Uncharted Waters

ROBIN RILEY, WEB EDITOR, robin.riley@photonics.com

As the saying goes, "difficult is done at once; the impossible takes a little longer."

It took Xi-Cheng Zhang, an optics professor at the University of Rochester, nearly a decade to generate a terahertz wave from water — an achievement that many in the scientific community considered unattainable. This point of view did not discourage Zhang and his team.

“It is a scientific curiosity,” he told Photonics Media. “Why can only three states in matter — solid, gas and plasma — be used to generate a THz wave, but not a liquid? Among liquids, including water, is it just due to the large absorption coefficient?

“We had been working on THz air photonics for several years, and recently on THz micro-plasma,” he added. “These research projects provided us with the background and training for such a project. I remember that I mentioned it during several group meetings: THz waves must be generated in liquid water — we just need to find a good way to couple it out and measure it.”

THz waves had previously been formed through the interaction between a laser beam and targets of solid crystals, metals, air plasma and water vapor; but until now, no one had produced THz pulses from liquid water.

In this case, the target was a film of water about 200 microns thick, suspended by surface tension between two aluminum wires. When the laser was focused into this extremely thin film, it acted as an emitter for THz radiation output.

“Water was considered the enemy of terahertz waves because of water’s strong absorption,” said Zhang. “We always tried to avoid water, but it is a surprisingly efficient terahertz source.”

Researchers use lasers to generate terahertz pulses via interaction with a target. In this case, the target was an extremely thin water film — approximately 200 microns or about the thickness of two pieces of paper — created using water suspended between two aluminum wires. Courtesy of University of Rochester/Kaia Williams.

The thickness of the water film was just 177 ± 8 μm, and could be adjusted by throttling the water flow rate. The flow rate of the water was about 1.3 m/s.

To confirm that the THz radiation was primarily emitted from the water film, with no assistance from the air plasma, researchers translated the water film along the direction of laser propagation. THz signals from air plasmas could be clearly observed if the thickness of the water film was reduced to 100 μm or less.

One challenge was to create a film of water that was thin enough so that it would not absorb the terahertz photons generated by the laser beam, but thick enough to withstand the laser’s energy. The team spent months optimizing the thickness of the water film and the incident angle, intensity and pulse duration of the laser beam.

Due to the strong absorption characteristics of liquid water in the THz frequency regime, Zhang and his team found that only one THz photon at a time could go through water that was 1 mm thick. To mitigate the considerable loss of THz waves, the researchers determined that water with much less than 1 mm thickness was needed to study THz wave generation.

“We increased the thickness of the water a little bit, and gradually increased the laser, and just kept trying until we could make it work,” said researcher Qi Jin.

Gravity-driven, free-flowing water films were used owing to their simple design and ability to generate a thin, continuous, stable film of liquid water in free space.

When the researchers measured the THz waves generated by the water, they found that they were 1.8 times stronger than THz waves that were generated from air plasma under comparable experimental conditions. Compared with THz radiation generated from the air plasma, the THz radiation from liquid water had a distinct response to various optical pulse durations and showed linear energy dependence on incident laser pulses.

The experimental setup used to generate terahertz waves from liquid water. Researchers focus the optical pump beam into the water film and use a series of filters and off-axis parabolic mirrors (OAPMs) to detect the terahertz signal and block any other light waves simultaneously generated from the water film. Courtesy of University of Rochester/Xi-Cheng Zhang Lab.

“Since we have demonstrated the use of water to generate THz waves, our next step is to check the possibility of using water as an intense THz source under an ultra-intense laser beam,” Zhang told Photonics Media. “Unlike any solid, intense laser will damage it. But flowing water is not an issue, since each fresh laser shot is a new spot in the flowing water film. Water also has very high density; it offers a much larger cross section for laser-matter interaction than gas.”

Because THz waves are non-ionizing, they do not have the same harmful effects on human tissue and DNA as x-rays do. THz waves are able to nondestructively pass through solid objects and produce interior images, making them useful for a range of applications.

“Terahertz waves have a capacity to see through clothing, which is why you have these sub-terahertz body scanners at airports,” said Zhang. “These waves can help to identify if an object is explosive, chemical or biological, even if they can’t tell exactly what the object is.”

The researchers note that their reported results cannot be fully interpreted through science’s existing understanding of the mechanism of THz wave generation. The team’s observations could support further research in the fields of THz and IR radiation. Additionally, their work could contribute to the exploration of laser-liquid interactions and their future as THz sources.

“Almost everyone thought we wouldn’t be able to get a signal from water,” said Jin. “At first, I didn’t believe it either.”

But in Zhang’s group, innovation and breakthrough are driving forces. He and his team have worked for nearly a decade to solve this scientific puzzle.

“One hundred years ago, Einstein provided Einstein relation for the Einstein coefficients. Einstein relations said that the emission coefficient is related to the absorption coefficient. Since water has a strong absorption, then we should check its emission. This relation might not directly apply to our model; however, it helped us to not give it up completely,” said Zhang.

“I always tell my students and researchers here, 'If you try something, you might not get the result you wanted. But if you never try it, you definitely won't get it,'” he said.

The research was published in Applied Physics Letters (doi: org/10.1063/1.4990824).