Scientists convert infrared rays to blue light for sterilization

OSAKA, Japan, June 25, 2021 — A microcavity device that converts infrared (IR) radiation into blue light could enable safe, daily use of deep ultraviolet (DUV) light for disinfection and sterilization. 

The device was developed by a search group from Osaka University and was built without a polarity-inverted structure. the shortage of reliance on birefringence or the periodically polarization inverted structure expands the pliability within the selection of the device structures and therefore the materials — giving the researchers more flexibility within the selection of structures and materials for the wavelength conversion to convert to DUV light. 
A DUV wavelength range of 220 to 230 nm is desirable if DUV light is to be used safely as a disinfectant. Although wavelength conversion offers a possible solution to achieving this as-yet-unrealized wavelength, conventional ferroelectric wavelength conversion materials can't be used, thanks to the absorption edge. 

To realize DUV light for bactericidal use, the researchers constructed a monolithic, microcavity, second-harmonic-generation (SHG) device employing a low-birefringence paraelectric material and a dielectric material. They used two high-reflectivity distributed Bragg reflectors (DBRs) to double the frequency of sunshine coming into the device. The DBRs strengthened the intensity of a fundamental wave within the microcavity. Counter-propagating SHG waves were efficiently generated during a very short region on the brink of a coherence length. 

As a primary step toward the application of their approach, the researchers built a gallium nitride (GaN) micro cavity device using microfabrication technology, including dry etching and anisotropic wet etching for vertical and smooth DBR sidewalls. once they demonstrated wavelength conversion within the GaN microcavity, they observed a 428-nm blue SHG wave within the absence of a polarity-inverted structure. 

Nitride semiconductors like GaN and aluminum nitride (AlN) have relatively high optical nonlinearity, making them suitable to be used in wavelength conversion devices; AlN is especially suitable for DUV wavelength conversion devices, thanks to its transparency to 210 nm. However, realizing structures with periodically inverted polarity, just like the structures in conventional ferroelectric wavelength conversion devices, has proven to be difficult using these materials. 

The researchers hope their flexible approach to wavelength conversion will make future nonlinear optical devices easier to construct.

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