Modes In Dielectric Waveguides Research Articles

Thin film lithium niobate electro-optic modulator with reduced half-wave voltage length product through design

This paper aims at shortening electrode spacing in a thin film lithium niobate (TFLN) electro-optic modulator (EOM) while avoiding an increase in metal absorption loss, thereby reducing the half-wave voltage length product (V π ⋅L). Through numerical simulations, we find that metal absorption loss reaches its peak values when the optical modes of the metal-clad dielectric waveguide and ridged waveguide hybridize. This negative effect can be mitigated by adjusting the electrode width to modify the optical mode of the metal-clad dielectric waveguide. In addition, we raise the vertical position of the electrodes to further mitigate metal absorption loss and reduce the electrode spacing. By calculating the optimal buffer layer thickness for two crystal axis orientations, our findings reveal a 19% reduction in V π ⋅L at conventional crystal axis orientation (θ=±90∘) and a 16% decrease at unconventional crystal axis orientation (θ=54∘). Notably, V π ⋅L at unconventional crystal axis orientation is 5% lower than at conventional crystal axis orientation. These findings demonstrate the effectiveness of geometric configuration optimization toward enhancing the efficiency and performance of the TFLN EOM.

Directional Coupling to a λ/5000 Nanowaveguide.

Silicon-based microdevices are considered promising candidates for consolidating several terahertz technologies into a common and practical platform. The practicality stems from the relatively low loss, device compactness, ease of fabrication, and wide range of available passive and active functionalities. Nevertheless, typical device footprints are limited by diffraction to several hundreds of micrometers, which hinders emerging nanoscale applications at terahertz frequencies. While metallic gap modes provide nanoscale terahertz confinement, efficiently coupling to them is difficult. Here, we present and experimentally demonstrate a strategy for efficiently interfacing subterahertz radiation (λ = 1 mm) to a waveguide formed by a nanogap, etched in a gold film, that is 200 nm (λ/5000) wide and up to 4.5 mm long. The design principle relies on phase matching dielectric and nanogap waveguide modes, resulting in efficient directional coupling between them when they are placed side-by-side. Broadband far-field terahertz transmission experiments through the dielectric waveguide reveal a transmission dip near the designed wavelength due to resonant coupling. Near-field measurements on the surface of the gold layer confirm that such a dip is accompanied by a transfer of power to the nanogap, with an estimated coupling efficiency of ∼10%. Our approach efficiently interfaces millimeter waves with nanoscale waveguides in a tailored and controllable manner, with important implications for on-chip nanospectroscopy, telecommunications, and quantum technologies.

Exhaustive Design and Realization for In-Line Topology Quasi-TEM Mode Dielectric Waveguide Filter With Dispersive Couplings

Exhaustive Design and Realization for In-Line Topology Quasi-TEM Mode Dielectric Waveguide Filter With Dispersive Couplings

Compact and efficient lossy mode resonant refractive index sensor for aqueous environment

A very short length high sensitivity, large figure of merit and very high resolution integrated-photonic refractometer for aqueous environment operating in visible region of wavelength is proposed. The sensor design depends on the periodic coupling of the guided dielectric optical waveguide mode field and the lossy mode field of the conducting indium-tin-oxide thin film. Various layer thicknesses can be optimized to provide power transfer to the lossy layer and lossy mode resonance, resulting in a strong guided mode power absorption in the lossy layer occurs. The sensor has been designed to operate in both the TE and TM polarizations with different optimized layer frthicknesses. The optimized thicknesses are different for TE and TM polarizations. The obtained numerical results show that a spectral sensitivity of 2200 nm/RIU/2645 nm/RIU for TE/TM mode could be achieved with a very high resolution. Also, the sensors can operate in power interrogation mode with a maximum sensitivity nearly 5 × 107 dB RIU−1.

