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. 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. 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. 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).
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