Abstract:
Acoustic waves generated by ultrashort optical pulses provide a novel approach to materials characterization. Frequencies in the gigahertz to terahertz regime, corresponding to acoustic wavelengths from sub-10 nm to ~1 (mu)m, can be used to investigate thin layers and nanostructures of commensurate dimensions. Elastic constants, ultrasonic attenuation, and thin film bonding can be probed down to the limit at which continuum acoustics ceases to apply. In addition, the polarity and shape of the generated acoustic pulses provides information on the nonequilibrium photoexcited carrier dynamics. Optical detection methods such as those based on reflectance modulation allow noncontact operation, and further sample characterization can be simultaneously carried out by analyzing signals due to thermal wave propagation. In the present studies, reflectance modulation and beam deflection techniques were used at visible wavelengths to probe the longitudinal optoacoustic generation in thin film and bulk semiconductors and metals. The beam deflection technique allows a direct measure of the acoustic strain pulse shape to be obtained in many cases, allowing simple comparison with the theory of acoustic generation. In metals, for example, the broadening of optically generated acoustic pulses by ultrafast electron diffusion can be confirmed experimentally.