Lasing Actions in GaN Tiny Hexagonal Nanoring Resonators

We achieved optically pumped lasing actions of extremely small optical resonators, which have 1.4-2-μm-diameter GaN hexagonal nanorings with wall widths of 60-140 nm. The hexagonal nanorings were grown by rf-plasma molecular beam epitaxy (rf-MBE) with Ti-mask selective area growth (SAG). Due to the nanometer scale of the wall width, the propagation of threading dislocations through the wall was suppressed, and high crystalline quality of the nanoring resonators was achieved. The hexagonal nanorings possessed a fine cavity configuration of crystallographically flat outer walls and consisted of hexagonally connected sixfold side sections. The periodic boundary condition for each section with a short length limits the number of longitudinal resonance wavelengths, and narrowing the wall suppresses the occurrence of lasing in higher transverse modes. Therefore, both contribute to the lasing action in a single longitudinal mode of the hexagonal nanoring.