Asymmetric Waveguide Research Articles

Theoretical Investigation of a Highly Sensitive Refractive-Index Sensor Based on TM₀ Waveguide Mode Resonance Excited in an Asymmetric Metal-Cladding Dielectric Waveguide Structure.

This study proposes a highly sensitive refractive-index (RI) sensor based on a TM0 waveguide mode resonance excited in an asymmetric metal-cladding dielectric waveguide structure, where the analyte serves as the guiding layer. By scanning the wavelength at fixed angles of incidence, the reflection spectra of the sensor were obtained. The results showed that the resonance wavelength redshifted dramatically with increases in the analyte RI, which indicates that this approach can be used to sense both the resonance wavelength and the analyte RI. Based on this approach, we investigated the sensing properties, including the sensitivity and figure of merit, at fixed incident angles of 60° and 45°, at which the sensitivity of the sensor reached 7724.9 nm/RIU (refractive index units) and 1339 nm/RIU, respectively. Compared with surface plasmon resonance sensors, which are based on a similar structure, the proposed sensor can accept a more flexible range of incident angles and a wider sensing range of analyte RI. This approach thus has tremendous potential for use in numerous sensing domains, such as biochemical and medical analyses.

Rotating polarization using Berry's phase in asymmetric silicon strip waveguides.

Light propagating in an out-of-plane curvilinear waveguide acquires a Berry's phase, which rotates optical polarization. The effect is promising for realizing waveguide polarization controllers. In high index contrast platforms, such as silicon-on-insulator, however, small waveguide cross-sectional asymmetries reduce the amount of polarization rotation. To overcome this, we present a method based on the periodic spatial modulation of Berry's phase. Ninety degree polarization rotation is achieved, even in the presence of waveguide asymmetry. Using a numerical model based on Jones calculus, we demonstrate the approach with 303×300 nm2 asymmetric silicon waveguides. We convert polarization from transverse electric to transverse magnetic with a polarization extinction ratio (PER) greater than 20dB PER over a 100nm bandwidth in a 110×240 μm2footprint.

Fano Resonance in an Asymmetric MIM Waveguide Structure and Its Application in a Refractive Index Nanosensor.

Herein, the design for a tunable plasmonic refractive index nanosensor is presented. The sensor is composed of a metal–insulator–metal waveguide with a baffle and a circular split-ring resonator cavity. Analysis of transmission characteristics of the sensor structures was performed using the finite element method, and the influence of the structure parameters on the sensing characteristics of the sensor is studied in detail. The calculation results show that the structure can realize dual Fano resonance, and the structural parameters of the sensor have different effects on Fano resonance. The peak position and the line shape of the resonance can be adjusted by altering the sensitive parameters. The maximum value of structural sensitivity was found to be 1114.3 nm/RIU, with a figure of merit of 55.71. The results indicate that the proposed structure can be applied to optical integrated circuits, particularly in high sensitivity nanosensors.

Theoretical realization of three-dimensional nanolattice structure fabrication based on high-order waveguide-mode interference and sample rotation

A theoretical method of using high-order waveguide-mode interference combined with sample rotation is proposed to fabricate different kinds of three-dimensional nanolattice structures. The lithography sample is assisted by an asymmetric metal-cladding dielectric waveguide structure. High-order waveguide modes can be excited in the photoresist layer by both transverse magnetic and transverse electric polarized light. By utilizing multiple exposures of high-order waveguide-mode interference combined with sample-rotation, various three-dimensional nanolattice structures can be obtained. The resulting optical field distributions are simulated using the finite element method in this study, and sample rotation is expressed by coordinate matrix transformation. As examples, the three-dimensional optical field distributions resulting from fifth-order waveguide-mode interference were simulated with 90° and 60° sample rotations and multiple exposures. The results show that a quasi-cuboid structure with a simple tetragonal arrangement and a quasi-hexagonal structure with a hexagonal close-packed lattice can be obtained. Moreover, the numerical simulation results revealed that the shapes, sizes, arrangements, and periods of the structures can be controlled by the rotation method, photoresist thickness, waveguide modes used for exposure, and so on.

