Terahertz Radiation from Two-Color Laser-Induced Gas Plasma Filament Under a Wide Range of Pressure

Terahertz radiation from two-color femtosecond laser-induced gas filament is investigated in a wide range of the ambient gas pressure from 0 to 606 kPa (i.e., 0–6 atm). We observed that the output energy of terahertz radiation first increased quickly to the strongest strength at the optimal pressure around 25 kPa then decreased gradually with increase of the gas pressure. We found a linear dependence of the time delay of the terahertz signals on increasing gas pressure under a wide range of gas pressure and nearly unchangeable plasma density profile of the filament in the high pressure range from ~ 50 kPa to 606 kPa if keeping the same pump laser energy. Our numerical simulation results reproducing the experimental dependence of decreasing terahertz strength on the increasing gas pressure in this high pressure range indicate that this dependence could be attributed mainly to the decreasing coupling of the two-color laser pulses in the plasma filament due to different changes of their propagating velocities in the ambient gas with increasing pressure. Our results would be helpful for understanding deeply the mechanism of terahertz radiation and remind a possible strategy for improvement of terahertz output under higher gas pressures and pumped by more laser energies.