Surface Waveguide Modes Research Articles

Evanescent coupling between surface and linear-defect guided modes in phononic crystals

Evanescent coupling between surface and linear-defect waveguide modes in a two-dimensional phononic crystal of steel cylinders in air is numerically demonstrated. When the ratio of scatterer radii to the lattice constant is set to 0.47 in the square phononic crystal, the two types of modes start interacting if there is one-row separation between the surface and waveguide. Supercell band structure computations through the Finite Element Method suggest that the waveguide band is displaced significantly, whereas the surface band remains almost intact when the waveguide and surface are in close proximity. The two resultant hybrid bands are such that the coupling length, which varies between 8 and 22 periods, initially changes linearly with frequency, while a much sharper variation is observed towards the top of the lower hybrid band. Such small values facilitate the design of compact devices based on heterogeneous coupling. Finite-element simulations demonstrate bilateral coupling behaviour, where waves incident from either the surface or waveguide can efficiently couple to the other side. The coupling lengths calculated from simulation results are in agreement with the values predicted from the supercell band structure. The possible utilisation of the coupling scheme in sensing applications, especially in acoustic Doppler velocimetry, is discussed.

Infrared reflection and attenuated total reflection spectra in the Bi2Se3 topological insulator

Infrared reflection and attenuated total reflection spectra are measured in the (111)Si/Bi2Se3 topological insulator film. The characteristic parameters of plasmons and phonons in the near-surface layers close to the Si–film interface are obtained from the dispersion analysis of the reflection spectra. It is found that the charge carrier density near the interface far exceeds that in the bulk. The dispersion laws for surface polaritons and waveguide modes are determined.

Lifshitz formula by a spectral summation method

The Lifshitz formula is derived by making use of the spectral summation method which is a mathematically rigorous simultaneous application of both the mode-by-mode summation technique and scattering formalism. The contributions to the Casimir energy of electromagnetic excitations of different types (surface modes, waveguide modes, and photonic modes) are clearly retraced. A correct transition to imaginary frequencies is accomplished with allowance for all the peculiarities of the frequency equations and pertinent scattering data in the complex $\omega $ plane, including, in particular, the cuts connecting the branch points and complex roots of the frequency equations (quasi-normal modes). The principal novelty of our approach is a special choice of appropriate passes in the contour integrals, which are used for transition to imaginary frequencies. As a result, the long standing problem of cuts in the complex $\omega $ plane is solved completely. Inconsistencies of some previous derivations of the Lifshitz formula are traced briefly. For completeness of the presentation, the necessary mathematical facts are also stated, namely, solution of the Maxwell equations for configurations under consideration, scattering formalism for parallel plane interfaces, determination of the frequency equation roots, and others.

Numerical and analytical evaluations of the sensing sensitivity of waveguide mode in one-dimensional metallic gratings

We study numerically and analytically the refractive index sensing sensitivities of surface plasmon (SSP) and waveguide (SWG) modes arising from one-dimensional Au gratings. By using rigorous coupled wave analysis, we find that while SSP is mainly controlled by the periodicity of the grating, the shape of the groove governs SWG. As a result, it is possible to increase SWG to 1000 nm/RIU and figure of merit to 24 by tailoring the height and width of the groove. Finally, a simple analytical expression is derived to describe SWG and it agrees well with the numerical data. This easy-to-use expression not only reveals the origin of waveguide mode sensitivity, but also provides useful guidance for the theoretical design and experimental realization of high-sensitivity metallic-gratings-based biosensors.

Propagation of surface waves and waveguide modes guided by a dielectric slab inserted in a sculptured nematic thin film

Wave propagation guided by a dielectric slab inserted in a sculptured nematic thin film (SNTF) was studied theoretically. Two types of guided waves can be identified: (i) surface (Dyakonov--Tamm) waves guided by one or both of the two planar interfaces of the dielectric slab and the SNTF, and (ii) waveguide modes in the dielectric waveguide formed by the slab with the SNTF as the cladding. As the thickness of the dielectric slab is increased, the number of waveguide modes increases. If the slab thickness is less than twice the e-folding distance into the dielectric slab, the Dyakonov--Tamm waves propagate coupled to both interfaces; the coupling decreases and eventually vanishes as the slab thickness increases, so that Dyakonov--Tamm waves are guided by the individual dielectric-SNTF interfaces independently. The chosen structure supports the propagation of Dyakonov--Tamm waves in all directions, in contrast to the restricted range of propagation supported by a single SNTF-dielectric interface. Propagation of both Dyakonov--Tamm waves and waveguide modes should occur in practice with negligible attenuation, in contrast to that of surface-plasmon-polariton waves that are guided when the dielectric slab is replaced by a metal slab.

Photonic-crystal surface modes found from impedances

We present a method for finding surface modes at interfaces between two-dimensional photonic crystals (PCs), in which the surface modes are represented as superpositions of the PCs' propagating and evanescent Bloch modes. We derive an existence condition for surface modes at an air-PC interface in terms of numerically calculated PC impedance matrices, and use the condition to find surface modes in the partial band gap of a PC. We also derive a condition for modes of a three-layer structure with two interfaces, and find both coupled surface modes and waveguide modes. We show that some waveguide modes cross the band edge and become coupled surface modes.

