Silicon Wire Waveguide Research Articles

Influence of discontinuities on photonic waveguides.

Fabrication-induced imperfections in photonic wire waveguides, such as roughness, stitching errors, and discontinuities, degrade their performance and thereby lower the yield of large-scale systems. This degradation is primarily due to the high insertion losses induced by imperfections, which scale nonlinearly with the index contrast in wire waveguides. Here we investigate the influence of discontinuities in photonic waveguides and later show a platform that is robust to fabrication imperfections. Our platform is based on an array of silicon nano-pillars, arranged to form a sub-wavelength (SW) grating waveguide. We focus on investigating the robustness by considering an abrupt break in the waveguide, as an extreme case of discontinuity. We show that sub-wavelength silicon waveguides are robust against unwanted large discontinuities relative to the operating wavelength. We measure a transmission loss of <2.2 dB at 1550 n m, for a discontinuity of length 2.1 μ m, when compared to more than 7 d B of loss in conventional silicon wire waveguides for the same discontinuity. Our results show that this mode of protection is broadband, covering the entire telecommunication band (λ =1500-1600 nm). We believe that this investigation of the influence of discontinuities in photonic waveguides could be a step toward the realization of low-loss optical waveguides.

Broadband-Tunable Vanadium Dioxide (VO2)-Based Linear Optical Cavity Sensor.

Sensors fabricated by using a silicon-on-insulator (SOI) platform provide promising solutions to issues such as size, power consumption, wavelength-specific nature of end reflectors and difficulty to detect ternary mixture. To address these limitations, we proposed and investigated a broadband-thermally tunable vanadium dioxide (VO2)-based linear optical cavity sensor model using a finite element method. The proposed structure consists of a silicon wire waveguide on a silicon-on-insulator (SOI) platform terminated with phase-change vanadium oxide (VO2) on each side to provide light confinement. A smooth transmission modulation range of 0.8 (VO2 in the insulator state) and 0.03 (VO2 in the conductive phase state) in the 125 to 230 THz spectral region was obtained due to the of Fabry-Pérot (FP) effect. For the 3.84 μm cavity length, the presented sensor resulted in a sensitivity of 20.2 THz/RIU or 179.56 nm/RIU, which is approximately two orders of magnitude higher than its counterparts in the literature. The sensitivity of the 2D model showed direct relation with the length of the optical cavity. Moreover, the change in the resonating mode line width Δν of approximately 6.94 THz/RIU or 59.96 nm/RIU was also observed when the sensor was subjected to the change of the imaginary part k of complex refractive index (RI). This property of the sensor equips it for the sensing of aternary mixture without using any chemical surface modification. The proposed sensor haspotential applications in the areas of chemical industries, environmental monitoring and biomedical sensing.

The Low-Loss Spot Size Converter for Alignment with Cleaved Single Mode Fiber

Integrating the graded index (GRIN) waveguide with the 2-D inversely-tapered waveguide, a new design of spot size converter (SSC) has been proposed to couple the light beam between the cleaved single-mode fiber (SMF) and the silicon (Si) wire waveguide on the silicon-on-insulator (SOI) platform. The device demonstrates a low coupling loss of 0.27 dB when it is coupled to a cleaved SMF. The polarization-dependent loss (PDL) is less than 0.25 dB, and the 1-dB alignment tolerance is about −2.0~2.0 µm for both vertical and horizontal directions. At the same time, the starting tip width of the tapered waveguide is kept at the level of the lithography limitation at the current commercial silicon photonics fab. This integrated SSC could be an improved design for reducing the coupling loss between the cleaved SMF and the Si waveguide.

Design of the Bimodal Grating Sensor with a Built-In Mode Demultiplexer.

