Waveguide Geometry Research Articles

Волноводные логические вентили для магнитооптических кубитов

Results of magneto-optical waveguide logic gates properties investigation are presented. Simple logic operations can be realized using photon properties in modulated magnetic field. Changing the magnetic field amplitude and its orientation relating to light propagation direction, choosing polarizer and analyzer orientation and proper waveguide geometry, we can design logic gates avoiding small coherence time of regular optic qubits.

Recent advances in nanowire quantum dot (NWQD) single-photon emitters

Future development of quantum technologies is dependent upon physical implementation of quantum systems. Photonic platforms have gained significant attention owing to the appealing properties that they offer, namely small probabilities of decoherence emergence, as well as being readily manipulable. In this regard, single-photon emitters play an integral role, and advancements in the design of single-photon sources, providing an anti-bunching photon correlation, have transitioned from its formerly proof-of-concept status to engineering attempts, and this has realized thanks to the ever-increasing improvements in the development of promising material platforms. In the race toward the realization of on-demand single-photon sources, solid-state host systems and in particular the quantum dot (QD)-based schemes, have very-well taken advantage of the recent headways in the bottom-up nanofabrication procedures. In this contribution, a review is presented on the recent progress and the existing challenges in fabrication of single-photon emitters, based on QD-containing nanowires (NWs) and their principles of operation. More specifically, we will consider how variation in values of different parameters, such as the purity, height and diameter of the NW waveguides, the electric-dipole orientation of the embedded QD, as well as different waveguide geometries, fabrication techniques and conditions exploited in their manufacturing, will affect the desired outputs in the system, i.e., the narrow photoluminescence spectrum, ideal single-photon emission of sufficient lifetime, and also the Gaussian profile in the far-field distribution. Additional focus is also given to different areas that the NWQD single-photon emitters have found application for quantum information processing purposes.

Optical analog of particle production in gravitational fields

A strong enough electric field is predicted to spontaneously create electron-positron pairs in vacuum via the Schwinger effect. A somewhat similar effect is predicted to occur in sufficiently strong inhomogeneous gravitational fields. However, due to the necessity of very large field strength, these effects were never observed in the experiment. Here we demonstrate that optical analogs of very strong gravitational fields (up to ) and very strong gravitational field gradients (up to ) may be created in electromagnetic metamaterial waveguides, leading to an optical analog of particle production in gravitational fields. Such waveguide geometries may also potentially be used to search for axion-like particles weakly interacting with an electromagnetic field.

Edge-plasmon assisted electro-optical modulator

An efficient electro-optical modulation has been demonstrated here by using an edge plasmon mode specific for the hybrid plasmonic waveguide. Our approach addresses a major obstacle of the integrated microwave photonics caused by the polarization constraints of both active and passive components. In addition to sub-wavelength confinement, typical for surface plasmon polaritons, the edge plasmon modes enable exact matching of the polarization requirements for silicon based input/output grating couplers, waveguides and electro-optical modulators. A concept of the hybrid waveguide, implemented in a sandwich-like structure, implies a coupling of propagating plasmon modes with a waveguide mode. The vertically arranged sandwich includes a thin layer of epsilon-near-zero material (indium tin oxide) providing an efficient modulation at small length scales. Employed edge plasmons possess a mixed polarization state and can be excited with horizontally polarized waveguide modes. It allows the resulting modulator to work directly with efficient grating couplers and avoid using bulky and lossy polarization converters. A 3D optical model based on Maxwell equations combined with drift-diffusion semiconductor equations is developed. Numerically heavy computations involving the optimization of materials and geometry have been performed. Effective modes, stationary state field distribution, an extinction coefficient, optical losses and charge transport properties are computed and analyzed. In addition to the polarization matching, the advantages of the proposed model include the compact planar geometry of the silicon waveguide, reduced active electric resistance R and a relatively simple design, attractive for experimental realization.

Hybrid plasmonic waveguide race-track µ-ring resonator: Analysis of dielectric and hybrid mode for refractive index sensing applications

We studied a hybrid plasmonic waveguide structure which is capable of supporting both dielectric and hybrid modes. It consists of a thin metal layer separated from a high index material by a low index interlay. Recently, this kind of waveguide has attracted a considerable amount of interest. Finite element method simulations suggest that waveguide geometry plays a vital role in the mode confinement and minimizing their propagation losses. The waveguide design is optimized such that both modes have a confinement factor of above 65% with minimum propagation losses. In the end, race-track µ-ring resonators are designed based on two different configurations of the hybrid plasmonic waveguide and their sensitivity is compared for refractive index sensing applications.

