The spectrum of THz radiation from plasma produced by strong multicolor optical fields

In the present work we explore, using a rigorous propagation model developed in[3], the possibilities to control the spectrum of the generated THz field by changing the frequency shift between two harmonics, as well as by changing the number of pulse components. In particular we also consider, in contrast to a "classical" two color scheme, a three-frequency pump provided by optical parametric amplification (see Fig. 1), as well as more complicated four-frequency schemes. We show that the proper choice of the pump configuration can improve the performances of THz generation, by producing ultrabroad spectra (with frequency 1 ÷150 THz), or alternatively, increasing the THz efficiency up to five times, compared with classical two-color pulse setups. We demonstrate that the THz spectrum is determined by the stepwise character of the ionization process occurring at the sub femtosecond timescale, and elucidate the role of pump propagation effects in this process.