This new sensor design provides good volume sensitivity (around 1600 nm/RIU) via collinear diffraction by the asymmetric grating placed in the waveguide vicinity. It provides the mode transformation between the fundamental TE0 and the first TE1 modes of the silicon wire (0.22 μm by a 0.580 μm cross-section) in the water environment. In order to provide the wavelength interrogation with a better extinction ratio for the measuring signal, the grating design is incorporated with the mode filter/demultiplexer. It selects, by the compact directional coupler (maximum 4 μm wide and 14 μm long), only the first guided mode (close to the cutoff) and transmits it with small excess loss (about -0.5 dB) to the fundamental TE0 mode of the neighboring single mode silicon wire, having variable curvature and width ranging from 0.26 μm to 0.45 μm. At the same time, the parasitic crosstalk of the input TE0 mode is below -42 dB, and that provides the option of simple and accurate wavelength sensor interrogation. The environment index is measured by the spectral peak position of the transmitted TE0 mode power in the output single mode silicon wire waveguide of the directional coupler. This type of optical sensor is of high sensitivity (iLOD~ 2.1 × 10-4 RIU for taking into account the water absorption at 1550 nm) and could be manufactured by modern technology and a single-step etching process.

Experimental research on ultracompact silicon hybrid plasmonic nanofocusing device

Silicon-based optoelectronics, using the mature silicon-based microelectronic complementary metal oxide semiconductor (CMOS) manufacturing process, is a large-scale optoelectronic integration platform that has attracted much attention. Surface plasmonic devices have also received extensive attention in the past decades, and especially the silicon-based surface plasmonic nanofocusing devices have become a research hotspot. Typical nanofocusing structures include chirped surface gratings, plasmonic Fresnel zone plate, nano-slit array, tapered metal tips. However, there occur some inevitable problems in these devices, such as the fine structure being too complex to be fabricated and too large transmission loss of metal slot waveguide. In this work, an ultra-compact hybrid surface plasmon nanofocusing device is designed and fabricated by the silicon-based optoelectronic technology, and the nanofocusing performance of the device is also experimentally verified. The hybrid surface plasmon nanofocusing devices are fabricated on a silicon-on-insulator (SOI) wafer by electron beam lithography (EBL) system. The silicon wire waveguides, tapers and the thin silicon strips in the middle of nanofocusing regions are patterned in only one step EBL. The gold layer is formed by a deposition and lift-off process, and then a partially etching process is introduced to make the thickness of the middle thin silicon strips the same as that of the gold layer. With a 1.23-μm-long tapered structure, our nanofocusing devices focus the light field of a silicon strip waveguide into a hybrid surface plasmon waveguide, making non-resonant optical field increase 20 times in the 1550 nm near-infrared band experimentally. The entire insertion loss is about 4.6 dB, and the mode area of the nanofocusing area is about &lt;inline-formula&gt;&lt;tex-math id="M1"&gt;\begin{document}${\left( {\lambda /n} \right)^2}/640$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="19-20212340_M1.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="19-20212340_M1.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; which is over 300 times smaller than that of the input silicon waveguide. When the middle slot silicon waveguide width &lt;i&gt;W&lt;/i&gt;&lt;sub&gt;Si&lt;/sub&gt; = 120 nm, the insertion loss reaches a minimum value of 2.8 dB. In our design, we adopt the design of silicon-based hybrid plasmonic waveguides. In this design, a layer of material with low refractive index is inserted between the metal layer and the silicon layer to act as a “container” of light field, which makes this silicon-based hybrid plasmonic waveguides have less loss than the traditional metal plasmonic waveguides, and can still maintain high optical field localization. Such silicon-based hybrid surface plasmon nanofocusing devices with simple structures and excellent performances are promising alternatives for future applications in optical field manipulation, optical sensing, nonlinear optical devices, and optical phase-change storage.

Nonlinear loss characterization of continuous wave guiding in silicon wire waveguides

Accurate propagation loss characterization of silicon waveguides is increasingly demanded for silicon-photonics-(Si-Ph) applications with high-power continuous-wave-(CW) light sources. We report on nonlinear loss parameters of silicon wire waveguides for 1.31 μm wavelength CW light extracted from transmission data measured for different lengths and polarizations. Such parameters were, so far, unavailable, although they are required for accurately modeling Si-Ph optical circuits. More-than-ten-times enhancement of two-photon absorption from prior results for short pulse light was observed at power densities ranging up to 4.7 × 1011 W m−2 while free carrier absorption was suppressed. We estimate the nonlinear loss of the waveguides using the parameter values obtained.

