Acoustic wave determination by degenerate four-wave-mixing using ultrafast laser pulses

摘要

Using degrenerate four-wave-mixing(DFWM) technique the acoustic wave produced in transparent microstructured optical materials can be determined. The light source used in this paper was a mode-locked, Q-switched Nd: YAG laser operated at 532 nm with puse width of approx20 ps. The laser beam was split into three pulses. By spatially overlapping inside the sample with an angle in a backward propagating scheme. A laser induced periodic interference pattern which serves as grating was formed in the sample. The third, "probe", beam incident on the region in the direction satisfied by the Bragg condition. The diffracted signal brings the information of the produced acoustic wave. The incident laser pulses couple to the acoustic field of the material through effects of thermally induced acoustic strain and eelctrostrictive coupling. This results in a generation of two counterpropagating ultrasonic acoustic waves in the grating wave vector direction, which is so-called the Laser Induced Phonon Spectroscopy. The wavevector of the generated acoustic mode is equal to the grating wavevector and related to the frequency of the mode through the speed of sound in the material. Changes in the refractive index induced by these acoustic waves cause a temporally periodic scattering of the probe beam in the Bragg direction. In the experiments it was found that an increase in the grating spacing due to a decrease in the pump beam crossing angle 2theta results in a decrease of the acoustic phonon frequency omega in terms of nu=lambda_(acoustic)omega. This will manifest in an increase in the period of the acoustic signal. The origin of the signal could arise from both, the heating mechanism due to optical absorption into high-lying levels followed by rapid radiationless relaxation, and the mechanism of electrostriction coupling. For the high input intensities of the two pump-beams (> 10~8 W/cm~2), two photon absorption was found to be the cause of the generated heat giving rise to a periodic temperature distribution along the grating direction. Using this technique, the acoustic wave can be precisely determined even in a microstructured materials.