Coherent Phonon Generation by Periodic Excitation of Transient Grating with a Picosecond Light Pulse Train

用调Q锁模的脉冲序列周期激励瞬变光栅，观察到增强相干声子(超声波)的产生。当满足共振条件nu=nf时，发生增强，其中nu是超声波的频率，f是锁模激光脉冲的重复频率(在我们的情况下是82 MHz)，n是整数。频率nu由光栅节距Lamda(=/(2sin(θ/2)，：光的波长，θ：激光束的交叉角)和声速v确定为nu=v/lamda。在n=1~4(即nu=82~328 MHz)范围内，在乙醇、甲醇、乙二醇和含有少量染料(Na-荧光素等)的水中观察到相当大的增强作用。在1.06 GHz(n=13)范围内观察到超声波的产生，但在较高频率时增强作用变小。对液晶MBBA和DOBMMB也进行了类似的研究。对于普通探测灯和非常探测灯，检测到了截然不同的超声波和热栅行为。这是由于这些光的折射率与温度的关系不同。还观察到了DOBMMB在相变温度下的声速变化。

Generation of enhanced coherent phonons (ultrasonic waves) was observed by using periodic excitation of transient gratings with a pulse train from a Q-Switched and mode-locked Nd:YAG laser. The enhancement occurs when the resonance condition nu=nf is satisfied, where nu is the frequency of the ultrasonic wave, f is the repetition frequency (82MHz in our case) of the mode-locked laser pulses and n is an integer. The frequency nu is determined from the grating pitch lamda (= /(2sin(theta/2), : wavelength of the light, theta: crossing angle of the laser beams) and the sound velocity v as nu=v/lamda. Detection of the ultrasonic waves was made by using diffraction of a cw or pulsed probe laser beam.Considerable enhancement was observed in ethanol, methanol, ethylene-glycol and water containing small amount of dye ( Na -fluorescein etc., 10^<-2>mol/l) for n=1 to 4(i.e., nu=82 to 328MHz). Generation of ultrasonic waves up to 1.06GHz (n=13) was observed but the enhancement became smaller at higher frequencies.Similar studies were made for liquid crystals MBBA and DOBMMB. Quite different behaviors of ultrasonic waves and thermal gratings were detected for the ordinary and extraordinary probe lights. This is due to the different temperature dependence of refractive indices of these lights. Changes of sound velocity in DOBMMB at phase transition temperatures were also observed.