Progress in Theoretical Research on the Mechanism of Liquid Terahertz Wave Generation by Precision Measurement Institute

Terahertz waves have significant application value in communication and imaging. The nonlinear interaction between strong field ultrafast laser and matter is one of the important ways to generate terahertz waves. The experimental and theoretical research related to terahertz generation media such as plasma, gas, and crystal is relatively sufficient. However, liquid water is a strong absorbing medium for terahertz waves, and there have been no reports of its generation of terahertz waves. In 2017, experiments found that when the thickness of a liquid film or the diameter of a liquid beam decreased to the micrometer level, the radiation of terahertz waves was greater than absorption. This opens up a new direction in the study of liquid terahertz waves.

In recent years, there have been experimental reports in the field of liquid terahertz waves, but many phenomena observed in experiments are different from the results of other media. For example, a monochromatic laser field can effectively generate liquid terahertz waves, while a gas medium requires a specific phase difference of a bicolor laser; The yield of liquid terahertz waves is directly proportional to the energy driving the laser, while in gas media it is a square relationship; The yield of liquid terahertz waves increases with the increase of laser pulse width within a certain range, while the opposite is true for gas media; Under the drive of dual color laser, non modulated signals appear in liquid terahertz waves, but similar signals are not observed in gas media. Theoretical research on complex and disordered liquid phase systems has always been a challenge, and the above phenomena are difficult to explain with existing theories. Researchers can only explain some macroscopic experimental results under high light intensity based on previous plasma models and interface effects.

Recently, Bian Xuebin, a researcher at the Institute of Precision Measurement Science and Technology Innovation of the Chinese Academy of Sciences, and Li Zhengliang, a doctoral candidate, proposed a displacement current model that generates liquid terahertz waves, which can systematically explain a series of anomalies observed in the above experiments. The physical image of the micro mechanism model is shown in the figure: the disordered structure of the liquid localizes the electron wave packet, and the energy of the outer electrons of different molecules is affected by the environment and moves. Under the action of strong field lasers, the outer electrons of different molecules undergo transitions, generating displacement currents in asymmetric systems. The energy difference of these transitions is in the terahertz energy region, which in turn radiates terahertz waves. Meanwhile, this work demonstrates that the quantum effects of atomic nuclei play a crucial role and predicts that terahertz radiation can be used to study the isotopic effects of liquids.

The above achievements are another theoretical advancement of Bian Xuebin's team in the field of liquid phase strong field ultrafast dynamics research, following the high-order harmonic statistical fluctuation model. The related research findings are titled Terahertz radiation induced by shift currents in liquids and published in the Journal of the National Academy of Sciences (PNAS) in the United States. The research work was supported by the National Key Research and Development Program, the National Natural Science Foundation of China, and the Chinese Academy of Sciences Youth Team Program for Stable Support in Basic Research.

Schematic diagram of liquid terahertz wave generation

Source: OFweek