Lateral terahertz wave emission from laser induced plasma in liquid water line

Laser ionized liquid has been considered as a promising broadband terahertz (THz) source recently. The ponderomotive force induced dipole model can be used to describe the mechanism of THz generation, implying that the lateral THz emission is stronger than the forward emission. Thus, we systemically investigate the lateral THz emission from laser induced plasma in liquid water line. THz energy about 4.4-times of that from the forward emission is achieved from the lateral emission scheme. Results on the angular distribution of coherent THz radiation indicate that the maximal THz radiation falls between 120° and 150° with respect to the pump laser propagation direction. Moreover, dependence of collected THz energy on the relative position between laser focus and the water line shows a counterintuitive “two peaks” trend, which can be explained by the impact of cylindrical water surface on the divergence angle of the generated THz wave. Therefore, the enhancement in THz energy can be attributed to the high-efficiency lateral collection scheme and the tradeoff between the THz wave converging effect of the cylindrical surface and the absorption in water. Our work helps to further optimize the coupling efficiency of generated THz wave out of liquid samples.