Inverse Problem Researching For Femtosecond Laser Heating On Metal Film

With the development of ultrafast femtosecond laser, the femtosecond laser pump-probe technique becomes an effective measure to observe microscale, especially in the process of time microscale. The femtosecond laser pump-probe technique can be utilized to measure the temperature within the very short time after the metal film had been heated by a femtosecond laser pulse, nonequilibrium transmission between electron and lattice will happen which is often described by the parabolic two-step model(PTS). To draw a conclusion, by combining the experimental results and the parabolic two-step model, it is an inverse problem to get microscale thermal physical property.This inverse problem is solve by the genetic algorithm in this paper. The genetic algorithms in this thesis adopt the commonly used proportional model, arithmetic crossover operation, uniform mutation operation, and C++language. To overcome the disadvantages of the low speed of genetic algorithm local search, this paper adopted simulated annealing method.The finite difference method is utilize to solve the positive problem of parabolic two-step model, and the C++language was adopted to develop the program. Utilizing the numerical solution of PTS model, this paper analyze the influence that the electronic volume specific heat capacity coefficient, initial temperature electronic coefficient of thermal conductivity, electricity-phonon coupling factor and the lattice volume specific heat capacity effected on metal film surface normalized electronic temperature.According to those influence, the solving steps of inversion problem of metal heating process is propose in this paper. The first step is to solve the electronic volume specific heat capacity coefficient and electric-phonon coupling factor, and then solve initial temperature of electronic coefficient of thermal conductivity. Finally, example of Au is give to validate the feasibility of solving steps.This study has important significance for promoting the transformation of femtosecond laser pump-probe thermo reflectance experiment from qualitative measurement to quantitative measurement.