Analysis of low-frequency THz emission from monolayer graphene irradiated by a long two-color laser pulse

Terahertz (THz) radiations from graphene are expected to provide a powerful light source for their wide applications. However, their conversion efficiencies are limited with either long-duration or few-cycle single-color laser pulses. Here, we theoretically demonstrate that THz waves can be efficiently generated from monolayer graphene by using a long-duration two-color laser pulse at normal incidence. Our simulated results show that low-frequency THz emissions are sensitive to the phase difference between two colors, the laser intensity, and the fundamental wavelength. Their dependence on these parameters can be very well reproduced by asymmetry parameters accounting for electron populations of conduction and valence bands. On the contrary, a newly defined σ parameter including the Landau-Zener tunneling probability cannot precisely predict such dependence. Furthermore, the waveform of THz electric field driven by two-color laser pulses exhibits the typical feature of a half-cycle pulse.