Active microring and microdisk optical resonators on indium phosphide
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Photonic or optical logic holds the promise of ultra-fast logic circuits with capability for speeds beyond what is possible using conventional silicon electronics. However, the jump from theory to practice has a high barrier set by critical issues such as integration, scalability and power requirements. Optical micro-resonator based schemes have the potential of addressing some of these issues. This thesis focuses on the development of active InGaAsP/InP microdisk and microring optical resonators to lower that barrier a little. Microrings and disks provide a compact and cascadable device platform to achieve resonance enhancement of optical non-linearity. By incorporating gain in such devices, the optical power needed for carrying out switching can be greatly reduced. Electrically pumped microring and microdisk resonators are fabricated on indium phosphide in both vertically and laterally coupled bus-waveguide configurations. The gain saturation non-linearity is used to demonstrate all-optical switching and bistable operation at optical powers more than two orders of magnitude lower compared to passive devices. The shift in the ring/disk resonances caused by the refractive index change due to a pump beam is used to switch a weaker probe beam tuned to one of the resonances. The non-linear response and switching mechanism is modeled numerically. A novel pseudodisk configuration that combines the best of microdisks and microrings is used to minimize device heating and surface recombination as well as provide near single-mode operation. Additionally, optical amplifiers based on microrings are also developed for cascading passive optical gates. Optical amplification up to 10 db in pulsed mode has been observed for 20 µm radius microrings. The control of surface recombination on the microring sidewalls is critical to avoid carrier loss and device heating. A sulfur passivation scheme is used to reduce the surface recombination velocity. The lateral carrier transport and surface recombination in microrings is analyzed by an ambipolar diffusion model.