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.
We have developed a novel conductivity based selective electrochemical etching to introduce nanometer sized pores into GaN. By controlling the doping and electrochemical etching bias, we are able to control the pore morphology. The nanoporous (NP) GaN can be considered a new form of GaN with an unprecedented tunability in optical index We show the potential of this NPGaN to overcome the optical and epitaxial limitations of AlGaN, which has been the bottleneck for GaN-based laser diodes for decades. The advantages of NP-GaN for both vertical surfaceemitting laser diodes (VCSEL) and edge-emitting laser diodes are exhibited in subsequent sections. We first demonstrate the record high reflectances (R > 99.5 %) from epitaxial NP-GaN mirrors, which are used in a low threshold optically pumped VCSEL. We then show a two-fold increase of modal gain in an edge-emitting waveguide geometry with optical confinement Γ engineering. The increase of the Γ also leads to a two-fold reduction in the threshold power density under optical pumping with a threshold material gain of 400 cm-1, which is more than two times lower than previously reported (> 1,000 cm-1).