Surface-Enhanced Raman Spectroscopy Based on Plasmonic Slot Waveguides With Free-Space Oblique Illumination

We report a novel on-chip approach for surface-enhanced Raman spectroscopy (SERS) using a plasmonic slot waveguide. The Raman signal is excited via free-space excitation and is collected by the waveguide. A significant improvement of the signal-to-background-ratio (SBR), as compared to the case of back-scattered Raman spectroscopy with waveguide-mode excitation, is demonstrated using a silicon nitride plasmonic slot waveguide with a 7-mm access waveguide. The flexible adjustment of the incident angle in free space and modification of waveguide geometry enables the further optimization of the pump-to-Stokes conversion efficiency. The combination of high SBR and long access waveguide allows the integration with additional photonics devices (sources, filters, spectrometers) on the same chip.

Regimes of two-color light bullet formation in a gradient waveguide

In this paper we concentrate on the remarkable role of a gradient waveguide in two-color light bullet formation. We study generation of the second harmonic in such an inhomogeneous nonlinear medium, taking into account diffraction and relatively weak temporal dispersion. Using the averaged Lagrangian method we consider all possible combinations of the range of group velocities (normal or anomalous dispersion) and waveguide geometry (focusing or defocusing waveguide). Stability conditions for a propagating two-color light bullet are derived analytically. We demonstrate the formation of a stable two-component light bullet in a parabolic planar quadratically nonlinear waveguide either at anomalous or at normal group velocity dispersion. We discuss also the results of numerical simulation confirming our analytical findings. Besides that, simulation allows us to expand the scope of the study and to show light bullet propagation at a certain phase mismatching and the formation of a stable coupled localized structure from a signal at the fundamental frequency as well.

High power GaInNAs superluminescent diodes emitting over 400 mW in the 1.2 μm wavelength range

A high-power superluminescent diode emitting over 400 mW in the 1.2 μm range is reported. The active region is based on a single GaInNAs/GaAs quantum well positioned within a low-confinement vertical waveguide and a lateral ridge waveguide geometry, ensuring single transverse mode operation. The peak wall-plug efficiency and the differential efficiency in the linear region were 22.8% and 0.38 W/A, respectively. The full width at half-maximum spectral width for the maximum output power was 22 nm, corresponding to a spectral power density of 19 mW/nm, a threefold increase compared to continuous wave superluminescent diodes based on a quantum dot active region operating in the same wavelength range. Besides exhibiting excellent optical and electrical properties, the GaInNAs active region enhances operation at elevated temperatures. In this respect, an output power of about 210 mW is demonstrated at operation temperatures as high as 60 °C, while 150 mW is still emitted at 70 °C. The unique combination of parameters demonstrated makes these GaInNAs QW-based superluminescent diodes particularly attractive for hybrid integration with silicon photonic circuitry, enabling the demonstration of compact solutions for sensing, optical coherence tomography, and other emerging concepts exploiting photonic integration technology and requiring single transversal mode operation, good efficiency, broadband high spectral power density, and uncooled operation at elevated temperatures.

Standing Surface Acoustic Wave-Assisted Fabrication of Region-Selective Microstructures via User-Defined Waveguides.

Polymer-based substrate with region-selective microstructures are crucial for many biomedical applications. Here, we explored a novel method based on standing surface acoustic waves (SSAWs) for the fabrication of localized polymer-based microstructures via a predefined waveguide. When the SSAWs are excited, the generated acoustic pressure field can be controlled in a predetermined region of the fluid surface through controlling the size and shape of the waveguide geometry. On the basis of the capillary wave motion, the generated acoustic pressure field can excite microwavy patterned structures on the surface. Then with use of ultraviolet (UV) solidification, the polymer-based substrates with region-selective patterned microstructures can be successfully fabricated. Both finite element modeling and experimental studies demonstrated that the polymer substrate with different region-selective microstructures can be achieved by selecting the pairs of interdigital transducers (IDTs) and shapes of the predefined waveguides. The results showed that the proposed method is effective for fabricating polymer-based substrate with region-selective microstructures and may have potential in cell-laden chips for tissue engineering, cell-cell interactions, and other biomedical applications.