Enhancing and confining light in hybrid plasmonic nanowire-integrated V-groove silicon waveguides

In recent years, the field of dielectric-plasmonic photonics has made remarkable strides, leading to the successful development of various technologies. The realization of sophisticated optical circuits on a single platform has become increasingly viable. Here we propose and investigate a hybrid dielectric waveguide integrated with plasmonics. This hybrid optical waveguide comprises a copper nanowire situated in close proximity to a silicon V-groove channel, separated by a nanoscale gap. This configuration is particularly advantageous, as achieving precise alignment of the nanowire within the V-groove addresses a fundamental challenge in engineering a fully functional integrated component. Additionally, a silicon nitride film coats the V-groove. Utilizing finite element analysis, we conduct numerical simulations to analyze field properties and modal propagation at a specific wavelength of 1550 nm. Our simulations reveal that meticulous optimization of the nanowire and V-groove channel’s geometrical parameters enables effective tailoring of the hybrid mode. This optimization results in strong mode coupling between the dielectric waveguide mode and the surface plasmon, leading to substantial field enhancement, confinement, and extended propagation length. These waveguides also hold promise for sensing applications, facilitating the detection of sample variations and locations due to pronounced mode characteristics. The proposed hybrid approach demonstrates potential for integration into high-level photonic circuits and on-chip optical computing systems.

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.

Unidirectional efficient hybrid coupler for an integrated single-photon source

Hybrid integrated quantum photonic circuits possess the potential to scale up the number of quantum nodes with distributed quantum-information-processing units at affordable resources. One of the key requirements is to achieve high-efficiency and unidirectional coupling of single quantum emitters into the low loss dielectric photonic waveguide modes. Plasmonic waveguides have the capability to have high coupling efficiency due to enhanced light–matter interactions. However, they suffer from significant propagation losses. Here we design and numerically demonstrate an on-chip hybrid plasmonic–photonic integrated single-photon source that enhances the emission rate of a quantum emitter and possesses unidirectional emission to a waveguide with high coupling efficiency. Currently, the emission rate of the single emitter is enhanced by a factor of 20, and the coupling efficiency from the single emitter into the photonic circuit exceeds 70%. The integrated hybrid single-photon source could find applications in quantum-information technologies.

Magnetic field sensor with Fano resonance in resonant plasmonic and planar waveguide structures

By introducing a waveguide layer into the surface plasmon resonance (SPR)-supported Kretschmann configuration, a Fano resonance (FR) sensor has been proposed and suggested for magnetic field sensing. FR with higher quality factor (Q) can be manipulated through the coupling between surface plasmon polariton (SPP) and dielectric planar waveguide (PWG) modes. The design procedures for determining and optimizing the performance of the magnetic field sensor (MFS) are elaborated through the numerical calculation with rigorous coupled wave analysis (RCWA) method. The sensor’s performance has been analyzed by considering the intensity sensitivity and figure of merit (FOM). By optimizing the structural parameters, the intensity sensitivity and FOM can reach 3002∕T and 1.68 × 104/T, respectively, which have been greatly increased compared to that obtained using the conventional SPR technique in state of-the-art sensors.

Total absorption and coherent perfect absorption in metal-dielectric-metal resonators integrated into a slab waveguide.

We propose and investigate integrated metal-dielectric-metal (MDM) resonators operating with semi-guided waves (guided modes of dielectric slab waveguides). The MDM resonators are constituted by two metal strips "buried" in the waveguide core layer and separated by a dielectric waveguide segment. We theoretically prove and numerically demonstrate that by a proper choice of the mode incidence geometry, the widths of the metal strips, and the distance between them, it is possible to achieve either total absorption of the incident wave or coherent perfect absorption (in the case of symmetric incidence of two modes on the structure). The proposed planar MDM resonators may find application as absorbers or filters in integrated optical circuits.

