Ultrafast all-optical switching via grating-based fabry-perot resonators and surface normal fiber-to-waveguide couplers
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Electronic microchips have now firmly plateaued in switching speed. A promising solution for increasing performance on unparallelizable tasks is to switch digital data purely in the optical domain on a photonic chip, as all-optical switching can reach up to terahertz speeds and beyond. Because ultrafast optical effects are weak phenomena, materials with extremely high nonlinear sensitivity must be developed, and very high optical intensities and coupling efficiencies are required to adequately switch data streams of light. In this thesis, unique all-optical platforms, waveguides, fiber-waveguide grating couplers, and an ultrafast optical switch were experimentally demonstrated as proofs-of-concept for the validity of densely integrated all-optical switches. Two horizontal slot waveguiding structures were designed and fabricated from scratch: a multiple horizontal slot waveguide with polycrystalline silicon sandwiching third-order nonlinear slots and a nonlinear cover-cladding with slot-like behavior over a thin crystalline silicon waveguide. Perfectly vertical grating couplers were then designed from a novel genetic algorithm, fabricated, and experimentally tested for both platforms with two promising nonlinear materials: silicon nanocrystals or a supra-molecular assembly, DDMEBT. Vertical grating couplers in the multiple horizontal slot waveguide achieved a theoretical coupling efficiency of 63% and an experimental coupling efficiency of 60%, which is the highest coupling efficiency into nonlinear slot waveguides to date. Vertical grating couplers for the cover-slot waveguide experimentally demonstrated a coupling efficiency of 38% and an extrapolated 1 dB bandwidth of 66 nm, the largest grating-coupled 1 dB bandwidth obtained for slot waveguides to date. A grating coupler was then designed to be included as one of two grating reflectors in a nonlinear resonator switch. Coupled mode theory and vectorial eigenmode propagation simulations were used to optimally design the grating coupler/resonator device, resulting in a record low footprint of 710 μm² per combined switch and fiber coupler device. The third-order nonlinear molecular material, DDMEBT, was, for the first time, successfully spun onto pre-patterned silicon-on-insulator chips with repeatable, defect-free results. Extremely sensitive experimental autocorrelation of the resonator's impulse response yielded output pulse durations as low as 600 femtoseconds. At high power and low pulse repetition rates, the switch's resonances redshifted by 4 nm with 4 dB of switching contrast, revealing an ultrafast Kerr effect which matched previous works. The resonator switch is therefore capable of modulating a single optical carrier frequency at 1 THz and switching an optical data stream at 500 GHz. These are the fastest switching speeds demonstrated by an integrated all-optical switch and validate the proof-of-concept needed for a future of densely integrated all-optical processing.