Measurements Of Sound Speed In Media Using Implusive Stimulated Light Scattering

Speed of sound is the velocity of compressive wave propagation in elastic medium, which is the basic physical quantity of describing the vibration characteristics and mechanical response. Sound speed measurements is of great significance in different states and systems. But traditional ultrasonic measuring method, due to the use of ultrasonic transducer, is difficult to realize the non-contact measurement at high temperature and high pressure environment loaded by diamond anvil cell. In optical noncontact detection, brillouin scattering method is not widely used, because of the subtle frequency shift and the weak signal. Impulsive stimulated light scattering is a technology of thermal grating generating ultrasonic based on nonlinear optical effect, which is a combination of laser ultrasounds and light scattering. This method stimulates the transient grating in the media by using two coherent beams of ultrashort pulse laser at an angle of sample surface Through the analysis of the diffraction dynamic signal of the transient grating, velocity of ultrasonic is calculated.First, This article focuses on the theoretical analysis of formation mechanism and characteristics of transient grating in impulsive stimulated light scattering. The modulation and polarization of transient grating is described by light in the form of electromagnetic field. Bragg diffraction of transient grating is also analysed detailly. The frequency and emergent direction of the signal light is discussed, and we demonstrate the feasibility of optical heterodyne detection.Then, we set up an experiment facility of impulsive stimulated light scattering excited by nanosecond laser, a preliminary exploration on the stimulated scattering. Thermal grating stimulated in several kinds of organic solution and dielectric film is researched. Then we analyzes that the optical heterodyne detection technique makes an significant amplification effect on the diffraction signal of single wavelength stimulated scattering experiment.Finally, combining the existing experience, we set up the femtosecond laser stimulating the impulsive stimulated light scattering experiment facility with full bandwidth of 1 GHz. With the laser dye auxiliary, we generate several hundred megahertz ultrasonic wave in kinds of organic solution. Then with the transient grating period,the velocity of sound in the medium is calculated. And to expand to hydrostatic pressure experiment, we measured the sound velocity of ethanol within 500 MPa with the variety of pressure loaded by the diamond anvil cells.In this work, we generate the ultrasonic in media by using implsive stimulated light scattering, which is a mean of all-optical non-contact method. And we successfully applied it to the sound speed measurements of nonopaque media, which absorb the pump light at high static pressure. We provide a new method for the research of media property at extreme condition.