Interaction of a dielectric waveguide mode with a resonant scatterer: Scattering properties, propelling force, and torque

A system of a resonant scatterer described as a two-level atom coupled to a circular dielectric waveguide is studied. The system is characterized by the scattering properties of a guided mode (guided transmission and reflection, bulk radiation) and optomechanical properties (propelling force and torque on the scatterer). The results are based on a self-consistent solution of the scattering problem using the Green's functions and cover the cases of scatterers inside and outside waveguides. It is shown that while a scatterer with a fixed dipole orientation gives rise to scattering spectra described by Lorentzian line shapes, an isotropic scatterer can produce non-Lorentzian line shapes. The parameters defining the line shape are analyzed. The complicated line shapes are likely to arise in the coherent characterization of an emerging class of photonic devices based on photon emitters coupled to well-confined modes of dielectric waveguides. It is obtained that the propelling force and axial torque on the scatterer at resonance can be directly expressed in terms of the spontaneous emission coupling factor $(\ensuremath{\beta})$ of the system. The balances of the linear and angular momentum in the system are discussed. Numerical results are presented for strongly confined guided modes which allow efficient mode scattering and conversion of the linear and angular mode momentum to the force and torque, respectively.

Simulation of a Thermally Efficient Heat-Assisted Magnetic Recording Ridge Waveguide NFT on an AlN Heat Sink

A thermally efficient ridge waveguide near-field transducer (NFT) design on an AlN heat sink is presented. Through simulation, we found our design can achieve thermal efficiency (TE), defined as the ratio of peak temperature rise in the medium to that in the NFT as high as 2.93. The ridge waveguide NFT is placed on a heat sink of high thermal conductivity material AlN, while otherwise surrounded by SiO2 as the top cladding and cavity filling material. The relatively large metallic waveguide structure can dissipate heat effectively even in the presence of finite interface thermal conductance between the NFT and the AlN. Different NFT excitation methods are examined, with an impinging dielectric waveguide mode leading to the highest TE. Some challenges are identified in using excitation by resonant disk structures. We note that surface plasmon is confined inside the NFT cavity, while heat escapes through the outer surface of the NFT, separating the dielectric interfaces responsible for optical propagation and heat transfer. This allows for further reduction in thermal interface resistance by adding an interface adhesion layer like Cr at the NFT/AlN interface without sacrificing optical properties.

Fano Resonance Hybrid Waveguide-Coupled Plasmonic Sensor Using Transparent Conductive Oxide in the Near-Infrared Range

We proposed an ultra-sensitive refractive index sensor by using indium-doped cadmium oxide as a plasmonic material operating in near-infrared based on Fano resonance. The proposed sensor has a hybrid multilayer waveguide structure that supports both a long-range surface plasmon polariton (LRSPP) mode and a dielectric waveguide (DWG) mode. The design strategy of the structure parameters of the inner layers is elaborated in detail through the numerical analysis of the two modes. By suitably tailoring the thickness of the coupling layer, a strong mode coupling between the two modes could be achieved, leading to a sharp asymmetric Fano resonance. With the designed optimal physical parameters, our proposed sensor could achieve a maximum intensity sensitivity of 19,909 RIU−1, a 193-fold enhancement than that of a conventional long-range SPR (LRSPR) based scheme. The proposed design can be a promising platform for biochemical sensing in the near-infrared region.

Terahertz hybrid plasmonic waveguides with ultra-long propagation lengths based on multilayer graphene-dielectric stacks.

To develop on-chip photonic devices capable of transmitting terahertz signals beyond the propagation distance of millimeter while keeping deep subwavelength field confinement has been a challenging task. Herein, we propose a novel multilayer graphene-based hybrid plasmonic waveguide (MLGHPW) consisting of a cylindrical dielectric waveguide and hyperbolic metamaterials. The device is based on alternating graphene and dielectric layers on a rib substrate, operating in the terahertz range (f = 3 THz). We couple the fundamental dielectric waveguide mode with the fundamental volume plasmon polarition modes originated from the coupling of plasmon polaritons at individual graphene sheets. The resulting hybrid mode shows ultra-low loss compared with the conventional GHPW modes at the comparable mode sizes. The present MLGHPW demonstrated a few millimeters of propagation length while keeping the mode area of 10-3A0, where A0 is the diffraction-limited area, thus possessing a thirty times larger figure of merit (FoM) compared to other GHPWs. The additional degree of freedom (the number of graphene layers) makes the proposed MLGHPW more flexible to control the mode properties. We investigated the geometry and physical parameters of the device and identified optimal FoM. Moreover, we analyzed the crosstalk between waveguides and confirmed the potential to construct compact on-chip terahertz devices. The present design might have the possible extensibility to other graphene-like materials, like silicene, germanen, stanene etc.

