Impulse Photopyroelectric Depth Profiling of Multilayers. Part II: Experiments with Amplitude- and Phase-Modulated Wideband Spectrometry

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Abstract
Impulse response photopyroelectric effect spectrometry (PPES) was implemented with the use of wideband excitation for the recovery of depth profiles of optical absorption in a series of multilayered thin-film polymer samples. The wideband PPES technique intensity modulates a cw laser beam with a waveform whose power spectrum is flat over the photothermal response bandwidth, enabling the recovery of the PPES impulse response (short pulse equivalent) by correlation and spectral analysis techniques. The impulse response measurement recovers a depth profile of subsurface optical absorption through the spatial dependence of the heat flux deposited in the sample by a short optical pulse. It shows excellent potential for use as a nondestructive evaluation tool in the analysis of polymers. The impulse response data recovered in this work could be accounted for by means of a theoretical model which neglected thermal reflections between the constituent sample layers. Absorption coefficient measurements on a single-layer sample were recovered through the time delay dependence of the PPES impulse response. The data were corrected for the effect of reflectivity at the thin-film pyroelectric detector surface, which significantly affects the time dependence of the impulse response. Several types of measurements were demonstrated on optically inhomogeneous samples. These included measurements of the thickness of transparent overlayers deposited on opaque substrates, depth-resolved spectroscopy, and techniques for improving multilayer contrast. The role of thermal wave reflections at the gas/solid interface was identified as a key factor in determining the depth profile contrast for subsurface spectral contributions, as well as the dependence of the impulse response on the thickness of nonabsorbing overlayers. Several approaches for optimizing photothermal depth profile contrast were demonstrated.