Scientists Demonstrate that Liquid Water can Generate THz Waves

Illuminating a thin film of water with ultrashort laser pulses.

Scientists Demonstrate that Liquid Water can Generate THz Waves
Experimental setup for generating THz waves by focusing a laser beam onto a free-flowing water film. Credit: Jin et al. ©2017 American Institute of Physics

It was previously impossible to generate THz waves through the water. Even a research has shown that THz waves can be generated from several other sources, including air plasma. They have shown that water can emit light at other frequencies, including white light, and that water vapor can emit THz waves.

In a new study, scientists demonstrated that liquid water can generate THz waves. They discovered it by just illuminating a thin film of water with ultrashort laser pulses.

During the study, scientists focused femtosecond laser pulses inside the water film. The laser pulses create a plasma in the water film, which ionizes the water molecules. It thus generated free electrons, and eventually emitting THz radiation.

As bulk water is a strong absorber of light in the THz frequency range, the result surprises everyone. And since a long time, it seemed very unlikely that liquid water could serve as a source of THz radiation.

Research led, Xi-Cheng Zhang said, “Although it might be too early to predict any industrial or commercial applications for our work at this moment, I think it provides the last piece of the puzzle. Solid, gas, and plasma have been used to generate THz waves already, but not liquid. With liquid, especially water, four states of matter have now been used as THz sources.”

In this study, scientists found that the THz radiation from water exhibits very different characteristics compared to the THz radiation from air plasma. Scientists also found that, when positioned away from the normal incident angle of optical excitation, the strength of the THz radiation from water depends on the polarization of the laser beam, whereas the THz radiation from the air plasma is polarization-independent.

Scientists explained, “these observations cannot be fully explained by physicists’ current understanding of the mechanisms involved in THz wave generation. We need further research for better understanding of the physics behind the new observations.”

Zhang said, “Water is the most important element in the human body and on our earth. Using water to generate THz waves is one step toward understanding the role of water in this frequency range. Previously, we tried to avoid water in most THz applications. Now we will likely use water for the generation of intense THz waves.”