Strip Waveguides Research Articles

Streamlined line-defect taper design for broadband and efficient mode coupling: connecting a silicon strip with a planar valley photonic crystal zigzag interface.

The integration of planar valley photonic crystal (VPC) interfaces into high-speed data communication chips markedly improves data rates and system robustness. This Letter presents a novel, to the best of our knowledge, edge coupler, termed the line-defect taper, which is crucial for efficient and broadband light delivery to planar VPC interfaces via silicon strip waveguides. The coupling performance of the line-defect taper is evaluated through full-wave three-dimensional finite-element simulations. The results demonstrate a -3 dB transmission bandwidth of 65.5 nm, covering 41.2% of the topological bandgap, and a -1 dB transmission bandwidth of 16.3 nm, accounting for 10.3%. With its compact design (only 3.6 µm in length), simplicity, and scalability, the line-defect taper is a promising candidate for integration into densely packed chips, highlighting its potential in advancing on-chip devices.

Enhancing stimulated Brillouin scattering in suspended silicon waveguides through subwavelength nanostructuration [Invited

Brillouin optomechanics is playing a key role in the development of groundbreaking devices and novel functionalities in integrated silicon photonics, such as narrow linewidth filtering and lasers, tunable frequency, non-reciprocity, etc. Most silicon-based optomechanical waveguides, which use anchoring arms or perforated slabs to ensure mechanical stability and operate for transverse-electric polarized light, face challenges with acoustic mode leakage into the lateral Si slab, limiting the photon-phonon overlap and the Brillouin gain. Here, we propose new waveguide designs based on subwavelength nanostructuration to tailor near-infrared photons and GHz phonons and maximize the Brillouin gain. We introduce six different geometries suitable for both membrane or fully suspended configurations (i.e., without transversal arms anchoring the core to the Si slab). Our three-dimensional optomechanical simulations predict that subwavelength silicon membranes with strip, slot, and SWG slot core waveguides achieve gains up to 12257 W-1m-1 at mechanical frequencies of 12-13 GHz. Moreover, suspended silicon waveguides with SWG slots achieve a high gain of 43542 W-1m-1 at 4.45 GHz, with the ability to adjust the mechanical frequency from 4 to 9 GHz. Further enhancements in the Brillouin gain are studied by integrating side arms to amplify the moving boundaries effect in the suspended SWG slot waveguides and leveraging the slow light regime, which can significantly increase the Brillouin gain up to 17 × 106 W-1m-1 for a mechanical mode at 11.18 GHz.

Preparation and loss analysis of Ge on Si SWIR optical strip waveguides

On-chip integrated photonics with operating wavelength at short-wave infrared region is becoming popular for its potential advantages in extending the telecommunication bandwidth and the application in gas sensing. As an important component of integrated optoelectronic chips, high-performance waveguides have attracted widespread attention. In this work, high-performance Ge waveguides were designed and fabricated on the Ge-on-Si platform, in which germanium films were epitaxially grown by the chemical vapor deposition process. The propagation loss for the waveguide was measured to be 2.34 dB/cm at the wavelength of 2 μm through the cut-back method. The quantitative relationship between the waveguide propagation loss and sidewall roughness as well as material defect density was established by introducing an empirical coefficient m. The m value was calculated to be 3.8 × 10−4 dB for the waveguide with a fixed Ge thickness of 1.5 μm by fitting the experimental propagation loss data at an operating wavelength of 2 μm. This formula provides a significant reference for future design of waveguide dimension structures, where defect induced absorption loss is non-negligible.

Ultrahigh Q SOI ring resonator with a Hermite or B-spline curved waveguide

Two new, to the best of our knowledge, designs for a huge, single-mode operation, ultrahigh quality factor SOI ring resonators based on the 90° bend of the B-spline curve and the Hermite curve were demonstrated. The ring resonators comprise multimode straight strip waveguides (MMWG), single-mode strip waveguides (SMWG), 90° bends, and tapers. The TE0-TE0 transmission loss of the bends can reach 0.000013 dB and 0.000091 dB, and the TE0-TE1 excitation ratios reach −48dB and −50dB, respectively, when the bends were designed using the Hermite curve or B-spline curve. Two SOI ring resonators of about 3.00 cm length consisting of the structures mentioned above are manufactured. The first was made up of Hermite curves with 90° bends. It could achieve a Q factor of 4.65×106 and a free spectrum range (FSR) of 0.025 nm. The second one employed B-spline curves with 90° bends. It could reach a Q factor of 4.37×106 and FSR of 0.022 nm.

Subwavelength Grating Cascaded Microring Resonator Biochemical Sensors with Record-High Sensitivity.