Optical-force laws for guided light in linear media

The mechanical response of transparent materials to optical forces is a topic that concerns a wide range of fields, from the manipulation of biological material by optical tweezers to the design of nano-optomechanical systems (NOMS). However, the fundamental aspects of such forces have always been surrounded by controversies, and several different formulations have been proposed. In this work, we focus on the specific case of light propagating as a superposition of guided modes in lossless dielectric waveguides as a physical example upon which to build a general stress tensor. We use this formalism to calculate optical forces for straight and curved waveguide sections and all possible excitation configurations for a given set of coupled eigenmodes, and then compare the results for each of the known proposed optical force laws as well as a novel one derived from this general stress tensor. We show that proper use of the divergence theorem is crucial to account for all force terms, many of which vanish if the procedure most commonly used is applied for situations other than eigenmodes in straight waveguides. A better understanding of how different stress tensors predict very different forces for certain waveguide geometries opens a pathway for new experimental tests of each formulation.

Numerical model for the determination of the reduced electric field in a CO2 microwave plasma derived by the principle of impedance matching

Three dimensional electromagnetic modelling of a free-standing CO2 microwave plasma has been performed, by describing the plasma as a dielectric medium. The relative permittivity and conductivity of the medium are parametrised. The waveguide geometry from the experiment, including the tuner, is put into the model, knowing that this corresponds to maximum power transfer of the microwave generator to the plasma under plasma impedance matching conditions. Two CO2 plasma discharge regimes, differing mainly in pressure, input power and temperature, have been studied. The model’s validity has been checked through study of materials of known conductivity. From measurements of the neutral gas temperature and the plasma electron density profile, the reduced electric field is determined. From the parametrisation of the dielectric properties, a range for the effective electron-neutral collision frequency for momentum transfer is estimated. The results for the reduced electric field and the range of the electron neutral collision frequency obtained, are consistent as verified by simulations using BOLSIG+. In addition, from this comparison it is possible to narrow down the range of the collision frequencies, and to estimate the electron temperature. The reduced electric field lies between 80 and 180 Td for the relatively low pressure, low input power, the so-called ‘diffuse’ regime. For the relatively high pressure, high input power (‘contracted’) regime it lies between 10 and 60 Td. The normalised collision frequency lies between 1.6 and 2.3 for the diffuse regime, while for the contracted regime it lies between 2 and 3. The electron temperature ranges from 2 to 3 eV for the diffuse regime, and from 0.5 to 1 eV for the contracted regime.

Optical Planar Waveguide Sensor with Integrated Digitally-Printed Light Coupling-in and Readout Elements.

Optical planar waveguide sensors, able to detect and process information from the environment in a fast, cost-effective, and remote fashion, are of great interest currently in different application areas including security, metrology, automotive, aerospace, consumer electronics, energy, environment, or health. Integration of networks of these systems together with other optical elements, such as light sources, readout, or detection systems, in a planar waveguide geometry is greatly demanded towards more compact, portable, and versatile sensing platforms. Herein, we report an optical temperature sensor with a planar waveguide architecture integrating inkjet-printed luminescent light coupling-in and readout elements with matched emission and excitation. The first luminescent element, when illuminated with light in its absorption band, emits light that is partially coupled into the propagation modes of the planar waveguide. Remote excitation of this element can be performed without the need for special alignment of the light source. A thermoresponsive liquid crystal-based film regulates the amount of light coupled out from the planar waveguide at the sensing location. The second luminescent element partly absorbs the waveguided light that reaches its location and emits at longer wavelengths, serving as a temperature readout element through luminescence intensity measurements. Overall, the ability of inkjet technology to digitally print luminescent elements demonstrates great potential for the integration and miniaturization of light coupling-in and readout elements in optical planar waveguide sensing platforms.

Study of crystalline defect induced optical scattering loss inside photonic waveguides in UV-visible spectral wavelengths using volume current method.

In this work, we study the crystalline defect induced optical scattering loss inside photonic waveguide. Volume current method is implemented with a close form of dyadic Green's function derived. More specifically, threading dislocation induced scattering loss inside AlN waveguides in UV-visible spectrum wavelengths are studied since this material is intrinsically accompanied with high densities of dislocations (typically on order of 108-1010cm-2). The results from this study reveal that threading dislocations contribute significant amount of scattering loss when material is not MOCVD grown. Additionally, the scattering loss is strongly dependent on polarization and waveguide geometries: TM modes exhibit higher scattering loss compared with TE modes, and the multimode large core waveguides are more susceptible to threading dislocations compared with single mode waveguides and high-aspect-ratio waveguides. Conclusions from this work can be supported by several recently published investigations on III-N based photonic devices. The model derived from this work can also be easily altered to fit other material systems with other types of crystalline defects.

