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carrier original scattering, carrier-lattice thermal , carrier relaxation and scattering. Original scattering is a nonequilibrium carriers from excited state transition to subequilibrum state, and electron-electron nonelastic scattering was very important to reflectivity curve. Carrier-lattice thermal equilibrium process was due to electron-phonon interaction transfer energy from carrier to lattice. Recovering was a slow process. The number of carrier significant decrease include radiation recombination, defect recobination，auger recombination. Diffusion make carriers move away from ecxited region and carrier thickness grads. Fig. 4 was carrier relaxation curve of GaAs in 500ps, in order to observe recovery character detected with transient thermoreflectance technique (TTR). Original scattering of carrier-carrier correspond to MO , due to electron absorb energy of pump pulse with a character time t1 was about 100fs. Carrier-lattice thermal equilibrium correspond to AO due to electron-phonon interact with character time t2 was about 1. 5ps. Carrier relaxation process corresponds to AN with character time tr was about 500ps. This curve include three region which were the original fast change, subsequent slowness decay and a constant value. Conclusion In this article , we give the theory of pump-probe technique, preparation sample and experiment setup. We demonstrate the theory of strain generation and detection. Carrier of GaAs excited and recovered by femtosecond was investigated. We found the curve include three region which were the original fast change, subsequent slowness decay and a constant value. Acknowledgements The authors would like to acknowledge the Open Fund of Jiangsu Key Laboratory of Marine Biotechnology (Grant No. 2011HS010) for financial support of this work. Referances.
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