Modes In Slab Waveguide Research Articles

Guided waves in asymmetric hyperbolic slab waveguides: the TM mode case

Nonlinear guided wave modes in an asymmetric slab waveguide formed by an isotropic dielectric layer placed on a linear or nonlinear substrate and covered by a hyperbolic material are investigated. Optical axis is normal to the slab plane. The dispersion relations for TM waves are found. It is shown that there are additional cut-off frequencies for each TM mode. The effects of the nonlinearity on the dispersion relations are investigated and discussed. There are the modes, which are corresponded with situation where the peak of electric field is localized in the nonlinear substrate. These modes are absent in the linear waveguide. To excite these modes the power must exceed certain threshold value.

Optical modes in slab waveguides with magnetoelectric effect

Optical modes in anisotropic slab waveguides with topological and chiral magnetoelectric effects are investigated analytically, by deriving the closed-form characteristic equations of the modes and hence computing the dispersion-diagrams. In order to compute the characteristic equations, a vector-potential approach is introduced by incorporating a generalized Lorentz gauge, and the Helmholtz equations are derived correspondingly. It will be shown that the formation of the complex modes and hybridization of the optical modes in such slab waveguides is inevitable. Moreover, when the tensorial form of the permittivity in the waveguide allows for a hyperbolic dispersion, complex transition from the photonic kinds of modes to the plasmonic modes is expected.

Randomly Distributed Fabry-Pérot-type Metal Nanowire Resonators and Their Lasing Action

Optical feedback mechanisms are often obtained from well-defined resonator structures fabricated by top-down processes. Here, we demonstrate that two-dimensional networks of metallic nanowires dispersed on the semiconductor slab can provide strong in-plane optical feedback and, thus, form randomly-distributed Fabry-Pérot-type resonators that can achieve multi- or single-mode lasing action in the near infrared wavelengths. Albeit with their subwavelength-scale cross-sections and uncontrolled inter-nanowire distances, a cluster of nearly parallel metal nanowires acts as an effective in-situ reflector for the semiconductor-metal slab waveguide modes for coherent optical feedback in the lateral direction. Fabry-Pérot type resonance can be readily developed by a pair of such clusters coincidentally formed in the solution-processed random nanowire network. Our low-cost and large-area approach for opportunistic random cavity formation would open a new pathway for integrated planar light sources for low-coherence imaging and sensing applications.

Nonclassical effects in plasmonics: An energy perspective to quantify nonclassical effects

Plasmons are commonly interpreted with classical electrodynamics, while nonclassical effects may influence the dynamics of plasmon resonances as the plasmon confinement approaches the few-nanometer scale. However, an unambiguous approach to quantify the degree of nonclassical dynamics remains. We propose a nonclassical-impact parameter (NCI) to characterize the degree of nonclassical effects from an energy perspective, i.e., which fraction of the total electromagnetic energy is attributed to classical electrodynamic terms and which fraction is correspondingly to be assigned to nonclassical degrees of freedom? We show that the NCI relates directly to two fundamental parameters of plasmon resonances: the loss function and the quality factor. Guided by the NCI, we discuss the nonclassical effects of plasmon waveguiding modes of metallic slab waveguides, and highlight the general features of the nonclassical effects at different microscopic levels by contrasting the numerical results from the semiclassical hydrodynamic Drude model (HDM) and the microscopic random-phase approximation (RPA). The formal relation between the HDM and the RPA is also established for metals by exploring the limit of an infinite work function.

Propagation of TM surface modes in a taper slab waveguide with anisotropic metamaterials

The TM surface mode propagation properties in a taper slab waveguide with an anisotropic metamaterial core and common material claddings are investigated. Through Maxwell's equations, the dispersion, reflection coefficient, transmission coefficient and power loss were extracted for the TM surface modes. Then, some characteristic curves are plotted for different metal filling ratios or different frequencies. From these curves, it is found that: (1) as waveguide length L=7.7μm, the reflection coefficient is near 0.99; (2) the transmission coefficient ct2≤0.1 or even ct2≤0.05; (3) the power loss Δp/p≥0.98 or even Δp/p≥0.99, which means extra strong reflection or extra big energy loss can exist in the taper waveguide. These new properties for TM surface modes propagation will benefit us to design extra strong reflectors or novel absorbers in the future.

Broadband high reflectivity in subwavelength-grating slab waveguides.

