Photoacoustic/photothermal characterization of metallic Interfaces

Focused laser beams with beam waists as small as a few micrometers can be used to locally heat a metallic sample via photothermal conversion. Detection of local variations in thermoelastic properties can be accomplished by measuring temperature variations using photothermal optical-beam deflection (PTOBD) or by measuring variations in the generation of elastic disturbances using a piezoelectric transducer attached to the sample. Since the limiting factor in definition/resolution is not the thermal diffusion length,1,2 features of the order of the size of the pump beam or smaller can be investigated, even at relatively low pump-beam modulation frequencies. PTOBD images are formed by measuring the deflection of a probelaser beam while the sample is scanned under computer control. Photoacoustic (PA) images are formed by measuring the signal from the attached transducer while the sample is scanned. These techniques have been applied to the characterization of interfaces in metallic crystals. Thermoelastic properties in the neighborhood of an interface are affected by defect structures such as dislocations, vacancies, and impurities. Results and comparison of PA and PTOBD images and the relation to physical properties of the interfaces are discussed.