Optical Switching Dynamics of a Nonlinear Bragg Reflector
In this thesis we present an experimental and numerical analysis of nonlinear dynamical effects in a silicon-on-insulator periodic (SOI) waveguide structure. Before this investigation the study of optical interactions in nonlinear periodic media had been primarily theoretical. The dynamical effects predicted include optical bistability, all-optical switching, gap soliton excitation, and unstable behavior at high intensities. Our investigation focused on observing and understanding these effects in corrugated SOI waveguides. The fabrication of the periodic structures was the initial task and possibly the most pertinent phase of this work. Bragg gratings and couplers were fabricated using electron beam lithography at the National Nanofabrication Facility at Cornell University. The Bragg gratings were designed to effectively reflect the lowest order transverse electric (TE) mode of the waveguide structure at 1.064 mum. The couplers were designed to efficiently couple either TE or transverse magnetic (TM) polarized light into the guides. The experiments were done in conjunction with the numerical analysis of the nonlinear dynamical effects. A Q-switched, Nd:YAG laser was used to couple laser pulses into the guide to demonstrate some of the predicted switching effects. Measurements were taken over various Bragg grating depths, with pulse widths raging from 23 nsec to 75 nsec. The results verify optical bistable behavior, all-optical switching effects, and unstable behavior at high intensities. Pump -probe measurements were carried out using a nonresonant TM pump beam and resonant TE probe beam to measure the temporal response of the nonlinearity. We additionally demonstrated that a nonresonant. TM pump beam can modulate a weak TE resonant probe, indicating the possibility of three terminal switching using a nonlinear Bragg reflector geometry. The numerical results clarify the contrast between our experimental data and the previous theoretical analysis. The previous work assumed a Kerr-like nonlinearity that instantaneously follows the field. However, in SOI structures the temporal response of the nonlinearity alters the dynamical behavior usually associated with Bragg reflectors. In our numerical analysis we considered the changes in the refractive index due to the generation of free carriers and lattice heating. The numerical model helped us to use the pump-probe measurements to estimate both the free carrier lifetime and the thermal decay. From these results we were able to more accurately match the experimental optical switching profiles with our numerical model. We conclude that for (low duty cycle) longer pulse interactions, lattice heating can destroy the hysteresis of the system. For this reason the laser pulse width needs to be 25 nsec or less in order to observe optical switching.
- Pub Date:
- WAVEGUIDE STRUCTURES;
- PERIODIC MEDIA;
- Physics: Optics