Zhao Yi-Ting, Jiang Shi-Cheng, Zhao Xi, Chen Ji-Gen, Yang Yu-Jun
Opt Lett. 2020 May 15;45(10):2874-2877. doi: 10.1364/OL.389787.
A series of theoretical and experimental results has proved that harmonics below/above the band gap are produced mainly by the intraband current/interband polarization for solids in strong mid-infrared laser pulses. However, which mechanism dominates the harmonic process is still debated. In this work, based on simulating high-order-harmonic generation from an MgO crystal in a linearly polarized mid-infrared laser by solving semiconductor Bloch equations, we demonstrate that harmonics just below the band gap originate from the interference between intraband and interband currents. Furthermore, it is found that intensities of harmonics just below the band gap are apparently enhanced with an increase in the incident laser's strength. By analyzing the band dispersion and the transition dipole moment of the 001-cut MgO crystal, this can be attributed to the interband polarization between two conduction bands.
一系列理论和实验结果证明，对于处于强中红外激光脉冲中的固体，带隙以下/以上的谐波主要由带内电流/带间极化产生。然而，哪种机制主导谐波过程仍存在争议。在这项工作中，通过求解半导体布洛赫方程，基于对线偏振中红外激光作用下MgO晶体的高次谐波产生进行模拟，我们证明了刚好低于带隙的谐波源于带内电流与带间电流之间的干涉。此外，发现刚好低于带隙的谐波强度会随着入射激光强度的增加而明显增强。通过分析001切割MgO晶体的能带色散和跃迁偶极矩，这可归因于两个导带之间的带间极化。
