Study On Characteristics Of Interaction Between Intense Field And Crystals

The interaction of an intense laser pulse with crystals gives rise to a set of nonlinear optical phenomena, including high-order harmonic generation(HHG), above-threshold ionization(ATI) etc. Until now, there two main theoretical models: the many-electron model and single-electron model. former is based on the semiconductor Bloch equation(SBE), which has been successfully applied in solids and semiconductor physics. The single-electron model is based on energy-band theory, which treats motions of each electron in solids independently in an effective potential.In the past years, interaction between intense laser field and molecules, and interaction between intense laser field and atoms have been extensively investigated. Sources of HHG have been well explained. However, sources of HHG from crystals are still investigating. Until now, some scientists think inter- and intra-band current lead to HHG from crystals. The way about solving related problems are based on the two models, although many-electron model can solve the related problem accurately, we have to consider interaction between electrons, it will spend much time. Single-electron model is more simple, as long as the effective potential is determined, the related problems can be solved directly.The main goal of this paper is to investigate interaction between crystals and intense laser pulses. Firstly, energy bands are numerically constructed using the modified Mathieu-type model. Time-dependent Schrodinger equation can be solved by B-splines basis. In real space, the HHG show the two-plateau structure, then we use Bloch basis and Houston basis to solve time-dependent Schrodinger equation, we get HHG in wave vector space. By analysis, primary plateau is due to the coupling of the valence band to the first conduction band and a weaker second plateau is due to coupling to higher-lying conduction bands, and both of the cutoff energies of the two plateaus in the harmonic spectrum scale linearly with the field strength. Furthermore, by considering the crystal driven by the few-cycle laser pulse, the cutoff energy of the second plateau changes monotonously with carrier-envelope phases. Finally, we study further the HHG from crystals driven by the chirped laser and find that it has a great influence on the HHG, and the second plateau ofHHG is sensitive to the chirp parameter. According to this phenomenon, we propose a novel way that is capable of greatly improving the emission efficiency of the second plateau by changing the chirp parameter of the driving laser.