Filamentation-assisted μJ THz generation by 2-TW laser pulses in a low-pressure gas

A 2-TW, 30-fs, 800-nm laser radiation is combined with its second harmonic to yield a two-color filament in a loose-focusing geometry inside a low-pressure gas cell. With multiple filamentation and excessive ionization avoided in our experiments through an appropriate choice of the sort and the pressure of the gas, as well as focusing conditions, this filament provides a bright source of terahertz pulses, at least with microjoule-level, for part of the spectrum below 2 THz. Ten-fold increase of THz yield is observed for 1-50 mbar gas pressure in comparison with ambient case. This THz output is focusable, as our experiments show, to field intensities high enough to induce a 16% transmission increase of a silicon wafer, indicating THz-field strengths up to 0.2 MV/cm in the beam-waist region.