Monolithic multi-wavelength lasing from multi-sized microdisk lasers.

This paper demonstrates monolithic multi-wavelength lasing through fabrication of multi-sized microdisks on a green-emitting thin film sample. The different dimensions of the microdisks incur different extent of strain relaxation, thus changing the emission/gain spectra due to the reduction of the quantum confined Stark effect. Under room-temperature optical pumping, lasing thresholds of 15.1 mJ/cm2, 2.9 mJ/cm2, and 5.3 mJ/cm2 with Q factors of 2370, 2060, and 4308 are realized, respectively, for fabricated microdisks with diameters of 950 nm, 6 µm, and 10 µm. By exciting the microdisks with a pump laser spot diameter of 2 mm, simultaneous multi-wavelength lasing action is thus observed. The strain relaxation effect is confirmed by the shift of the E2 (high) Raman peak from 563.2 cm-1 to 561.5 cm-1 as the diameter of the fabricated microdisk reduces.

Influence of surface roughness on the lasing characteristics of optically pumped thin-film GaN microdisks.

Optically pumped whispering-gallery mode (WGM) lasing is observed from a thin-film GaN microdisk processed from GaN-on-Si InGaN/GaN multi-quantum well wafers by selective wet-etch removal of the substrate. Compared with thin-film microdisks processed from GaN-on-sapphire wafers through laser lift-off of the sapphire substrate, the exposed surface is significantly smoother as laser-induced damage is avoided, with a root-mean-square roughness of 1.3 nm compared with 5.8 nm of the latter wafer. The ∼8-µm diameter microdisks, fabricated by pattern transfer from a silica microsphere and dry etching, benefit from the surface smoothness to offer superior optical confinement within the cavity. WGM lasing thresholds of ∼2.9 mJ/cm2 and ∼3.5 mJ/cm2 with quality (Q)-factors of ∼3100 and ∼1700 are observed at the peak lasing wavelengths of ∼453 nm and ∼532 nm, respectively, which are significantly better than thin-film microdisks processed from GaN-on-sapphire wafers despite lower internal quantum efficiency, highlighting the importance of surface smoothness in such optical cavities.