Picosecond Holographic Grating Generation of Ultrasonic Waves

The picosecond holographic grating generation and detection of ultrasonic waves are described. Two mechanisms, a heating mechanism and a stimulated Brillouin scattering mechanism, are described. These permit the generation of tunable ultrasonic waves in the MHz to GHz range. It is demonstrated that the two mechanisms can be distinguished by the distinct time dependences of the diffraction of a variably delayed probe pulse from the acoustic gratings. In addition, the acoustic waves can be optically manipulated, i.e., amplified, cancelled, and phase shifted. The use of the technique for the detection of weak ground state absorptions and excited state-excited state absorptions is illustrated. Mixed excited state and acoustic gratings are discussed. The phase grating contribution to diffraction from an excited state grating is displayed, and the interference between the acoustic phase grating and the excited state phase grating is described and experimentally illustrated. Contributions to the acoustic grating diffraction from spectral shifts when the probe is near a strong, narrow transition are considered. Grating experiments on various phases of liquid crystal thin films are presented. It is shown ttiat in addition to an acoustic grating, a Kerr effect grating is produced. The Kerr grating can be separated from the acoustic grating using polarization grating excitation. The Kerr grating reveals nonexponential relaxation of orientation order in liquid crystals on a short time scale (< 1 ns).