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Compared with conventional edge-emitting laser diodes, vertical-cavity surface-emitting lasers (VCSELs) have numerous advantages such as the compatibility with mainstream semiconductor microfabrication, the possibility of on-wafer testing, a better beam profile, the feasibility of arrayed and coherent operation and the possibility of low-power and single-mode operation. While, in the short wavelength, from green and blue to near ultraviolet (530 to 360 nm), only edge-emitting laser diodes are commercially available, the commercialization of III-nitride VCSELs are expected to enable unique applications in mobile display and projection, virtual, augmented, and/or mixed reality, adaptive beam steering, and high-resolution laser printing. The main challenge in the technology of III-nitride VCSELs, however, is the fabrication of planar distributed Bragg reflectors (DBRs) that provide near-unity reflection and thus support the formation of high-quality vertical cavity. While, over the past decade, numerous groups reported VCSEL operations (only in visible spectrum) using epitaxial DBRs (AlGaN/GaN, AlN/GaN, AlInN/GaN) and dielectric DBRs (involving lift-offs or epitaxial lateral overgrowth), the devices commercialization is still hampered by challenging epitaxial growth, complicated fabrication or low thermal management. We report in this paper the use of a nanoporous (Al)GaN medium as an alternative method in the formation of III-nitride VCSELs. Specifically, we will focus on the development of VCSELs with an emission wavelength at 369 nm which is crucial for the realization of next-generation compact atomic clocks. We note that VCSEL operation below 400 nm has never been demonstrated, and we will discuss factors of importance toward this realization, including the formation of near-unity DBR mirrors using nanoporous AlGaN/AlGaN structures, the investigation and control of cavity loss including absorption and scattering, and the formation of intra-cavity current injection, optically-transparent pathways. So far clear single-mode spontaneous emission at 369 nm (linewidth < 2 nm) is observed. We will report the latest result including pulsed characterizations under high level injection.
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