Gallium nitride (GaN)-based vertical cavity surface-emitting lasers (VCSELs) exhibit luminescence wavelengths that span the entire visible spectrum. They present several advantages, including a reduced threshold current, narrower divergence angle, single longitudinal mode operation, and a circularly symmetric output beam. GaN-based VCSELs have the potential to supersede conventional light-emitting diodes (LEDs) and edge-emitting lasers (EELs) as optimal light sources for applications such as semiconductor laser illumination, laser displays, high-density optical storage, optical interconnects, and underwater communications. Over the past two decades, significant advancements in technology have positioned they as a focal point of research for next-generation semiconductor lasers.

The resonant cavity structures of GaN-based VCSELs are primarily classified into two types: hybrid distributed Bragg reflector (DBR) and dual-dielectric film DBR structures. In the hybrid DBR configuration, the upper reflector comprises a dielectric-film DBR, whereas the lower reflector consists of a nitride DBR. In contrast, the dual-dielectric-film DBR structure utilizes dielectric-film DBRs for both the upper and lower reflectors. Despite the significant advancements in the prevalent DBR configurations of GaN-based VCSELs, several technical challenges persist. For instance, the hybrid DBR structure faces issues such as complex epitaxial growth of the nitride DBR, extended growth durations, and high costs. Conversely, the dual-dielectric film DBR structure encounters issues such as elaborate fabrication processes, suboptimal quality of heteroepitaxial crystals, insufficient uniformity of cavity length, and challenges in mass production. To address these challenges, Hmaguchi et al. (2018) introduced an innovative GaN-based VCSEL featuring a curved mirror, marking the first instance of room-temperature pulsed lasing with electrically injected curved-mirror GaN-based VCSELs. This design mitigates the need for the demanding and expensive nitride DBR epitaxial growth process prevalent in hybrid DBR structures. Additionally, curved-mirror GaN-based VCSELs eliminate the complex substrate transfer process associated with dual-dielectric film DBR structures, facilitating homoepitaxial growth on GaN single-crystal substrates. This advancement yields high-quality active regions, which are crucial for high-performance laser devices. The planar-concave stable cavity structure of the curved mirror GaN-based VCSELs demonstrates extremely low diffraction loss, accommodating an extended cavity length (20?50 μm). This characteristic not only simplifies GaN substrate polishing and thinning, but also significantly enhances the thermal performance of the devices. The advantages of the curved-mirror structure suggest a promising trajectory for the commercialization of GaN-based VCSELs. In 2019, the same research group achieved the inaugural room-temperature continuous lasing of a GaN-based VCSEL with a curved DBR structure, achieving a threshold current of only 0.25 mA. Concurrently, Nakajima et al. increased the output power to 7.1 mW by optimizing the curvature radius of the curved DBR. In 2020, Hamaguchi et al. demonstrated room-temperature continuous lasing of a green VCSEL with a curved mirror on semi-polar GaN substrates and pioneered preliminary white-light display systems using blue and green VCSELs with curved mirrors, alongside GaAs-based red VCSELs. In 2023, Ito et al. further enhanced the wall-plug efficiency (WPE) of a curved GaN-based VCSEL to 13.4% while maintaining consistent performance across all devices in the VCSEL array. Palmquist et al. achieved continuous room-temperature lasing of a GaN-based VCSEL with a curved DBR structure incorporating a top-surface lens, thus eliminating the need for a curved mirror support process. The following year, they fabricated an m-plane GaN-based VCSEL with a SiO₂ curved top lens, achieving a maximum side-mode suppression ratio of 30 dB.

The GaN-based VCSEL with a curved-mirror structure, while exhibiting commendable attributes such as high stability, uniformity, low threshold, and substantial output power, is nonetheless confronted with several technical challenges: 1) the extended cavity length of the device, which results in a very narrow longitudinal mode spacing, complicates the attainment of single longitudinal mode operation and induces mode hopping in the emission spectrum with variations in injection current; 2) the substrate employed for the epitaxial growth of the device is Si-doped GaN, where the optical absorption loss is directly proportional to the doping level. Consequently, precise control of the doping level in the GaN substrate is essential for minimizing internal losses within the resonator cavity; 3) the high mechanical hardness and pronounced chemical inertness of GaN crystals render conventional chemical-mechanical polishing techniques challenging. Consequently, achieving the required substrate thickness without inflicting damage remains a significant processing challenge ; 4) despite the robust lateral optical confinement provided by the curved-mirror structure, the laser divergence angle tends to be large due to the extended cavity length. Notwithstanding these technical challenges, the development of curved-mirror GaN-based VCSELs is promising. Addressing these issues is expected to facilitate the eventual commercialization of GaN-based VCSELs.