Efficient terahertz and Brunel harmonic generation from air plasma via mid-infrared coherent control

Nonlinear light conversion involves one or more bound bound, bound-free, free-free, and free-bound transitions. It is often challenging to interpret the exact conversion mechanisms. Here we use a femtosecond mid-infrared laser to enhance free-free transitions in terahertz and Brunel harmonic generation from air plasma. Microscopically, both THz and harmonics originate from a common source-ionization-induced plasma currents-and are greatly enhanced when driven by intense long-wavelength pulses. We observe 1% laser-to-terahertz conversion efficiency. Using two-color laser fields, we generate coherent radiation from terahertz to petahertz and investigate the interplay among tunneling ionization, terahertz, and harmonic generation with coherent control. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement