Analysis and integration of plasmonic wires and rings for VLSI photonics

Abstract

We report on the design and integration of nano-scale surface plasmonic polariton (SPP) wires and dielectric wires as applicable for VLSI photonic integrated circuits. We analyze the propagation characteristics of the slow plasmonic lightwave and the fast lightwaves. We examined three different structures of plasmonic wires: Single SPP, slot SPP, and stripe SPP. We design and integrate coupled structures of plasmonic nano-wires and examine the optical mismatch problem between the two wires of these structures. The plasmonic nano-wires are integrated with other dielectric wires and devices, such as silicon wires and polymer wires. The propagation loss of the SPP depends so much on the mode confinement that the SPP waveguide of ultra-small mode size suffers a large propagation loss. On the other hand, although a dielectric waveguide has a larger mode, its propagation loss is much smaller than that of the SPP waveguide. Hence, the interconnection between a SPP waveguide and a dielectric waveguide is considered useful in transmitting the lightwaves with reduced loss. We examine the matching conditions of the two wires in terms of optical impedance matching (effective refractive index matching) and polarization matching. For effective refractive index matching, we first analyze the effective refractive index of the silicon waveguide and that of the SPP waveguide using two-dimensional (2-D) move solver. We design a guided wave coupler, the so called directional coupler, matching the effective refractive indices between the two waveguides using three dimensional (3-D) finite element method. We calculate the mode field of the individual SPP nano-waveguide. We then calculate the coupled eigen-modes of even and odd eigen-modes directly to analyze the coupling between two nano-waveguides. We find that, for the even mode, the magnetic field has the same direction in all the position while the odd mode has opposite field direction on the two nano-waveguides. In terms of energy transfer, we find that the lightwave coming into a no-wire is transferred to the SPP wire due to the refractive index matching or the optical impedance matching. We study both the single SPP and the slot SPP wire excited by the electrical filed component normal to metal-dielectric interface. For a single SPP waveguide we find that the SPP modes are not excited by TE mode but by TM mode in the lateral guided wave coupler. In the slot SPP waveguide, on the other hand, we find that the SPP modes are not excited by TM mode but by TE mode in the lateral guided wave coupler. We also used surface plasmon-polaritons (SPPs) formed on a flat metal surface like silver or gold for micro/nano-photonic circuits and networks. We have designed and fabricated stripe waveguides, horizontal directional couplers, vertically integrated directional couplers, using plasmonic waveguides. The modes of both single and slot SPP waveguide are excited by the electrical filed component normal to metal-dielectric interface. An infinitely wide structure supports only two purely bound transverse magnetic modes that exhibit symmetry and asymmetry modes. Unlike the structure of infinite width, the pure TM modes are not supported by a thin metal strip. The modes are excited by two polarizations, transverse electric mode and transverse magnetic mode in the case of 3-dimensional geometry with finite width and finite height. We also examine the characteristics of the lightwave propagation of plasmonic-based micro-ring structures and use the results to examine the mismatch problems between a straight plasmonic waveguide and a micro-ring device. These results provide the basis of nano-scale photonic integration. The presentation will show the details of the calculation and analytical results of the optical mismatching and polarization mismatching studies for various combinations of plasmonic integration and will discuss their meanings for VLSI photonic integration.