Lossless Waveguide Research Articles

Compact Silicon-on-Insulator Dual-Microring Resonator Optimized for Sensing

We demonstrate a novel silicon-on-insulator dual-microring resonator for sensing application. Numerical analysis shows that the Q factor of dual-microring resonator is three orders of magnitude higher than that of conventional single-ring resonator with lossless waveguide and thereby as a sensor, the resolution and minimum level of detection is enhanced greatly. For the resonance peak of inner ring, a Q value of 1.07 × 107 is achieved theoretically in weak coupling conditions while the sensing area is kept below 50 × 50 μm2. Detection limit of refractive index change of 3.85 × 10-6 can be achieved for bulk refractive index sensing. Moreover, by enlarging bending radius of the outer ring, Q factor can be further improved. The analytical description of output spectrum response is derived. In addition, for low propagation loss waveguide, critical coupling condition of two rings and bus waveguides is obtained and it's essential for sensor device optimization.

Study on active surface plasmon waveguides and design of a nanoscale lossless surface plasmon waveguide

We studied active lossless single-interface surface plasmon polariton (SPP) waveguides with extremely short guided wavelengths and optimally designed a nanometer-size active lossless single-mode dielectric-loaded SPP waveguide (DLSPPW). The sufficient condition, under which both lossless propagation and extremely short guided wavelength can be achieved simultaneously in active single-interface SPP waveguides, is presented. Under the same condition, the gain coefficient of the active medium reaches its maximum. These maximum gain coefficients are calculated in cases when the metals Au, Ag, and Cu are used, respectively, operating in wavelengths ranging from 330 to 1600 nm. From the above calculation results, the working wavelength and metal are selected for the active DLSPPW. A single-mode lossless DLSPPW is designed and optimized using the effective index and finite element methods in vacuum wavelength 450 nm. In this waveguide, 95% of the energy flux is constrained within a core area with thickness of 20 nm and width of 22 nm.

Localization of light in evanescently coupled disordered waveguide lattices: Dependence on the input beam profile

We investigate how the transverse localization of light in evanescently coupled, disordered, lossless waveguide lattices depends on the shape and size of an input beam. Our detailed numerical study not only reveals waveguide-like propagation of the localized state inside such a disordered discrete medium but also shows that a specific localized state is independent of the spatial profile of the input beam. Dependence of the localized state on input beam parameters and lattice parameters is also reported. Our results should be of interest for engineering light propagation with discrete diffractive optics in practical optical geometries (e.g., microstructured arrays of optical waveguides, fiber arrays, etc.) and for realizing waveguide-like (without any diffractive spread) propagation even in the presence of structural disorders and refractive index perturbations.

Backward Coupling of Modes in a Left-Handed Metamaterial Tapered Waveguide

The wave propagation in an adiabatic tapered waveguide with left-handed metamaterial (LHM) core and air cladding is investigated by mode-matching method. The results show that the forward guided modes will always couple to the backward modes at the 'critical' thickness in the lossless waveguide. So, the propagating wave will not be completely standstill at the 'critical' thickness as proposed in previous reports, but be 'bounced' back. Besides, the propagation of wave in a lossy LHM tapered waveguide is also investigated.

Backward and forward modes guided by metal-dielectric-metal plasmonic waveguides

We revisited the problem of the existence of plasmonic modes guided by metal-dielectric-metal slot waveguides. For the case of lossless slot waveguides, we classify the guided modes in the structure with the metal dispersion and found that, in a certain parameter range, three different guided modes coexist at a fixed frequency, two (symmetric and antisymmetric) forward propagating modes and the third, backward propagating antisymmetric mode. We study the properties of the forward and backward plasmonic guided modes in the presence of realistic losses, and discuss the importance of evanescent modes in lossy structures.

Modeling of optical properties of nanosize metal-dielectric gratings within the eigenmode approach

We show that the wide-spread concept of optical eigenmodes in lossless waveguide systems, which assumes the separation into propagating and evanescent modes, has to be revised in the case of metal-dielectric structures. In addition, there exists a sequence of new eigenstates with complex values of the propagation constant. Taking these anomalous modes into account we have developed an original approach for the modeling of optical properties of subwavelength metal gratings with nanosize slits. The obtained spectra of transmittance and reflectance demonstrate the application potential of such structures for efficient spectral filtration of optical signals.

