PHOTOTHERMAL DEFLECTION IN A SUPERCRITICAL-FLUID

Photothermal deflection is among the most sensitive techniques availab le for the measurement of small, localized heating, such as that from the absorption of a focused laser beam in the bulk or surface of a mat erial. A thin optical probe beam is deflected by the refractive-index gradients arising from the heating, and the size of the deflection pro vides the measure of the heating. We describe the use of a critical fl uid to enhance the sensitivity of the technique by at least 10(3). The diverging coefficient of thermal expansion of a pure fluid near the g as-liquid critical point gives this dramatic enhancement when used as a sensing fluid. With sensitivity calculations and measurements in sup ercritical xenon, T-c approximate to 16.7 degrees C, we show that the noise floor of our apparatus when used for surface absorption measurem ents corresponds to a fractional power absorbed of P-absorbed/P-incide nt = 10(-10), while the noise floor for bulk measurements corresponds to an absorption coefficient alpha = 10(-13) cm(-1). We report the fir st measurements of the surface absorption of superpolished surfaces of sapphire and fused quartz, P-a/P-i approximate to 2 x 10(-5), and the first measurements of the bulk absorption in xenon, alpha approximate to 2 x 10(-6) cm(-1). We also show how the present work fits into the current status of absorption measurement techniques and describe the effects of the peculiar properties of critical fluids on the execution of photothermal deflection measurements.