Circularly-polarized THz wave emission from a micro-thin water flow

Intense THz wave sources are highly expected for further progresses in nonlinear THz science and practical implementation of non-ionizing radiation in sensing and communications. Solid-based sources have inherent limits of material breakdown, while intense laser irradiation of liquids is a promising emerging technique for THz wave and hard X-ray emission. Water-based THz emission shows intensity enhancements up to 10 3 times when laser-pulse pairs with nanosecond delay are used. Here we show circularly- polarized THz wave emission from thin water flow irradiated by two time-separated and linearly-polarized femtosecond laser pulses. THz time-domain spectroscopy reveals the circularly-polarized THz emission dominates 4.7 ns after the first pulse irradiation. THz wave detection delay in the spectroscopy and time-resolved micrography indicate that the THz wave emission originates from the rarefied volume in front of the flow. Radial relaxation of charges (currents) in the focal volume where ponderomotive charge depletion occurred is the origin for the circular polarization; tight focusing localized THz wave emission to the sub-wavelength region.