The Focal Length Effect on Energy Absorption and Terahertz Generation upon Focusing Two-Color Radiation in Air

Generation of terahertz radiation upon filamentation in air of two-color radiation in loose focusing conditions with focal lengths from 30 to 312 cm and a beam diameter of 0.8 cm was studied. Two-color radiation was generated in a nonlinear BBO crystal irradiated by a converging laser beam. Measurements show that the energy input into the medium increases dramatically with a decrease in the focal length to 50 cm or less; at F = 30 cm, the energy of a terahertz pulse linearly depends on the laser pulse energy and increases significantly at F = 50 cm. Terahertz radiation is still not observed at longer focal lengths, which is apparently due to a spatiotemporal mismatch of the radiation of the first and second harmonics.