Integrated Graphene/Silicon Quantum Photonics Waveguides with Polarization Control

Graphene can be used to fabricate hybrid photonic nanodevices for the coherent and dynamically control photon polarization in photonic waveguides. In these nanodevices the graphene conductivity regime can be precisely tuned by modifying the Fermi level with a gate voltage, moving in a regime where the inter-band chiral transitions, rather than the surface plasmon polariton, dominate the optical polarization. We exploit this property to design integrated Graphene/Silicon quantum photonics waveguides with polarization control, performing a numerical investigation of the use of graphene nanoribbons placed on top of a silicon-on insulator (SOI). These devices are intended for they implementation in silicon photonics polarization-encoded Quantum Key Distribution (QKD) systems working in the telecom C-band, exploiting the advantage of the Si based technology as regards cost, scalability, power consumption and reliability. Graphene can selectively sustain both TE and TM polarization modes, depending on the energies of the photon energy and on the graphene chemical potential of graphene, with a broadband operation due to its gapless optical spectrum. We found that two factors mainly determine the polarization control: (i) the graphene chemical potential and (ii) the geometrical parameters of the waveguide, such as the waveguide and nanoribbon widths and distance. We revealed that the graphene chemical potential influences both TE and TM polarizations in a similar manner, and that the use of a tapered waveguide allows both TE-pass and TM-pass polarizing functionalities. The increase of the the thickness of an oxide spacer placed between the Si waveguide and the graphene nanoribbon allows a reduction of the device insertion losses, while maintaining preserving high polarization extinction ratio. Our numerical analysis show that the tuning of the chemical potential affects the TE and TM polarizations in similar way but induces different optical losses. We found that that the tuning of the chemical potential at energy above 0.5 eV leads to a remarkable reduction of the losses to reduce the optical losses. We also found that increasing the thickness of the oxide spacer the device insertion losses are reduced, keeping a a reasonably low polarization extinction ratio.