Nanoscale C‐Rich SixC1−x Bus/Ring Waveguide Based Cross‐Wavelength Data Converter

AbstractThe carbon‐rich silicon carbide (C‐rich SixC1−x) micro‐ring channel waveguide with asymmetric core aspect is demonstrated for all‐optical cross‐wavelength pulsed return‐to‐zero on‐off keying (PRZ‐OOK) data conversion. Enhanced nonlinear optical Kerr switching enables 12‐Gbit per second data processing with optimized modulation depth. The inverse tapered waveguide at end‐face further enlarges the edge‐coupling efficiency, and the asymmetric channel waveguide distinguishes the polarization modes. To prevent data shape distortion, the bus/ring gap spacing is adjusted to control the quality factor (Q‐factor) of the micro‐ring. Designing the waveguide cross section at 500 × 350 nm2 provides the C‐rich SixC1−x channel waveguide to induce strong transverse electric mode (TE‐mode) confinement with a large Kerr nonlinearity of 2.44 × 10−12 cm2 W−1. Owing to the trade‐off between the Q‐factor and the on/off extinction ratio, the optimized bus/ring gap spacing of 1400 nm is selected to provide a coupling ratio at 5–6% for compromising the modulation depth and the switching throughput. Such a C‐rich SixC1−x micro‐ring with asymmetric channel waveguide greatly enhances the cross‐wavelength data conversion efficiency to favor its on‐chip all‐optical data processing applications for future optoelectronic interconnect circuits.

Vertical Coaxial-to-Ridge Waveguide Transitions for Ridge and Ridge Gap Waveguides With 4:1 Bandwidth

An ultra-wideband vertical coaxial-to-single-ridge waveguide transition is designed to cover Ku-, K-, and Ka-bands simultaneously. The simulated and measured back-to-back transitions have achieved better than 15-dB return loss within 12.8–40 GHz. The transition can also be a direct transition to the ridge gap waveguide (RGW). In addition, a similar concept is used to design a wideband coaxial-to-double-ridge waveguide transition to feed symmetric and asymmetric double ridge waveguide on a bandwidth ratio of more than 4:1. These transitions will be used to feed broadband ridge and RGW power splitters.

Purcell effect in GaN-based waveguiding structures

We provide an analysis of Purcell coefficient dependence on frequency, wavevector and emitter position in nitride-based waveguide structures. Was shown that spontaneous emission of emitter placed in one-dimensional slab waveguide structure could lead to a modification of spontaneous emission rate in the both considered structures. Results for symmetric and asymmetric nitride-based waveguides in TE polarization case was demonstrated.

Advanced Passive Silicon Photonic Devices With Asymmetric Waveguide Structures

Various passive photonic integrated devices have been developed successfully with silicon-on-insulator (SOI) nanowires in the past decade. It is well known that SOI-nanowire waveguides have ultrahigh index contrast ( $\Delta $ ) and ultrahigh birefringence. As a result, the structures and the design rules of a silicon photonic device are probably very different from the conventional case of using low- $\Delta $ optical waveguides. For example, some asymmetric waveguide structures have been used often to realize many silicon photonic devices developed recently. Furthermore, higher order modes have been involved in some of these silicon photonic devices. This paper discusses the special mode properties of silicon nanophotonic waveguides, including birefringence, mode dispersion, and mode hybridness. A review is then given on recent progress of these advanced passive silicon photonic devices with structural asymmetry, including on-chip polarization-handling devices, mode converters/(de)multiplexers, microring-resonator optical filters/switches, and Mach..Zehnder interferometer optical switches. Silicon photonic integrated circuits with these building blocks are also reviewed, including hybrid (de)multiplexers and reconfigurable optical add..drop multiplexers.

