Frequency tuning for broadband terahertz emission from two-color laser-induced air plasma

Effective manipulation of broadband terahertz emission, especially on spectrum tuning, is of great importance for many applications. We demonstrate a method to realize frequency tuning of terahertz emissions from two-color laser-induced air plasmas. The terahertz central frequency is switched from 0.56 to 0.82 THz by changing the polarization state of the fundamental wave with a quarter-wave plate. Based on numerical simulation, it is found that this frequency tuning is due to the birefringence effect induced by the fundamental wave on the second harmonic inside the filament, which leads to a discrepancy on the polarization chirality of the two-color laser components. Two-color lasers with opposite chirality will emit terahertz radiation with higher central frequency compared to two-color lasers with the same chirality at moderate laser intensity.