We computationally study a subwavelength dielectric grating structure, show that slab waveguide modes can be used to obtain broadband high reflectivity, and analyze how slab waveguide modes influence reflection. A structure showing interference between Fabry-Perot modes, slab waveguide modes, and waveguide array modes is designed with ultra-broadband high reflectivity. Owing to the coupling of guided modes, the region with reflectivity R > 0.99 has an ultra-high bandwidth (Δf / ̅f > 30%). The incident-angle region with R > 0.99 extends over a range greater than 40°. Moreover, an asymmetric waveguide structure with a semiconductor substrate is studied.

Localized and nonlocalized plasmon resonance enhanced light absorption in metal-insulator-metal nanostructures

Multiple absorption bands in a metal-insulator-metal (MIM) nanostructure are comprehensively investigated in the visible and near-infrared (near-IR) regime. The MIM nanostructure consists of patterned gold squares with systematically varying size and period atop a thin aluminum nitride dielectric layer on a thick gold film. Both the transverse-electric (TE) slab-waveguide mode and the transverse-magnetic (TM) surface plasmon-polariton mode can be excited in the nanostructure. Grating coupling of the incident light into nonlocalized traveling waves is found from electromagnetic field patterns and from the linear dispersion of the absorption modes with period. A localized mode strengthens the near-IR absorption band for small square sizes, and the resonance shifts to red for large square sizes.

各向异性左手材料劈形平面波导传输特性

各向异性左手材料劈形平面波导传输特性

Silicon-based metal-loaded plasmonic waveguides for low-loss nanofocusing.

We introduce plasmonic waveguides based on metal loading of silicon-on-insulator (SOI) substrates. Here slab waveguide modes hybridize with the plasmonic modes of either a metal nanowire or a slot in a metal film. By tapering a single dimension of either structure, the resulting hybrid mode can be converted from photon-like to plasmon-like, allowing up to millimeter-range transport and rapid nanoscale focusing down to mode areas ∼λ2/400. Metal loading is achievable with a single lithography step directly on SOI without the need for etching and, thus, opens practical possibilities for silicon nanoplasmonics.

Possible behaviours of TE modes in a left-handed slab waveguide

We present a theoretical study of a slab waveguide made of a left-handed material in order to clarify the action of electric plasma frequency on the guided dispersion characteristics. We also examined the dispersion properties of the guided waves and discuss the existence of new modes. We show that it is possible to increase the domain of existence of the mode and control the energy flow for a suitable choice of parameters. That leads to an exhibition of new types of modes unusually seen in slab waveguide only composed by right-handed material.

Dispersion characteristics of surface plasmon polariton modes in a metallic slab waveguide with nonlinear magnetic cladding

We investigate the propagation characteristics of surface plasmon polaritons along a slab waveguide with a nonlinear magnetic cladding characterized by permeability μ =μl+μnlH2 and a linear substrate. The dispersion relation for the surface plasmon polariton modes supported by the structure was derived and numerically solved. The dependance of mode behavior on input power was studied. As the input power increases, the propagation constant of the modes increases. The effect of self focussed peak on the mode confinement near the interface and the resulting propagation characteristics of the plasmon modes were investigated. Increase in metal film thickness lifts the degeneracy of nonlinear plasmon modes supported by the waveguide. It has been observed that the propagation length is not a single valued function of input power which is crucial in understanding optical nano-switches.

Nonlinear equation of the modes in circular slab waveguides and its application

In this paper, circularly curved inhomogeneous waveguides are transformed into straight inhomogeneous waveguides first by a conformal mapping. Then, the differential transfer matrix method is introduced and adopted to deduce the exact dispersion relation for modes. This relation itself is complex and difficult to solve, but it can be approximated by a simpler nonlinear equation in practical applications, which is close to the exact relation and quite easy to analyze. Afterward, optimized asymptotic solutions are obtained and act as initial guesses for the following Newton's iteration. Finally, very accurate solutions are achieved in the numerical experiment.

Enhanced light–matter interaction at nanoscale by utilizing high-aspect-ratio metallic gratings

We report the design, fabrication, and experimental characterization of high-aspect-ratio metallic gratings integrated with nanoscale semiconductor structures, which enable efficient light-matter interaction at the nanoscale over interaction lengths as long as two times of the effective optical wavelength. The efficient light-matter interaction at the nanoscale is enabled by excitation of the guided modes of subwavelength slab waveguides formed by the high-aspect-ratio metallic gratings. By controlling the height of the high-aspect-ratio gratings, the wavelength of the guided modes through the nanoscale semiconductor structures is determined.

