Waveguide coupling of SAW resonators with different properties

In contrast to conventional transversely coupled resonator filters, waveguide coupling of two one-port resonators with different resonance frequencies is described. The different resonance frequencies are implemented by different central finger gaps or gratings differing by a scaling factor. Consequently, the equivalent fingers and reflecting strips of different resonators are shifted with respect to each other, and the waveguide modes are, therefore, no longer independent of each other. This effect is called mutual coupling of waveguide modes and requires a new type of modeling. The main characteristics of the new modeling method are described. The advantage of the design principle consists of a wider bandwidth without changing the waveguide parameters and different input and output impedance.     

Excitation of waveguide modes in organic light-emitting diode structures by classical dipole oscillators

Analytical techniques known in the literature are used to (i) identify all the planar waveguide modes in four top-emitting organic light-emitting diode (OLED) structures over the visible spectrum, and (ii) compute both TM and TE power spectra for classically radiating dipoles in the emissive layers of these OLED structures. Peaks in the computed power spectra are identified with the waveguide modes in the OLED devices, and areas associated with these peaks are used to estimate the excitation probability of the waveguide modes. In cases where ambiguities arise because of overlapping peaks, it is shown that computed power spectra can be approximated as sums of Lorentzian line shapes. It is found that for all four structures, the dipoles couple almost 80% of their radiant energy into TM modes with only about 20% going into TE modes. Furthermore, except for a narrow spectral band, the excited TM modes are primarily short-range surface plasmon polaritons. Excitations in the narrow spectral band correspond to TM and TE Fabry-Perot microcavity modes. Finally, the analysis shows that, in the absence of grating couplers, only light in the microcavity modes escapes into the air cover.     

Grating-coupled surface plasmon polaritons and waveguide modes in a silver-dielectric-silver structure

A silver-dielectric-silver structure that supports both waveguide modes and surface plasmon polaritons is explored. The upper interface between the dielectric and the silver is periodically corrugated to allow coupling of visible photons to both types of mode. Such a metallic microcavity leads to plasmonic and waveguide self-interacting bandgaps at Brillouin zone boundaries. In addition there are found other bandgaps from mode crossings within the Brillouin zone. This results specifically in a very flat photonic band due to anticrossings between a surface plasmon polariton and waveguide modes. Characterization of the observed modes in terms of their resonant electromagnetic fields is achieved by using a multilayer, multishape differential grating theory.     

A physical interpretation of elastic guided-wave reflection from normal ends of a waveguide

Studied in this paper are two-dimensional guided wave reflections from normal boundaries in an isotropic elastic media. By making use of the transverse resonance concept, the reflections of the waveguide modes from normal interfaces are interrogated. A general condition is obtained under which the guided waves in an isotropic medium will undergo no mode conversion when interaction occurs with a normal traction free or fixed end. Under some circumstances, similarities are obtained between waveguide modes and bulk-wave modes, for example, doubling of the displacement field at a free end and doubling of the stress field at a fixed end. The results obtained are applicable to all two-dimensional, guided-wave modes, along one waveguide direction with lossless boundaries on the surface(s) parallel to the waveguide direction, including all possible guided-wave modes, propagating and nonpropagating, in plates, one half space, interface of two different half spaces, layers on a half space, multilayer structures, and all axisymmetric modes in cylindrical structures. In addition, the function of displacement potentials is analyzed in the course of guided-wave mode conversion at a normal end.     

Phase effects in guided mode resonances: spectral phase

When a suitably designed waveguide grating couples an external wave to a local wave, a well-known (and well-studied) resonance anomaly can appear in the reflectance spectrum. We employ a guided mode resonance in a silicon-on-insulator waveguide to study the spectral phase response using backside illumination, in which interference fringes from a weak substrate reflection allows the extraction of the phase. Comparing a slab analysis, rigorous coupled wave analysis, and experiment shows that the strongest resonances in this structure are associated with modes confined in the silicon layer that are very close to cut-off.     

Holography with surface-plasmon-coupled waveguide modes

We report on an attempt to use the enhancement effect of surface-plasmon resonance to improve the image quality of a waveguide hologram. With a structure consisting of a waveguide medium sandwiched between a metal film and a hologram, we obtained holographic images reconstructed by surface-plasmon-coupled waveguide modes. Comparison of the holographic images reconstructed by TM and TE modes indicates that the surface-plasmon effect is responsible for better image quality in diffraction efficiency and image contrast.     

Recent developments in the design and fabrication of visible photonic band gap waveguide devices

In this paper, we present the design, fabrication and initial optical testing of dielectric waveguide devices which incorporate photonic crystals with photonic band gaps (PBG) in the visible region of the spectrum. In the design of our devices we use a full three-dimensional plane wave analysis to solve the photonic band structure simultaneously with the dielectric waveguide boundary conditions for a fixed lattice and waveguide geometry. This takes into account the finite thickness of the waveguide core, and the evanescent wave in the dielectric cladding layers. Furthermore, we explain how the effective Bloch mode index can be extracted from the results. This enables us to tackle important problems associated with mode coupling between the input waveguide and guided Bloch modes within the porous PBG region, such as Fresnel reflections at the interface and up-scattering from the holes. Finally, we present the recent fabrication of quasi-periodic photonic crystals and PBG waveguide bends.     

