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Abstract

Surface-emitting (SE) semiconductor lasers have become one of the most important building blocks in modern photonic technologies. Currently, SE semiconductor laser technologies are predominantly based on the vertical cavity surface-emitting laser (VCSEL) architecture. While VCSELs have been successful in the near-infrared (NIR) region, they have lagged severely at shorter wavelengths, especially the ultraviolet (UV) spectral range. A breakthrough in the SE UV laser development is pivotal, given their potential applications related to our everyday life including disinfection, medical diagnostics, and more. In this context, III-nitride nanowires have attracted significant interest for the SE laser development, leveraging the unique physical properties of III-nitrides including widely tunable bandgap energies, as well as the merits of semiconductor nanowires such as the large surface-to-volume ratio. In fact, to date III-nitrides are the only known material system suitable for all-semiconductor UV photonics.
In this thesis study, we fulfill the technological need for SE lasers with III-nitride-based epitaxial nanowire photonic crystals (epi-NPCs). Such lasers exploit the slow-light modes near the Gamma-point in the photonic band structure of nanowire photonic crystals. This is made possible thanks to the improved epitaxial growth using low-substrate temperature, i.e., low-temperature selective area epitaxy (LT-SAE) by molecular beam epitaxy (MBE). In contrast to conventional SAE at high temperatures, LT-SAE enables precise control over the nanowire growth and morphology. Using GaN epi-NPC grown by LT-SAE, ultralow threshold optically pumped SE UV lasing at 367 nm is demonstrated, with a threshold of merely 7 kW/cm2 —more than two orders of magnitude reduction compared to the previously reported conventional III-nitride-based UV VCSELs at similar lasing wavelengths. Moreover, by changing the III-nitride epi-NPC material composition (e.g., InGaN epi-NPCs) and further engineering the photonic crystal bands, ultralow threshold optically pumped SE UVlasers at various UV wavelengths are achieved.
This thesis work further demonstrates the electrically injected SE UVlaser. By employing transferred graphene as the top p-contact of the epi-NPC, SE UV lasing at 383 nm under a direct electrical current injection is successfully achieved. This thesis work not only represents an important step in the journey of the SE UV laser development, but more importantly, it lays the foundation for the wavelength-tunable SE lasers broadly from the NIR to UV spectral range.