Design of a promising silicon slot waveguide-based ultra-short low loss efficient polarization rotator for the mid-IR

We propose design of a promising ultra-short and highly efficient mid-IR polarization rotator (PR) on silicon-on-calcium-fluoride (SOCF) platform, whose rotation section is formed from an asymmetric slot waveguide for the working wavelength of 4.47 μm by exploiting mode coupling scheme. Numerical results demonstrate that within a conversion length of just ∼18 μm, the maximum achievable polarization conversion efficiency (PCE) for both the polarizations (TE-TM and TM-TE) is as high as ∼100% with significantly low loss of ∼ 0.15 dB. Achieved polarization extinction ratios (PERs) are as high as 43.3 dB and 34.9 dB for TE-TM and TM-TE polarizations, respectively. Furthermore, over a large bandwidth of 360 nm (4.29–4.65 μm), our designed PR exhibits very high PCE (> 98%) for both the TE-TM and vice versa with an appreciable PER of ∼18 dB and relatively low loss of ∼ 0.26 dB. Study on fabrication tolerances reveals that designed PR is robust to potential imperfections in fabrication. To the best of our knowledge, our designed PR is the shortest PR reported to date for the mid-IR wavelength regime with a healthy balance/compromise between PCE, PER, bandwidth, loss and fabrication tolerance.

275–500-GHz Wideband Waveguide SIS Mixers

We have successfully demonstrated low-noise and wideband SIS mixers for the 275–500-GHz radio-frequency (RF) band, aiming to cover Atacama Large Millimeter/submillimeter Array band 7 (275–373 GHz) and band 8 (385–500 GHz) simultaneously. The waveguide SIS mixers integrate a multisection impedance transformer with a parallel-connected-twin-junction or single-junction circuits based on Nb/AlN/Nb trilayers with a critical current density of 31 kA/cm2. In order to achieve the targeted broadband performance, we designed an asymmetric one-side waveguide probe placed across a reduced height waveguide with dimensions 100 μm × 580 μm. The shape of the waveguide probe was optimized to have nearly constant impedance at around 35 Ω in the desired RF range. For evaluation of the receiver performance, we have established a dual-band noise temperature measurement system. The system implements a broadband corrugated horn antenna and a 15-dB RF/local oscillator (LO) waveguide coupler based on WR-2.3 (290 μm × 580 μm) waveguide size. The target bandwidth is too wide for typical LO sources at these high frequencies, and, therefore, we applied a dual-band LO injection system utilizing two different LO sources and an LO diplexer. The IF chain uses a cryogenic isolator and a low-noise amplifier with a typical noise temperature of 2.1 K over the 4–8-GHz range. The measured double sideband receiver noise temperature is two to three times the quantum noise at all LO frequencies for both the twin-junction and the single-junction circuits.

Optical analog of Rashba spin-orbit interaction in asymmetric polariton waveguides

We demonstrate that for a one-dimensional asymmetric polariton waveguide, coupling between the fundamental and the excited modes leads to the emergence of splitting in circular polarizations. This splitting scales linearly with the polariton momentum along the channel and the corresponding effective magnetic field is perpendicular to both the direction of propagation and the axis of asymmetry of the system. The effect thus represents a full optical analog of the Rashba spin-orbit interaction.

Sequential Self‐Assembly of 1D Branched Organic Homostructures with Optical Logic Gate Function

AbstractIntegrated microstructures composed of organic micro/nanocrystals have gained impressive interest recently as probable candidate for constructing optoelectronic circuits. However, the fabrication of these designed composite structures remains a huge challenge. Herein, the sequentially self‐assembled branched homostructures based on 1D single‐crystalline organic microrods of 1,4‐bis((E)‐4‐iodostyryl)‐2,5‐dimethoxybenzene through the facile solution‐processing approach are demonstrated. The growth mechanism of these dendritic microcrystals with an angle of 75° between trunk and branch is attributed to the facet‐selective growth principle between the (020) and (110) crystal planes, for which the interplanar spacing mismatch rate is as low as 6.1%. More significantly, this branched microstructure exhibits an asymmetric optical waveguide and can function as an optical logic gate with an ON/OFF ratio of 10.5. Therefore, the approach to build this organized multilevel structure could provide further applications as building blocks in integrated optoelectronic circuits.

