Dielectric Waveguide Research Articles

Surface-enhanced Raman spectroscopy long-range detection investigation based on the alternating coupling of a surface plasmonic array and dielectric waveguide.

Surface-enhanced Raman spectroscopy (SERS) is widely used to detect low-concentration samples in biology, medicine, etc. We design and theoretically investigate a SERS sensor with a surface plasmonic array coupled alternately with a dielectric waveguide. The effect of the incident angle on the coupling efficiency of an evanescent field is systematically studied. The results show that the maximum evanescent field coupling efficiency can be obtained at an incident angle of 66°. The proposed SERS sensor has a transmission length of 1.027cm and a high enhancement performance with an enhancement factor of 1.574×104 at a wavelength of 633nm. The integration of this SERS sensor with a metal array and a dielectric waveguide prevents the direct illumination of the sample molecules by the excited light. Furthermore, the long-range nondestructive detection of the SERS signals of the low-concentration sample molecules can be achieved.

Efficient optimal design of mosaic-like PPDW devices for THz application using the adjoint variable method.

In the development of THz-wave circuits, parallel plate dielectric waveguide (PPDW) is a promising platform and recently some fundamental devices have been reported. In order to realize high performance PPDW devices, optimal design methods are crucial and as out-of-plane radiation does not occur in PPDW, mosaic-like optimal design appears to be appropriate for PPDW platform. In this paper, we present a novel and efficient mosaic-like design approach based on gradient method with adjoint variable method (AVM) to realize high performance PPDW devices for THz circuit applications. The design variables in the design of PPDW devices are efficiently optimized by utilizing the gradient method. The mosaic structure in the design region is expressed by using density method with an appropriate initial solution. In the optimization process, AVM is employed for an efficient sensitivity analysis. The usefulness of our mosaic-like design approach is confirmed by designing several PPDW devices, T-branch, three branch, mode splitting device, and THz bandpass filter. The proposed mosaic-like PPDW devices except bandpass filter achieved high transmission efficiencies at single frequency operation as well as at broadband operation. Furthermore, the designed THz bandpass filter achieved the desired flat top transmission property at the targeted frequency band.

Terahertz Oscillator Chips Backside-Coupled to Unclad Microphotonics

Terahertz technology is largely dependent upon planar on-chip antennas that radiate downwards through the substrate, and so an effective means to interface these antennas with integrated waveguides is an attractive prospect. We present a viable methodology for backside coupling between a terahertz oscillator chip and a broadband all-intrinsic-silicon dielectric waveguide. Our investigation employs resonant tunneling diodes as compact two-terminal electronic terahertz oscillators, and terahertz waves are observed from 270 GHz to 404 GHz. The fact that this power is accessed via a curved length of silicon waveguide validates successful backside coupling.

Weak optical modes for high-density and low-loss photonic circuits

Dielectric optical waveguides constitute the main building blocks of photonic integrated circuits (PICs). Channels with high refractive index contrast can provide very compact PIC components, whereas structures with lower index exhibit less propagation loss. A hybrid concept that can combine the best of high- and low-index materials is highly required. Here, we devise a new approach to realize compact and low-loss hybrid optical waveguides based on the interaction of weak optical modes. This is a rather universal approach that can be applied to a wide range of optical materials. To prove the principle, the hybrid waveguide structure is formed by combining a low-index polymer and a thin layer of silicon nitride. For this material combination, a minimum bending radius of 90 µm (for a bending loss of 0.005 dB/90°) and a propagation loss of 0.7 dB/cm are achieved. The viability of this platform is demonstrated through a series of high-performance novel PIC components. This hybrid waveguide platform enabled by a powerful and simple design concept holds great promise for high-density and low-loss PICs.

Research on a Novel Ka-Band Low Insertion Loss Transmission Line Based on H-Guide

H-guide and nonradiative dielectric (NRD) waveguide have more favorable prospects in the millimeter wave and even terahertz bands comparing to other waveguides, such as surface plasmon polaritons transmission lines and Goubau line. Basically, the H-guide described in this letter can avoid many of the disadvantages of the other more-conventional guides, such as high losses, low <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$</tex-math> </inline-formula> -factors, poor field confinement and leakage, low power rating, small dimensions leading to severe tolerance problems, and expensive fabrication. In this letter, we analyze the characteristics of H-guide and design an adapter from microstrip line to H-guide. The experimental results are in good agreement with the simulation ones. The measured insertion loss of H-guide with back-to-back transitions ranges from 1.8 to 3.1 dB during 33–37 GHz, where the length of the H-guide reaches 200 mm.

