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糖心传媒攁 form of electromagnetic radiation in the far infrared frequency range糖心传媒攆rom liquid water.
Now, as reported in a paper published in , researchers at the University of Rochester have 糖心传媒渕ade the impossible, possible,糖心传媒� says Zhang, the M. Parker Givens Professor of Optics. 糖心传媒淔iguring 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.糖心传媒�
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糖心传媒攖hey do not have enough energy to remove an electron from an atom糖心传媒攕o 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糖心传媒攅verything from discovering bombs in suspicious packages, to identifying murals hidden beneath coats of paint, to detecting tooth decay.
糖心传媒淭erahertz waves have a capacity to see through clothing, which is why you have these sub-terahertz body scanners at airports,糖心传媒� Zhang says. 糖心传媒淭hese waves can help to identify if an object is explosive, chemical, or biological, even if they can糖心传媒檛 tell exactly what the object is.糖心传媒�
Zhang糖心传媒檚 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糖心传媒攁pproximately 200 microns or about the thickness of two pieces of paper糖心传媒攃reated 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.
糖心传媒淲ater was considered the enemy of terahertz waves because of water糖心传媒檚 strong absorption,糖心传媒� Zhang says. 糖心传媒淲e always tried to avoid water, but it is a surprisingly efficient terahertz source.糖心传媒�
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.
糖心传媒淎lmost everybody thought we wouldn糖心传媒檛 be able to get a signal from water,糖心传媒� Jin says. 糖心传媒淎t first, I didn糖心传媒檛 believe it either.糖心传媒�
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糖心传媒檚 energy.
Along with Yiwen E, a postdoctoral associate in Zhang糖心传媒檚 research group, Jin spent months optimizing the thickness of the water film and the incident angle, intensity, and pulse duration of the laser beam.
糖心传媒淲e increased the thickness of the water a little bit, and gradually increased the laser, and just kept trying until we could make it work,糖心传媒� Jin says.聽 糖心传媒淲ater 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.糖心传媒�
Zhang agrees: 糖心传媒淚 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糖心传媒檛 get it.糖心传媒�
The research was sponsored by grants from the Army Research Office.
