Fig. 1. SEM images of (a) top view and (b) side view morphology of hexagonal GaN microdisk grown on graphene films. The top of the microdisk is rough and the semi-polar surface is present at the edge. (c) Raman spectrum of GaN/MLG/sapphire. (d) CL and PL spectra of the GaN microdisk, respectively.

Fig. 2. (a) Schematic diagram of μ-PL system. Illustration is an experimental optical microscope image of μ-PL. (b) The μ-PL spectrum of GaN microdisk. The inserted image shows an enlargement in the red area of spectrum. Blue line is the experimental data fitting line, and the red and green lines in the illustration are Lorentz fitting of a single peak. (c-e) Schematics of possible resonant cavity modes in GaN disk, which are 6-WGM, 3-WGM and D3-WGM, respectively.

Fig. 3. (a) Excitation power dependent μ-PL spectra recorded at RT on a single GaN microdisk. The inserted image on the upper left is an enlarged view of the multi-peak position. (b) The plot of integrated PL intensity and FWHM as a function of excitation power. PE is the excitation threshold energy of about 11.5 μW.

Fig. 4. (a) The intense optical field distributed of hexagonal resonant was calculated by 2D-COMSOL. The white frame is the excitation area. (b) Schematic of the D3-WGM formed in GaN microdisk. The purple dotted line is the original path equivalent optical path. (c) The plot of the normalized quality factor as function of cavity polygonal order m for reflectivity of 76.3%, under different the radius of the circumscribed circle. Three modes in the same microdisk are marked in the image.

Fig. 5. The light response spectra and the intense optical field distributed at 374.5 nm for a 3-WGM and 376.5 nm for a D3-WGM were obtained by simulation for the excitation region with a side length of 0.7 μm (a-c) and 1.8 μm (d-f). As the excitation area increases, the number of modes decreases significantly, the light path narrows, and D3-WGM disappears.