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Investigation of Ultrafast Reflectivity and Nonequilibrium Relaxation in Chromium Thin Films by Examining the Nonequilibrium Transient Thermoreflectance Twice
X-F. Xu, L. Lv, Y-F. Gao, M. Zhou and N-F. Ren

The femtosecond optical pumped transient thermoreflectance technique is an effective tool to investigate the ultrafast dynamics in metal thin films. In this paper both the transient reflectivity response and nonequilibrium relaxation time of electron-phonon interaction in chromium thin films are studied by using femtosecond laser pump-probe thermoreflectance experimental technology. The results show that the transient reflectivity relaxation times of chromium thin films is independent of the thicknesses of films, while the recovery time of electron-phonon relaxation from nonequilibrium state to equilibrium state is proportional to the thickness of the metal films, and the transient reflectivity of detected signals shows that changes are larger with the laser pumped power increasing. In addition, two transient reflectivity signals on glass substrate films are found which are different from those films on silicon substrates.

Keywords: Femtosecond laser, chromium film, transient reflectivity, nonequilibrium relaxation

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