Realizing multi-functional all-optical data processing on nanoscale SiC waveguides

All-optical logics are realized on nanoscale SiC waveguides with add-drop micro-ring functionality, including the TE/TM polarized data decoding, the dual-port Kerr switching and gating beyond 12 Gbit/s. With employing the C-C bond enriched SiC thin film upon thermal oxide, the nonlinear refractive index of up to 2.44 × 10−12 cm2/W enables the asymmetric waveguide with polarization distinguishable transmission, which provides a polarization-selectivity to discreminate the TE/TM polarized data decoding with an nearly 9-dB extinction ratio. The TE/TM polarized decoding performance is comparable with a state-of-the-art fiberized in-line polarizer. The complementary transmission in the bus waveguide port facilitates the dual-port Kerr switching for data format conversion/inversion in both add/drop channels. Owing to the TE/TM polarization discriminated throughput, the asymmetric add-drop waveguide micro-ring also permits all-optical AND logic gating functions, where the ON-state outputs only if the pump bit is set at ON state and the probe bit with matched polarization. These results reveal the multi-functionality of the nanoscale SiC add-drop micro-ring waveguide for future photonic logics on chip.

Investigation of wide-range refractive index sensor based on asymmetric metal-cladding dielectric waveguide structure

A refractive index (RI) sensor based on an asymmetric metal-cladding dielectric waveguide structure, in which an analyte is used as the guiding layer, is proposed in this report. The angular scanning attenuated total reflection spectra of the sensor and optical field distributions under waveguide mode resonance conditions were simulated, which confirmed that multiple waveguide modes can be used for sensing, and that different waveguide modes exist in different analyte RI ranges. The curves of both waveguide mode resonance angles and the sensitivity as a function of the analyte RI demonstrated that a wide sensing range can be achieved by combining waveguide modes of different orders. The proposed sensor could be applied to numerous fields, such as biological and chemical detection.

Miniaturised triple‐band antenna loaded with complementary concentric closed ring resonators with asymmetric coplanar waveguide‐fed based on epsilon negative transmission line

This study presents an open-ended triple-band miniaturised metamaterial (MTM) inspired antenna based on the epsilon negative-transmission line (ENG-TL). The zeroth-order resonance (ZOR) and first-order resonance (FOR) characteristics are implemented using the ENG-TL theory. The proposed MTM antenna consists of rectangular and square-shaped complementary concentric closed ring resonator (CCCRR) in the left and right half of the asymmetric coplanar waveguide (CPW) ground plane, respectively. The CCCRR generates an extra coupling capacitance and inductance, which help for improving the gain at ZOR mode and better impedance matching at higher-order mode. The resonance characteristic of the antenna is controlled by the shunt elements due to its open-ended boundary condition. The proposed triple-band MTM antenna offers measured -10 dB impedance bandwidths of 8.54% (1.12-1.22 GHz), 10.25% (2.22-2.46 GHz) and 35.75% (2.94-4.22 GHz). All these three bands show good impedance matching with appropriate gain and high radiation efficiency and also having an omnidirectional pattern in the xz- plane and dipolar type pattern in the yz -plane. The designed antenna shows an overall dimension of 0.11 λ 0 × 0.11 λ 0 × 0.006 λ 0 at ZOR frequency (1.16 GHz).

Theoretical study of multiexposure zeroth-order waveguide mode interference lithography

A new nanolithography technique in which sample rotation is incorporated into zeroth-order waveguide mode interference lithography is proposed in this report. A 325-nm laser was used to excite zeroth-order waveguide modes, which were loaded by an asymmetric metal-cladding dielectric waveguide structure. The optical field intensity distribution of zeroth-order waveguide modes interference is numerically simulated using the finite element method. The lithography sample consisted of a glass substrate, Al film, and photoresist film, and the rotation operation on the sample is expressed in coordinate matrix transformation. Various subwavelength structures, such as two-dimensional square lattices, two-dimensional hexagonal closed-packed lattices,and circular gratings, were obtained through double, triple, and continuous exposure. These subwavelength structures with different sizes can be produced by changing the thickness of the photoresist. The subwavelength structures simulated with various shapes and sizes can be applied to the field of nano-optics. The proposed technique provides a flexible and promising approach for interference nanolithography because of its simplicity and low cost.