Investigation of Hybrid Silicon-Nitride/Polymer Waveguides for Second-Harmonic Generation

We propose a hybrid silicon-nitride (Si3N4 or SiN for short)/nonlinear polymer waveguide to achieve second-harmonic generation of an input 1550 nm signal by modal phase-matching between zero-order spatial mode at 1550 nm and second-order mode at 775 nm. We explore and optimize two cases, single channel waveguide and slot waveguide, by varying the waveguide dimensions. For TE polarized light, the optimum slot waveguide exhibits an effective interaction area A eff of about 16 μ m2, which is an order-of-magnitude improvement over the single channel waveguide and can translate to a high value of effective nonlinearity. We also show that the optimum slot waveguide geometry is robust with respect to fabrication uncertainties and bending effects.

Generation of midinfrared and visible radiation in a multiband phase-matched subwavelength LiNbO3 slab waveguide

An on-chip lithium niobate (LiNbO3) slab waveguide platform is proposed that simultaneously produces electric field pulses through sum and difference frequency generation. By controlling the geometry of the slab waveguide, phase-matching is achieved that produces radiation in the visible and midinfrared spectral bands. The slab waveguide confines the sum frequency generation wavelengths to the waveguide core and allows the difference frequency generation wavelengths to be emitted as Cherenkov waves. A finite-difference time-domain analysis is performed to study this multiband generation, where the LiNbO3 slab waveguide produces electric field pulses at a conversion efficiency of 14×10−5 and 0.5×10−5 through sum and difference frequency generation, respectively. This slab waveguide platform provides suitable design flexibility because the emission wavelengths are adjustable through proper choice of the waveguide core width and the excitation wavelengths. This source allows for better utilization of on-chip space by reducing the total number of devices necessary to achieve multiband light generation.

Wideband Epsilon‐Near‐Zero Supercoupling Control through Substrate‐Integrated Impedance Surface

AbstractThe supercoupling phenomenon of electromagnetic wave through arbitrary shaped epsilon‐near‐zero (ENZ) material channels has attracted considerable attention recently, leading to numerous intriguing applications. It has been demonstrated that around the cutoff frequency of fundamental TE10 mode, waveguide behaves as the 2D homogenous ENZ material. However, for a given waveguide geometry, the ENZ operating bandwidth is inherently narrow and the center frequency is fixed. Here, a reconfigurable ENZ medium is proposed and experimentally demonstrated with a wideband‐tunable supercoupling effect by means of inserting a metasurface inside the waveguide, named as substrate‐integrated impedance surface (SIIS). With different insertion depth (which can be mechanically tuned) of SIIS, the ENZ supercoupling frequency can be precisely controlled. The proposed technique not only achieves wideband tuning of the ENZ supercoupling, especially to control the enhanced group delay of the electromagnetic waves, thereby offering a promising way toward for tunable slow‐light devices, such as light storage and processing components.

The Investigation of Subwavelength Grating Waveguides for Photonic Integrated Circuit Based Sensor Applications

Approaches to provide chip-scale aqueous chemical sensing for environmental and health monitoring via a photonic integrated circuit (PIC) technology are showing promise. To meet future U.S. Army demand, we have investigated waveguide structures compatible with feature sizes offered by modern mass-manufacturing PIC foundries. We have found that wide subwavelength grating (SWG) waveguide structures can maximally interact with an encapsulating analyte capture material with low propagation loss. We describe the analyte/light interaction component of our general integrated photonics chemical detector sensitivity model and use it to compare the effectiveness of waveguide geometries for sensing. The structures we have investigated afford superior detection to strip and slot waveguides. Finally, we introduce a novel tapered SWG waveguide geometry with improved transmission and similar sensitivity to nontapered SWG designs.

