Photothermal deflection in a supercritical fluid

Abstract

Photothermal deflection is among the most sensitive techniques available 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 material. A thin optical probe beam is deflected by the refractive-index gradients arising from the heating, and the size of the deflection provides the measure of the heating. We describe the use of a critical fluid to enhance the sensitivity of the technique by at least 103. The diverging coefficient of thermal expansion of a pure fluid near the gas-liquid critical point gives this dramatic enhancement when used as a sensing fluid. With sensitivity calculations and measurements in supercritical xenon,Tc≈16.7‡C, we show that the noise floor of our apparatus when used for surface absorption measurements corresponds to a fractional power absorbed ofPabsorbed/Pincident=10−10, while the noise floor for bulk measurements corresponds to an absorption coefficientα=10−13 cm−1. We report the first measurements of the surface absorption of superpolished surfaces of sapphire and fused quartz,Pa/Pi≈2×10−5, and the first measurements of the bulk absorption in xenon,α≈2×10−6 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.