Huang, Hsin-huiJuodkazis, SauliusGamaly, Eugene GNagashima, TakeshiYonezawa, TetsuHatanaka, Koji2026-01-232026-01-232399-3650https://hdl.handle.net/1885/733804860Intense THz sources are expected for further progresses in nonlinear THz science and technology. Liquids like water are durable and continuously-reusable under intense laser irradiation for THz emission though such studies on THz emission from water targets are so far limited. Polarisation fine control of THz emission is demonstrated with a tilted micro-thin water flow by the irradiation of two cross-linearly-polarised femtosecond laser pulses (800nm, 35fs, transform-limited) with spatio-temporal offsets. With an optimized horizontal offset at ∼11 μm between the ∼8 μm focal spots and time delay at 4.7ns, circularly-polarised THz emission is 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 7.1 x 10−3 is achieved with the optimisation of the double pulse irradiation. Polarisation-resolved THz time-domain spectroscopy and time-resolved shadowgraphy imaging reveal that the circularly-polarised 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 pre-pulse with a laser wake-field along the optical path of the main pulse is responsible for the point-like single-cycle THz emission.S.J. is grateful for partial support by the ARC DP190103284, LP190100505 and JST CREST JPMJCR19I3 grants. T.N. is grateful for the support by JSPS KAKENHI Grant Number 20K05371. T.Y. is grateful for the support by Grant-in-Aid for Scientific Research for Fostering Joint International Research (B) (18KK0159). K. H. is grateful for the supports by the Ministry of Science and Technology (MOST) of Taiwan (107-2112-M-001-014-MY3, 110-2112-M-001-054), the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”, Nanotechnology Platform (Hokkaido University), and the Collaborative Research Projects of Laboratory for Materials and Structures, Institute of Innovative Research (Tokyo Institute of Technology). K.H. also acknowledges the Japan Science and Technology Agency (JST) PRESTO (Precursory Research for Embryonic Science and Technology) Program (SAKIGAKE, Innovative use of light and materials/life) for its supports on the original project on X-ray/THz simultaneous emission, “Ultrawide band light conversion by controlling structures of microdroplets and ultrashort laser pulse (2009–2013)” and for the laser facilities for the current project.application/pdfen-AU© 2022 The Author(s)http://creativecommons.org/licenses/by/4.0/202210.1038/s42005-022-00914-22023-10-22Creative Commons Attribution 4.0 International License