Long-period gratings in planar optical waveguides

We present a theoretical analysis of light propagation in a four-layer planar waveguide that consists of a long-period grating (LPG) having a period of the order of 100 microm. By means of the coupled-mode theory, we show that such a structure is capable of coupling light from the fundamental guided mode to the cladding modes at specific wavelengths (resonance wavelengths) and thus results in sharp rejection bands in the transmission spectrum of the waveguide. Our numerical results show that the resonance wavelengths as well as the transmission spectrum can be significantly changed with the waveguide and grating parameters. A waveguide-based LPG should provide a useful approach to the design of a wide range of integrated-optic devices, including wavelength-tunable filters, switches, and environmental sensors.     

Dispersion relation of guided-mode resonances in multimode grating waveguide structures

It is well known that the location of guided-mode resonance (GMR) in grating waveguide structures closely tracks the leaky mode dispersion curves. In this paper, taking Bragg reflection due to periodicity and interaction between different modes into account, we first present a schematic diagram of the dispersion relations of leaky modes in multimode grating waveguide structures, both for s-polarized (TE mode) and p-polarized (TM mode) incident waves. Due to the perturbation of the grating layer, the interaction between different resonance modes (transverse standing waves) is inevitable. This transverse interference will result in the non-Bragg nature resonance band gaps in the dispersion curves. Exploiting the characteristics of leaky mode dispersions over the full range of the first Brillouin zone, we hoped we could gain some insight into the relationship among the mode interactions, band gaps, and their benefits to optical elements utilizing the GMR effect in grating waveguide structures. Finally, a specific structure is analyzed.     

Numerical characterization of whispering-gallery mode optical microcavities

We characterize planar microcavities in whispering-gallery mode optical resonances. The microcavity consists of a waveguide and a microdisk, and a nanoscale gap separates the waveguide and the microdisk. The devices can be fabricated on Si-based thin films by using conventional microelectronics techniques. To characterize these types of cavity, we study a broad range of resonator configuration parameters including the size of the microdisk, the width of the gap, and the waveguide dimensions. The finite-element method is used for solving Maxwell's equations. The electric fields and the energy density distributions are obtained and compared between the on-resonance and off-resonance situations. A brilliant ring with a strong electric field and a high-energy density is found inside the periphery of the microdisk under first-order resonance. While under second-order resonance, there are two bright rings, and the light intensity in the inner ring is stronger than that in the outer ring. The resonant frequencies and their free spectral ranges are predominantly determined by the size of the microdisk. The gap effect on the resonant frequencies is observable, although it is minor. The gap strongly affects the full width at half-maximum (FWHM), finesse, and quality factor of the resonances. With an increase in the gap width from 100 to 300 nm, both the Q value and finesse increase substantially, while the FWHM decreases. The waveguide width has a visible influence on the Q value, FWHM, and finesse as well.     

单层膜多模导模共振滤光片的电场增强效应研究

本文研究了单层膜多模导模共振滤光片的电场增强效应.单层膜导模共振滤光片同时兼具波导和相位匹配功能的物理机制得到了论证.当光栅厚度不断增加将出现多模共振效应,导致高阶泄漏模电场增强效应的产生.电场增强振幅的最大值反映导模共振滤光片的泄漏程度.通常在同一共振位置处电场增强振幅的最大值越大,其带宽就越小.     

Rarefaction of the spectrum of H-type whispering-gallery modes in a hemispherical dielectric resonator

A dense spectrum of H-type resonance oscillations (whispering-gallery modes) is observed in a hemispherical dielectric resonator excited by a capacitive slit situated on the equatorial metal mirror surface. The spectrum can be significantly rarefied by exciting the WG modes in the resonator by means of distributed coupling to a dielectric waveguide arranged in a certain special position relative to the hemisphere base.     

Prism Coupling of Light to Discrete Substrate Modes

Abstract

The most common coupling between a prism and a waveguide requires a gap between the prism and the waveguide. The modes in the waveguide are guided modes. We show that discrete substrate modes may be excited if the waveguide film is directly deposited onto the prism. These discrete substrate modes have many similarities to guided modes.     

Polarization eccentricity of the transverse field for modes in chiral core planar waveguides.

The general solution for modes in an asymmetric planar waveguide with a homogeneous and isotropic chiral core is given in terms of a pair of parameters related to the eccentricity of the polarization ellipse for the transverse electric field. This formulation provides insight into the transition, with increasing chirality of the core, from TE/TM modes to right-handed and left-handed circular polarization modes. Mode polarization as a function of waveguide thickness and of frequency is discussed in detail. Beyond a mode-dependent maximum thickness (or frequency), the left-handed elliptical modes consist of a slow-wave component whose cutoff properties are examined. The limiting case of a symmetric waveguide is also discussed.     

