| In a general picosecond ultrasonic experiment, a sub-picosecond laser pulse (the pump) excites the sample and causes a change in optical reflectivity of the sample, and a time-delayed laser pulse (the probe) is directed to the excited region to measure the time-dependence of this change. The time delay is usually obtained by means of a conventional optical path and can be made as long as several nanoseconds. In this thesis we present a variation of the picosecond ultrasonic technique, in which an extra delay is achieved by using a later pulse from the laser as a probe. By doing so we can measure the effects that happen at a relatively long time scale compared with that in other picosecond ultrasonic experiments.; The first application is the measurement of the dispersion of long-wavelength longitudinal acoustic phonons in Ge, Si, GaAs, z-cut quartz, and sapphire. In these experiments the pump light pulse generated a strain pulse at one surface of the sample. After propagation through the sample, the shape of this strain pulse was modified because of the phonon dispersion. From this change in shape, the magnitude of the dispersion could be determined. The results are compared with various lattice dynamical models.; In the second application, by increasing the pulse energy of the pump pulse, we have studied the deformation of longitudinal acoustic pulses propagating in solid crystals due to dispersion and non-linearity. We found that the evolution of the one-dimensional wave can be described by Korteweg-de Vries equation. The experiments have been performed on Si, MgO, α-quartz, and sapphire. In each case we have observed the evidence of acoustic solitons and the results agree reasonably well with our computer simulations.; In the end, we also report on measurements of the temperature dependence of the sound velocity and the acoustic attenuation. |
