Water coaxed to emit terahertz radiation

Electromagnetic radiation in the terahertz (THz) range has been generated from a variety of materials, including solids, plasmas and water vapor. However, investigators doubted that liquid water, which strongly absorbs in the THz range, could be coaxed to emit THz radiation. A team of investigators led by University of Rochester physicist Xi-Cheng Zhang has developed a new method to generate THz radiation using a liquid water film and laser, as reported in Applied Physics Letters.

Zhang’s group predicted that a thin film of water might avoid absorption problems. An 800-nm laser pulsed at a repetition rate of 1kHz was focused onto a flowing thin film of water through a parabolic mirror. The film was created using two aluminum wires separated by 4mm, which produced a 177-micron thick water film. They successfully generated THZ waves and demonstrated that the waves are significantly sensitive to the relative position of the beam’s focus and the film.

The wavelength of THz radiation emitted from water is between that of infrared light and microwaves, and thereby has less energy than the band gap of many nonmetallic materials, enabling it to penetrate and make imaging of most solid objects possible. This could offer specialized uses in manufacturing, medical imaging and security, such as in airport scanners. Moreover, since THz radiation has low photon energy, it is unlikely to lead to damage from photo-ionization, in contrast to commonly used X-ray imaging, which also has a large penetration depth but with much higher photon energies.

The interaction between laser light and water is still not fully understood, Zhang says, and he hopes that his group, together with their collaborators from China and Russia, will soon achieve a more detailed understanding of the physics involved.

Source: “Observation of broadband terahertz wave generation from liquid water,” by Qi Jin, Yiwen E, Kaia Williams, Jianming Dai, and X.-C. Zhang, Applied Physics Letters (2017). The article can be accessed at https://doi.org/10.1063/1.4990824 .