H. Sonoda, H. Kobayashi, R. Kanamoto, H. Odashima, and M. Tachikawa
Our spectroscopic method using laser trapping and heating has demonstrated that thermal emission from a metal oxide microsphere is enhanced at frequencies resonant with the whispering gallery modes of the spherical resonator. Only a mode series of a specific order effectively emits thermal photons, and spectral peaks shift from higher-order whispering gallery modes to fundamental whispering gallery modes as the size parameter decreases. These spectral profiles are analyzed with the Mie scattering theory and a semiclassical rate-equation model. The observed mode selectivity in thermal radiation is attributed to a matching between the rates of cavity damping and internal absorption. Excellent reproducibility of the observed spectral profiles leads to a precise determination of optical constants of extremely hot materials.

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