Dephasing Of Coherent Phonon In Sapphire With Time Resolved CARS

As a common infrared window material sapphire is widely used in infrared military and aerospace device because of its excellent mechanical, physical and chemical property. There will be new color centers in the sapphire in the irradiation of theγ-ray, and the presence of these defects will affect the crystal structure and properties. The subtle change of the crystal structure can be detected by the vibration spectrum, so the femtosecond time resolved Coherent Anti-Stockes Raman Spectroscopy (CARS) is proposed to research the defect in the crystal through measuring the dephasing time of the coherent phonon in the crystal.The theory of Coherent Anti-Stockes Raman Spectroscopy is introduced first in this thesis. A simple pendulum model is used to describe the dephasing and the energy relaxation process of the coherent phonon in the crystal. The behavior of coherent phonon with coherent optical excitation is described, the change of the CARS signal with the intensity of the coherent phonon in the probe process is represented, and it is presented employing the time resolved CARS to measure the dephasing time of the coherent phonon in the crystal.The time resolved CARS measurement system is set up based on the femtosecond laser system, the white-light continuum (WLC) is introduced as the Stokes in the experimental system. The mechanism and application of WLC is mentioned. The WLC created in our lab is tested through the four-wave mixing experiment, the results show that the coherence and intensity of the WLC is to meet the needs of time resolved CARS.Three sapphire samples undergo different does ofγ?ray were test using the CARS measurement system. The results show that there is a obvious decay process with the delay extension of the probe pulse, and the depahsing time of coherent phonon in sapphire is successfully obtained through the femtosecond time resolved CARS. It is proved that the dephasing time is basically inversely proportional to the does of theγ-ray after comparing the decay process and the simulation of the three samples.