Surface plasmon coupled fluorescence in the ultraviolet and visible spectral regions using zinc thin films

The use of zinc thin films deposited onto glass supports for surface plasmon coupled fluorescence (SPCF) over a broad 200 nm wavelength range is demonstrated. Fresnel calculations performed in the ultraviolet and visible spectral range are predicted to generate surface plasmon modes in 30 nm zinc thin films. In this spectral range, the extent of coupling of light to zinc thin films was shown to be significant as compared to similar aluminum, gold, and silver thin films. The experimental demonstration of SPCF using 30 nm zinc thin films in the ultraviolet and visible spectral regions was undertaken using three different fluorophores 2-AP, POPOP, and FITC, respectively. Surface plasmon coupled fluorescence from zinc thin films was p-polarized and highly directional with lambda max conferred at an angle of 58, 68, and 60 degrees for FITC, POPOP, and 2-AP, respectively. s-Polarized emission from zinc thin films was negligible for all fluorophores except for a sample spin coated from a 10% PVA solution, which resulted in significant s-polarized emission due to the generation of waveguide modes. The experimental results are consistent with reflectivity curves that are theoretically predicted using Fresnel calculations. Given the growing use and utility of plasmon-enhanced fluorescence in the analytical and biological sciences, our findings will serve as a useful tool for workers in the ultraviolet and visible spectral regions.     

Waveguide confinement of cerenkov second-harmonic generation through a graded-index grating coupler: Electromagnetic optimization

Abstract

The feasibility of making a frequency doubler for integrated optics is studied with the electromagnetic theory of gratings and graded-index waveguides as a tool. The device consists of a first waveguide filled with a sol-gel nonlinear material doped or grafted with a nonlinear chromophore whose thickness is chosen to generate a second-harmonic Cerenkov radiation in a dispersive glass substrate. The Cerenkov radiation is coupled into a second waveguide through a graded-index layer produced by ion exchange into glass, lying on top of an ion-etched grating coupler. The aim of the study is to optimize the optogeometrical parameters of the device in order to obtain a resonance line of the second waveguide modes that has an angular width large enough to match the experimental constraints, and which leads to a good enough coupling coefficient. The electromagnetic theory of grating couplers is developed into an S-matrix propagation algorithm form in order to be combined with the electromagnetic analysis of the thick graded-index waveguide with a view to analysing the device.     

Radiation of a charge moving in a waveguide filled with a dielectric medium possessing resonance dispersion

The radiation of a point charged particle moving along the axis of a round waveguide filled with a dielectric medium possessing resonance dispersion is theoretically studied. It is shown that the dispersion significantly influences the frequencies of excited harmonic modes and the energy of radiation. Allowance for dispersion leads to suppression of all excited harmonics to an extent increasing with the order of harmonics.     

Design and characteristics of polarization-insensitive resonant gratings for color filtering

A method of designing polarization-insensitive color filters with guided-mode resonance grating is presented. The influence of incident conditions on exciting waveguide mode is investigated, and we find that polarization insensitivity may occur at the full conical incidence. Using rigorous coupled-wave analysis, we mainly analyze the effects of waveguide thickness and fill factor on the diffracted efficiency of the grating filter. The final structural parameters of the devices for three primary colors are collected after optimization, and the calculated results show that the spectral reflectance of each color filter is basically identical for different polarization states of incident light. Moreover, field analysis by the finite-difference time-domain technique indicates that two symmetric modes are excited between the waveguide layer and substrate under full conical incidence, which is coincident with the previous theoretical study. The reported work will eliminate the restriction of polarization-dependence of color filters and greatly expand their application range.     

Multimode solution for the reflection properties of an open-endedrectangular waveguide radiating into a dielectric half-space: theforward and inverse problems

Open-ended rectangular waveguides are extensively used in nondestructive dielectric material evaluation. The dielectric properties of an infinite-half space of a material are calculated from the measured reflection properties referenced to the waveguide aperture. This calculation relies on a theoretical and numerical derivation of the reflection coefficient likewise referenced to the waveguide aperture. Most of these derivations assume the dominant mode field distribution across the waveguide aperture. However, when dealing with low permittivity and low loss dielectric materials, there may exist significant errors when calculating the dielectric properties from the measured reflection coefficient. These errors have also shown to be more significant in the upper frequency portion of a waveguide band. More accurate results are obtained when higher order modes are considered in addition to the dominant waveguide mode. However, most studies incorporating higher-order modes have used various approximations when calculating the reflection properties and have not provided a full discussion on the influences of dielectric properties of the infinite-half space and the frequency of operation. This paper gives a rigorous and exact formulation in which the dominant mode and the evanescent higher-order modes are used as basis functions to obtain the solution for the reflection coefficient at the waveguide aperture. The analytic formulation uses Fourier analysis in addition to the forcing of the necessary boundary conditions at the waveguide aperture. The solution also readily accounts for the complex contributions of both TE and TM higher-order modes. Finally, the influences of the dielectric properties of the infinite-half space and the frequency of operation are investigated