Optimal generation and systematic analysis of tunable terahertz emissions from single-layer graphene using two-color laser pulses with different durations

We propose that frequency-tunable terahertz (THz) emissions can be efficiently generated from single-layer graphene at normal incidence by optimizing two-color laser pulses. For three different pulse durations, a genetic algorithm (GA) is applied to search for optimal laser parameters, which show quite different characteristics. Resulted THz spectra have different spectral shapes, and their dependence on the relative phase between two colors is dramatically changed with pulse duration. We develop an interference model to explain the THz emissions for three durations, employ a four-wave mixing model to obtain the optimal wavelength of second color for long duration, and use an asymmetry parameter model to describe the THz yields with the relative phase for short duration. We reveal that the dominant mechanism for the THz generation is shifted from the symmetry of laser pulse to the four-wave mixing when the duration of two-color laser is increased. We identify that the shorter duration is better for generating intense THz emission while the longer duration is preferable for selecting the THz frequency. Furthermore, we optimize the two-color laser parameters for maximizing the THz emissions from gapped graphene with the GA. And we also examine the temporal THz waveforms by varying the parameters in the two-color laser pulses.