Abstract
Ionization process in strong optical field attracts much attention in the context of high-order harmonic generation, which is produced by electrons returning to their parent ions. Recently it was found that the ionization process itself, without returning-back mechanism, can produce a strong radiation in another edge of the spectrum, namely in the THz range [1,2]. The resulting THz pulses are short (down to single cycle durations), coherent, intense and broadband. Such THz radiation can be of high demand for the ultrafast THz and far-infrared spectroscopy. A setup often used to produce such THz emission is rather simple: a strong optical pulse with center frequency ω (typically corresponding to 800 nm wavelength) is sent to a BBO crystal in order to produce its second harmonic 2ω and strongly focused afterwards to create a plasma spot in a noble gas. Due to the asymmetry of the resulting field, the photoionization plasma current develops a low-frequency component in the THz range. This leads to an electromagnetic radiation which also contains a low-frequency part. Comparison of numerical simulations with experiments [3] shows that in this typical setup the plasma mechanism overcomes the other possible ones, such as four wave mixing.
© 2011 Optical Society of America
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