In an article recently published in the journal Applied Physics Express, researchers designed and fabricated a human-safe transverse quasi-phase-matched (QPM) aluminum nitride (AlN) channel waveguide with vertical polarity inversion that can kill pathogens on surfaces.
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Importance of Far-ultraviolet (UV) Light
UV light sources are utilized in different applications, including material processing and sensing. In recent years, far-UV light in the 210–230 nm wavelength range has gained significant attention for bacterial disinfection and viral inactivation without harming the exposed human eyes and skin due to its strong absorption by non-living surface tissues.
Although Excimer lamps at 222 nm wavelength are commercially available, they have several disadvantages, including short lifetime, low efficiency, and the use of expensive gases. Similarly, the efficiency of light-emitting diodes (LEDs) reduces significantly at wavelengths shorter than 250 nm.
However, an LED with the shortest emission wavelength at 210 nm has been realized using AlN with a wide band gap of ≈6.0 eV. In aluminum gallium nitride (AlGaN) laser diodes (LDs), the shortest lasing wavelength has been 271.8 nm. The insulating nature of AlGaN is a major challenge in realizing far-UV LEDs and LDs, which necessitated the development of novel far-UV light sources.
Wavelength Conversion to Realize Far-UV Light
Far-UV light sources can be realized using wavelength conversion technology. Far-UV wavelength conversion systems using bulk crystals, such as cesium lithium borate (CLBO), are used extensively for industrial applications. Although these crystals possess small nonlinear optical constants, high conversion efficiency can still be achieved under high-power excitation.
In waveguide devices with QPM structures that possess periodic polarity inversion in the longitudinal direction, the high normalized conversion efficiency can be attained owing to long interaction lengths while maintaining a high power density under low power excitation.
Ferroelectric crystals, such as lithium tantalate and lithium niobate, are used to manufacture these devices for wavelengths longer than 300 nm due to their large optical nonlinearity.
Although lanthanum barium germanate is transparent till 190 nm wavelength, high-efficiency wavelength conversion cannot be achieved using it due to small optical nonlinearity.
AlN is a suitable material for far-UV light generation owing to its large optical nonlinearity and transparency to far-UV light. UV second harmonic generation (SHG) has been demonstrated in an AlN waveguide with a thickness satisfying a modal dispersion phase matching (MDPM) condition at 306 nm without the QPM structure.
A third-order QPM structure was also synthesized in the AlN channel waveguide to increase the efficiency and UV SHG was displayed at 344 nm. No SHG has been reported in AlN waveguides with first-order QPM structures fabricated for 250–275 nm UV light.
The efficiency of wavelength conversion can be increased using a vertically polarity-inverted multilayer structure that satisfies the MDPM condition/transverse QPM structure.
The Proposed Approach
In this study, researchers designed and fabricated a transverse QPM SHG channel waveguide with a vertical polarity inverted AlN bilayer. 229 nm far-UV SHG was demonstrated successfully from the channel waveguide consisting of a −c/+c-AlN bilayer/ upper −c-AlN and lower +c-AlN layers on a c-plane sapphire substrate fabricated using sputtering and post-deposition face-to-face annealing (FFA).
The prototype device fabrication method for the generation of far-UV light was primarily based on techniques from semiconductor processing, which enabled precise control of the AlN crystal orientation.
Initially, a 210 nm-thick +c-AlN layer was deposited on a c-plane sapphire substrate using radio frequency (RF) sputtering with a sintered AlN target, followed by a post-deposition FFA. Then, a 280 nm-thick −c-AlN layer was deposited on the +c-AlN layer using RF sputtering with a metallic Al target, followed by another post-deposition FFA.
Inductively coupled plasma-reactive ion etching (ICP-RIE) was used with chlorine and boron trichloride gases to reduce the thickness of the AlN bilayer, with boron trichloride gas being used to eliminate unintentionally formed oxides at the surface.
Additionally, the bias and antenna powers were set at 10 W and 250 W, respectively, which were lower compared to the powers used during the channel formation process to decrease the etching rate for precise thickness control, and a polarity-inverted −c-AlN/+c-AlN layer with 352 nm thickness was obtained.
Subsequently, a 200 nm-thick silicon dioxide layer was deposited using plasma-enhanced chemical vapor deposition (PECVD). The silicon dioxide masks were formed using capacitively coupled plasma RIE (CCP-RIE) with carbon tetrafluoride/hydrogen gas.
ICP-RIE was used to fabricate the channel waveguides at bias and antenna powers of 50 W and 400 W, respectively. Eventually, a one μm thick silicon dioxide cladding layer was deposited using PECVD and the channel waveguide end faces were formed by dicing and polishing using diamond slurry. The fabricated waveguide length was approximately two mm.
A tunable frequency-doubled femtosecond titanium (Ti):sapphire laser with a beta-barium borate (BBO) crystal as a pump source was used to perform the optical experiments. The power and polarization of the pump light were adjusted using a Glan–Thomson prism and a half-wave plate.
In the fabricated transverse QPM SHG device with a vertical polarity inverted AlN bilayer, the silicon dioxide cladding layer was formed to protect the channel waveguide during end face formation. However, the layer led to a slight reduction of optical confinement.
Additionally, the channel waveguide cross-sectional dimension was designed to satisfy the MDPM condition between an SH wave of a high-order transverse magnetic (TM0q) mode and a fundamental wave of a first-order TM00-guided mode. The TM-polarized fundamental wave was selected to utilize the largest nonlinear optical tensor component of d33 of AlN, and the TM-polarized SH wave was generated.
Significance of the Study
Researchers successfully obtained a far-UV light with a central wavelength of 229 nm. Far-UV SHG through the largest nonlinear optical tensor component d33 was confirmed under ultrashort pulse laser excitation from the wavelength spectra and pump power dependence of an SH intensity.
The full widths at half maximum of the SH spectra and pump were 1.0 nm and 2.7 nm, respectively. Additionally, the SH light peak intensity was proportional to the average pump power. These observations confirmed the far-UV SHG in the vertical polarity inverted AlN bilayer channel waveguide.
Moreover, the wavelength spectra of the SH lights for transverse electric (TE)- and TM-polarized pump lights displayed that the SH intensity for the TM pump light was significantly stronger compared to the SH intensity for the TE pump light, which indicated that SHG was achieved through the nonlinear optical tensor component of d33.
To summarize, the wavelength of the UV light emitted by the prototype device can effectively kill germs without posing any risk to human health, which demonstrates that efficiency and compactness can be successfully realized in far-UV disinfection tools without compromising human safety.
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References and Further Reading
Katayama, R., Tanikawa, T., Uemukai, M., Tonouchi, M., Murakami, H., Serita, K., Fujiwara, Y., Tatebayashi, J., Ichikawa, S., Miyake, H., Shojiki, K., Umeda, S., Honda, H. (2023). 229 nm far-ultraviolet second harmonic generation in a vertical polarity inverted AlN bilayer channel waveguide. Applied Physics Express, 16, 062006. https://doi.org/10.35848/1882-0786/acda79
Researchers create optical device that can kill pathogens on surfaces while remaining safe for humans. [Online] Available at https://phys.org/news/2023-09-optical-device-pathogens-surfaces-safe.html
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