Spectral tailoring of the terahertz radiation from air plasma excited by two-color femtosecond pulses

We analyzed both experimentally and theoretically the spectrum of terahertz emission from air plasma excited by phase-controlled two-color femtosecond pulses. It was found that the low frequency component (<5 THz) exhibits high energy modulation contrast (>90%) for varied phase shift between the fundamental light field and its second harmonic, while the high frequency component (5-20 THz) was just slightly modulated with the phase shift. This proofs that the shape of the THz spectrum can be effectively tailored by changing the relative phase. To explain the experimental results, we developed a nonlocal plasma current approach, which was employed to calculate broadband spectral THz shapes and study their dependence on pulse parameters. It was revealed that the modulation contrast in the high-frequency part of the THz spectrum decreases with the pulse duration and the frequency detuning between the doubled fundamental frequency and the central frequency of the second harmonic field in the focal region. The obtained results indicate that the shape of the THz spectrum is affected by fine details of the frequency and phase structure of the two-color pulse, which can be used both for simple experimental tailoring of the THz spectrum and probing the two-color pulses undergoing lengthening and spectral shifts while propagating.