Transient optical relaxation of the Cu–BaO thin film

The pump–probe technique was used for measuring the transient optical relaxation of Cu–BaO composite thin film which was stimulated by femtosecond laser pulses. The experimental results were obtained that the optical transmissivity of the thin film rapidly decreased and a negative peak emerged during the time delay. The relaxation process consisted of two components: a fast recovery with a time of 500 fs and a subsequent slow one with over several picoseconds. An amended theoretical value of the electron–phonon coupling constant g of Cu nanometer particles in the composite thin film is given as 1.3×1017 W/m3 K according to the experimental results. The optical relaxation is a process in which nonequilibrium electrons, originating from Cu nanometer particles and excited by ultrashort laser pulses, return to the equilibrium state.