Optical Mode Localization in Air-Gap-Coupled GaN Nanorod Dimer Cavities

We report on the demonstration of dielectric amplified spontaneous emission (ASE) from the gallium nitride (GaN) nanorod dimer, in which localized optical modes arise from strong coupling across a nanoscale air gap. The nanorod dimer forms a self-aligned cavity that supports spatially confined resonant modes, enabling ASE under continuous-wave excitation at room temperature. The system achieves a narrow emission line width of approximately 3 nm and a low ASE threshold of ∼26.98 kW/cm2. Photoluminescence (PL) measurements reveal distinct spectral narrowing and polarization-dependent emission as the injection power increases. Time-resolved PL and quality factor analysis confirm enhanced photon confinement and coherence in the dimer configuration, with the Q-factor increasing by nearly 8-fold compared to single nanorods. Finite-difference time-domain (3D-FDTD) simulations support these observations by showing electric field localization within the dimer cavity and modal selectivity between TE and TM modes. Furthermore, the air-gap cavity exhibits characteristics of a leaky-mode resonance, facilitating directional emission and improved gain accumulation. These findings highlight a fabrication-compatible route toward compact, low-threshold nanophotonic light sources based on the dielectric confinement in semiconductor nanostructures.