Novel Low-Loss Fiber-Chip Edge Coupler for Coupling Standard Single Mode Fibers to Silicon Photonic Wire Waveguides

Fiber-to-chip optical interconnects is a big challenge in silicon photonics application scenarios such as data centers and optical transmission systems. An edge coupler, compared to optical grating, is appealing to in the application of silicon photonics due to the high coupling efficiency between standard optical fibers (SMF-28) and the sub-micron silicon wire waveguides. In this work, we proposed a novel fiber–chip edge coupler approach with a large mode size for silicon photonic wire waveguides. The edge coupler consists of a multiple structure which was fulfilled by multiple silicon nitride layers embedded in SiO2 upper cladding, curved waveguides and two adiabatic spot size converter (SSC) sections. The multiple structure can allow light directly coupling from large mode size fiber-to-chip coupler, and then the curved waveguides and SSCs transmit the evanescent field to a 220 nm-thick silicon wire waveguide based on the silicon-on-insulator (SOI) platform. The edge coupler, designed for a standard SMF-28 fiber with 8.2 μm mode field diameter (MFD) at a wavelength of 1550 nm, exhibits a mode overlap efficiency exceeding 95% at the chip facet and the overall coupling exceeding 90%. The proposed edge coupler is fully compatible with standard microfabrication processes.

Topological converter for high-efficiency coupling between Si wire waveguide and topological waveguide.

Replacing part of a conventional optical circuit with a topological photonic system allows for various controls of optical vortices in the optical circuit. As an underlying technology for this, in this study, we have realized a topological converter that provides high coupling efficiency between a normal silicon wire waveguide and a topological edge waveguide. After expanding the waveguide width while maintaining single-mode transmission from the Si wire waveguide, the waveguides are gradually narrowed from both sides by using a structure in which nanoholes with C6 symmetry are arranged in a honeycomb lattice. On the basis of the analysis using the three-dimensional finite-difference time-domain method, we actually fabricated a device in which a Si wire waveguide and a topological edge waveguide were connected via the proposed topological converter and evaluated its transmission characteristics. The resulting coupling efficiency between the Si wire waveguide and the topological edge waveguide through the converter was -4.49 dB/taper, and the coupling efficiency was improved by 5.12 dB/taper compared to the case where the Si wire waveguide and the topological edge waveguide were connected directly.

A High Finesse Serial Coupled Microresonator Filter

AbstractA serially coupled double-ring resonator using the low loss multilayer silicon wire waveguide is presented that has featured high out-of-band rejection and high finesse. The round trip loss and Q factor has also been calculated and found moderate. However, extremely low loss waveguide has shown reduced coupling between the ring and the waveguide.

An Ultracompact GRIN‐Lens‐Based Spot Size Converter using Subwavelength Grating Metamaterials

AbstractGraded‐index materials offer virtually complete control over light propagation in integrated photonic chips but can be challenging to implement. Here, an anisotropic graded‐index metamaterial, synthesized with fully etched silicon subwavelength structures, is proposed. Based on this material, a spot size converter that expands the transverse electric (TE) mode field profile from a 0.5 µm wide silicon wire waveguide to a 15 µm wide waveguide within a length of only 14 µm is designed. Measured insertion losses are below 1 dB in an unprecedented 130 nm bandwidth, limited by the measurement setup, with full 3D finite‐difference time‐domain (FDTD) simulations predicting a bandwidth in excess of 300 nm. Furthermore, the device is well suited to feed fiber‐to‐chip grating couplers, while requiring a footprint ten times smaller than conventional adiabatic tapers.

Demonstration of slow-light effect in silicon-wire waveguides combined with metamaterials.

We demonstrated a novel slow-light Si-wire waveguide combined with metamaterials, which can be easily integrated with other Si photonics devices. The slow-light effect can be produced simply by placing metamaterials at an appropriate position on a Si-wire waveguide. It was confirmed that the large group index of more than 40 could be obtained because of a steep and discontinuous change of dispersion relation near the resonance frequency of metamaterials.

Design of silicon-wire waveguide ultracompact racetrack resonators: geometrical parameters for optimal coupling.

