Fig. 1. Experimental setup. The laser pulse travels along the z axis. The β-BBO crystal with 0.3 mm thickness is used to generate dual-color pulse, a 0.1-mm-thick α-BBO crystal is applied to compensate the phase delay of the dual-color pulse, and a 0.04-mm-thick DWP is a dual-wavelength plate. The “sample” is the water wedge. EO Sampling, the electro-optical detection. Inset: schematic diagram of water wedge generation, a 1-mm-wide thin plate is on the left, and an aluminum wire is on the right. The surface asymmetry of the water wedge is changed by changing the diameter of the aluminum wires.

Fig. 2. (a) THz electric fields generated from the water film (black solid line) and the water wedge (red solid line). (b) Comparison of THz electric fields obtained from the water wedge produced by aluminum wires with different diameters.

Fig. 3. (a) The detected THz electric fields from the water wedge as a function of the distance the water moves. (The position after moving 600 μm is where the laser focus is closest to the front surface when THz can be detected.) (b) The detected THz spectra after moving 300 μm, 500 μm, and 600 μm.

Fig. 4. The detected THz electric fields from the water wedge as a function of the total excitation laser pulse (ω and 2ω) energy. (Blue, the error bar of nine measurements. Red, the quadratic fitting line.)