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Free-Standing Integrated Optics in Silicon
Sun, Peng

2012, Doctor of Philosophy, Ohio State University, Electrical and Computer Engineering.
A technique of releasing silicon nanophotonic waveguides from substrates and engineering residual stress in silicon dioxide cladding is developed. By modifying the optical, mechanical, or thermal properties of silicon nanophotonic waveguides, the technique can improve the performance of existing photonic devices and enable unprecedented devices. In this dissertation, the technique is applied to optical coupling, optical switching, and polarization control. Optical couplers using free-standing cantilevers for fiber-to-chip coupling to silicon nanophotonic waveguides are demonstrated. Cantilevers that consist of silicon strip waveguides embedded at the centers of silicon dioxide claddings are defined and released from the bulk silicon substrate using electron/ion beam lithography and dry etching. The residual stress in the silicon dioxide cladding deflects the cantilevers out-of-plane and enables butt-coupling to tapered optical fibers. The cantilever coupling scheme enables direct access to photonic devices on an entire wafer surface without dicing or cleaving. Coupling losses of 1.6 dB per connection for TE polarization and 2 dB per connection for TM polarization are achieved at 1550 nm wavelength. The cantilever coupling scheme can also be employed to couple light between separate photonic chips. Vertical chip-to-chip light coupling using silicon strip waveguide cantilever couplers are demonstrated, where the free-standing cantilevers deflect 90° out-of-plane. At 1550 nm wavelength, a chip-to-chip coupling loss of 2.5 dB per connection is measured for TE polarization and 1.1 dB for TM polarization. Thermo-optic switches employing free-standing silicon waveguides are demonstrated with improved efficiencies. Submilliwatt switching power and submillisecond switching time are achieved in Mach-Zehnder switches where the interferometer arms are released from the substrate. The air gap provides thermal isolation between the waveguide interferometer arms and the silicon substrate. This approach effectively trades off switching speed for reduced switching power. At 1550 nm wavelength, measurements of fabricated devices demonstrate a switching power of 540 µW and a 10%-90% rise time of 141 µs. Optical bistability using pump power in the range of tens of microwatts in free-standing silicon ring resonators is demonstrated. By etching an air gap between the ring resonator and the substrate, heat conduction from the silicon ring to the surrounding environment is greatly reduced. Similar non-thermally isolated resonators at the same detuning wavelength do not exhibit a bistable mode for input powers less than 2 mW. Polarization controllers that are wavelength-independent, tunable, and easy-to-fabricate, are enabled in photonic integrated circuits by employing the stress-engineered free-standing waveguide structures. Manifestation of geometric phase in silicon nanophotonic waveguides as the dependence of light polarization on light trajectory is proposed. One necessary condition for the existence of nontrivial geometric phase requires manipulating waveguide shapes in three-dimensions, which is satisfied via the technique of releasing silicon waveguides and engineering stress in silicon dioxide cladding. Finite-element method simulation results are in good agreement with theory prediction.
Ronald M. Reano, PhD (Advisor)
Betty L. Anderson, PhD (Committee Member)
Roberto Rojas-Teran, PhD (Committee Member)
Fernando L. Teixeira, PhD (Committee Member)
159 p.

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