Waveguide Surface Research Articles

Study of Refractive Index and Birefringence Variation near the Surface of LiNbO3 Open Type Optical Waveguide using Point Dipole Approximation

In view of the immense use of LiNbO3 (LN) in the Photonic industry, a need arises to develop a theory to study the variation of Refractive Indices near the surface of LN device. The “Local” fields as experienced by ions on or near the surface of a dielectric medium ought to be different from those in the interior, thus one expects a Refractive Index variation near the surface of the material. In the studies involving thin films and optical waveguides, this variation could possibly alter the propagation characteristics. The variation of ordinary and extraordinary refractive indices (no, ne) from the surface to the interior of depth 69.315A° near the surface for X-cut and Z-cut LN waveguides are evaluated using theoretical Point Dipole Approximation (PDA) method. For an X-cut waveguide, no is found to decrease from 2.9696 on the surface to 2.2864 in the interior whereas, ne is found to increase from 1.4849 to 2.2024. The birefringence (dn=ne–no) is negative and observed to be increase from -1.4847 to -0.0840. For a Z-cut waveguide, no is found to increase from 1.8046 to 2.2864 whereas ne is found to decrease from 2.7005 to 2.2024. Here the birefringence is positive, decreasing from 0.8958 to zero to a depth of 60.5A° and further decreasing to -0.0840 to a depth of 69.315A°. It is observed that the variation of refractive indices no , ne and dn variation with depth near the surface for both the cases are quite opposite in nature. This is primarily due to the dipole orientation and their relative distributions in the lattice being different for the two directions of the electric vectors of light as is evident from the values.

Tunable surface waves supported by graphene-covered left-handed material structures

The active tuning of surface waves supported by left-handed materials (LHMs) is a major challenge for their practical applications and usability, and graphene has been proposed as an agent for the active tuning of surface waves. In this study, tunable surface waves supported by graphene-layered LHM structures are investigated theoretically. The analytical and numerical results are computed for transverse electric (TE) and transverse magnetic (TM)-polarized surface waves. According to the spectral range, two configurations of LHMs (i.e., gigahertz (GHz)-LHM and terahertz (THz)-LHM) are discussed. The physical modeling of graphene is done using the conductivity model based on random phase approximation-based Kubo formalism. The split ring resonator (SRR) model and Kramers–Kronig relations-based causality principle are used for the physical realization of LHMs. To simulate the graphene-layered LHM interface, impedance boundary conditions (IBCs) are applied. Numerical results are computed for dispersion curves, effective wave numbers, and field profiles of surface waves supported by graphene-covered GHz-LHM and THz-LHM. The influence of graphene parameters on the resonance frequency, dispersion curves, and confinement characteristics of surface waves is analyzed for both configurations of LHMs. It is concluded that the graphene-covered GHz-LHM structure supports the tunable surface waves as surface plasmon polaritons (SPPs) support TM-polarized waves. However, with the TE-polarized surface wave, the structure supports the surface polaritons, which do not depend upon the graphene parameters. For the case of the graphene-covered THz-LHM structure, it is concluded that the structure supports the plasmon modes for both polarized waves and the resonance frequency ranges from THz to infrared (IR). The confinement of the TE-polarized surface waves on the graphene layer can be enhanced up to fourfold as compared to the suspended monolayer graphene by using a THz-LHM substrate. Further, the numerical results are found to be consistent with the literature under special conditions. The proposed graphene-layered LHM structure may have potential uses in the active tuning of surface waves, near-field communication and imaging devices, surface waveguide design, and LHM-based metasurfaces.

Fiber-Optic Surface Waveguide Modes Excited by Inter/Intra Mode Transition for Refractometric Sensitivity Enhancement

The generation of surface waveguide modes in telecommunication-grade optical fibers is highly desirable, since it would enable the development of novel all-fiber devices with high performance. Herein, we carry out a comprehensive theoretical investigation on the intrinsic excitation mechanism and sensing performance of fiber-optic surface waveguide modes supported by a tilted fiber grating (TFG) with nanometric-scale gold and TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films. The results reveal that both P-and S-polarized surface waveguide modes originate from the mode transition tuned by the Au-TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> composite, including inter mode transition associated with two adjacent modes and intra mode transition only related to the mode itself. During the mode transition, the power carried by cladding modes is absorbed by the Au-TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> composite, thereby yielding a strong surface-localized evanescent field that is sensitive to external medium. It is found that for both polarizations the surface waveguide modes induced by the inter/intra mode transition present almost the same refractometric sensitivity, but with the P-polarization (605.6 nm/RIU) being more sensitive than the S-counterpart (339.2 nm/RIU), the state-of-the-art surface plasmon resonance (539.7 nm/RIU), and also the approach enhanced by high-refractive-index film (7.4 nm/RIU). The results attained here represent a breakthrough since they would open new pathways toward single molecular detection.

