Design of Transverse Quasi-Phase-Matched Non-Polar/AlN Waveguides for 230-nm Far-UV Second Harmonic Generation

Far-ultraviolet (far-UV) light near 230 nm in wavelength can be used for disinfection and sterilization without harming the human body because it is absorbed by the stratum corneum, which is an outer layer of the skin without nucleic acids [1]. Since AlN has both strong optical nonlinearity and high transparency in the UV region, it is useful for wavelength conversion device application including far-UV light-emitting devices. We have proposed a transverse quasi-phase matched (T-QPM) waveguide wavelength conversion device in which the nonlinear optical coefficient is spatially modulated in the vertical direction by stacking the polarity-inverted nitride semiconductor layers. In our previous study, the polarity-inverted AlN T-QPM device was fabricated and blue second harmonic generation (SHG) was demonstrated [2]. Recently, a method for inverting the polarity of AlN by epitaxial crystal growth has been developed [3]. In the polarity-inverted AlN T-QPM device for far-UV SHG, the thickness of the AlN is about 300 nm, but it is difficult to fabricate such a thin polarity-inverted structure by epitaxial crystal growth. On the other hand, the enhancement of the efficiency can be also expected by employing an upper amorphous core instead of the upper polarity inverted AlN core in comparison with a single AlN core waveguide. In this paper, we propose the non-polar/polar T-QPM waveguide consisting of an amorphous upper core and an AlN lower core and designed for 230 nm far-UV.<br/>The design was carried out using a channel waveguide consisting of HfO₂ as the non-polar material, and <i>c</i>-oriented AlN as the polar material. Sapphire and SiO₂ were used as the substrate and the cladding, respectively. To use the largest nonlinear optical tensor component <i>d</i>₃₃ of AlN, the TM molarizations were used for both fundamental and SH waves. The thickness of the T-QPM waveguides is determined so that it satisfies the modal dispersion phase matching condition between the 460 nm fundamental wave of TM₀₀ mode (TM₀₀^F) and the 230 nm SH wave of <i>n</i>th-order TM mode (TM_0<i>n</i>^SH, <i>n</i> ≥ 1). The nonlinear coupling coefficient <i>κ</i> between TM₀₀^F and TM_0<i>n</i>^SH is calculated by <i>κ</i>=2<i>ωε</i>₀/4∬[<i>E</i>^F(<i>x</i>,<i>y</i>)]²<i>d</i>₃₃<i>E</i>^SH(<i>x</i>,<i>y</i>)<i>dxdy</i>,(1) where <i>ω</i> is an angular frequency of the fundamental wave, and <i>E</i>^F and <i>E</i>^SH are the transverse electric field distributions of TM₀₀^F and TM_0<i>n</i>^SH, respectively. In a single AlN waveguide, <i>κ </i>becomes almost zero because <i>E</i>^SH has positive and negative antinodes. High <i>κ </i>is expected by spatially modulating <i>d</i>₃₃ with inverting the polarity of AlN at one node of the <i>E</i>^SH. In the non-polar/polar T-QPM waveguide with a channel width of 1.2 µm, the maximum <i>κ</i> of 1.6 W^−1/2cm⁻¹ was obtained for TM₀₂^SH . In this design, the thicknesses of the upper HfO₂ core and the lower AlN core were 120 nm and 75 nm, respectively. For comparison, the maximum <i>κ</i> of the polarity inverted AlN T-QPM waveguide was calculated as 4.4 W^−1/2cm⁻¹for TM₀₃^SH.<br/>With pumping by an InGaN laser with a wall plug efficiency (WPE) of 40%, the conversion efficiency <i>η</i> of 75% is required to obtain the total WPE of 30%, which is comparable to that of light-emitting diodes. It was found that a device length of 2.6 cm was required to obtain the <i>η</i> of 75% with the HfO₂/AlN T-QPM waveguide excited at 100 mW. It can be expected to realize a compact and high efficiency far-UV light source by combining a InGaN laser and the HfO₂/AlN T-QPM waveguide.<br/>This work was partly supported by JSPS KAKENHI grant numbers JP16H06415, JP17H01063, JP17H05335, and JP19H02631.<br/>[1] T. Fukui <i>et al</i>., PLoS ONE <b>15</b>, e0235948 (2020).<br/>[2] A. Yamauchi <i>et al</i>., LEDIA 2019, 8-3 (2019).<br/>[3] K, Shojiki et al., CGCT-8, C01-01-05, (2021)