Numerical proposal to solve Maxwell’s equations in a toroidal waveguide

In this contribution, we propose a numerical solution to the Maxwell equations to determine the electromagnetic fields in a toroidal dielectric wave-guide. Analyzing the relationship between the size of the cross-section of the toroid and its bending radius. We considered a sinusoidal time dependence of the electromagnetic field, then we just solve the Helmholtz equation in the waveguide. We propose a numerical way to solve this equation and analyze for to find the power radiation in each synchronous mode. We show a numerical method by calculating the frequencies and the loss factors for any mode in square, triangular, and round waveguides.

Numerical evaluation of an external environment effectiveness to the surface plasmon mode in Au slot waveguide for design the sensor simulation

The effective permittivity of gold thin films with an external thin dielectric layer on the top and core of the slot waveguide was evaluated as an effective compound semi-metal layer of the surface plasmonic polariton slot waveguide that will vary due to some environmental factors. By uses simple parallel and series permittivity of compound material, this value need for the computation of the effective propagating vector βsp. This takes for a sensor model designing which the external environment effectiveness was changed by the surface plasmonic mode in its slot waveguide. So the resonance length and grating period are evaluated for the sensor model in the first. Further, the propagation of the SPP surface wave mode along the interface between the compound cladding and Si dielectric waveguide is computed, while the effective refractive index of the compound cladding of the slot waveguide is increased from 1 − 1.50. The computation results showed that the TM0 mode power transfer and propagation length seem to vary with this effective refractive index of the compound cladding on the surface plasmonic polariton waveguide. The 140 nm gap width and 3.70 mm long waveguide is also simulated by the finite different time domain method in the optiwave program, where the TM0 mode transition output is corrected, while the refractive index of the environment layer is changed. The results of the simulation showed good agreement with the computation in that the TM0 mode depends on the external refractive of index and input wavelength. This result shows that loss peak ships vary with the external refractive index thin film layer and input wavelength. Thus, they seem to be useful for sensor applications such as the solution concentration analytic.

Robust and non-robust bound states in the continuum in rotationally symmetric periodic waveguides.

A fiber grating and a one-dimensional (1D) periodic array of spheres are examples of rotationally symmetric periodic (RSP) waveguides. It is well known that bound states in the continuum (BICs) may exist in lossless dielectric RSP waveguides. Any guided mode in an RSP waveguide is characterized by an azimuthal index m, the frequency ω, and Bloch wavenumber β. A BIC is a guided mode, but for the same m, ω and β, cylindrical waves can propagate to or from infinity in the surrounding homogeneous medium. In this paper, we investigate the robustness of nondegenerate BICs in lossless dielectric RSP waveguides. The question is whether a BIC in an RSP waveguide with a reflection symmetry along its axis z, can continue its existence when the waveguide is perturbed by small but arbitrary structural perturbations that preserve the periodicity and the reflection symmetry in z. It is shown that for m = 0 and m ≠ 0, generic BICs with only a single propagating diffraction order are robust and non-robust, respectively, and a non-robust BIC with m ≠ 0 can continue to exist if the perturbation contains one tunable parameter. The theory is established by proving the existence of a BIC in the perturbed structure mathematically, where the perturbation is small but arbitrary, and contains an extra tunable parameter for the case of m ≠ 0. The theory is validated by numerical examples for propagating BICs with m ≠ 0 and β ≠ 0 in fiber gratings and 1D arrays of circular disks.

Artificial Gauge Field Enabled Low‐Crosstalk, Broadband, Half‐Wavelength Pitched Waveguide Arrays

AbstractDense waveguide arrays with half‐wavelength pitch, low‐crosstalk, broadband, and flexible routing capability are essential for integrated photonics. However, achieving such performance is challenging due to the relatively weaker confinement of dielectric waveguides and the increased interactions among densely packed waveguides. Here, leveraging the artificial gauge field mechanism, half‐wavelength‐pitched dense waveguide arrays, consisting of 64 waveguides, in silicon with −30 dB crosstalk suppression from 1480 to 1550 nm are demonstrated. The waveguide array features negligible insertion loss for 90° bending. This approach enables flexibly routing a large‐scale dense waveguide array that significantly reduces on‐chip estate, leading to a high‐density photonic integrated circuit, and may open up opportunities for important device performance improvement, such as half‐wavelength‐pitch optical phased array and ultra‐dense space‐division multiplexing.

Plasmonic nanoparticle doped medium for frequency selective waveguide coupling

We present a powerful technique for frequency selective coupling of dielectric waveguides by incorporating plasmonic nanoparticles within the structure. This is achieved by the inclusion of an optimal density of subwavelength plasmonic nanoparticles within the cladding layer between the coupling waveguides. Frequency selectivity of the coupling is possible by careful manipulation of the nanoparticle characteristics that govern the effective refractive index of the doped layer that supports plasmonic excitations within a narrow frequency range. The efficient coupling arises from inducing modes within the nanoparticle doped cladding layer of the waveguide in the spectral and spatial vicinities of the selected waveguide modes. Our observations are modeled with the transfer matrix method and corroborated with full-wave simulations.

