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 <i>et al</i>. [Opt. Lett. <b>25</b>, 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 <i>β</i> -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.
© 2004 Optical Society of America
(190.4380) Nonlinear optics : Nonlinear optics, four-wave mixing
(260.3090) Physical optics : Infrared, far
(320.2250) Ultrafast optics : Femtosecond phenomena
(320.7120) Ultrafast optics : Ultrafast phenomena
Markus Kress, Torsten Löffler, Susanne Eden, Mark Thomson, and Hartmut G. Roskos, "Terahertz-pulse generation by photoionization of air with laser pulses composed of both fundamental and second-harmonic waves," Opt. Lett. 29, 1120-1122 (2004)