Tapered racetrack microring resonator for single nanoparticle detection

Rapid detection of a single nanoparticle has always been challenging and demanding. Several methods and approaches have been employed over the last few years. In this work, we have devised an integrated photonic platform to detect a single nanoparticle (NP) such as gold (Au) nanoparticle. The proposed platforms make use of a racetrack micro-ring resonator where a small region in the lower arm (non-coupled) of the resonator has been tapered to enhance the evanescent field strength many folds, which enhances the sensitivity of the device. Numerical simulations have been performed using finite difference time domain that reveal the splitting of a resonant mode in the presence of Au nanoparticle (AuNP) of radius 100 nm. The proposed approach has the capability to distinguish between different sizes and numbers of NPs. We have also studied the effect of assorted NPs in the vicinity of the tapered region. Numerical results also confirm the unique potentiality to detect NPs even if they are not adjacent to waveguide surface as well detection of a single NP of infinitesimally small size. We believe that the proposed architecture may pave a pivotal role in the sensing of biological objects such as novel viruses whose size is similar to the size of AuNP as studied in this work.

Distributed feedback laser with methylammonium lead bromide embedded in channel-type waveguides

Distributed feedback (DFB) lasing is achieved with CH3NH3PbBr3 perovskite embedded in channel-type waveguides. By using a focused ion beam, DFB gratings are engraved within the bottom surface of channel-type waveguides etched on a SiO2/Si wafer. The oriented CH3NH3PbBr3 crystallized inside the waveguide with the DFB gratings shows second-order DFB lasing mode in lateral and normal directions above a threshold excitation density of 47 μJ cm−2 which is one order of magnitude lower than that in the non-DFB laser. The splitting DFB peaks are well reproduced by finite-differential time-domain calculation. The peaks change to a power-broadened amplified spontaneous emission as the excitation fluence is increased far above the threshold.

Electromagnetic Energy Surface Modes in Metamaterial-Filled Bi-layer Graphene Structures

This paper presents a theoretical study on the propagation of electromagnetic surface waves supported by the metamaterial-filled bi-layered graphene structure. The effective surface conductivity approach based upon the Kubo’s formalism is used to model the atomically thick graphene layer, while the analytical modeling of the metamaterials used the framework of causality principle-based Kramers-Kroing relations. With respect to the index of refraction, the two configurations of metamaterials have been considered—i.e., double positive (DPS) and double negative (DNG). The impedance boundary conditions (IBCs) have been employed at the interface for the computation of characteristics equations of odd and even surface modes under DPS and DNG configurations. The numerical results regarding the dispersion relations, effective mode index, and phase speed for the odd and even modes of electromagnetic surface waves supported by each configuration of structure have been presented. Further, the influence of chemical potential and thickness of metamaterial filling in parallel graphene layers on the wave propagation characteristics has been analyzed. To check the accuracy of the numerical results, special conditions were employed, which converge to the published results. The proposed work may have potential applications in plasmonic based logic designs, renewable energy devices, THz surface wave guides, and near-field communication devices.

Model evaluations of multipactor suppression in rectangular waveguides through grooved surfaces and static magnetic field

Mitigation of multipactor in waveguides is of importance, and strategies have included the addition of external fields, materials engineering, or surface modifications. Here, geometry modifications of rectangular waveguide surfaces and the application of an axial magnetic field are investigated for suppressing multipactor growth. A Monte Carlo approach has been used to simulate electron dynamics. The empirical secondary electrons yield is modeled based on a modified Vaughan approach. The electric fields driving electron transport were derived from separate electromagnetic calculations to adequately include field perturbations due to the presence of surface patterns in the rectangular waveguide structure. Combinations of grooves and a DC magnetic field are shown to effectively mitigate multipactor growth at field strengths up to ∼105 V/m. Finding optimal combinations for an arbitrary field and operating frequency requires further work.

