Eu,O-Codoped GaN-Based High-Q Two-Dimensional Photonic Crystal Cabvities in the Red Region

Eu,O-codoped GaN (GaN:Eu,O) is an efficient red light emitting material based on the GaN platform. We have demonstrated red light-emitting diodes using GaN:Eu,O grown by organometallic vapor phase epitaxy method and reported a high light output power of 1.25 mW at 20 mA operation with a maximum external quantum efficiency over 9% [1,2]. Furthermore, we have shown that GaN:Eu,O has a moderate value of gain of 6.6 cm^-1 [3]. This observation suggests the possibility to develop a novel GaN:Eu,O-based red laser diodes (LDs). The emission wavelength from Eu³⁺ ions is highly stable with respect to the surroundings, thus GaN:Eu,O-based red LDs have a large potential applications including high-resolution laser displays and optical communications. In this contribution, we designed and fabricated an optical cavity with a high <i>Q</i>-factor in the red region, which is required for the realization of a laser based on this material [4].<br/>Although two-dimensional photonic crystal (2D-PhC) cavities are prominent candidates to achieve high <i>Q</i>-factors, they typically show relatively low <i>Q</i>-factors (&lt; 5000) due to structural disorder that is inevitably introduced during the fabrication stages [5]. From an extensive analysis of the influence of such disorder for various cavity designs, we have shown that especially H3-type cavities are much less sensitive to optical losses, when compared to linear (L<i>N</i>) cavities [4]. We have fabricated H3-type cavities and demonstrated a highest value of <i>Q</i> of 7900. This value is much higher than previously reported <i>Q</i>-factors (~5500) of III-nitride-based 2D-PhC cavities in the visible range, and approaches the theoretical <i>Q</i>-factor of this cavity design. These results demonstrate that H3-type cavities can indeed effectively suppress the disorder-induced optical losses, and that a further improvement of the design is required with a higher theoretical <i>Q</i>-factor, in order to achieve higher experimental values.<br/>We propose a 2D-heterostructure with core, trans and clad region. By designing the radii as <i>r</i>_core &gt; <i>r</i>_trans &gt;<i> r</i>_outer, the photonic band structure is modified and light at the photonic band-edge is well confined in the core region. For the optimal design parameters, a theoretical value of the <i>Q</i>-factor of 1.1×10⁵was obtained.<br/>The fabricated structure of this design shows a few resonant modes, with the mode with the largest <i>Q</i>-factor reaching a value of <i>Q</i> = 10500 [6], which is almost twice that of previously reported values. Thus, we conclude that this type of III-nitride-based 2D-PhC cavity is effective for confining light, and has a great promise for obtaining ultrahigh <i>Q</i>-factors in the visible region. This study shows the guideline to achieve high <i>Q</i>-factors with III-nitride-based 2D-PhC cavities in the red region, which would open up a larger potential application.<br/><b>Acknowledgments</b><br/>This work was supported by JSPS KAKENHI Grant Nos. 18H05212, 22K14614 and No.23H05449, and by Nanotechnology Platform of MEXT, Grant Number JPMXP09F20OS0026 and JPMXP09S20OS0021.<br/><b>References </b><br/>[1] A. Nishikawa, Y. Fujiwara <i>et al.</i>, <i>Appl. Phys. Express</i> <b>2</b>, 071004 (2009).<br/>[2] B. Mitchell, Y. Fujiwara <i>et al.</i>, <i>J. </i><i>Appl. Phys. </i><b>123</b>, 160901 (2018).<br/>[3] A. Takeo, Y. Fujiwara <i>et al.</i>, <i>Jpn. J. Appl. Phys. </i><b>60</b>, 120905 (2021).<br/>[4] T. Iwaya, Y. Fujiwara,<i> et al.</i>, <i>Appl. Phys. Express</i> <b>14</b>, 122002, (2021).<br/>[5] R. Butté and N. Grandjean, <i>Nanophotonics</i> <b>9</b>, 569, (2020).<br/>[6] T. Iwaya, Y. Fujiwara,<i> et al.</i>, <i>Opt. Express</i> <b>30</b>, 28853, (2022).