Book Description

"Developing efficient and robust terahertz (THz) sources is of incessant interest in the THz community for their wide applications. For instance, THz waves are widely applied to nondestructive evaluation and biomedical imaging. Intense THz fields can be used for nonlinear interactions and electron acceleration. With successive efforts in past decades, numerous groups have achieved THz wave generation from solids, gases, and plasmas. However, liquid, especially liquid water has never been demonstrated as a THz source. One main reason leading the impediment is that water has strong absorption characteristics in the THz frequency regime. In this dissertation, a thin water film under intense laser excitation was introduced as the THz source to mitigate the considerable loss of THz waves from the absorption. Laserinduced plasma formation associated with a ponderomotive force-induced dipole model was proposed to explain the generation process. Compared with gas targets, water has a lower photon-ionization energy, but three orders of magnitude higher molecular density. Compared with solid targets, water does not suffer from permanent damage caused by the intense laser pulses. For the one-color excitation scheme, the water film generates a higher THz electric field than the air does under the identical experimental condition. Unlike the case of air, THz wave generation from liquid water prefers a subpicosecond (200-800 fs) laser pulse rather than a femtosecond pulse (~50 fs). This observation results from the plasma generation process in water. For the two-color excitation scheme, the THz electric field is enhanced by one-order of magnitude in comparison with the one-color case. Meanwhile, coherent control of the THz field is achieved by adjusting the relative phase between the fundamental pulse and the second-harmonic pulse. To eliminate the total internal reflection of THz waves at the water-air interface of a water film, a water line produced by a syringe needle was used to emit THz waves. As expected, more THz radiation can be coupled out and detected. As a supplement to the research of Terahertz Aqueous Photonics, THz wave generation from other liquids were also tested. For example, the THz electric field obtained from ?-Pinene is 1.8-times stronger than that from water under the same experimental condition, which mainly arises from the much weaker absorption of ?-Pinene for THz waves. Historically liquid water has been sworn off as a THz source. This dissertation not only demonstrates that water can be used to generate THz waves but also contributes to enthralling insight into the development of intense THz sources. The work also indicates that THz waves can be a great tool for the investigation of laser-liquid interaction"--Pages xii-xiii.