PHOTOACOUSTIC TECHNIQUES FOR THE QUANTITATIVE CHARACTERIZATION OF MATERIALS (PHOTOTHERMAL, SEMICONDUCTOR, THERMAL IMAGING, SURFACE WAVE).

摘要

In this dissertation several novel photoacoustic techniques are investigated, along with their application to the quantitative characterization of materials. The majority of the dissertation involves a new measurement technique in which the periodic phase perturbation of an externally generated acoustic wave is detected, due to propagation of the wave through a region of material irradiated by a modulated laser beam. A detailed theory of the acoustic perturbation is developed, based on the reciprocity theorem.;The direct photoacoustic effect is also discussed. Reciprocity theory is used to predict photoacoustic generation in a very general manner. A photoacoustic measurement in air at 2 MHz is described. Applications of the measurement include the determination of thin film thicknesses, and the imaging of surface topography. The sensitivity of the 2 MHz photoacoustic measurement is investigated in detail.;Finally, photoacoustic effects in silicon are discussed. It is shown that photoinjected carriers directly contribute to the phase perturbation of a Rayleigh wave progating along the surface of an illuminated silicon sample. Measurement of the Rayleigh wave perturbation is found to yield information concerning the photocarrier lifetime. It is furthermore determined that the mechanical strain due directly to the injection of photocarriers dominates photoacoustic generation in silicon, at 1 MHz. The effect of the surface recombination of photoinjected carriers on the periodic surface heating of an illuminated silicon sample is also investigated.;In most materials, the acoustic phase perturbation arises from the periodic heating of the illuminated sample, and the phase perturbation measurement may be viewed as a probe of the local heating. An experiment is described in which the perturbation technique is used to determine he absolute optical absorption coefficient of a fluid. Experiments involving the phase perturbation of Rayleigh surface waves are discussed. Also, a noncontacting version of the measurement technique is introduced, in which the phase perturbation of an acoustic wave propagating in the air above an illuminated sample is detected. Application of this measurement to thermal imaging and to the thermal characterization of subsurface features is described.