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, and it is often challenging to interpret the exact conversion mechanisms. Here we use a femtosecond mid-infrared laser to dominantly enhance free-free transitions in terahertz and harmonic generation from air plasma. We find both radiation originate from a common source, ionization-induced plasma currents, and are greatly enhanced when driven at mid-infrared wavelengths. We observe 1% laser-to-terahertz conversion efficiency. With two-color laser fields, we not only generate ultra-broadband radiation from terahertz to petahertz but also investigate the interplay between tunneling ionization, terahertz, and harmonic generation with coherent control.