Terahertz emission driven by two-color laser pulses at various frequency ratios

We present a simulation study of terahertz radiation from a gas driven by two-color laser pulses in a broad range of frequency ratios ω1/ω0. Our particle-in-cell simulation results show that there are three series with ω1/ω0=2n, n+1/2, n±1/3 (n is a positive integer) for high-efficiency and stable radiation generation. The radiation strength basically decreases with the increasing ω1 and scales linearly with the laser wavelength. These rules are broken when ω1/ω0<1 and much stronger radiation may be generated at any ω1/ω0. These results can be explained with a model based on gas ionization by two linear-superposition laser fields, rather than a multiwave mixing model.