Experimental characterization of bending effects for solid and hollow dielectric waveguides at V-band

Mm-wave dielectric waveguides are a promising and low-cost technology for the transmission of ultra-high data rates. Besides the attenuation (losses) and group delay, the bending loss of the dielectric waveguides is also one of the key parameters to establish the capacity and energy efficiency of such wired links, when deployed in realistic scenarios. In this context, we report the experimental characterizations of bending effects for various solid and hollow commercially available dielectric waveguides at V-band (50–75 GHz). A wide-band transition has been designed to carry out the measurements using a Vector Network Analyzer (VNA) and extension modules. The measured results are in very good agreement with full-wave simulations. Our experimental results show an average bending loss of 1.46 dB over the entire V-band for the fundamental {HE}_{11}^{y} mode of a PTFE solid dielectric waveguide (core diameter of 3.06 mm) with a 90° bending angle and 25 mm radius of curvature. This value rises up to 2.88 dB (or 3.25 dB) when bending radius is changed to 15 mm (or bending angle grows up to 140°). The measurements also show that the measured bending losses increase significantly for hollow dielectric waveguides, in particular when the inner to outer diameter ratio gets larger.

Self-Referenced Refractive Index Biosensing with Graphene Fano Resonance Modes.

A hybrid structure composed of periodic monolayer graphene nanoribbons and a dielectric multilayer structure was designed to generate a Fano resonance (FR). The strong interaction between the surface plasmon resonance of graphene and the dielectric waveguide mode results in the FR. The finite element method is utilized to investigate the behaviors of the FR, and it matches well with the theoretical calculations using rigorous coupled wave theory. The results demonstrate that the profile of the FR can be passively tuned by the period of the graphene nanoribbons and actively tuned by the Fermi level of the graphene. The decoupled nature of the FR gives it potential applications as a self-calibrated refractive index biosensor, and the sensitivity can reach as high as 4.615 μm/RIU. Thus, this work provides a new idea for an excellent self-referencing refractive index biosensor.

In Situ Monitoring of Rolling Circle Amplification on a Solid Support by Surface Plasmon Resonance and Optical Waveguide Spectroscopy.

The growth of surface-attached single-stranded deoxyribonucleic acid (ssDNA) chains is monitored in situ using an evanescent wave optical biosensor that combines surface plasmon resonance (SPR) and optical waveguide spectroscopy (OWS). The "grafting-from" growth of ssDNA chains is facilitated by rolling circle amplification (RCA), and the gradual prolongation of ssDNA chains anchored to a gold sensor surface is optically tracked in time. At a sufficient density of the polymer chains, the ssDNA takes on a brush architecture with a thickness exceeding 10 μm, supporting a spectrum of guided optical waves traveling along the metallic sensor surface. The simultaneous probing of this interface with the confined optical field of surface plasmons and additional more delocalized dielectric optical waveguide modes enables accurate in situ measurement of the ssDNA brush thickness, polymer volume content, and density gradients. We report for the first time on the utilization of the SPR/OWS technique for the measurement of the RCA speed on a solid surface that can be compared to that in bulk solutions. In addition, the control of ssDNA brush properties by changing the grafting density and ionic strength and post-modification via affinity reaction with complementary short ssDNA staples is discussed. These observations may provide important leads for tailoring RCA toward sensitive and rapid assays in affinity-based biosensors.