Terahertz amplification and lasing by using transverse electric modes in a two-layer-graphene-dielectric waveguide structure with direct current

We study for the first time the interaction between the waveguide modes of graphene structure and freely propagating terahertz (THz) electromagnetic waves (this interaction takes place within the light cone). We revealed a new and rather unexpected physical phenomenon by showing that freely incident THz electromagnetic waves can resonate with the surface transverse electric (TE) modes of the graphene waveguide in virtue of these modes having their dispersions in the vicinity of the light cone. The dispersion and amplification of surface TE modes in a dielectric waveguide covered with two graphene layers biased by direct current (DC), as well the amplification and lasing of incident THz wave by excitation of TE mode resonances, are investigated. The DC flows perpendicular to the direction of the surface wave propagation and creates the capacitive complex conductivity of graphene at THz frequencies, which is necessary for the existence of surface TE modes in graphene. The real part of graphene conductivity can be negative at THz frequencies due to DC in graphene which leads to amplification and lasing of THz radiation. Such structure can be of great practical importance because an external THz wave can be amplified or generated in lasing process without using special coupling elements commonly needed for ensuring the interaction between external THz wave and surface waveguide modes. The use of a two-layer graphene structure makes it possible to reduce the charge–carrier drift velocity required for reaching the lasing threshold at those resonances, as compared to a structure with a single graphene layer.

A Boundary Integral Method for 3-D Nonuniform Dielectric Waveguide Problems via the Windowed Green Function

This communication proposes an efficient boundary-integral-based “windowed Green function” (WGF) methodology for the numerical solution of 3-D general electromagnetic problems containing dielectric waveguides. The approach, which generalizes a recently introduced 2-D version of the method, provides a highly effective solver for such problems. In particular, using an auxiliary integral representation, the proposed method is able to accurately model incident mode excitation. On the basis of a smooth window function, the integral operators along the infinite waveguide boundaries are smoothly truncated, resulting in errors that decay faster than any negative power of the window size.

Photonic spin Hall effect with high coupling efficiency in the combined structure of periodic dielectric waveguides and photonic crystal waveguide

In this paper, we discuss the photonic spin Hall effect (PSHE) in the combined structure of periodic dielectric waveguides (PDWs) and photonic crystal waveguide (PCW). By controlling the polarization of the dipole source, we have successfully achieved the arbitrary manipulation of PSHE for the PDW guided mode in our structure. The advantage of PDW is that it can be designed to different shapes without the geometric constraints, and still maintains a relatively high transmission rate. Therefore, by further changing the structure, including the adjustment of the intersection angle between PDW and PCW, and the alteration of the straight PDW to the bending or bifurcated shapes, we find that the output of highly unidirectional PSHE is mainly determined by the intersection angle between PDW and PCW and almost independent of the detailed shape of PDW.

Контрольоване нагрівання циліндричної плазми з використанням особливостей виняткової точки

The paper proposes a method of controlled heating of a cylindrical plasma using the features of the Exceptional point. It is shown that the coupled system of plasma and dielectric waveguides is capable of generating exceptional points where their dispersion curves cross. By controlling the connection (distance) between the waveguides, it is possible to control the distribution of the electromagnetic field, both in the plasma and in the dielectric waveguides around the exceptional point. It is also shown that in the presence of dissipative losses in the plasma, the degree of heating of the plasma waveguide can be controlled by tuning the distribution and intensity of the exciting electromagnetic field in the coupled waveguide system, which gives a potential advantage among other methods of plasma heating. The results obtained in the work can be considered as an example of a new method of controlled plasma heating, which can be used to overcome the existing problems of controlled thermonuclear fusion.

Function expansion based topology optimization of NRD guide device using hybrid method of harmony search and gradient method

In recent years, optimization technique has attracted a lot of attention to develop high performance electromagnetic devices and various kinds of design strategies have been reported. Among them, topology optimization method has potential to develop innovative devices. In this paper, the function expansion based topology optimization method, which has been developed for design of photonic devices, is extended to be applied to design of nonradiative dielectric waveguide (NRD guide) devices. In the optimal design of NRD guide devices, solution search sometimes tends to fall into local minimum compared with the design of photonic devices. Therefore, we develop a novel hybrid optimization method combining harmony search which is one of the evolutionary algorithm and gradient method using adjoint variable method for robust and efficient design of NRD guide devices.