Photonic integrated circuit biochemical and biomedical sensors show promising applications in medical diagnosis, food security, healthcare, and environmental monitoring. Silicon-on-insulator subwavelength grating waveguides and cascaded microring resonator structures enhance photon-analyte interaction, offering superior sensing performance (higher sensitivity with lower limit of detection and larger free spectral range) compared to traditional strip and slot waveguide microring resonator structures. In this study, we design, simulate, and experimentally demonstrate a novel and compact biochemical sensor integrating subwavelength grating cascaded microring resonators and multibox subwavelength grating straight waveguides on a silicon-on-insulator platform. We achieve a record-high refractive index sensitivity of 810 nm/RIU with a limit of detection value of 2.04 × 10-5 RIU. The measured concentration sensitivity for sodium chloride solutions is 1430 pm/% with a limit of detection of 0.04%. The free spectral range is 35.8 nm, and the measured Q factor is 1.9 × 103. By combining the advantages of cascaded microring resonators with those subwavelength gratings, our sensor offers unprecedented sensitivity for biochemical sensing applications, promising significant enhancements in healthcare diagnostics and environmental monitoring systems.

Broadband and low-loss power splitter with polarization manipulation using subwavelength gratings

A broadband and low-loss power splitter with polarization manipulation using subwavelength gratings (SWGs) is proposed and analyzed, where SWGs are embedded in input taper/output inverse tapered waveguides in the bottom layer to form an SWG-based structure, and a silicon nitride layer is placed above the bottom layer to form a low-index guiding structure. In the bottom layer, the TE mode is cut off as the input strip waveguide is tapered down. In this way, the injected TE mode is evenly coupled to two adjacent output ports with high efficiency by the assistance of SWGs. By contrast, the input TM mode is well supported by the above silicon nitride layer and directly transmitted to the Through port, which is almost not influenced by the bottom structures. Consequently, both power splitting and polarization handling are achieved in the designed device simultaneously. Simulation results show that an extinction ratio (ER) of 32.63 dB (18.56 dB) and insertion loss (IL) of 0.16 dB (0.16 dB) for the TE (TM) mode are obtained at 1.55 µm. The bandwidth is up to 191 nm (1450–1641 nm) for the TE mode and 200 nm (1450–1650 nm) for the TM mode with an ER exceeding 15 dB and IL below 0.3 dB. Furthermore, fabrication tolerances and power distribution with polarization manipulation through the device are also presented.

Ultrawide dual contact strip fabricated quantum cascade lasers with reduced beam divergence angle.

Power scaling through the broadening of the laser core in Quantum Cascade Lasers can extract greater average power from devices at the cost of mode quality. To improve transverse mode quality for ultrawide (>100 µm) devices, a dual contact strip waveguide geometry is designed and demonstrated, utilizing a reduced top cladding thickness, and modifying the gold electrical epi-side contact to affect mode competition. A 200 µm wide Dual Contact Strip laser emitting at 4.6 µm wavelength is measured with far-field full-width half-maximum of 2.5° in a central single lobe. Relative an identical waveguide without the dual contact strips, peak power was 30% higher and far field was 32% narrower. This demonstrates the ability to achieve fundamental-like far field behavior in very broad quantum cascadel asers while potentially simplifying the fabrication method.

Efficient Optical Coupling between Dielectric Strip Waveguides and a Plasmonic Trench Waveguide

Efficient Optical Coupling between Dielectric Strip Waveguides and a Plasmonic Trench Waveguide

Theoretical and Experimental Analysis of Single-Arm Bimodal Plasmo-Photonic Refractive Index Sensors.

In this paper, we study both theoretically and experimentally the sensitivity of bimodal interferometric sensors where interference occurs between two plasmonic modes with different properties propagating in the same physical waveguide. In contrast to the well-known Mach-Zehnder interferometric (MZI) sensor, we show for the first time that the sensitivity of the bimodal sensor is independent of the sensing area length. This is validated by applying the theory to an integrated plasmo-photonic bimodal sensor that comprises an aluminum (Al) plasmonic stripe waveguide co-integrated between two accessible SU-8 photonic waveguides. A series of such bimodal sensors utilizing plasmonic stripes of different lengths were numerically simulated, demonstrating bulk refractive index (RI) sensitivities around 5700 nm/RIU for all sensor variants, confirming the theoretical results. The theoretical and numerical results were also validated experimentally through chip-level RI sensing experiments on three fabricated SU-8/Al bimodal sensors with plasmonic sensing lengths of 50, 75, and 100 μm. The obtained experimental RI sensitivities were found to be very close and equal to 4464, 4386, and 4362 nm/RIU, respectively, confirming that the sensing length has no effect on the bimodal sensor sensitivity. The above outcome alleviates the design and optical loss constraints, paving the way for more compact and powerful sensors that can achieve high sensitivity values at ultra-short sensing lengths.

