Nanophotonic and nanoplasmonic couplers: Analysis and fabrication

Mode mismatch between waveguides of different geometries and propagation mechanisms causes radiation and back reflection, which results in significant loss of optical power. This is considered one of the obstacles that prevents multiple applications of the optical integrated circuits. In this dissertation, we design, fabricate, and experimentally demonstrate four novel photonic couplers that achieve mode matching between hybrid waveguides. These hybrid waveguides include conventional optical waveguides, photonic crystal (PC) waveguides, and plasmonic waveguides. First, we propose a novel method to enhance the coupling efficiency between a dielectric waveguide and a planar PC. This method is based on introducing structural imperfections that cause a change in the mode size and shape inside the taper to match that of the PC line-defect waveguide. These imperfections are introduced by changing the size and position of the inner taper rods. Our results show that introducing the structural imperfections increases the coupling to 96% without affecting the transmission spectrum of the structure. Second, we demonstrate through numerical simulations and experiments that low crosstalk between two crossed line-defect waveguides formed in a square lattice PC structure can be achieved by using a resonant cavity at the intersection area. The PC resonator consists of cubic air-holes in silicon. The theoretical and experimental crosstalk values are approximately -40 dB and -20 dB, respectively. Third, we introduce a novel silicon microring vertical coupler that efficiently couples light into a silicon-on-insulator (SOI) waveguide. A specific mode is excited to match the effective index of the SOI guided mode by oblique incidence. The vertical leakage from the microring forms gradual coupling into the SOI slab. Coupling efficiency up to 91% is demonstrated numerically. The coupler is fabricated and tested to confirm the analytical results. Fourth, we present a novel design, analysis, and fabrication of an ultracompact coupler and a 1 × 2 splitter based on plasmonic waveguides. In addition, we present two nano-scale plasmonic devices: a directional coupler and a Mach-Zehnder interferometer. The devices are embedded between two dielectric waveguides. Our simulation results show a coupling efficiency of 88% for the coupler, 45% for each splitter\u27s branch, 37% for a 2 × 2 directional coupler switch, and above 50% for the proposed designs of the Mach-Zehnder interferometer. In order to confirm the analytical results, the plasmonic air-slot coupler and splitter are fabricated and tested

Pinned modes in two-dimensional lossy lattices with local gain and nonlinearity

We introduce a system with one or two amplified nonlinear sites ("hot spots", HSs) embedded into a two-dimensional linear lossy lattice. The system describes an array of evanescently coupled optical or plasmonic waveguides, with gain applied at selected HS cores. The subject of the analysis is discrete solitons pinned to the HSs. The shape of the localized modes is found in quasi-analytical and numerical forms, using a truncated lattice for the analytical consideration. Stability eigenvalues are computed numerically, and the results are supplemented by direct numerical simulations. In the case of self-focusing nonlinearity, the modes pinned to a single HS are stable or unstable when the nonlinearity includes the cubic loss or gain, respectively. If the nonlinearity is self-defocusing, the unsaturated cubic gain acting at the HS supports stable modes in a small parametric area, while weak cubic loss gives rise to a bistability of the discrete solitons. Symmetric and antisymmetric modes pinned to a symmetric set of two HSs are considered too.Comment: Philosophical Transactions of the Royal Society A, in press (a special issue on "Localized structures in dissipative media"

HIGH EFFICIENCY EDGE COUPLER, NOVEL NONLINEAR OPTICAL POLYMERS WITH LARGE KERR-COEFFICIENT AND AUTOMATIC LAYOUT GENERATION IN SILICON PHOTONICS

The potential of on-chip photonics is limited by the difficulty in coupling light from optical fibers to on-chip waveguides. Specifically, 3rd-order nonlinear on-chip photonics usually requires high optical power. Hence the first major focus of this research is to design high-efficiency edge couplers. To achieve this goal, loss mechanisms of basic inverse taper couplers are analyzed and experimentally verified. Then a cantilever-encapsulated inverse taper is demonstrated to further lower coupling loss compared to basic inverse tapers. Nonetheless, both couplers are designed to couple with lensed fibers. Hence for flat fibers with larger mode-field-diameter (MFD), a novel sub-wavelength grating based edge coupler is proposed and experimentally demonstrated to have 1.9dB/facet loss. Eventually a silicon multi-section taper with intermediate SU-8 waveguide cladding is proposed for flat fibers with even larger MFD and experimentally verified. Based on the result several suggestions are proposed for further improvement

