Development of opto-thermal phonon transducer driven by semiconductor laser

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Recently, we developed a new technique to excite high frequency coherent phonon in soft materials using semiconductor lasers. The output of the semiconductor laser is periodically modulated and the laser light illuminates a metal thin film fabricated on the solid substrate. The periodical thermal expansion thus induced in the film excites the mechanical vibration, which emits the coherent ultrasonic wave into the medium contacting with the metal film transducer. In this study, we successfully fabricated a new phonon transducer whose frequency band ranges from 1 MHz up to several hundreds of MHz. The transducer was employed as a phonon source for the ultrasonic spectroscopy measurement. The intensity of the induced phonon by the opto-thermal transducer is about 10,000 times larger than that of the thermal phonon. The sensitivity and the accuracy of the light scattering measurement of the phonon propagation was remarkably improved.We also tried to induce other excitation mode in the medium with lasers, such as the alignment of the molecular orientation. When the laser beam with linear polarization is incident to the liquid sample composed of anisotropic molecules, the molecular orientation is aligned into the direction of the optical polarization due to the Kerr effect. The induced molecular orientation can be sensitively observed with the technique of the optical ellipsometry. We developed a new optical system with the co-axial configuration of the pump and probe lasers. The system enables us to measure quantitatively the optical Kerr constant of various soft materials.The techniques of optical excitation of phonon is now applied to the another experiment of the observation of the Mountain component, which gives the direct information on the ultrasonic relaxation in high frequency range.