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Optical Depth Profiling of Thin Films by Impulse Mirage Effect Spectroscopy. Part II: Measurements Using Wide-Band Modulated Excitation

Volume 48, Number 9 (Sept. 1994) Page 1076-1087

Schweitzer, M.A.; Power, J.F.


Laser mirage effect (photothermal deflection) spectroscopy was used in this work to determine depth profiles of the optical absorptivity of thin polymer films whose thermal properties were approximately invariant with depth. A variety of well-characterized laminates and continuous samples were examined. The key experimental variables affecting optical depth profile resolution were the offset of the probe beam above the surface, the thermal properties of the fluid medium in which optical deflection occurs, and the relative depths of the subsurface features to be resolved. The accuracy of the experimental signal was found to depend on several key factors which included the electrical bandwidth of the electronic detection system and the linearity of the photothermal deflection signal. Experimental procedures for ensuring quantitative signal recovery are described, and a theoretical interpretation of the depth dependence of the mirage signal was made by means of a model presented in a companion paper.