Multi-beam plasma generation of terahertz radiation: experiments and modeling

We demonstrate two experimental schemes for plasma-based terahertz (THz) generation using ultrafast laser pulses. In one scheme three laser pulses with incommensurate wavelengths and comparable relative powers are used. The three colors include 800 nm and a variable combination of the IR signal and idler outputs from an optical parametric amplifier. Stable THz is only generated when all three colors are present, with little power or spectral dependence on the selection of signal and idler IR wavelengths (range of 1300-2000 nm). In another experiment, we show that the THz emission of a plasma formed by a commensurate two-color pulse (containing a near IR frequency and its second harmonic) can be enhanced by the addition of an 800-nm pulse. We observed enhancements of the THz electric field by a factor of up to 30. The dependence of the THz electric field enhancement factor on the powers of the two-color and 800-nm beams is considered. Numerical calculations done using the well-known photocurrent model are in good agreement with both experimental results. These studies help suggest future avenues of improvement in practical table-top plasma generation of THz.