Graphene-on-Silicon Suspended Membrane Planar Lightwave Circuits

Mid-infrared (mid-IR) silicon photonics has enormous opportunities and potential applications in chemical spectroscopic studies, clinical diagnosis, and environmental monitoring. Silicon-on-insulator (SOI) technology has been widely employed in planar lightwave circuits (PLCs) in the telecommunication band, but the buried oxide (BOX) has extremely strong absorption to the mid-IR light beyond 2.6 microm, which makes it not suitable for mid-IR applications. Now the mid-IR integrated platform is still under discussion.;In the thesis, a suspended membrane platform is introduced based on the commercial SOI wafer for mid-IR applications, in which the BOX is locally removed by the hydrofluoric acid. Focusing subwavelength gratings (SWGs) are proposed and demonstrated for coupling the near-IR and the mid-IR light into suspended membrane waveguides (SMWs). An apodized focusing SWG with the best coupling efficiency hitherto reported for transverse magnetic (TM) mode is reported. A "fishbone-like" SWG is designed and fabricated for broadband coupling to SMWs. We also report an apodized focusing SWG which can couple both transverse electric (TE) and TM modes to an identical SMW. SMWs, with ∼3 dB/cm minimum loss, are demonstrated at 2.75 microm wavelengths. The micro-ring resonators and micro-disk resonators are presented based on the suspended membrane platform. The dispersion and nonlinear coefficients of SMWs are also discussed numerically in the mid-IR wavelength range.;Graphene, with zero band gap and linear electron/hole dispersion, has been suggested as an excellent optoelectronic and nonlinear optical material with the spectral coverage from visible light to terahertz region. The two-dimensional nature makes it very suitable for integration on PLCs, which can dramatically increase the interaction between top layer graphene and waveguide evanescent field. The graphene-on-silicon SMW will be promising to explore the novel device with wide operation range covers from near-IR to mid-IR wavelengths.;The in-plane interaction between propagating light and top layer graphene is first studied systematically. Under low power inputs, the graphene plays as an integrated polarizer which introduces an excess loss for the TM mode than the TE mode. The graphene also induces a coupling profile red-shift. Under high power inputs, dynamics of optical absorption of graphene-on-silicon SMWs are investigated using a pump-probe experiment. Saturable absorption in the graphene is observed at the beginning of pump pulse followed by an increase in absorption. The increase in absorption builds up over several microseconds, and is the experimental evidence that free carriers generated by the pump absorption in the graphene can transfer into the silicon waveguide. Free carrier absorption in the silicon waveguide eventually dominates the optical loss after several microseconds. The spectral hole burning of graphene-on-silicon waveguides has been also observed experimentally.;A broadband graphene-on-silicon heterostructure SMW photodetector is finally demonstrated. A large responsivity as high as 0.13 A/W at 1.5V bias for 2.75 microm wavelength light at room temperature is achieved. Different polarities of photocurrent are observed, and may be attributed to two distinct mechanisms for absorption, namely, direct and indirect transitions in the graphene at 1.55 microm and 2.75 microm wavelengths, respectively.