Development of opto-thermal phonon transducer driven by semiconductor laser

最近，我们开发了一种利用半导体激光器激发软材料中高频相干声子的新技术。所述半导体激光器的输出被周期性调制，所述激光照射在所述固体衬底上制备的金属薄膜。在金属薄膜中引起的周期性热膨胀激发机械振动，机械振动将相干超声波发射到与金属薄膜换能器接触的介质中。在这项研究中，我们成功地制作了一种新的声子换能器，其频带范围从1mhz到数百MHz。该换能器作为声子源用于超声光谱测量。光热换能器诱导声子的强度约为热声子的1万倍。该方法显著提高了声子传播光散射测量的灵敏度和精度。我们还尝试用激光在介质中诱导其他激发模式，如分子取向的排列。当线偏振的激光束入射到各向异性分子组成的液体样品时，由于克尔效应，分子取向向光偏振方向排列。利用光学椭偏技术可以灵敏地观察到分子的诱导取向。我们开发了一个新的光学系统，泵浦和探头激光器的同轴配置。该系统使我们能够定量测量各种软质材料的光学克尔常数。将声子光激发技术应用到另一个山分量观测实验中，得到了超声高频松弛的直接信息。

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.