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

Abstract

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- μ m 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.

Enhancing Optical Confinement of InGaN Thin-Film Microdisk Lasers with Hybrid Omnidirectional Reflectors

“…Multimode lasing is expected for an 8-μm diameter GaN microdisk based on both theory , and our previous experimental observations. − However, single-mode lasing is observed under a low excitation power, which remains dominant until it reaches saturation. This is attributed to spatial hole burning (SHB), where the mode with the highest gain depletes all the inverted carriers near the mode’s antinodes within the resonant cavity .…”

“…Radiation loss refers to the energy leakage of light during propagation at the interface, and the integration of the hybrid ODR structure in our microdisk plays an important role in minimizing such losses. On the other hand, the smoothness of the top surface of the microdisk laser as observed from the SEM image of Figure b, is ensured by the wet etch removal of the Si substrate as compared with thin-film microdisk lasers processed by laser liftoff . Together with the smoothness of the sidewalls, as observed from the SEM image of Figure c, scattering losses are minimized.…”

“…However, compared to the undercut structure, the lasing threshold and Q factor of these thin-film microdisks with metallic mirrors did not improve significantly due to strong optical absorption of the metal. 18 A previous study explored the utilization of epitaxial 25-pair AlN/AlGaN distributed Bragg reflector (DBR) instead of the undercut structure to achieve optical isolation from the substrate, realizing an optical-pumped ultraviolet GaN microdisk laser with a Q factor of approximately 400. 22 Furthermore, due to the angular dependency of DBR reflection, especially when applied to cavities of high refractive index materials, the reflectance drops sharply as the angle of incidence increases, making it challenging to achieve omnidirectional high reflectivity which is needed for microdisks.…”

“…To address these issues, thin-film GaN microdisks have been developed utilizing wafer bonding, substrate removal, and thinning techniques, ,− enabling optimization of the microdisk thickness without altering the growth process to maintain the high internal quantum efficiency (IQE) of InGaN/GaN multiquantum wells (MQWs). In this approach, the metallic bonding layer serves as the bottom mirror to provide optical confinement for the microdisks.…”

Comparison of lasing characteristics of GaN microdisks with different structures

“…More recently, to reduce the surface roughness caused by laser-induced damage from the LLO process [49], a thinfilm microdisk is fabricated from GaN-on-Si wafers through wet etch removal of the Si substrate [50]. Attributed to the improved smoothness of the microdisk surface, lower lasing threshold and higher Q has been achieved compared to its thin-film counterpart fabricated by LLO processes, in spite of lower internal quantum efficiencies (IQEs).…”