Comparison of broadband microwave generation from single and two-color ultrafast mid-IR laser-produced plasmas in atmosphere*

Nonlinear propagation of a high power ultrashort laser pulse through a transparent medium creates a plasma that radiates broadband electromagnetic waves. We present a direct comparison between the absolute emitted microwave frequency spectrum for plasmas produced with a ultrashort pulse at a wavelength of 3.9 microns focused in air, and a two-color pulse consisting of the 3.9 micron fundamental superimposed with its second harmonic. Comparison of the microwave emission for the single and two-color cases may reflect different current generation mechanisms in the plasma, namely the ponderomotive force, and direct current driven by the temporal asymmetry of the laser field oscillation that is enabled in a two-color scheme. The frequency content of the microwaves is measured as a function of viewing angle relative to the laser propagation direction up to a real-time bandwidth of 70 GHz. The frequency spectra are compensated for the instrument response of the receiver so that the absolute electric field amplitude incident on the receiver aperture is known.