Coherent control of terahertz supercontinuum generation in ultrafast laser-gas interactions

Frequency mixing an ultrafast-pulse laser's fundamental and second-harmonic fields in semiconductors, atomic gases, and on metal surfaces generates a directional electrical current for which the magnitude and polarity depend upon the relative phase between these two fields. As this current occurs on the timescale of the duration of the laser pulse, in the case of ultrafast lasers (<100 fs), this process can generate electromagnetic radiation at terahertz frequencies. Although such terahertz generation has been observed in semiconductors and air, the terahertz generation mechanism is not well understood and the terahertz yield has not been optimized. Here, we demonstrate a coherent control scheme to optimize terahertz generation in gases, yielding a new source of high-energy (>5 μJ), super-broadband terahertz radiation (∼75 THz) as well as an enhanced accompanying third harmonic. We also present a unifying explanation for such extremely broad electromagnetic radiation generation.