The impact of plasmonic electrodes on the photocarrier extraction of inverted organic bulk heterojunction solar cells

Nano-patterning the semiconducting photoactive layer/back electrode interface of organic photovoltaic devices is a widely accepted approach to enhance the power conversion efficiency through the exploitation of numerous photonic and plasmonic effects. Yet, nano-patterning the semiconductor/metal interface leads to intertwined effects that impact the optical as well as the electrical characteristic of solar cells. In this work we aim to disentangle the optical and electrical effects of a nano-structured semiconductor/metal interface on the device performance. For this, we use an inverted bulk heterojunction P3HT:PCBM solar cell structure, where the nano-patterned photoactive layer/back electrode interface is realized by patterning the active layer with sinusoidal grating profiles bearing a periodicity of 300 nm or 400 nm through imprint lithography while varying the photoactive layer thickness (LPAL) between 90 and 400 nm. The optical and electrical device characteristics of nano-patterned solar cells are compared to the characteristics of control devices, featuring a planar photoactive layer/back electrode interface. We find that patterned solar cells show for an enhanced photocurrent generation for a LPAL above 284 nm, which is not observed when using thinner active layer thicknesses. Simulating the optical characteristic of planar and patterned devices through a finite-difference time-domain approach proves for an increased light absorption in presence of a patterned electrode interface, originating from the excitation of propagating surface plasmon and dielectric waveguide modes. Evaluation of the external quantum efficiency characteristic and the voltage dependent charge extraction characteristics of fabricated planar and patterned solar cells reveals, however, that the increased photocurrents of patterned devices do not stem from an optical enhancement but from an improved charge carrier extraction efficiency in the space charge limited extraction regime. Presented findings clearly demonstrate that the improved charge extraction efficiency of patterned solar cells is linked to the periodic surface corrugation of the (back) electrode interface.

Complex-amplitude modulation of surface waves based on a metasurface coupler.

Simultaneous and independent modulation of the amplitude and phase of surface waves (SWs) is critical in photonics and plasmonics. Here, we propose a method for flexible complex-amplitude modulation of SWs based on a metasurface coupler. Benefiting from the full range complex-amplitude modulation ability of the meta-atoms over the transmitted field, the coupler can convert the incident wave into a driven surface wave (DSW) with an arbitrary combination of amplitude and initial phase. By placing a dielectric waveguide that supports guided SWs below the coupler, the DSWs can resonantly couple to SWs while preserving complex-amplitude modulation. The proposed scheme provides a practical way for freely tailoring the phase and amplitude profiles of SWs wavefronts. As verification, meta-devices for normal and deflected SW Airy beam generation and SW dual focusing are designed and characterized in the microwave regime. Our findings may stimulate various advanced surface optical meta-devices.

Perfect Invisibility Modes in Dielectric Nanofibers

With the help of the original mathematical method for solving Maxwell’s equations, it is shown that in dielectric waveguides along with usual waveguides and quasi-normal modes, there are perfect invisibility modes or perfect non-scattering modes. In contrast to the usual waveguide modes, at eigenfrequencies of the perfect invisibility modes, light can propagate in free space. The properties of the invisibility modes in waveguides of circular and elliptical cross-sections are analyzed in detail. It is shown that at the eigenfrequencies of the perfect invisibility modes, the power of the light scattered from the waveguide tends to zero and the optical fiber becomes invisible. The found modes can be used to create highly sensitive nanosensors and other optical nanodevices, where radiation and scattering losses should be minimal.

Creation and preservation of superoscillation in a dielectric optical waveguide.

Superoscillation refers to a phenomenon where a band-limited wave locally oscillates faster than its highest Fourier component. Current research on optical superoscillations predominantly lies on the basis of free-space waves. As the optical waveguides play a key role in energy and information transportation, guided waves with precisely controlled deep-subwavelength features offers unprecedented flexibility for applications. In this Letter, we numerically show that, by superimposing eigenmodes of a multimode SiO2 waveguide and forcing the resultant field to pass through a set of predetermined points, superoscillatory fields in various shapes can be formed in preset cross-sectional planes. Furthermore, by padding prescribed intensities in multiple cross sections, we successfully create a persistent superoscillatory saddle.

Terahertz fiber link using dielectric silicon waveguide interface.

Nascent data-intensive emerging technologies are mandating low-loss, short-range interconnects, whereas existing interconnects suffer from high losses and low aggregate data throughput owing to a lack of efficient interfaces. Here, we report an efficient 22-Gbit/s terahertz fiber link using a tapered silicon interface that serves as a coupler between the dielectric waveguide and hollow core fiber. We investigated the fundamental optical properties of hollow-core fibers by considering fibers with 0.7-mm and 1-mm core diameters. We achieved a coupling efficiency of ∼ 60% with a 3-dB bandwidth of 150 GHz in the 0.3-THz band over a 10 cm fiber.