Sung-Un Kim

Programmable photonic computing requires nanoscale cavities that can be driven between distinct optical states using simple and physically controllable optical inputs. Here, we demonstrate logic-compatible all-optical modal switching in GaN nanorod cavities through artificial refractive-index engineering under ultraviolet excitation. At room temperature, the nanorods exhibit lasing at 369.4 nm. As the pump fluence increases above the threshold, the initial single lasing line evolves into reproducibly split modes around the original emission wavelength. Power-dependent micro-photoluminescence measurements, laser-heating simulations, and temperature-dependent finite-difference time-domain calculations collectively show that localized optical heating increases the GaN refractive index and reorganizes the cavity modes. The resulting thermo-optic perturbation detunes the initial triangle-like whispering-gallery mode and promotes linearly polarized-like modes at higher excitation, producing reversible modal switching during repeated pump cycling without progressive spectral drift. These results identify the physical origin of pump-induced peak splitting in GaN ultraviolet nanorod cavities and establish a compact III-nitride nanolaser platform whose cavity states can be optically programmed. Such controllable modal-state engineering offers a promising route toward ultraviolet nanophotonic switches, optical-state encoders, and prospective building blocks for all-optical logic and next-generation photonic computing.

Direct photoelectrochemical splitting of natural seawater offers a scalable route to green hydrogen but is constrained by an interfacial paradox: alkaline conditions that accelerate hydrogen evolution also induce Mg/Ca hydroxide scaling, rapidly blocking semiconductor surfaces. Here, we report a membrane-defined III–nitride photocathode that stabilizes the seawater interface and enables catalyst-free hydrogen evolution directly in Yellow Sea seawater. N-polar InGaN quantum pyramids integrated on vertical GaN nanowires provide broadband (UV–NIR) absorption and a composition-graded “quantum-sliding” band profile, funneling photogenerated electrons toward the pyramid apex and the catalytically active N-polar surface. A conformal graphene-oxide/polylactic-acid (GO/PLA) nano-membrane laminated on the nanowire forest serves as an ion-sieving transport layer, preserving water access while suppressing multivalent-ion-driven precipitation and electrode blockage. X-ray photoelectron spectroscopy quantifies heterointerfacial band offsets, while Raman spectroscopy and electron microscopy confirm conformal encapsulation and mitigated deposits after operation. The photocathode sustains ~0.5 mA cm−2 at −0.45 V versus RHE for over 200 h in untreated Yellow Sea seawater (pH 7) and reaches ~−16.51 mA cm−2 in Yellow Sea water containing 0.5 M NaOH, with onset potentials of 0.26–0.29 V versus RHE. This membrane–band co-engineering strategy decouples semiconductor photophysics from electrolyte complexity, enabling practical solar hydrogen production from unprocessed seawater.

High‐efficient red emission in III‐nitride semiconductors remains a significant challenge due to crystal degradation caused by high In content and non‐radiative recombination. In this study, we propose a 405 nm laser‐induced re‐crystallization (Re‐Cryst) process to enhance the optical performance of 665 nm red‐emitting core‐shell InGaN/GaN nanowires grown by metal‐organic chemical vapor deposition. Unlike conventional thermal annealing, this laser‐based approach enables localized defect healing and improved lattice ordering while minimizing In decomposition. Micro‐photoluminescence measurements revealed a 1.92‐fold increase in band edge emission intensity at a shifted peak wavelength of 624 nm, attributed to strain relaxation and partial In decomposition. To evaluate the improvement in radiative recombination, we introduce a new figure of merit, referred to as the Radiative‐PL‐Rate (R _RPL ), which is defined as the rate of PL intensity at low excitation to that at saturation. The R _RPL increased from 6.21% in the as‐grown sample to 11.59% in the Re‐Cryst sample, indicating a substantial suppression of non‐radiative recombination. Furthermore, we demonstrated the mechanisms behind improved crystal quality and radiative efficiency, showing that the Re‐Cryst process selectively heals defects, enhances red emission, and advances high‐performance micro‐displays for next‐generation AR/VR applications.

A membrane-defined N-polar InGaN/GaN quantum pyramid photocathode, encapsulated by a selective GO/PLA nano-membrane, enables catalyst-free, long-term stable hydrogen evolution directly in untreated natural seawater.

Despite the considerable potential of AlGaN-based ultraviolet-B light-emitting diodes (UV-B LEDs) in various applications such as phototherapy, UV curing, plant growth, and analytical technology, their development is still ongoing due to low luminescence efficiency. In this study, we introduced a novel epitaxial growth mechanism to effectively control the height and thickness of AlGaN multiple wells (MWs) on AlGaN nanorod structures using horizontal reactor-based metal-organic chemical vapor deposition (MOCVD). By adjusting the H₂ carrier gas flow rate, we could control the growth boundary layer's thickness, successfully separating the AlGaN well and p-AlGaN layer from the substrate. Cathodoluminescence (CL) measurements confirmed the stability of the core-shell AlGaN quantum wells as a highly stable nonpolarized structure, with the wavelength peak remaining almost unchanged under various injection currents. Furthermore, transmission electron microscopy (TEM) provided clear evidence of differentiation, highlighting the distinct formation of the 275 nm AlGaN core and the 295 nm AlGaN shell structure. The developed AlGaN MW structure, characterized by these rectification features, not only demonstrated a significantly improved electroluminescence (EL) peak intensity but also exhibited a much lower leakage current compared to the conventional core-shell AlGaN structure. The newly proposed growth mechanism and advanced nonpolarized core-shell AlGaN structure are expected to serve as excellent alternatives for substantially enhancing the efficiency of the next generation of high-efficiency UV LEDs.

GaN‐based materials are the hottest research topic in UV photodetectors (PDs) because of their low operating voltage, small volume, long lifetime, high‐temperature resistance, and low energy consumption. However, there are still fundamental issues to be overcome, and the most important issue is to get a photoconductive gain. In this paper, the following new approaches are provided to innovatively improve the photoconductive gain of UV PDs in GaN‐based materials. First, the aspect ratio of the 1D GaN microwire (MW) structure is dramatically improved by analyzing the pulse growth mechanism using the metal‐organic vapor deposition system. Second, the comprehensive strain behavior in the MW epitaxial growth system is successfully analyzed. Third, the fabricated metal‐semiconductor‐metal‐based MW UV PD shows photoresponsivity and sensitivity of 28.365 A W⁻¹ and 93.16%, respectively, at the −2 V bias, which significantly outperforms the conventional structures in the UV region. Finally, a trap‐assisted Poole–Frenkel effect‐based energy bandgap mechanism, that allows the defect level formed by lattice mismatch between the substrate and GaN to be used as an electron carrier path, is newly defined. This study will present the direction of future UV PDs by providing a new MW structure based on GaN materials, a third‐generation semiconductor.