Guided surface modes in a hollow slab waveguide with a left-handed material substrate or cover

The structural dispersion characteristics of guided surface modes in a hollow slab waveguide with a left-handed material substrate or cover are investigated. Dispersion relation is derived by using normalized parameters, and universal dispersion curves have been obtained analytically, by solving transcendental dispersion equations in a reverse way. Existence condition, mode degeneracy and other dispersion properties of guided surface modes have been discussed for different ɛ or μ of three layers for this substrate or cover layer.

Narrowband mid-infrared transmission filtering of a single layer dielectric grating

We experimentally demonstrate a simple narrowband transmission filter using a silicon/air grating, exhibiting broadband high-reflectance between 8 and 14 μm and narrow transmission peaks at slightly off-normal incidence. We explain the response in terms of guided modes in a dielectric slab waveguide, with numerical modal analysis corroborating an intuitive slab waveguide picture and the experimental results. The filtered wavelength depends predictably on the grating period, making wavelength selection straightforward, while the length-scale independent operation principle shows promise for enabling similar filtering capabilities at higher frequencies.

Study on the Dispersion Characteristics of Surface TM Modes in Hollow Slab Waveguide with a Left-Handed Material Cover

The dispersion characteristics of surface TM modes in a hollow slab waveguide which consists of the core and substrate of right-handed material (RHM) and the cover of left-handed material (LHM) are investigated. Normalized frequency, normalized propagation constant and asymmetry measure are introduced to the left-handed material hollow slab waveguide. The dispersion relation is derived by using these normalized parameters, and universal dispersion curves are obtained by solving transcendental dispersion equations analytically. Existence condition and other dispersion characteristics of guided surface TM modes have been discussed in detail for different permittivity of three layers for this left-handed material hollow dielectric slab waveguide.

Enhancement of SOI Photodiode Sensitivity by Aluminum Grating

Aluminum grating was applied to silicon-on-insulator (SOI) pn-junction photodiode to increase the light sensitivity. For 100-nm-thick diode, quantum efficiency (QE) of 26% (x15 enhancement) was attained at the wavelength of 700 nm. Wavelengths of the QE peaks for various grating pitches and light incident angles were examined, and it was confirmed that the coupling between the diffracted light from the grating and the propagation modes in the SOI slab waveguide caused the enhancement.

Light Extracting Properties of Buried Photonic Quasi-Crystal Slabs in InGaN/GaN LEDs

Detailed analysis of experimentally observed far-field diffraction patterns from LEDs containing a buried photonic quasi-crystals (PQC) slab are presented for devices with PQC layer depths of 300 to 1300 nm and pitch 400 nm. Ewald constructions allowed identification of the features in experimental data to be correlated with identified waveguide modes from which light is extracted. Effective indices for the slab waveguide modes calculated by a modal expansion method are found to be in close agreement with values extracted from the experimental results, allowing validation of both an effective medium model for the PQC slabs and a design and analysis method for LED structures with diffractive buried photonic crystals.

Enhanced Visible Light Sensitivity by Gold Line-and-Space Grating Gate Electrode in Thin Silicon-On-Insulator p-n Junction Photodiode

A new type of thin silicon-on-insulator (SOI) p-n junction photodiode (PD) with gold (Au) line-and-space grating gate electrode is proposed and demonstrated to have high quantum efficiency (QE) at visible wavelengths. This PD is similar to our previous one in the use of the grating and the metal-oxide-semiconductor structure but is very different in the use of the separate binary grating instead of the continuous one and the thick gate oxide. As a result, the diffracted light from the grating efficiently couples with the lateral propagation mode in the SOI slab waveguide, leading to an external QE one order of magnitude higher than that without the grating. In addition, the importance of the proper setting of the gate and substrate biases is shown so as to realize the wide depletion region and resultant high QE.

Resonant Modes of Single Silicon Nanocavities Excited by Electron Irradiation

High-index dielectric or semiconductor nanoparticles support strong Mie-like geometrical resonances in the visible spectral range. We use 30 keV angle-resolved cathodoluminescence imaging spectroscopy to excite and detect these resonant modes in single silicon nanocylinders with diameters ranging from 60 to 350 nm. Resonances are observed with wavelengths in the range of 400-700 nm, with quality factors in the range Q = 9-77, and show a strong red shift with increasing cylinder diameter. The photonic wave function of all modes is determined at deep-subwavelength resolution and shows good correspondence with numerical simulations. An analytical model is developed that describes the resonant Mie-like optical eigenmodes in the silicon cylinders using an effective index of a slab waveguide mode. It shows good overall agreement with the experimental results and enables qualification of all resonances with azimuthal (m = 0-4) and radial (q = 1-4) quantum numbers. The single resonant Si nanocylinders show characteristic angular radiation distributions in agreement with the modal symmetry.