Design and modeling of a photonic integrated device for optical vortex generation in a silicon waveguide

We propose and numerically verify a design of the photonic integrated circuit for in-plane generation of a 1st azimuthal order vortex mode in dielectric rectangular waveguides. Radiation is introduced into the proposed structure in a standard way through two grating couplers. Applying a mode coupling and specific phase shift, a field with the required amplitude-phase distribution is formed directly in the output waveguide. The geometric dimensions of the device are simulated and optimized to fit the technological parameters of the silicon-on-insulator platform.

Dynamically Switchable Polarization-Independent Triple-Band Perfect Metamaterial Absorber Using a Phase-Change Material in the Mid-Infrared (MIR) Region.

A tunable metamaterial absorber (MMA) by reversible phase transitions in a mid-infrared regime is theoretically investigated. The absorber is composed of a molybdenum (Mo)-germanium-antimony-tellurium (Ge2Sb2Te5, GST)-Mo nanodisk structure superimposed on the GST-Al2O3 (aluminum oxide)-Mo film. Studies have shown that the combination of the inlaid metal-medium dielectric waveguide mode and the resonant cavity mode and the excitation of the propagating surface plasmon mode are the main reasons for the formation of the triple-band high absorption. Additionally, through the reversible phase change, the transition from high absorption to high reflection in the mid-infrared region is realized. The symmetry of the absorber eliminates the polarization dependence, and the near unity absorption efficiency can be maintained by incidence angles up to 60°. The presented method will enhance the functionality of the absorber and has the potential for the applications that require active control over light absorption.

Edge‐Orientation Dependent Nanoimaging of Mid‐Infrared Waveguide Modes in High‐Index PtSe2

AbstractAll‐dielectric nanophotonics is a rapidly developing field that employs high‐index low‐loss dielectric materials to boost light–matter interactions at the nanoscale. This is owing to their potential in achieving low‐loss optical responses and the coexistence of strong enhancement of electric and magnetic fields, which are absent in their plasmonic counterparts. Transition‐metal dichalcogenides (TMDCs) have been utilized for high‐index dielectric Mie nanoresonators in the visible to near‐infrared spectral range due to their high in‐plane and out‐of‐plane optical anisotropy, excitonic effect, and high‐index properties. However, high‐index materials for mid‐infrared (MIR) nanophotonics are still highly sought after. Here, it is shown that PtSe2, a group‐10 TMDC, could support dielectric waveguide modes in MIR despite its semimetallic nature. It is revealed that PtSe2optically acts as a dielectric material with a high refractive index of ≈5 and a low extinction coefficient in the MIR region. The value is among the highest in the low‐loss TMDCs and mainstream dielectric materials. A comprehensive sample‐edge‐orientation dependent nanoimaging together with spectroscopic nanoimaging characterization of the PtSe2 allows the extraction of the dispersion relations of the modes, from which the velocity parameters could be determined. This work paves the way for high‐performance MIR devices via high‐index PtSe2‐based nanostructures.

Fabrication-Tolerant and Low-Loss Hybrid Plasmonic Slot Waveguide Mode Converter

Integrated plasmonic structures can concentrate light to deep-subwavelength volumes below the diffraction limit and have been explored in many applications including label-free nanophotonic sensing with single molecule sensitivity, nonlinear conversion, and lasing. However, conversion between the dielectric waveguide mode and the plasmonic mode typically suffers from large mode conversion loss and requires high nanofabrication precision because of the mode mismatch and the precise alignment requirements with the plasmonic slot structure, respectively. Here, we report low-loss conversion between a dielectric waveguide mode and a hybrid plasmonic slot waveguide mode in a structure with a large fabrication tolerance. The measured conversion loss between the dielectric waveguide mode to the hybrid plasmonic slot waveguide mode can be <; 2 dB in the wavelength range of 1.54-1.60 μm under the optimal phase match condition. The loss remains low with a lateral misalignment tolerance of >500 nm. We confirmed enhanced light-matter interaction in the plasmonic slot waveguide region by measuring the enhancement in optical absorption of graphene with and without the plasmonic slot structure. Our results demonstrate a new approach to achieve deep-subwavelength light concentration by using an easy-to-fabricate interface with dielectric waveguides.