Inferring Current Formation from Microwave Secondary Radiation in Two-Color Laser-Air Interactions

Low-frequency currents induced in laser-produced air plasmas can arise from multiple sources of energy imparted to electrons by the laser field. While efforts have made progress to map out the sequence of events, starting with ionization, that lead to the emission of secondary radiation in the terahertz, microwave, or radiofrequency ranges, for a general sub-relativistic laser-gas interaction the explanation remains incomplete. We use two-color laser pulses, where the fundamental and second harmonic wavelengths are coherently superimposed, in order to correlate changes in the laser field’s waveform with low-frequency plasma currents that radiate microwaves. Measurements of the far-field microwave emission as a function of the relative phase between the driving laser harmonics show that the angular distribution and total radiated microwave energy change simultaneously. A possible explanation for our observations is interference between multiple current sources in the plasma, which is not consistent with proposed mechanisms that fit the single-color case.