THE PROPAGATION PROBLEM IN A BI-ISOTROPIC WAVEGUIDE

We investigate the problem of deflning propagating constants and modes in metallic waveguides of an arbitrary cross section, fllled with a homogeneous bi-isotropic material. The approach follows the guidelines of the classical theory for the isotropic, homogeneous, lossless waveguide: starting with the Maxwell system, we formulate a spectral problem where the square of the propagation constant shows up as the eigenvalue and the corresponding mode as the eigenvector. The di-culty that arises, and this is a feature of chirality, is that the eigenvalue is involved in the boundary conditions. The main result is that the problem is solvable whenever the Dirichlet problem for the Helmholtz equation in the cross section is solvable and two technical hypotheses are fulfllled. Our method, inspired by the null-fleld method, is quite general and has a good potential to work in various geometries.

Fabrication of Periodically-Inverted AlGaAs Waveguides for Quasi-Phase-Matched Wavelength Conversion at 1.55 µm

We have improved fabrication processes of periodically inverted AlGaAs waveguides for highly efficient quasi-phase-matched wavelength conversion. In order to reduce propagation losses in waveguiding devices fabricated based on sublattice reversal epitaxy, we have introduced template planarization by chemical mechanical polishing and low-temperature regrowth technique. Corrugations at the core/clad interface and the propagation loss at 1.55 µm has been reduced to 40 nm and 9 dB/cm, respectively. First-order quasi-phase-matched second-harmonic generation was demonstrated at 1.55 µm fundamental wavelength with a 1.47-mm-long device. Obtained internal conversion efficiency is 2.2%/W which is one order of magnitude lower than the theoretical value for an ideal lossless waveguide. A significant increase in the conversion efficiency is expected with a redesign of the device and further optimization of the fabrication processes.

Nanoscale optical field localization by resonantly focused plasmons

We experimentally demonstrate use of plasmonic resonant phenomena combined with strong field localization to enhance efficiency of confining optical fields in a Si waveguide. Our approach utilizes a plasmonic resonant nano-focusing-antenna (RNFA), that simultaneously supports several focusing mechanisms in a single nanostructure, integrated with a lossless Si waveguide utilized with silicon-on-insulator (SOI) technology, to achieve a sub-diffraction limited focusing with a nanoscale (deeply subwavelength) spot size. The metallic RNFA effectively converts an incoming propagating waveguide mode to a localized resonant plasmon mode in an ultrasmall volume in all 3 dimensions. The near-field optical measurements of the fabricated RNFA using heterodyne near-field scanning optical microscope (H-NSOM) validate the theoretical predictions showing strong optical field localization.

Spinwave propagation in lossless cylindrical magnonic waveguides

Spinwave propagation in clad cylindrical magnonic waveguides is investigated under linear approximation. With the assumption of no magnetic damping, characteristic equation to determine the bound spinwave modes has been obtained based on the structural and magnetic properties of the waveguides. The study is then applied to homogenous magnetic nanowires with no cladding. Spinwave characteristics and properties, such as the dispersion relationship and group velocity, can be described analytically.

Spinwave propagation and coupling in magnonic waveguides

In this study, we start with the Landau-Lifshitz-Gilbert equation to describe the spinwaves in magnetic medium using the linearization approach. First, the reflection and refraction behaviors of spinwaves on an interface of a nonuniform magnetic medium is investigated. The study is then extended to spinwave propagation in lossless planar magnonic waveguides. Spinwave modes have been characterized based on the magnetic properties of the waveguides. Coupling between spinwaves in the structure of double magnonic waveguides is found to be possible due to the dipole interaction of the spinwaves. The spinwave coupling and power transfer associated with it is totally dependent on the waveguide structure and properties and the spinwave modes.

Fabrication and direct bonding of photosensitive multicomponent silicate glasses for lossless planar waveguide splitters

In this paper the fabrication, characterization, and thermal assisted bonding of two photosensitive silicate glasses is presented, as part of a wider process aiming to obtain a planar waveguide lossless splitter for telecom applications. Two glass compositions are investigated: an Er–Yb doped silicate glass, which amplifies the signals when pumped at 980 μm and a matchable passive glass, which is designed to host the waveguide splitter. The two parts were bonded successfully using direct bonding technique at 605 ± 20 °C for 240 min under a pressure of 28 kPa. Vickers hardness measurements were made on the bulk glasses and at the bonded interface, which showed no cracks propagating along the interface.

Trapezoidal waveguides: first-order propagation equivalence with rectangular waveguides

In the present paper we present in detail a new and improved version of the method of perturbative equivalence between the propagation characteristics of lossless trapezoidal cross-section waveguides and of rectangular cross-section waveguides. The analysis is carried out within the framework of perturbation theory, and to the first order we develop a very simple geometrical argument that allows one to construct for any given waveguide with trapezoidal cross-section an equivalent waveguide with rectangular cross-section. The numerical investigation of our arguments shows excellent agreement between the propagation characteristics of the two equivalent structures.

