Towards directional nanophotonics with parity-time symmetric plasmonics [electronic resource]
Keywords: Materials Science and Engineering.

The energy challenges of the future are generally cast as a need for more efficient and carbon neutral energy sources, but equally important is the need for a more efficient use of that power. The internet consumes more than 2% of the world's total power, with projections of considerable growth as more users come online and more services are offered online. To stem this upsurge in power use, light-based information systems offer better energy efficiencies and higher bandwidths. However, making these systems directional and chip-scale remains a challenge. Optical systems that obey parity- and time- (PT) symmetry offer a route towards directional optical devices. Here, I describe the computational design of several plasmonic architectures that obey PT-symmetry and enable unidirectional photon propagation in a deeply subwavelength footprint. First, I describe a PT-symmetric plasmonic modulator that operates as either an amplifier or directional near-perfect absorber, tuned by the phase of incident light. The device consists of a subwavelength planar metal film pierced by two dielectric channels with a sub-100-nm cross section. Introduction of balanced but asymmetric loss and gain into the insulating layers renders the structure PT-symmetric. While addition of pure gain or pure loss enables realization of amplifiers and absorbers respectively, the combination of loss and gain enables a plasmonic device that can perform as both an absorber and amplifier. Secondly, I describe a PT-symmetric plasmonic architecture for nanoscale mode division multiplexing. This system consists of a coaxial cable with a 25 nm dielectric ring and metallic core and cladding. The passive coaxial waveguide supports a number of degenerate optical modes, which become either amplified or absorbed with addition of PT-symmetric configurations of loss and gain. This thresholdless PT-plasmonic waveguide enables selective writing or reading from previously indistinguishable (degenerate) modes, providing one possible route for multiplexing in a deeply subwavelength coaxial channel. Thirdly, I describe a nanoscale PT-symmetric plasmonic structure for reconfigurable and unity-efficiency polarization conversion. This coaxial resonator supports two orthogonal modes, one associated with amplification and the other with absorption. This mode distribution enables conversion of circular polarized light to linearly polarized light and rotation of linear polarized light, while amplifying the output. These three PT-symmetric plasmonic systems highlight the utility of loss in optical systems and provide a foundation for directional and potentially non-reciprocal integrated nanophotonic circuits. Finally, I outline some topics that may be of interest for the future of plasmonic PT-symmetry. Routes towards the fabrication of the proposed polarization converter in the dissertation is presented, along with future considerations for theoretical work (inclusion of non-linearities and better modeling the electron populations in the gain and absorbing media), and new PT-symmetric devices.