Nonlinear Applications using Silicon Nanophotonic Wires

de Xiaoping Liu

This thesis is concerned with an emerging set of nonlinear-optical applications using silicon nanophotonic "wires" fabricated on a silicon-on-insulator photonic chip. These deeply scaled silicon nanophotonic wires are capable of confining the telecom and mid-infrared (mid-IR) light tightly into an optical-modal area ~ 0.1 μm2. The tight optical confinement leads to many advantageous physical properties including enhanced effective nonlinearity, flexible control of waveguide dispersion, and short free-carrier lifetime. All these advantages make silicon nanophotonic wires an ideal platform for a variety of nonlinear applications. The first part of my thesis study is focused on nonlinear applications in the telecom bands. In Chapter 3, I study the frequency dependence of optical nonlinearity in silicon nanophotonic wires, and its influence on the propagation of ultra-short optical pulses in such wires. I show that silicon nanophotonic wires possess a remarkably large characteristic time associated with the self-steepening effect and optical-shock formation. In Chapter 4, I present an experimental demonstration of an ultrafast cross-phase-modulation-based wavelength-conversion (XPM-WC) technique for telecom RZ-OOK data. I also investigate the effect of pump-probe detuning on the efficacy of this XPM-WC technique. In Chapter 5, I show a (primarily) numerical study of a method for dispersion-engineering of silicon nanophotonic wires using a conformal thin-silicon-nitride dielectric film deposited around the silicon wire core. My simulation results show that this approach may be used to achieve the dispersion characteristics required for broadband phase-matched four-wave-mixing processes, while simultaneously maintaining strong modal confinement within the silicon core for high effective nonlinearity. The second part of my thesis is devoted to investigations of nonlinear applications in mid-IR spectral region, in which nonlinear optical loss due to parasitic two-photon absorption can be significantly reduced and therefore a large nonlinear figure of merit can be achieved in order to facilitate efficient nonlinear processes. In Chapter 6, I present an experimental demonstration of a mid-IR-silicon-nanophotonic-wire optical parametric amplifier with 25.4 dB on-chip gain. This gain achieved with only a 4-mm-long silicon nanophotonic wire is sufficient enough to overcome all the insertion loss, resulting in 13 dB net off-chip amplification. In addition, I show, on the same waveguide, efficient generation of 4 orders of cascaded FWM products enabled by the large on-chip gain. In Chapter 7, I report a comprehensive study of the propagation characteristics of a picosecond pulse through a 4-mm-long silicon nanophotonic wire with normal dispersion with excitation wavelengths crossing the mid-infrared two-photon absorption edge at λ = 2200 nm. Significant reduction in nonlinear loss due to two-photon absorption is demonstrated as the excitation wavelengths approach 2200 nm. Self-phase modulation at high input power is also observed. Analysis of experimental data and comparison with numerical simulations illustrates that the two-photon absorption coefficient obtained from nanophotonic wire measurements is in reasonable agreement with prior measurements of bulk silicon crystals, and that bulk silicon values of the nonlinear refractive index can be confidently incorporated in the modeling of pulse propagation in deeply-scaled waveguide structures. In Chapter 8, I investigate a higher-order phase matching technique utilizing the 4th-order dispersion term for realizing a broadband or discrete band parametric process in silicon nanophotonic wires. I demonstrate experimentally, on a silicon nanophotonic wire designed to exhibit a desired 2nd-order and 4th-order dispersion, broadband/discrete-band modulation instability and 50 dB Raman assisted parametric gain.