Mode Composite Waveguide Based on Hybrid Substrate Integrated Waveguide and Spoof Surface Plasmon Polariton Structure

In this paper, a novel mode composite waveguide based on hybrid substrate integrated waveguide and spoof surface plasmon polariton (SIW-SSPP) is proposed. Through different feeding methods, the odd mode and even mode of SSPP are excited simultaneously and operate at the same frequency band. Thus, co-frequency mode multiplexing is achieved for the proposed odd-even mode composite waveguide (OEMCW). The design principle and characteristics of the OEMCW are analyzed in this work. A prototype is fabricated and measured to demonstrate its transmission performance. The measured results indicate that the proposed waveguide can operate from 10.3 GHz to 18 GHz in two modes with the isolation larger than 18 dB.

Sensing performance of surface waveguide modes excited in long-period fiber grating with gold-silicon nanocoatings.

We carry out a very early theoretical study on surface waveguide modes excited in a long-period fiber grating (LPFG) coated with gold-silicon thin films for refractive index sensing. The surface waveguide modes originate from the intermode transition of EH cladding modes and present a very strong evanescent field penetrating into the surrounding medium, which makes them ultrasensitive to external changes. By tracking the dual resonances of surface waveguide modes, ultrahigh sensitivity up to 7267.7nm/RIU around 1.315 is obtained, which is 76-/4-fold higher than the case of bare LPFG/LPFG-assisted surface plasmon resonance, together with the capability of self-referenced measurement. This new, to the best of our knowledge, concept is expected to find wide applications based on refractive index sensing.

Propagation of Hybrid Surface Waves in Ferrite Anisotropic Plasma Planar Structures

This manuscript presents a theoretical analysis of hybrid surface wave propagation at the ferrite–anisotropic plasma planar interface. Maxwell’s equations in differential forms and impedance boundary conditions are used to solve the problem. The influence of the plasma frequency, cyclotron frequency, and the different permittivities of ferrite films on the dispersion curve are analyzed. Moreover, the propagation length, normalized phase speed, and normalized field distribution are also analyzed. The presented work may have potential applications in surface waveguide control, optical sensing, and wave propagation in the terahertz frequency range such as measurement of electron bunches, remote sensing, material characterization, medical and biological imaging.

Utilization of Trapped Optical Modes for White Perovskite Light-Emitting Diodes with Efficiency over 12%

<h2>Summary</h2> The inferior light extraction efficiency (LEE), which is generally less than 20%, based on optical modeling, and the difficulty in achieving white emission are the two main challenges in the metal-halide-perovskite light-emitting diode (PeLED) field. Herein, we report a simple and efficient approach to construct high-performance white PeLEDs with much-enhanced LEE by coupling a blue PeLED with a layer of red perovskite nanocrystal (PeNC) down-converter through a rationally designed multilayer semitransparent electrode (LiF/Al/Ag/LiF). The red PeNC layer allows the extraction of the trapped waveguide mode and surface plasmon polariton mode in a blue PeLED and converts them to red emission, resulting in over 50% LEE improvement. Simultaneously, the complementary emission spectrum of blue photons and down-converting red photons contributes to a white PeLED with a high external quantum efficiency and luminance of more than 12% and approximately 2,000 cd m⁻², respectively, which represent state-of-the-art results in this field.

BIO bragg gratings on microfibers for label-free biosensing

Discovering nanoscale phenomena to sense biorecognition events introduces new perspectives to exploit nanoscience and nanotechnology for bioanalytical purposes. Here we present Bio Bragg Gratings (BBGs), a novel biosensing approach that consists of diffractive structures of protein bioreceptors patterned on the surface of optical waveguides, and tailored to transduce the magnitude of biorecognition assays into the intensity of single peaks in the reflection spectrum. This work addresses the design, fabrication, and optimization of this system by both theoretical and experimental studies to explore the fundamental physicochemical parameters involved. Functional biomolecular gratings are fabricated by microcontact printing on the surface of tapered optical microfibers, and their structural features were characterized. The transduction principle is experimentally demonstrated, and its quantitative bioanalytical prospects are assessed in a representative immunoassay, based on patterned protein probes and selective IgG targets, in label-free conditions. This biosensing system involves appealing perspectives to avoid unwanted signal contributions from non-specific binding, herein investigated in human serum samples. The work also proves how the optical response of the system can be easily tuned, and it provides insights into the relevance of this feature to conceive multiplexed BBG systems capable to perform multiple label-free biorecognition assays in a single device.

