Generating terahertz radiation from water makes 'the impossible, possible'

phys.org ^ | 09-28-2017 | Provided by: University of Rochester

[Red Badger]

Xi-Cheng Zhang has worked for nearly a decade to solve a scientific puzzle that many in the research community believed to be impossible: producing terahertz waves—a form of electromagnetic radiation in the far infrared frequency range—from liquid water.

Now, as reported in a paper published in Applied Physics Letters, researchers at the University of Rochester have "made the impossible, possible," says Zhang, the M. Parker Givens Professor of Optics. "Figuring out how to generate terahertz waves from liquid water is a fundamental breakthrough because water is such an important element in the human body and on Earth."

Terahertz waves have attracted increased attention recently because of their ability to nondestructively pass through solid objects, including those made of cloth, paper, wood, plastic, and ceramics, and produce images of the interiors of the objects. Additionally, the energy of a terahertz photon is weaker than an x-ray photon. Unlike x-rays, terahertz waves are non-ionizing—they do not have enough energy to remove an electron from an atom—so they do not have the same harmful effects on human tissue and DNA.

Because of these abilities, terahertz waves have unique applications in imaging and spectroscopy—everything from discovering bombs in suspicious packages, to identifying murals hidden beneath coats of paint, to detecting tooth decay.

"Terahertz waves have a capacity to see through clothing, which is why you have these sub-terahertz body scanners at airports," Zhang says. "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."

Zhang's research group uses lasers to generate terahertz pulses via interaction with a target. In this case, the target is an extremely thin film of water—approximately 200 microns or about the thickness of two pieces of paper—created using water suspended by surface tension between two aluminum wires. Researchers focus a laser into the water film, which acts as an emitter for the terahertz radiation output.

Previous researchers have generated terahertz waves from targets of solid crystals, metals, air plasma, and water vapor, but, until now, liquid water has proved elusive.

The experimental set-up 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. Credit: University of Rochester / Xi-Cheng Zhang Lab

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

In fact, when researchers measured the terahertz waves generated by the water, they found they were 1.8 times stronger than the terahertz waves generated from air plasma under comparable experimental conditions.

Because water is such a strong absorber, however, many people in the research community believed it would be impossible to use water as a target. Zhang himself has spent years attempting a solution, and he found a likewise stalwart in Qi Jin, now a PhD candidate in optics at Rochester, and the lead author on the paper.

"Almost everybody thought we wouldn't be able to get a signal from water," Jin says. "At first, I didn't believe it either."

One of the challenges was creating a film of water thin enough that the terahertz photons generated by the laser beam would not be absorbed, but thick enough to withstand the laser's energy.

Along with Yiwen E, a postdoctoral associate in Zhang's research group, Jin spent months optimizing the thickness of the water film and the incident angle, intensity, and pulse duration of the laser beam.

"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," Jin says. "Water is one of the richest resources on Earth, so it was really important for us to be able to generate these waves from water. There were many times I wanted to give up on this, but people in the lab kept encouraging me."

Zhang agrees: "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."

Explore further: Once thought impossible, scientists demonstrate that liquid water can generate THz waves

More information: Qi Jin et al. Observation of broadband terahertz wave generation from liquid water, Applied Physics Letters (2017). DOI: 10.1063/1.4990824

Journal reference: Applied Physics Letters search and more info website

[Red Badger]

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. Credit: University of Rochester photo / Kaia Williams

[Red Badger]

[posterchild]

It’s a good thing he didn’t believe anyone telling him ‘the science is settled.’

[TexasGator]

” “Water is one of the richest resources on Earth, so it was really important for us to be able to generate these waves from water. “

I am not understanding this.

[Paladin2]

The Scientific Consensus was wrong? Again?

Does ManBearPig know?

[tired&retired]

Very Interesting....

What about Holy Water?

[Red Badger]

IOW, no exotic and therefore expensive, materials required..............

[Red Badger]

Then you get Holy Tera-hertz..................

[tired&retired]

I wonder if this is how I feel density differences several feet from my hands...

I’ve used this ability in the past when screwing drywall to the studs. I just move my open hand and I feel the stud location behind the drywall.

It also works on brain tumor locations....

I read about this technology several years ago in Japanese research. They were using it to determine contents of rooms in the pyramids. Kind of like an energy ultrasound.

[TexasGator]

“IOW, no exotic and therefore expensive, materials required..............”

It’s only a few microns thick. This cost of this part of the generator is minor compared to the rest of the equipment.

[tired&retired]

Per Wiki:

Terahertz radiation – also known as submillimeter radiation, terahertz waves, tremendously high frequency[1] (THF), T-rays, T-waves, T-light, T-lux or THz – consists of electromagnetic waves within the ITU-designated band of frequencies from 0.3 to 3 terahertz (THz; 1 THz = 1012 Hz). Wavelengths of radiation in the terahertz band correspondingly range from 1 mm to 0.1 mm (or 100 μm). Because terahertz radiation begins at a wavelength of one millimeter and proceeds into shorter wavelengths, it is sometimes known as the submillimeter band, and its radiation as submillimeter waves, especially in astronomy.

Photon energy in the THz regime is less than the band-gap energy of non-metallic materials and thus THz radiation can penetrate such materials. THz beams transmitted through materials can be used for material characterization, layer inspection, and as an alternative to X-rays for producing high resolution images of the interior of solid objects.[2]

Terahertz radiation occupies a middle ground between microwaves and infrared light waves known as the “terahertz gap”, where technology for its generation and manipulation is in its infancy. It represents the region in the electromagnetic spectrum where the frequency of electromagnetic radiation becomes too high to be measured digitally via electronic counters, so must be measured by proxy using the properties of wavelength and energy. Similarly, the generation and modulation of coherent electromagnetic signals in this frequency range ceases to be possible by the conventional electronic devices used to generate radio waves and microwaves, requiring the development of new devices and techniques.

[tired&retired]

This does explain what I feel with my hands:

Here is a research study:

PHYSICS IN MEDICINE AND BIOLOGY Phys. Med. Biol. 49 (2004) 1595–1607

In vivo study of human skin using pulsed terahertz radiation

Abstract

Studies in terahertz (THz) imaging have revealed a significant difference between skin cancer (basal cell carcinoma) and healthy tissue. Since water has strong absorptions at THz frequencies and tumours tend to have different water content from normal tissue, a likely contrast mechanism is variation in water content. Thus, we have previously devised a finite difference timedomain (FDTD) model which is able to closely simulate the interaction of THz radiation with water. In this work we investigate the interaction of THz radiation with normal human skin on the forearm and palm of the hand in vivo. We conduct the first ever systematic in vivo study of the response of THz radiation to normal skin.

[Red Badger]

The simplest explanation is Temperature Differential.

The human skin can detect temperature differences of 0.1°F.

You feel the wall and the temperature of where the stud is versus where the empty air space is detected by your hand as a location device.....................

[Windflier]

I’ve used this ability in the past when screwing drywall to the studs. I just move my open hand and I feel the stud location behind the drywall.

I believe you. I've experienced similar kinds of heightened perception now and again in life.