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

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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.