Double Narrow Notch Spectral Filter Design Using Waveguide Grating on Metal Substrate

A double narrow notch spectral filter design using planar dielectric grating diffraction coupled resonant thin dielectric waveguide on metal substrate is numerically studied in this article. Due to excitation and coupling of guided resonance mode in thin dielectric waveguide layer and surface plasmon mode on the interface between the waveguide and the metal substrate, double deep and narrow reflection spectrum dip can be obtained. This physical explanation is confirmed by the momentum matching conditions of resonance and the field distribution calculation. As an example, double notch filter design with full width half maximum less than 2 nm centered at 549 nm and 651 nm is presented.

Kinetic analysis of antibody binding to integral membrane proteins stabilized in SMALPs

The fundamental importance of membrane protein (MP) targets in central biological and cellular events has driven a marked increase in the use of membrane mimetics for exploring these proteins as therapeutic targets. The main challenge associated with biophysical analysis of membrane protein is the need for detergent extraction from the bilayer environment, which in many cases causes the proteins to become insoluble, unstable or display altered structure or activity. Recent technological advances have tried to limit the exposure of purified membrane protein to detergents. One such method involves the amphipathic co-polymer of styrene and maleic acid (SMA), which can release lipids and integral membrane proteins into water soluble native particles (or vesicles) termed SMALPs (Styrene Maleic Acid Lipid Particles). In this study, assay conditions that leverage SMA for membrane protein stabilization were developed to perform kinetic analysis of antibody binding to integral membrane protein and complexes in SMALPs in both purified and complex mixture settings using multiple biosensor platforms. To develop a robust and flexible platform using SMALPs technology, we optimized various SPR assay formats to analyze SMALPs produced with cell membrane pellets as well as whole cell lysates from the cell lines overexpressing membrane protein of interest. Here we emphasize the extraction of model membrane proteins of diverse architecture and function from native environments to encapsulate with SMALPs. Given the importance of selected membrane targets in central biological events and therapeutic relevance, MP-specific or tag-specific antibodies were used as a proof-of-principal to validate the SMALPs platform for ligand binding studies to support drug discovery or tool generation processes. MP-SMALPs that retain specific binding capability in multiple assay formats and biosensors, such as waveguide interferometry and surface plasmon resonance, would be a versatile platform for a wide range of downstream applications.

Classical Analogy of Double Electromagnetically Induced Transparencies from Interactions of Surface Plasmon Resonances and Waveguide Modes

Classical Analogy of Double Electromagnetically Induced Transparencies from Interactions of Surface Plasmon Resonances and Waveguide Modes

Numerical techniques of the estimation the amount of waveguides surface roughness for the THz regime

The article is devoted to the problem of the efficient design of THz waveguides devices. Extreme small sizes of THz devices such as subharmonic pumped mixers for radiometer witch waveguides are integrated in copper block covered with 3 μm gold are presented. The three numerical techniques and direct measurements in the THz regime have been used to predict the effective conductivity reduction due to protrusions. Efficient design of THz systems requires the ability to predict effective conductivity, allowing engineers to determine the amount of surface roughness for acceptable reflection losses prior to fabrication. Full-wave numerical technique to predict the field enhancement factors for both the rf electric field and the rf magnetic field on the protrusion above smooth sample in the THz regime has been mentioned. The electric field and magnetic field enhancement factors on the hemispherical protrusion should be excluded in case the protrusions size are smaller than δ/50 and δ/1.5 correspondingly (δ is skin depth). Mie-Scattering-based approach approximates a unit cell of the grating as a circular cylinder protrusion the additional loss due to the protrusion compared with a perfectly flat surface of the same material. Hammerstad–Bekkadal Model demonstrates an analytical function which depends of sheet resistance for a sample with surface roughness on the surface roughness and skin depth of the metal. Numerical calculations using this technique predict 10% power absorption due to surface features 35 nm at 400 GHz. A layout of the apparatus for direct measurements of the effective conductivity of samples with roughness features using an open quasi-optical resonator is given. The results of theoretical calculation and empirical evaluation have been compared with computer simulation software taking into consideration of the effective conductivity only due to the change in the surface geometry. The comparison empirical results and numerical calculations full-wave numerical technique are more accurate for samples that are smooth relative to the skin depth. For surface features greater than the skin depth, we found the Hammerstad and Bekkadal model to be a better. The observed methods can be used as a rapid means to estimate waveguides surface roughness in terms of power loss in the THz regime.

Design and fabrication of GeAsSeS chalcogenide waveguides with thermal annealing* *Project supported by the National Natural Science Foundation of China (Grant Nos. 61904091 and 61775111), Zhejiang Provincial Natural Science Foundation of China (Grant No. LR18E010002), the Natural Science Foundation of Ningbo City, China (Grant No. 2019A610065), the International Cooperation Project of Ningbo City, China (Grant No. 2017D10009), and K. C.

