Researchers have developed an optical device that can kill pathogens on surfaces while remaining safe for humans

While it has long been known that ultraviolet (UV) light can help kill pathogens, the COVID-19 pandemic has put a spotlight on how this technology can remove pathogens from the environment. However, excimer lamps and LEDs that can directly emit light at the desired depth of UV wavelengths are generally less efficient or suffer from a short life. In fact, UV light of the wrong wavelength can be harmful to human cells.

Now, a team led by researchers from Osaka University has shown how an optical device made of aluminum nitride can be used to produce deep UV light in a completely different way from previous methods. The team used a process called “second harmonic generation,” which relies on the fact that the frequency of a photon, or particle of light, is proportional to its energy. The study was published in the journal Applied Physics Express.

Most visible materials are considered “linear” in terms of their response to light, that is, photons cannot interact with each other. However, within certain “non-physical” materials, two photons can be combined into a single photon with twice the energy, and thus, twice as much.

In this case, two visible photons can be combined into one deep UV photon inside an aluminum nitride waveguide less than one micron in diameter. A waveguide is a channel of transparent material with a physical size chosen so that light of desired frequencies can travel easily. The waveguide helps to take advantage of the nonlinear optical characteristics of the material, so that the generation of the second harmonic can occur with high efficiency.

“Our new method of producing deep-UV light lends itself to semiconductor processing, which allows precise control of the orientation of the aluminum-nitride crystal. This was difficult to achieve in the past,” explains lead author Hiroto Honda.

The wavelength of UV light created by the prototype device is in a very narrow range that is strong enough to kill germs but remains harmless to humans.

“The results of our project help demonstrate that compactness and efficiency are possible in deep UV disinfection devices, without sacrificing human safety,” said senior author Ryuji Katayama. Researchers hope to refine this method to produce commercial devices that use less energy than previous options.