Light extraction from surface plasmons and waveguide modes in an organic light-emitting layer by nanoimprinted gratings

Organic light-emitting diodes (OLEDs) usually exhibit a low light outcoupling efficiency because a large fraction of power is lost to surface plasmons (SPs) and waveguide modes. In this paper it is demonstrated that periodic grating structures with almost µm-scale can be used to extract SPs as well as waveguide modes and therefore enhance the outcoupling efficiency in light-emitting thin film structures. The gratings are fabricated by nanoimprint lithography using a commercially available diffraction grating as a mold which is pressed into a polymer resist. The outcoupling of SPs and waveguide modes is detected in fluorescent organic films adjacent to a thin metal layer in angular dependent photoluminescence measurements. Scattering up to 5th-order is observed and the extracted modes are identified by comparison to the SP and waveguide dispersion obtained from optical simulations. In order to demonstrate the low-cost, high quality and large area applicability of grating structures in optoelectronic devices, we also present SP extraction using a grating structure fabricated by a common DVD stamp.

Ultrashort excitations of surface polaritons and waveguide modes in semiconductors

Polarization-dependent structures have been formed on the silicon surface under the action of femtosecond laser pulses. Some model concepts are proposed to describe changes in the response of the semiconductor surface caused by the generation of a nonequilibrium electron-hole plasma and explain the excitation of surface polaritons and waveguide modes during a femtosecond laser pulse.

High sensitivity sensors made of perforated waveguides

Sensors based on surface plasmons or waveguide modes are at the focus of interest for applications in biological or environmental chemistry. Waveguide-mode spectra of 1 mum-thick pure and perforated silica films comprising isolated nanometric holes with great aspect ratio were measured before and after adhesion of streptavidin at concentrations of 500 nM. The shift of the angular position for guided modes was nine times higher in perforated films than in bulk films. Capturing of streptavidin in the nanoholes is at the origin of that largely enhanced shift in the angular position as the amplitude of the guided mode in the waveguide perfectly overlaps with the perturbation caused by the molecules. Hence, the device allows for strongly confined modes and their strong perturbation to enable ultra-sensitive sensor applications.

Surface modes in two-dimensional photonic crystal slabs with a flat dielectric margin

We report numerical simulations of surface modes in two-dimensional high-index-contrast photonic crystal slabs with a flat dielectric margin of width on the order of the photonic crystal periodicity. Our calculations using plane wave expansion method reveal multiple surface guided modes within the photonic band gap, with some high-order modes exhibit relatively flat dispersion curves. We calculate the finite-length surface waveguide modes transmission and field patterns using two dimensional finite-difference time-domain method. We verify the surface mode dispersion curves by using spatial Fourier transform of the mode field patterns. Our study on surface modes under small ambient refractive index changes (5 x 10(-3)) shows that lower order modes exhibit larger wavelength shifts on the order of 1 nm. We also design a 4-port 3-channel bidirectional coupler using a conventional dielectric waveguide side coupled to the multimode surface waveguide.

Extraction of the surface waves and waveguide modes of the Green's function in layered anisotropic dielectrics between two ground planes

In layered lossless dielectrics between two ground planes there exist the complicated waveguide modes as well as the surface waves in the electromagnetic field induced by a horizontal electric dipole. All these modes can be characterized by the rational parts with the real poles of the spectral domain vector and scalar potentials. The accurate extraction of these modes plays an important role in the evaluation of the Green's function in spatial domain. In our previous work [Song and Hong, 2002] a new algorithm based on rational approximation is presented for the extraction of surface waves and waveguide modes of the Green's function for layered isotropic dielectrics. In this paper, we extend the algorithm to the more complex case of layered anisotropic dielectrics; this algorithm can accurately extract all the real poles and their residues simultaneously. Thus we can get all the surface waves and waveguide modes, which can greatly help in the calculation of the spatial domain Green's function. The numerical results demonstrate the accuracy and efficiency of the proposed method.

Optimization of the emission characteristics of light emitting diodes by surface plasmons and surface waveguide modes

In this letter, the influence of metal films and dielectric waveguides on the radiation characteristics of light emitting diodes (LEDs) is investigated. Cross and hexagonal surface grating structures are used to excite surface plasmons and waveguide modes in these media. A beam divergence as small as 17° is achieved for a hexagonal grating coated with a 40 nm Ag film. The maximum quantum efficiency is found for a hexagonal grating with a 10-nm-thick Au film and a 250-nm-thick dielectric layer. The optical power emitted by this LED into the solid angle from −30° to 30° is 10% higher than that of an LED without metal film and dielectric layer.

Integrated optics for the characterization of photoreactive organic thin films

Abstract

Numerical Solution of Surface Waveguide Modes Using Transverse Field Components (Short Papers)

The computation of surface waveguide modes is facilitated by reducing the surface waveguide field problem to a conventional eigenvalue problem that has no spurious solutions. This is achieved by formulating the field problem in terms of transverse field components and by using impedance boundary conditions on an auxiliary boundary with a specified value of the exterior cutoff wavenumber.

Computation of surface-waveguide modes by use of reactance boundary conditions

A new numerical method for solving surface-waveguide problems is proposed. It leads to the solution of a related closed-waveguide problem with reactance boundary conditions The results from a simple application of the method are given.

Surface Waveguide Modes Along a Semi-Infinite Dielectric Fiber Excited by a Plane Wave

The power transmitted by surface waveguide modes propagating along a semi-infinite dielectric fiber is determined. For a plane wave normally incident on the end of a perfectly circular fiber only the HE1m modes are excited. Of these modes the majority of the power is contained in the HE11 mode. Far above the cutoff conditions for the HE13 mode, approximately 99.8% of the incident power is contained in the HE11, HE12, and HE13 modes.

Excitation of surface modes along a semi-infinite dielectric cylinder

Expressions for the amplitude and power of surface-waveguide modes excited by a plane wave along a semi-infinite dielectric cylinder are presented. Approximations for small dielectric differences are given, which result in a simple expression for the power of the modes. Application of these results to light propagating along the human retinal receptors is discussed.