Complex-k modes of plasmonic chain waveguides

Nanoparticle chain waveguide based on negative-epsilon material is investigated through a generic 3D finite-element Bloch-mode solver which derives complex propagation constant ($k$). Our study starts from waveguides made of non-dispersive material, which not only singles out "waveguide dispersion" but also motivates search of new materials to achieve guidance at unconventional wavelengths. Performances of gold or silver chain waveguides are then evaluated; a concise comparison of these two types of chain waveguides has been previously missing. Beyond these singly-plasmonic chain waveguides, we examine a hetero-plasmonic chain system with interlacing gold and silver particles, inspired by a recent proposal; the claimed enhanced energy transfer between gold particles appears to be a one-sided view of its hybridized waveguiding behavior --- energy transfer between silver particles worsens. Enabled by the versatile numerical method, we also discuss effects of inter-particle spacing, background medium, and presence of a substrate. Our extensive analyses show that the general route for reducing propagation loss of e.g. a gold chain waveguide is to lower chain-mode frequency with a proper geometry (e.g. smaller particle spacing) and background material setting (e.g. high-permittivity background or even foreign nanoparticles). In addition, the possibility of building mid-infrared chain waveguides using doped silicon is commented based on numerical simulation.Comment: 26 pages, many figures, now including "Supplementary Data". Accepted, Journal of Physics Communicatio

Chains of coupled square dielectric optical microcavities

Chains of coupled square dielectric cavities are investigated in a 2-D setting, by means of a quasi-analytical eigenmode expansion method. Resonant transfer of optical power can be achieved along quite arbitrary, moderately long rectangular paths (up to 9 coupled cavities are considered), even with individual standing-wave resonators of limited quality. We introduce an ab-initio coupled mode model, based on a simple superposition of slab mode profiles as a template for the field of individual cavities. Although no loss mechanisms are built in, the model can still help to interprete the results of the former numerical experiments

Soliton generation and control in engineered materials

Optical solitons provide unique opportunities for the control of light‐bylight. Today, the field of soliton formation in natural materials is mature, as the main properties of the possible soliton states are well understood. In particular, optical solitons have been observed experimentally in a variety of materials and physical settings, including media with cubic, quadratic, photorefractive, saturable, nonlocal and thermal nonlinearities. New opportunities for soliton generation, stability and control may become accessible in complex engineered, artificial materials, whose properties can be modified at will by, e.g., modulations of the material parameters or the application gain and absorption landscapes. In this way one may construct different types of linear and nonlinear optical lattices by transverse shallow modulations of the linear refractive index and the nonlinearity coefficient or complex amplifying structures in dissipative nonlinear media. The exploration of the existence, stability and dynamical properties of conservative and dissipative solitons in settings with spatially inhomogeneous linear refractive index, nonlinearity, gain or absorption, is the subject of this PhD Thesis. We address stable conservative fundamental and multipole solitons in complex engineered materials with an inhomogeneous linear refractive index and nonlinearity. We show that stable two‐dimensional solitons may exist in nonlinear lattices with transversally alternating domains with cubic and saturable nonlinearities. We consider multicomponent solitons in engineered materials, where one field component feels the modulation of the refractive index or nonlinearity while the other component propagates as in a uniform nonlinear medium. We study whether the cross‐phase‐modulation between two components allows the stabilization of the whole soliton state. Media with defocusing nonlinearity growing rapidly from the center to the periphery is another example of a complex engineered material. We study such systems and, in contrast to the common belief, we have found that stable bright solitons do exist when defocusing nonlinearity grows towards the periphery rapidly enough. We consider different nonlinearity landscapes and analyze the types of soliton solution available in each case. Nonlinear materials with complex spatial distributions of gain and losses also provide important opportunities for the generation of stable one‐ and multidimensional fundamental, multipole, and vortex solitons. We study onedimensional solitons in focusing and defocusing nonlinear dissipative materials with single‐ and double‐well absorption landscapes. In two‐dimensional geometries, stable vortex solitons and complexes of vortices could be observed. We not only address stationary vortex structures, but also steadily rotating vortex solitons with azimuthally modulated intensity distributions in radially symmetric gain landscapes. Finally, we study the possibility of forming stable topological light bullets in focusing nonlinear media with inhomogeneous gain landscapes and uniform twophoton absorption