In this paper, it is shown that a suitable choice of the geometrical parameters of a silicon-wire waveguide microracetrack resonator structure can lead to a substantial improvement in the control of coupling coefficients and, hence, the design of ultracompact devices for high-performance channel add-drop filters and all-optical switching applications. On the one hand, some simple theoretical arguments and simulation results indicate that the reduction of the silicon-wire rectangular waveguide cross-section area (width×height) is possible, from standard (450 nm×220 nm) to (380 nm×200 nm) on both the bus and the resonator waveguides; this action, apart from still guaranteeing a quasi-TE single-mode operation, would provide an effective improvement into scale-of-integration by a 1.30 factor per device volume. On the other hand, it will be shown by a semianalytical method (analytical calculation + numerical simulation) that achieving the waveguide-racetrack optimal coupling condition for a particular application can be reduced to a prime calculation of the main resonator geometrical parameters (bend radius, straight length, air gap and overall coupling length). In particular, the design of high-performance ultracompact waveguide-racetrack resonator structures, with pre-established Q factor (Q≥2000), free spectral range (FSR≥15 nm), full width at half-maximum (FWHM≤5 nm), finesse (F≥40) or extinction ratio signals (ER≥20 dB) can be systematically obtained with this procedure.

Silicon-Nitride-Assisted Edge Coupler Interfacing With High Numerical Aperture Fiber

Reducing the mode size mismatch between high-index-contrast silicon waveguide and single-mode fiber takes an important role in photonic integration. A low-loss and broadband silicon-nitride-assisted edge coupler for the coupling of light between silicon wire waveguides and high numerical aperture fiber is proposed and optimized for TE polarization. The mode size converter is composed of silicon inverse taper waveguides in the core and two silicon nitride interlayers spaced by silica in cladding layers. The edge coupler is characterized by cleaved high numerical aperture fiber (mode field diameter is about $6.5~\mu \text{m}$ ). With the help of refractive index matching oil ( $n =1.46$ ), the coupling loss for TE polarized light input is 0.35 dB at 1570 nm. The 1-dB bandwidth is 95 nm. The 1-dB alignment tolerances are higher than ± 2.5 and $\pm ~1~\mu \text{m}$ in the horizontal and vertical directions, respectively, and higher than $25~\mu \text{m}$ in the axial direction.

Numerical Modeling of Abnormal Blocking Effect for the Design of Novel Optical Sensor Element Constructed by Periodic Grating Strips Over Si/SiO2 Wire Waveguide

In this paper the numerical modeling of periodic structures for implementation of a novel type of photonic sensors by using the 3D finite difference time domain (FDTD) method is presented. The sensing is based on the optical phenomena occurring in the segmented grating which is placed on the thin silica buffer over the silicon wire waveguide in the silicon‐on‐insulator (SOI) structure. This design provides the effective resonance interaction of the guided wave with the virtual leaky wave supported by the segmented grating evanescently coupled with the silicon wire. The dropping wavelength of this interaction is strongly dependent on the grating environment and it provides a strong refractometricsensitivity (Sn &gt; 420 nm RIU−1) and surface sensitivity Sh &gt; 0.17, evaluated in the case of water. The modeling proves that the effect of abnormal blocking due to virtual leaky wave could be the base for the design of optical sensors with extremely high sensitivity.

Highly Nondegenerate Two-Photon Absorption in Silicon Wire Waveguides

Non-degenerate two-photon absorption (TPA) is investigated in a nanophotonic silicon waveguide in a configuration such that the dispersion of the nonlinear absorption and refraction cannot be neglected. It is shown that a signal wave can strongly be absorbed by cross-TPA by interaction with a low energy pump pulse (1.2\,pJ), close to the half-bandgap and experiencing low nonlinear absorption. The experiments are very well reproduced by numerical simulations of two-coupled generalized nonlinear Schr\"{o}dinger equations (GNLSE), validating the usual approximation made to compute the cross nonlinear coefficients in indirect-gap semiconductors. We show that the nonlinear dynamics can be well described by a single GNLSE despite the wavelength separation between the pump and the signal waves. We also demonstrate that in silicon wire waveguides and contrary to optical fibers, the dispersion of the nonlinear absorption is much larger than the dispersion of the Kerr effect. This could have an impact in the design of all-optical functions based on cross-TPA, as well as on the study of supercontinuum and frequency comb generation in integrated semiconductor-on-insulator platforms.

High-extinction-ratio directional-coupler-type polarization beam splitter with a bridged silicon wire waveguide.

