The Femtosecond Laser Si <sub> 3 </ Sub> N <sub> 4 </ Sub> Crystal Interaction Theory Research

Researching the theory that femtosecond laser damaged Si3N4 crystal, can provide theoretical support in the field of micro-machining etc, and both promote application prospect of femtosecond laser in a variety of areas,.it analysis and perfect in turn can also the interaction between femtosecond laser and Si3N4 crystal.In this paper, using perturbation theory combined the time and the energy state, the theory, of which the interaction between femtosecond laser and Si3N4 crystal, is deduced. The main contents are classified as follows:Firstly, because of pulse width of femtosecond laser is extremely short. Therefore, when discussing the interaction of femtosecond laser and Si3N4 crystal, the nonlinear process is discussed mainly. At the same time, this process is theoretically analyzed in detail. And to detailed analysis of the valence band electrons individual excitation, which may be contribute to absorption coefficients of the relevant model, of the valence band electrons as well as possible within the localized states.Furthermore, using steady state Boltzmann equation and the time-dependent perturbation theory, electronic relaxation time, that is, the time required for lattice back to the next equilibrium state, will be estimate roughly. And taking into account the change of a single electronic state, this amendment to the state's distribution of lattice vibration, since the thermal effect of crystal depended primarily on the lattice vibration when is not extremely low. By courtesy of this consideration and mean spacing of adjacent pulse in comparison with average impulse width, and spacing can be properly adjusted in order to further impair crystal significantly different from the long pulse.Finally, Density of energy state can be approximated with method of spherical wave. As a basis, equation, of which electron density of conduction band was changed with time and energy, was simplified.