Artificially induced refractive index engineering for all-optical modal switching in GaN nanorod cavities

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.