GaN hemispherical micro-cavities

GaN-based micro-dome optical cavities supported on Si pedestals have been demonstrated by dry etching through gradually shrinking microspheres followed by wet-etch undercutting. Optically pumped whispering-gallery modes (WGMs) have been observed in the near-ultraviolet within the mushroom-like cavities, which do not support Fabry-Perot resonances. The WGMs blue-shift monotonously as the excitation energies are around the lasing threshold. Concurrently, the mode-hopping effect is observed as the gain spectrum red-shifts under higher excitations. As the excitation energy density exceeds ∼15.1 mJ/cm2, amplified spontaneous emission followed by optical lasing is attained at room temperature, evident from a super-linear increase in emission intensity together with linewidth reduction to ∼0.7 nm for the dominant WGM. Optical behaviors within these WGM microcavities are further investigated using numerical computations and three-dimensional finite-difference time-domain simulations.