Spatio-Temporal Control of THz Emission

Abstract Polarization fine control of THz emission is demonstrated with a tilted micro-thin water flow by the irradiation of two cross-linearly-polarized femtosecond laser pulses (800 nm, 35 fs, transform-limited) with spatio-temporal offsets. With an optimized horizontal offset at ∼ 11 𝜇m between the ∼ 8 𝜇m focal spots and time delay at 4.7 ns, circularly-polarized THz emission was obtained with its intensity enhancement more than 1,500-times if compared with the single pulse irradiation. It is shown that the photon-number-based efficiency from the laser to THz at 7.1x10^−3 is achieved with the optimisation of the double pulse irradiation. Polarization-resolved THz time-domain spectroscopy and time-resolved shadowgraphy imaging revealed that the circularly-polarized THz emission originates from the focal volume in front of the water flow. Coupling between a shockwave due to air-breakdown and water ablation-mediated mass transport by the first pulse with a laser wake-field along the optical path of the main pulse is responsible for the point-like single-cycle THz source.