Terahertz Radiation in Liquids

Experimental setup to create terahertz radiation in liquids. (Source: ITMO)

A research team from ITMO Univer­sity in St. Petersburg and the Univer­sity of Rochester conducted a study on the formation of terahertz radiation in liquids. Previously, the gene­ration of such radiation in a liquid medium was considered impos­sible due to high absorption. However, in their new research, the scientists described this pheno­menon’s physical nature and demonstrated that liquid radiation sources can be equally effective to tradi­tional ones.

Terahertz electro­magnetic radiation can easily pass through most materials except metals and water. Today, it is widely used in security systems used to detect illicit drugs and weapons, as well as for bio­medical research. Most modern research involving terahertz radiation focuses on finding new, more stable, powerful and efficient sources. The most common sources of terahertz radiation are solid materials. In addition, there are sources based on femto­second laser fila­mentation in air and gases. In this case, a powerful laser beam creates a plasma in the gas medium by ionizing it so that free electrons generate electro­magnetic terahertz radiation.

Although doing the same in a liquid medium was until now considered impossible due to high absorp­tion, the research team showed the opposite. Their new study revealed that liquid, in fact, has a number of advan­tages over other sources such as gases. “Until our colleague, Xi-Cheng Zhang, had been able to detect terahertz radiation in a liquid, it was believed to be impossible. But we demon­strated that, in terms of efficiency, liquid sources can approach solid-state sources, which are now considered to be the standard. Moreover, liquids are much easier to obtain than crystals. They can also with­stand high pumping energy, which makes it possible to obtain a better output,” explains Anton Tsypkin, Head of the Labora­tory of Femto­second Optics and Femto­technology at ITMO University.

Usually, radiation is generated due to the release of free excited electrons during filamentation. The more electrons can be excited or ionized, the stronger the output terahertz radiation will be. The number of excited electrons of one molecule depends on the energy spent on the excitation of the medium. The dif­ference between the required pumping energies in gas and liquid is small. At the same time, the density of molecules in a liquid is much higher than in a gas, so that a comparable pump energy makes it possible to excite more electrons and make the radiation stronger.

Scientists inves­tigated the direction of terahertz radiation in the liquid. Experiments were conducted in parallel at two univer­sities so as to eliminate errors. Then, the scientists verified the inde­pendently obtained results and worked together on a theo­retical model to explain them. As a result, they managed to draw up and physically substan­tiate the radiation patterns of terahertz radiation in a liquid and its depen­dence on the angle at which the liquid collides with the pump radiation. According to the researchers, these results will be used in future work.

“A signi­ficant drawback of fluid is its large absorption. We plan to solve this problem by opti­mizing the type of fluid, the shape of the jet, the pump power and a number of other para­meters. We want to experi­mentally find the optimal parameters for the radiation gene­ration in different liquids, as well as to develop a theo­retical model based on this data. It can be used to create a proto­type device that will allow us to produce different types of terahertz radiation from liquids,” says Xi-Cheng Zhang from Inter­national Institute Photonics and Optical Infor­matics at ITMO Univer­sity. (Source: ITMO)

Reference: E. Yiwen et al.: Terahertz wave generation from liquid water films via laser-induced breakdown, Appl. Phys. Lett. 113, 181103 (2018); DOI: 10.1063/1.5054599

Link: Institute of Optics, University of Rochester, Rochester, USA • Laboratory of Femtosecond Optics and Femtotechnology, ITMO University, St. Petersburg, Russia

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