Sideway terahertz emission from a flowing water line

We compared the forward and sideway THz wave emission from a flowing water line excited by subpicosecond laser pulses. The peak field of THz sideway emission from a 210 μm waterline in is about twice stronger than that in forward direction when a 2-inch focal length lens is used to focus the optical beam. This observation indicates that the length of plasma in water is much shorter than that in air, resulting in more THz emission to sideway. Moreover, we measured the THz emission energy from water line with different diameters. The forward THz emission decreases greatly when the diameter of the water line increases from 0.3 mm to 1.4 mm. However, the diameter shows a much less influence on the THz emission to sideway, especially for the large diameter of waterline. Our study shows that the sideway detection of THz wave from an ionized waterline is a better scheme to collect more THz signal. These observations are helpful to understand the THz generation from water and laser-induced ionization in water.