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(1,2), atomic gases(3,4), and on metal surfaces(5) generates a directional electrical current for which the magnitude and polarity depend upon the relative phase between these two fields(1-5). 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(6) and air(7-13), 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 mu J), super-broadband terahertz radiation (similar to 75 THz) as well as an enhanced accompanying third harmonic. We also present a unifying explanation for such extremely broad electromagnetic radiation generation.