We demonstrate an integrated polarization beam splitter (PBS) of a simple and compact device geometry with high polarization extinction ratios (PERs). A silicon PBS based on a three-waveguide directional coupler is numerically simulated for optimum device parameters, and fabricated experimentally. The measured PER values of the fabricated PBSs are 40.74dB and 39.01dB for transverse electric and transverse magnetic modes, respectively, and their corresponding insertion losses are 0.35dB on average for a coupler length of about 29.4μm.

Enhanced Terahertz Fingerprint Detection beyond Refractive Index Sensing in a Periodic Silicon Waveguide Cavity

Resonance shifting due to refractive index changes is used quite often in terahertz sensing, but it does not show the advantages of substance identification of terahertz technology. Different from that approach, we explored the use of a cavity to enhance the sensitivity of terahertz sensing while retaining the original capability of substance identification. The defect mode of a one-dimensional photonic crystal cavity composed of periodic holes etched into a silicon wire waveguide was investigated for this purpose. The resonance of the defect mode was designed to match one characteristic absorption frequency of the sample. Due to the high dependence of the defect mode transmission on the material loss, the transmission sensitivity to the quantity of target was amplified significantly. The detection of α-lactose was used as an example, which demonstrates steady detection with its thickness of a few microns.

Numerical Simulation of a Novel Sensing Approach Based on Abnormal Blocking by Periodic Grating Strips near the Silicon Wire Waveguide

This paper discusses the physical nature and the numerical modeling of a novel approach of periodic structures for applications as photonic sensors. The sensing is based on the high sensitivity to the cover index change of the notch wavelength. This sensitivity is due to the effect of abnormal blocking of the guided wave propagating along the silicon wire with periodic strips overhead it through the silica buffer. The structure sensing is numerically modeled by 2D and 3D finite difference time domain (FDTD) method, taking into account the waveguide dispersion. The modeling of the long structures (more than 1000 strips) is accomplished by the 2D method of lines (MoL) with a maximal implementation of the analytical feature of the method. It is proved that the effect of abnormal blocking could be used for the construction of novel types of optical sensors.

Analysis of the slow-light effect in silicon wire waveguides with metamaterials

Slow-light devices have been attracting a great amount of attention owing to their potential applications in future optical networks and information-processing systems. In this paper we propose two types of silicon wire waveguides with metamaterials, which are potentially applicable to the deceleration of light speed, and we analyze the slow-light effect by using the homogenization-based mode analysis method. The derived dispersion relation indicates that a group index larger than 70 can be obtained with a propagation loss of ∼0.7 dB/μm near the resonant frequency of the metamaterial.

Performance Enhancement of Nanoscale VO2 Modulators Using Hybrid Plasmonics

Vanadium dioxide (${\rm VO}_2$) is a phase change material (PCM) that exhibits a large change in complex refractive index on the order of unity upon switching from its dielectric to its metallic phase. Although this property is key for the design of ultracompact optical modulators of only a few microns in footprint, the high absorption of ${\rm VO}_2$ leads to appreciable insertion loss (IL) that limits the modulator performance. In this paper, through theory and numerical modeling, we report on a new paradigm, which demonstrates how the use of a hybrid plasmonic waveguide to construct a ${\rm VO}_2$ based modulator can improve the performance by minimizing its IL while achieving high extinction ratio (ER) in comparison to a purely dielectric waveguide. The hybrid plasmonic waveguide that contains an additional metal layer with even higher loss than ${\rm VO}_2$ enables unique approaches to engineer the electric field (E-field) intensity distribution within the cross section of the modulator. The resulting figure-of-merit, FoM = ER/IL, is much higher than what is possible by simply incorporating ${\rm VO}_2$ into a silicon wire waveguide. A practical modulator design using this new approach, which also includes input and output couplers yields ER = 3.8 dB/{\mu}m and IL = 1.4 dB/{\mu}m (FoM = 2.7), with a 3-dB optical bandwidth >500 nm, in a device length = 2 {\mu}m, and cross-sectional dimensions = 200 nm $\times$ 450 nm. To the best of our knowledge, this is one of the smallest modulator designs proposed to date that also exhibits amongst the highest ER, FoM, and optical bandwidth, in comparison to existing designs. In addition to ${\rm VO}_2$, we investigate two other PCMs incorporated within the waveguide structure. The improvements obtained for ${\rm VO}_2$ modulators do not extend to other PCMs.