Fundamental guided electromagnetic dispersion characteristics in lossless dispersive metamaterial clad circular air-hole waveguides

The fundamental guided electromagnetic dispersion characteristics in lossless dispersive metamaterial clad circular air-hole waveguides are investigated. Two operating guided modes are found to exist: circular waveguide and surface plasmon polariton modes that support fast and slow waves, respectively. Hybrid mode classifications are also made in an empirical manner so that the guided modes can be sorted into either transverse magnetic (TM)-like or transverse electric (TE)-like modes in terms of their unique dispersion characteristics. Unusual dispersion characteristics, including multi-valued propagation constants at a single frequency, backward waves, and subwavelength guided propagations, are observed and discussed in relation to the air-hole radii and dielectric and magnetic constants of the metamaterial clad. A discussion comparing the extraordinary dispersion characteristics with those of other dielectric, plasmonic, and metamaterial waveguides is also included.

Eigenmodes for metal-dielectric light-transmitting nanostructures

We show that the concept of optical eigenmodes in lossless waveguide structures, which assumes the separation into propagating and evanescent modes, fails in the case of metal-dielectric structures. In addition to these modes, there is a sequence of new eigenstates with complex values of the propagation constant and nonvanishing lateral energy flows. The whole eigenmode problem ceases to be Hermitian because of changing sign of the optical dielectric constant. Particular examples of the waveguide structures include single slits, one-dimensional photonic crystals, and circular holes. The emphasis is made on the structures with nanoholes (whose sizes are considerably smaller than the light wavelength) possessing propagation modes and showing an extraordinary high light transmission. The results obtained form a necessary basis for modelling of this phenomenon.

Bidirectionality of Reciprocal, Lossy or Lossless, Uniform or Periodic Waveguides

Using the scattering-matrix equations for two antennas placed in the fields of a waveguide, it is proven that all reciprocal, lossy or lossless, uniform or periodic waveguides are bidirectional. Since Maxwell's equations imply directly that propagation constants on a lossless reciprocal waveguide come in pairs (beta,-beta*), "complex waves" on a lossless reciprocal, uniform or periodic waveguide come in quadruplets with propagation constants (beta,-beta, beta*,-beta*).

Polarization of the electromagnetic field of modes propagating in an open circular ferrite waveguide

An open circular longitudinally magnetized ferrite waveguide is considered. Both the frequency-dependent rotation angle of the polarization plane of the HE11 mode propagating in a lossless waveguide and the effect of ferrite loss on the field polarization is studied.

Advancement of Algebraic Function Approximation in Eigenvalue Problems of Lossless Metallic Waveguides to Infinite Dimensions, Part I: Properties of the Operator in Infinite Dimensions

In an attempt to advance finite dimensional algebraic function approximation technique in eigenvalue problems of lossless metallic guides filled with anisotropic and/or inhomogeneous media, to exact analysis in infinite dimensions, properties of the linear operator in infinite dimensions corresponding to Maxwell's equations, are investigated. Some function theoretic aspects of this operator formalism for guidance phenomenon are discussed. It is found that this operator can be considered bounded and holomorphic. If not, because of poles the medium constituent matrices ε and μ may have, its inverse, which will be bounded and holomorphic, can be used to assess propagation constant behavior in the neighborhood of the singularity.

Advancement of Algebraic Function Approximation in Eigenvalue Problems of Lossless Metallic Waveguides to Infinite Dimensions, Part III: Examples Verifying the Theory

Verifications of the theory for extension of the algebraic function approximation in closed, lossless guiding system eigenvalue problems to infinite dimensions, are given. Numerical verifications are made on a closed circular guide loaded with a coaxial dielectric rod. The defective nature of an eigenvalue at the onset of a complex wave mode is demonstrated both in the algebraic function approximation and in infinite dimensions. At a cutoff frequency, for the exact infinite system, analyticity of the eigenvalue, but singularity of the propagation constant, are checked numerically. It is observed that in exact analysis too, by itself a complex wave mode does not take part in energy transportation, but a pair of complex conjugate modes behaves as an evanescent wave.

Algebraic Function Approximation in Eigenvalue Problems of Lossless Metallic Waveguides: Examples

The method of algebraic function approximation in eigenvalue problems of closed lossless waveguides has been illustrated by means of two examples. The first example takes up a cylindrical guide with a concentric ferrite tube, magnetized axially. The second example consists in approximation of the eigenvalue (square of the propagation constant), by this technique in a cylindrical guide loaded coaxially by an isotropic dielectric rod. It is found that the type of singularities in the dispersion characteristics of the guiding system, can also be identified by the approach. It is shown that the method is both efficient and accurate numerically, in addition to the function theoretic insight it brings when used as a tool in analysis of the guiding system.