Internal-loss-limited 5.5% wall-plug efficiency in index-guided AlGaN UV-B laser diodes with polarization-doped cladding layers
Rintaro Miyake, Takumu Saito, Shion Kamiya, Ryota Watanabe, Yuma Miyamoto, Seiya Kato, Naoki Kitta, Rintaro Kobayashi, Tomoya Tanikawa, Kenta Kitagawa, Sho Iwayama, Hideto Miyake, Tetsuya Takeuchi, Satoshi Kamiyama, Motoaki IwayaThis study identifies the dominant factor governing wall-plug efficiency (WPE) in AlGaN-based UV-B laser diodes. While previous studies have mainly focused on optical confinement, the limiting mechanism has remained unclear. Here, we investigate index-guided ridge waveguide laser diodes and systematically analyze the relationship between internal optical loss (αi) and device performance. Index-guided operation is confirmed by effective index simulations and near-field measurements. We find that device performance is governed primarily by αi rather than optical confinement. By minimizing αi, the threshold current is reduced, and the slope efficiency is improved, leading to a maximum WPE of 5.5% in an uncoated device. These results reveal that reducing αi, rather than enhancing optical confinement, is the key design principle for improving WPE in AlGaN-based UV-B laser diodes.