Investigating properties of surfaces and thin films using microsphere whispering-gallery modes

We use a tunable diode laser operating near 1570 nm to investigate various effects of the heat transfer from fused-silica microspheres, with and without thin-film coatings, to the surrounding gas in a vacuum chamber. The resonance frequencies of microsphere whispering-gallery modes (WGMs), excited by a tapered-fiber coupler, shift with changing temperature (about -1.6 GHz/K at 1570 nm). This shift, primarily due to the temperature dependence of the refractive index of fused silica, enables the measurements whose results are reported here: determination of the thermal accommodation coefficient of air on different surfaces, and measurement of the optical absorption coefficients of surface water layers and of a thin film coating. Our method for determining thermal accommodation coefficients involves deducing the thermal conductivity of the air as a function of pressure by measuring the relaxation rate of an externally heated microsphere to room temperature. Then, in a separate experiment, by observing thermal optical bistability of the WGM resonances caused by absorption of the probe laser, the contribution of water or film absorption to the total loss is found.