Frequency modulated (FM) time delay photoacoustic and photothermal wave spectroscopies. Technique, instrumentation, and detection. Part II: Mirage effect spectrometer design and performance

Abstract

FM time delay laser excitation has been implemented in a photothermal deflection spectrometric apparatus with fast rise-time optics. The performance of the system was studied in both frequency and time delay domains. The spectrometer was further used in the study of thin quartz layers on an opaque substrate. In this work we have demonstrated the operation of the first frequency modulated (FM) time delay photothermal deflection (mirage effect) spectrometer with a ns rise time spectrally flat beam position detector unit. It was established by use of a blackbody reference that the spectrometer is responsive to impulsive heat sources in the sample with no measureable instrumental distortion of the frequency or time delay domain responses. The sensitivity of the device was tested successfully in the measurement of thermal parameters of thin quartz layers on absorbing backings. It was further shown that our spectrometer is sensitive to thin SiO2 layers on Si wafers. The data were largely consistent with Green’s function models of heat conduction in the liquid interface and bulk. The ability of the spectrometer to perform high-quality frequency response measurements via fast Fourier transformations of the input data in very short time compared to the conventional lock-in detection is an extremely useful feature for thermal-wave applications.