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The total losses due to absorption and scatter from the best optical coatings can be made as low as <EQ 3 ppm, at the limit of sensitivity of present optical-absorption measurement techniques. We show by measurement and calculation that a dramatic increase in the sensitivity of absorption measurements is obtained by using a supercritical fluid, instead of an ordinary (non-critical) fluid, as the sensing fluid in a collinear photothermal-deflection apparatus. The noise floor in our surface-absorption measurements using supercritical xenon, Tc equals 16.7 degree(s)C, corresponds to an absorptance A equals Pabsorbed/Pincident equals 10-10 under illumination of 1 W. Bulk absorption measurements are similarly enhanced: the noise floor corresponds to an absorption coefficient of (alpha) equals 10-13 cm-1 for 1 W of illumination in a sample of length 1 cm. These levels are three orders of magnitude more sensitive than any previously reported. The enhancement is brought about by the divergence in the coefficient of thermal expansion of a fluid near the critical point. In attempting to use this sensitivity to measure the absorption in transmission of low-absorbing (<EQ few ppm) anti-reflection coatings, we found that the bare superpolished fused-silica and sapphire substrates absorb at A approximately 2 X 10-5. The low-level absorption at uncoated polished optical surfaces thus appears to be an important question.
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