Measurement of the thermal wave in metal films using femtosecond laser thermoreflectance system

Abstract

The thermal wave phenomenon induced by the ultrafast laser heating in metal films has been studied by different experimental methods in the past. In our experiment, a femtosecond laser pump-probe system is used to study the ultrafast energy transport between the electrons and phonons. The experiments showed a weak wave peak in the electron temperature curve following the main peak caused by the laser pulse. The reproducibility of the results has been tested for different laser intensities and different film samples. This weak peak in electron temperature could be explained by the reflection of the thermal wave from the rear surface. The experimental results agree with the theoretical curves calculated by the hyperbolic two-step model. The speed of the thermal wave is about 5×105 m/s and the electron relaxation time is about 60 fs.