Circular Bragg Lasers & Supermode Si/III-V Lasers: From Theory to Devices
Surface-emitting lasers have attracted much attention because of their salient features such as low-threshold current, single-mode operation, and wafer-scale integration. Their low-divergence surface normal emission also facilitates output coupling and packaging. VCSELs though available commercially, their single-modedness and good emission patterns are guaranteed only over a very small mode area (diameter of ~µm). Attempts to further increase the emission aperture have failed mostly because of the contradictory requirements of large-area emission aperture and single-modedness. From this perspective, surface-emitting circular Bragg lasers, if properly designed, could circumvent this problem. Part I of this talk will be focused on a comprehensive and systematic study on the surface-emitting Hankel-phased circular Bragg lasers in various geometries. The analytical and numerical mode-solving methods will be described, followed by near- and above-threshold modal analyses and design guidelines.
The Si large-scale integration technology is approaching a bottleneck due to the increased ohmic heating and RC delay inherent in the metal interconnection. Hybrid Si/III-V integration provides a solution to this problem because optical communication can operate at much higher data rates. Among various integration schemes the most promising is the "hybrid Si evanescent platform,"; developed by UCSB and Intel researchers, based on a low-temperature wafer-bonding technique that is compatible with current CMOS processing. The reliance on the small evanescent tail penetrating into the III-V slab waveguide to obtain optical gain is a drawback that prevents it from being industrialized. In part II of the talk, I will introduce a novel mode-control method to such hybrid waveguide system to enhance the modal gain, which makes for more efficient and, more importantly, shorter devices that may hold the key to the photonics/electronics integration. We have developed the theory to design the shortest adiabatic mode transformer, experimentally demonstrated the principle in hybrid Si/InGaAsP laser devices, and now we are working on making c.w. devices that can hopefully be industrialized.
Xiankai Sun received his B.S. degree in physics (with honors) from the University of Science and Technology of China (USTC), Hefei, China in 2004, and his M.S. and Ph.D. degrees in applied physics from the California Institute of Technology (Caltech) in 2006 and 2010, respectively. His undergraduate research focused on the growth, characterization, and device fabrication of high-quality heteroepitaxial ZnO films on Si substrates. His doctoral thesis investigated the supermode Si/III-V lasers and circular Bragg lasers. In addition to his thesis topics, he was also involved in fabricating the first room-temperature continuous-wave electrically-pumped 2-D photonic crystal Bragg lasers in InGaAsP material. He has been an author or coauthor of over 30 peer-reviewed journal and conference papers, 3 invited book chapters, and a pending patent. He also serves as an active reviewer for major journals in optics and in physics.