Terahertz-pulse generation by photoionization of air with laser pulses composed of both fundamental and second-harmonic waves.

TLDR: A threshold for THz generation is identified that proves that generation of a plasma is required and that the nonlinearity of air is insufficient to explain measurements, and the plasma emitter is competitive with other state-of-the-art THz emitters.

Abstract: Intense radiation in the terahertz (THz) frequency range can be generated by focusing of an ultrashort laser pulse composed of both a fundamental wave and its second-harmonic field into air, as reported previously by Cook [Opt. Lett.25, 1210 (2000)]. We identify a threshold for THz generation that proves that generation of a plasma is required and that the nonlinearity of air is insufficient to explain our measurements. An additional THz field component generated in the type I β-barium borate crystal used for second-harmonic generation has to be considered if one is to avoid misinterpretation of this kind of experiment. We conclude with a comparison that shows that the plasma emitter is competitive with other state-of-the-art THz emitters.