КОГЕРЕНТНОЕ РАСПРОСТРАНЕНИЕ ЛАЗЕРНЫХ ПУЧКОВ В МАЛОРАЗМЕРНОЙ СИСТЕМЕ СЛАБОСВЯЗАННЫХ СВЕТОВОДОВ

The successful development of fiber-optic technologies in recent decades has stimulated research on the replacement of components of solid-state laser systems with fiber components, which can drastically change the attractiveness of the corresponding applied developments. Yielding on the energy characteristics of solid-state systems, fiber lasers and nonlinear optical devices have high efficiency of conversion of pump energy to radiation energy associated with waveguide geometry, high quality of the spatial profile of the laser beam, as well as low cost, compactness, lack of alignment in work process. Note that the maximum achievable radiation power in a single fiber is limited primarily by the process of self-focusing, which leads to fiber damage. The use of a multi-core fiber (MCF), consisting of identical equidistant weakly coupled optical fibers, makes it possible to realize initially coherent propagation of laser radiation with a total power noticeably higher than it can be transmitted in a single optical fiber. However, as theoretical and experimental studies have shown, such systems have its own critical power (Balakin et al., 2016) whereby the self-focusing of the quasihomogeneous distribution of the wave field and its separation into a set of incoherent structures occurs. Therefore, we have considered a small-sized optical system of 2N identical weakly coupled optical fibers arranged in a ring (Balakin et. al., 2018). In such systems, it is possible to find stable distributions of intense wave beams, which allow coherent radiation transport over long distances. The total radiation power in the found distributions can significantly (up to 2N times) exceed the critical self-focusing power in continuous media. This manifests itself most clearly for the distribution of un ~ (-1)n with antiphase fields in neighboring waveguides, which is stable at an arbitrary wave beam power. Direct numerical simulation of a nonlinear wave equation confirms the stability of the field distributions found. The research was supported by the RAS Presidium Program «Nonlinear dynamics: fundamental problems and applications».

Dual-wavelength generation of picosecond pulses with 9.8 GHz repetition rate in Nd : YAG waveguide laser with graphene

We report a new solid-state waveguide laser generating picosecond pulses with a GHz repetition rate, based on the use of graphene as a saturable absorber. Lasing at the main transverse mode is provided by the geometry of a cylindrical waveguide formed in the active crystal volume by the method of direct writing with a femtosecond laser beam. Fine tuning of the intracavity interferometer formed between the active medium and output mirror makes it possible to control the spectral-temporal parameters of output radiation and to smoothly tune the repetition rate of pulses having a duration of less than 20 ps. In particular, the possibility of dual-wavelength generation in the regime of continuous passive mode locking using a single saturable absorber based on graphene is demonstrated. By amplifying laser output radiation in the ytterbium fibre amplifier, an average output power of 530 mW is obtained.

Modeling of Label-Free Optical Waveguide Biosensors with Surfaces Covered Partially by Vertically Homogeneous and Inhomogeneous Films

Optical Waveguide Lightmode Spectroscopy (OWLS) is widely applied to monitor protein adsorption, polymer self-assembly, and living cells on the surface of the sensor in a label-free manner. Typically, to determine the optogeometrical parameters of the analyte layer (adlayer), the homogeneous and isotropic thin adlayer model is used to analyze the recorded OWLS data. However, in most practical situations, the analyte layer is neither homogeneous nor isotropic. Therefore, the measurement with two waveguide modes and the applied model cannot supply enough information about the parameters of the possible adlayer inhomogeneity and anisotropy. Only the so-called quasihomogeneous adlayer refractive index, layer thickness, and surface mass can be determined. In the present work, we construct an inhomogeneous adlayer model. In our model, the adlayer covers the waveguide surface only partially and it has a given refractive index profile perpendicular to the surface of the sensor. Using analytical and numerical model calculations, the step-index and exponential refractive index profiles are investigated with varying surface coverages from 0 to 100%. The relevant equations are summarized and three typically employed waveguide sensor structures are studied in detail. We predict the errors in the calculated optogeometrical parameters of the adlayer by simulating the OWLS measurement on an assumed inhomogeneous adlayer. We found that the surface coverage has negligible influence on the calculated refractive index below film thicknesses of 5 nm; the calculated refractive index is close to the refractive index of the adlayer islands. But the determined quasihomogeneous adlayer refractive index and surface mass are always underrated; the calculated quasihomogeneous thickness is heavily influenced by the surface coverage. Depending on the refractive index profile, waveguide geometry, and surface coverage, the thickness obtained from the homogeneous and isotropic modeling can even take negative and largely overestimated values, too. Therefore, experimentally obtained unrealistic adlayer values, which were dismissed previously, might be important indicators of layer structure.