As early as the mid-20th century, ultraviolet (UV) light has been used for sterilization and disinfection. With technological advancements, experts were able to develop UV bulbs and UV LEDs with smaller sizes and a reliable long lifespan, broadening the field where they can be used.
The low-pressure mercury vapor discharge lamp is the UV light source normally used today. There are also emerging deep-UV (DUV) chip-scale technologies that deploy disinfection systems using devices that emit in the deep-UV spectral band.
Deep UV Light Sterilization
The ultraviolet light, a component of the electromagnetic spectrum, is divided into three spectral bands, namely the UVA (320-400 nm), UVB (280-320 nm), and UVC (200-280 nm). Among these three, UVA is the one that is known to cause skin damage, while UVB causes sunburn in the daylight. UVA and UVB easily enter the Earth's atmosphere and harm our skin due to their long penetration depth into the human tissue.
On the other hand, UVC, also known as deep UV, has wavelengths blocked by the ozone in the Earth's atmosphere. Since it is not present in sunlight at the surface of our planet, pathogens have not evolved defenses against this type of radiation. Because of this, UVC wavelengths can exhibit a phenomenon known as ultraviolet germicidal irradiation (UVGI). It can destroy the ability of microorganisms to reproduce by causing photochemical changes in their nucleic acids.
Constructing UVC Light Source With Bactericidal Effect
Light sources in the wavelength range of UVC have not yet been developed. The excimer lamps and LEDs can directly emit light in the deep-UV wavelengths, but they either have low efficiency or short life spans.
To address this challenge, scientists from Osaka University developed an optical device to generate deep-UV light using a completely different approach. The team, led by Hiroto Honda, used the second harmonic generation process, which relies on the proportionality of a light particle (photon) and its energy.
With respect to their response to light, most transparent materials are considered "linear." However, inside certain "nonlinear" materials, two photons can be merged into a single photon with twice the energy and twice the frequency.
In this study, two visible photons are combined into a single deep-UV photon inside an aluminum nitride waveguide, which is less than one micron wide. A waveguide refers to a channel of transparent material with chosen physical dimensions that allow light of the desired frequency to travel easily. It helps the experts take advantage of the nonlinear optical attributes of the material in such a way that second harmonic generation can take place with the highest efficiency.
According to Honda, their novel fabrication method for deep-UV light generation was inspired by the techniques in semiconductor processing. These techniques enable the precise control of the orientation of the aluminum-nitride crystal.
Their prototype device creates UV light with a wavelength within a very narrow range, having enough energy to kill disease-causing pathogens while remaining harmless to humans.
Senior author Ryuji Katayama claims that the results of their project help demonstrate that compactness and efficiency are possible for deep-UV disinfection tools. In the future, the research team plans to refine their method to create commercial devices that consume less energy than previous methods.
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