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Daily-Use Light Source Converts IR to Blue Light for Sterilization, Disinfection

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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 research group from Osaka University and was built without a polarity-inverted structure. The lack of reliance on birefringence or the periodically polarization inverted structure expands the flexibility in the selection of the device structures and the materials — giving the researchers more flexibility in the selection of structures and materials for the wavelength conversion to convert to DUV light.

Digital camera image of blue second harmonic light emission with a wavelength of 428 nm using an IR cutoff filter. Courtesy of Osaka University.
Digital camera image of blue second-harmonic light emission with a wavelength of 428 nm using an IR cutoff filter. Courtesy of Osaka University.
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 potential solution to achieving this as-yet unrealized wavelength, conventional ferroelectric wavelength conversion materials cannot be used, due to absorption edge.

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

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

Schematic of GaN monolithic microcavity SHG device on an Si pedestal structure. Courtesy of Osaka University.
Schematic of GaN monolithic microcavity SHG device on an Si pedestal structure. Courtesy of Osaka University.
Nitride semiconductors such as GaN and aluminum nitride (AlN) have relatively high optical nonlinearity, making them suitable for use in wavelength conversion devices; AlN is particularly suitable for DUV wavelength conversion devices, due to its transparency to 210 nm. However, realizing structures with periodically inverted polarity, like the structures in conventional ferroelectric wavelength conversion devices, has proven to be difficult using these materials.

“Our device can be adapted to use a broader range of materials,” professor Masahiro Uemukai said. “They can be applied to deep ultraviolet light emission or even broadband photon pair generation.”

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

The research was published in Applied Physics Express (www.doi.org/10.35848/1882-0786/abff9e).

Published: June 2021
Glossary
deep ultraviolet
Deep ultraviolet (DUV or deep-UV) refers to a specific range of ultraviolet light with shorter wavelengths than those in the UV-A and UV-B regions. The exact wavelength range considered as DUV can vary, but it often includes wavelengths from around 100 to 300 nanometers. DUV light is shorter in wavelength and higher in energy than visible light. DUV light covers the spectrum from approximately 100 to 300 nanometers. This range is subdivided into different sub-bands, such as UV-C (100 to...
ultraviolet
That invisible region of the spectrum just beyond the violet end of the visible region. Wavelengths range from 1 to 400 nm.
polarization
Polarization refers to the orientation of oscillations in a transverse wave, such as light waves, radio waves, or other electromagnetic waves. In simpler terms, it describes the direction in which the electric field vector of a wave vibrates. Understanding polarization is important in various fields, including optics, telecommunications, and physics. Key points about polarization: Transverse waves: Polarization is a concept associated with transverse waves, where the oscillations occur...
birefringence
Birefringence is an optical property of certain materials that causes them to exhibit different refractive indices for light of different polarizations. In other words, when light passes through a birefringent material, it splits into two separate rays, each traveling with a different speed and direction. This phenomenon is also known as double refraction. The difference in refractive indices for the two polarizations is a characteristic feature of birefringent materials. The separation of...
second-harmonic generation
Second-harmonic generation (SHG) is a nonlinear optical process that occurs when two photons with the same frequency combine within a nonlinear material, resulting in the generation of a new photon with twice the frequency (and therefore half the wavelength) of the original photons. This phenomenon is a specific case of second-order nonlinear optical effects. Key points about second-harmonic generation include: Nonlinear optical process: SHG is a nonlinear optical effect, meaning that the...
Research & TechnologyeducationAsia PacificOsaka UniversityJapanUVDUVdeep ultravioletmedicalultravioletLight SourcesMaterialsOpticspolarizationwavelength conversionmicromicromachineslight conversioninfrared radiationnonlinear optical devicesnanoscale and nonlinear optical design and measurementstructuresbirefringencebirefringent componentssecond-harmonic generation

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