We reported a chalcogenide glass-based rib waveguide fabricated using photolithography and dry etching method. A commercial software (COMSOL Multiphysics) was used to optimize the waveguide structure and the distribution of the fundamental modes in the waveguide based on the complete vector finite component. We further employed thermal annealing to optimize the surface and sidewalls of the rib waveguides. It was found that the optimal annealing temperature for GeAsSeS films is 220 °C, and the roughness of the films could be significantly reduced by annealing. The zero-dispersion wavelength (ZDW) could be shifted to a short wavelength around ∼ 2.1 μm via waveguide structural optimization, which promotes supercontinuum generation with a short wavelength pump laser source. The insertion loss of the waveguides with cross-sectional areas of 4.0 μm × 3.5 μm and 6.0 μm × 3.5 μm was measured using lens fiber and the cut-back method. The propagation loss of the 220 °C annealed waveguides could be as low as 1.9 dB/cm at 1550 nm.

Flexibly Tunable Surface Waveguide Resonances in Cylindrical Waveguide-Metal-Waveguide Configuration Assisted by Tilted Fiber Grating

Surface waveguide modes/resonances (SWRs) are a promising and relatively new research route toward advanced photonic applications ranging from spectroscopy and sensing to communications. However, the conventional planar setups for exciting the SWRs suffer from its intrinsic shortcomings. Herein, we propose and demonstrate a new cylindrical waveguide-metal-waveguide (WMW) configuration consisting of a tilted fiber grating (TFG) coated with nanometric-scale gold and silicon films for exciting fiber-optic SWRs. The investigations are performed using the ray-optic approach and rigorous coupled mode theory. It is found that the surface waveguide modes essentially stem from polarization-dependent mode transition induced by the dielectric silicon film, including inter mode transition dependent on two adjacent modes for transverse magnetic (TM) polarization and intra mode transition related to only the mode itself for transverse electric (TE) polarization. This is confirmed by the dispersion characteristics, field localization and reflectivity/absorptivity of the metal-silicon composite. Tuned by the silicon film, different surface waveguide modes from low- to high-order are excited. When these surface waveguide modes are phase-matched with the core mode in the TFG, flexibly tunable TM-/TE-polarized SWRs are attained in a wide wavelength range. This work provides new prospects and general guidelines for developing novel robust fiber-optic SWR-based devices with huge potential applications.

Realization of exceeding 80% external quantum efficiency in organic light-emitting diodes using high-index substrates and highly horizontal emitters

Although both high-index substrates and horizontal-dipole emitters have been shown to be facile approaches for enhancing OLED (organic light emitting diode) light extraction, the full benefits and potential of their combination for OLED optical out-coupling have not been thoroughly studied and explored. Simulation studies indicate that very high optical coupling efficiency into substrates ϕsub (and perhaps similarly high OLED external quantum efficiencies) of ~90% can be possibly obtained with both high-index substrates (refractive index >1.8–1.9) and highly horizontal-dipole emitters (horizontal dipole ratio >85%), together with adoption of low-index or index-matching carrier transport layers and optimization of organic layer and transparent electrode thicknesses. With these judicious device design conditions, all waveguided modes and surface plasmon modes in devices can be effectively suppressed for optimal optical out-coupling. Finally, combining the sapphire substrate having high index of n~1.78, the recently developed OLED emitters having high horizontal emitting dipole ratio of up to 87%, and simple external extraction lens, OLED devices having external quantum efficiency of over 80% was successfully realized.

Bleustein–Gulyaev waves in a finite piezoelectric material loaded with a viscoelastic fluid

A generalized analytical approach for the propagation of Bleustein–Gulyaev wave in a piezoelectric material loaded on its surface with a viscoelastic fluid is established in this paper. The Bleustein–Gulyaev waveguide surface is subjected to various glycerol concentrations. The Maxwell and Kelvin–Voigt models are used to describe the viscoelasticity of this fluid. Exact dispersion equation is obtained in the cases of both electrically short circuit and open circuit by solving the equilibrium equations of piezoelectric materials and the Stokes equation of viscoelastic fluid. The effect on the phase velocity and attenuation for several frequencies is highlighted. The influence of key parameters such as substrate thickness and fluid thickness is also studied. These investigations can serve as benchmark solution in design of Bleustein–Gulyaev wave sensors.