Active polarization control of plasma-based THz wave emission from water flow
Hsin-hui Huang1*, Takeshi Nagashima2, Saulius Juodkazis3,4, Koji Hatanaka1,5,6
1Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan
2Faculty of Science and Engineering, Setsunan University, Osaka, Japan
3Center for Micro-Photonics, Swinburne University of Technology, Melbourne, Australia
4Melbourne Centre for Nanofabrication, the Victorian Node of the Australian National Fabrication Facility, Clayton, Australia
5College of Engineering, Chang Gung University, Taoyuan, Taiwan
6Department of Materials Science and Engineering, National Dong-Hwa University, Hualian, Taiwan
* Presenter:Hsin-hui Huang,
Intense THz wave emission, associated with hard X-ray emission, is induced under the double pulse excitation condition with nanoseconds time delay by tightly focusing femtosecond laser pulses (35 fs, 800 nm, 0.5 kHz) onto a thin water flow. Polarization-sensitive THz time-domain spectroscopy (TDS) shows that THz wave emission is linearly-polarized under the single pulse excitation, whereas under the double pulse excitation (Δt = 4.7 ns) the emission is circularly-polarized and is induced far from the flow surface in the upstream side in front of the flow. Time-resolved imaging experiments reveal the ablation dynamics in front of the water flow induced by the pre-pulse irradiation and support the hypothesis that THz wave emission is induced in the ablation plume in front of the flow. Further TDS experiments by changing the irradiation spatial position of the main pulse to the pre-pulse show that the polarization status depends on the relative position between the two pulses and changes from left-handed circular to right-handed circular through linear. Mechanisms related are discussed from the viewpoints of the ablation dynamics in front of the water flow and its interaction with the following irradiation of the main pulse. 

Keywords: THz, Water, Ablation, Double pulse excitation, Circularly polarization