Terahertz emission of ZnO crystal driven by two-color laser

Terahertz (THz) emission of crystal ZnO in a strong laser field and its second harmonics are simulated by solving the semiconductor Bloch equations (SBEs), which describes the coupled interband and intraband dynamics in two-band model. Due to the two-color laser field breaks the dynamic symmetry of the electronic wave packed, the THz yield is highly enhanced. The calculations show that the THz emission in solid is contributed by the directional current, which is named intraband mechanism. It can be seen that the even-harmonics oscillate two times, while THz shows a two-time oscillation in one fundamental cycle, and the optimal phase delay of THz pulse is 0.5 π. Also the dependence of THz amplitude on the amplitude of the fundamental beam is quite different from the atomic transient plasma current PC model. The dependence shows different region due to the different dynamics in solid structure. It is meaningful to investigate the channel closing during the Landau-Zener tunneling and the degree of asymmetric of the intraband current.