Recently, electromagnetic radiation in the terahertz (THz) frequency range has emerged as one of the most promising imaging techniques for a variety of applications in science and engineering.  The potential and suitability of the THz technology for practical applications such as the nondestructive testing field has been released by the recent progress in producing efficient sources and detectors. Thanks to the development of ultra-fast components in both photonics and electronics, the situation is evolving rapidly.

THz waves, residing at a relatively unexplored region between the microwave and infrared, roughly 0.1-10THz, is one of the last frontiers in the electromagnetic spectrum.  Unlike X-ray, THz is a non-ionizing radiation. It causes no known harm to the human body and the materials being examined.  Moreover, THz can penetrate many common gases, non-polar liquids, and non-metallic solids including air, plastics, gasoline, paper, plant material, clothing, fatty tissue, and composites.  However, all these promising potential application depends on reliable tools and especially the THz source.

“Terahertz waves have a capacity to see through clothing, which is why you have these sub-terahertz body scanners at airports […] 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.”

Building a THz generator is a real challenge. The most common way is to use a pulsed system with a laser. It works by splitting a beam from a femtosecond laser into two: the probe and the pump beams. The pump beam is used to generate the THz pulse, while the probe beam is used to sample and obtain the pulse profile. The THz waveform can be obtained by scanning the time delay between the two signals. To increase the sensitivity, the pump beam is modulated by an optical chopper, and the THz-induced modulation on the probe beam is extracted by a lock-in amplifier. In a recent work, X.-C. Zhang from The Institute of Optics, NY, USA proposed a new way to generate THz waves.

Experimental setup. Broadband THz wave is generated by tightly focusing the optical laser beam into a gravity-driven wire-guided freeflowing water film.

With successive development, numerous research groups have demonstrated terahertz waves generation from solid crystals, metals, gas plasmas or even water vapors. Until now liquid water was never used before as a possible source for THz radiation. One possible reason leading to the impediment is that liquid water has strong absorption characteristics in the THz frequency regime. And recently Zhang team demonstrated gravity-driven, free-flowing water films could be efficient with their simple design and almost unmatched ability to generate a thin, continuous, and stable film of liquid water in free space which offers us the liquid source for the THz radiation.

“We always tried to avoid water, but it is a surprisingly efficient terahertz source.[…] 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.[…] 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, The Institute of Optics, NY, USA

The team experimentally demonstrated the generation of broadband THz waves from liquid water excited by femtosecond laser pulses. Their measurements reveal the critical dependence of the THz field upon the relative position between the water film and the focal point of the laser beam. The THz radiation from liquid water shows distinct characteristics when compared with the THz radiation from air plasmas with single color optical excitation.


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