2D Organic Photonics: An Asymmetric Optical Waveguide in Self-Assembled Halogen-Bonded Cocrystals.

Anisotropic organic molecular construction and packing are crucial for the optoelectronic properties of organic crystals. Two-dimensional (2D) organic crystals with regular morphology and good photon confinement are potentially suitable for a chip-scale planar photonics system. Herein, through the bottom-up process, 2D halogen-bonded DPEpe-F4 DIB cocrystals were fabricated that exhibit an asymmetric optical waveguide with the optical-loss coefficients of RBackward =0.0346 dB μm-1 and RForward =0.0894 dB μm-1 along the [010] crystal direction, which can be attributed to the unidirectional total internal reflection caused by the anisotropic molecular packing mode. Based on this crystal direction-oriented asymmetric photon transport, these as-prepared 2D cocrystals have been demonstrated as a microscale optical logic gate with multiple input/out channels, which will offer potential applications as the 2D optical component for the integrated organic photonics.

2D Organic Photonics: An Asymmetric Optical Waveguide in Self‐Assembled Halogen‐Bonded Cocrystals

AbstractAnisotropic organic molecular construction and packing are crucial for the optoelectronic properties of organic crystals. Two‐dimensional (2D) organic crystals with regular morphology and good photon confinement are potentially suitable for a chip‐scale planar photonics system. Herein, through the bottom‐up process, 2D halogen‐bonded DPEpe‐F4DIB cocrystals were fabricated that exhibit an asymmetric optical waveguide with the optical‐loss coefficients of RBackward=0.0346 dB μm−1and RForward=0.0894 dB μm−1along the [010] crystal direction, which can be attributed to the unidirectional total internal reflection caused by the anisotropic molecular packing mode. Based on this crystal direction‐oriented asymmetric photon transport, these as‐prepared 2D cocrystals have been demonstrated as a microscale optical logic gate with multiple input/out channels, which will offer potential applications as the 2D optical component for the integrated organic photonics.

Efficient pump–signal combiner for stimulated Raman scattering in photonic crystal waveguide

A silicon photonic crystal (PhC)-based structure is proposed to efficiently couple power from two different wavelengths, targeted to the application of an integrable Raman amplifier. The coupler uses two asymmetrical PhC waveguides as input ports which are joined together and the coupled power is directed to a slotted photonic crystal waveguide-based output port. Coupling efficiency and the other performance metrics are evaluated using a three-dimensional full-vector finite-difference time domain simulation method, which is tested for accuracy using some existing experimental structures and corresponding results. The heuristic approach has been adopted to optimize the design for maximizing the coupling efficiency. Dependence of the coupling performance on different design parameters has also been investigated. Simulations exhibit ∼3-dB coupling losses for both arms in a small footprint as small as ∼54 μm2.

Polymer waveguide WDM channel selector operating over the entire C and L bands.

A tunable channel selector operating over both the C and L bands of wavelength-division-multiplexing optical network is proposed based on a thermo-optic tunable Bragg grating device in a polymer waveguide. A tilted Bragg grating cascaded with an asymmetric Y-branch waveguide effectively implements a small Bragg-reflection wavelength filter that does not require an external circulator. To increase the operating wavelength span of the channel selector, two Bragg gratings with different periods are fabricated on a single substrate for covering C and L bands, respectively. A wide tuning range over 80 nm along with a narrow bandwidth is demonstrated. Moreover, the polarization dependence of the reflection spectrum is observed to be less than 0.1 nm.