Abstract

The terahertz frequency range belongs to the “difficult” ones, that is, there are no good methods to generate high energy pulses in this range, especially of the arbitrary shape [1]. Several methods of THz generation are related to plasma creation in gases, exploiting strong field ionization effects. The most efficient known mechanism of such kind is based on the ionization in two-color and multi-color fields and is related to the asymmetry of the fast optical pump waveform [1]. This asymmetry leads to the asymmetry in the electron movement, so that at the end of the pulse a residual macroscopic current J is created, which is a source of the low frequency broadband field in THz range, E_J~dJ/dt. The mechanism requires strong intensities because ionization must be in the tunnel regime, that is, take place predominantly near the maxima of the electric field, and thus do not work for lower intensities where ionization is slower than optical cycle (multiphoton ionization regime) [2]. Practically, this makes it difficult to use pulses much longer than 100 fs, which puts limits on the spectral and temporal tunability.

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