Estimation of losses caused by sidewall roughness in thin-film lithium niobate rib and strip waveguides.

Samples of dielectric optical waveguides of rib or strip type in thin-film lithium niobate (TFLN) technology are characterized with respect to their optical loss using the Fabry-Pérot method. Attributing the losses mainly to sidewall roughness, we employ a simple perturbational procedure, based on rigorously computed mode profiles of idealized channels, to estimate the attenuation for waveguides with different cross sections. A single fit parameter suffices for an adequate modelling of the effect of the waveguide geometry on the loss levels.

Low-loss silicon waveguide and an ultrahigh-Q silicon microring resonator in the 2 µm wave band.

Silicon photonic-integrated circuits (PICs) operating in the 2 µm wave band are of great interest for spectroscopic sensing, nonlinear optics, and optical communication applications. However, the performance of silicon PICs in this wave band lags far behind the conventional optical communication band (1310/1550 nm). Here we report the realization of a low-loss waveguide and an ultrahigh-Q microring resonator in the 2 µm wave band on a standard 200 mm silicon photonic platform. The single-mode strip waveguide fabricated on a 220 nm-thick silicon device layer has a record-low propagation loss ∼0.2 dB/cm. Based on the low-loss waveguide, we demonstrate an ultrahigh-Q microring resonator with a measured loaded Q-factor as high as 1.1 × 106 and intrinsic Q-factor of 2 × 106, one order of magnitude higher than prior silicon resonators operating in the same wave band. The extinction ratio of the resonator is higher than 22 dB. These high-performance silicon photonic components pave the way for on-chip sensing applications and nonlinear optics in the 2 µm wave band.

High-quality on-chip entangled photon source with broad tunable range based on coupling compensation

This paper introduces a tunable and high-quality photon source that utilizes evanescent-wave coupling phase matching. By adjusting the coupling gap, the signal light can be tuned from 1307 nm to 1493.9 nm, and the idler light can be tuned from 1612.8 nm to 1907 nm. Throughout the entire tuning range, the purity of the photon pairs remains above 92%. In specific tuning ranges (signal photons from 1307 nm to 1421.7 nm and idler photons from 1706.4 nm to 1907 nm), the purity exceeds 99% and the full width at half-maximum of the generated photon spectra is less than 1.85 nm. The photon source is designed using a silicon–organic hybrid waveguide, which effectively minimizes the impact of two-photon absorption and achieves a 15 dB enhancement in four-wave mixing conversion efficiency compared to a strip waveguide. This design may promote efficient and precise generation of high-quality photons at desired frequencies, offering promising potential for various applications in quantum technologies.

High‐Efficiency Fiber‐Chip Edge Coupler for Near‐Ultraviolet Integrated Photonics

AbstractIntegrated photonics is demanded in applications operating at ultraviolet to visible wavelengths, such as atomic/quantum systems, on‐chip broadband receivers, and far‐field structured illumination autofluorescence microscopy. A fundamental challenge in these applications is efficient edge coupling from a single‐mode fiber (SMF) to on‐chip photonic components, which is critical for on‐chip integration. In this paper, a high‐efficiency edge coupler based on an alumina‐on‐insulator platform is introduced and experimentally validated. The coupler employs a symmetric double‐tip taper and a multimode interference (MMI)‐based optical combiner. The double‐tip taper effectively expands the mode field diameter at the chip facet to match that of the SMF at the initial stage. Then the MMI‐based combiner efficiently combines the two channels in the taper into a highly confined strip waveguide. A coupling loss of 2.85 dB/facet has been achieved for the transverse magnetic mode at the wavelength of 407 nm, which is the lowest insertion loss for fiber‐chip coupling on this platform to the best of the knowledge. The design can significantly reduce the insertion loss associated with fiber‐chip coupling, offering a key component for diverse areas ranging from atomic/quantum photonic integrated circuits and ultra‐high capacity communications to optical microscopes.

Dyakonov surface waves in dielectric crystals with negative anisotropy

Abstract Since the initial discovery of Dyakonov surface waves at a flat infinite interface of two dielectrics, at least one of which is positively anisotropic, extensive research has been conducted towards their theoretical and experimental studies in materials with positive anisotropy. The potential applications of these waves were initially limited due to the stringent conditions for their existence and the requirement for position anisotropy. In our study, we present the theoretical prediction and experimental observation of a novel type of Dyakonov surface waves that propagate along the flat strip of the interface between two dielectrics with negative anisotropy. We demonstrate that the conditions for surface waves are satisfied for negatively anisotropic dielectrics owing to the specific boundaries of the strip waveguide confined between two metallic plates. We study such modes theoretically by using the perturbation theory in the approximation of weak anisotropy and demonstrate that the electromagnetic field distribution in these modes is chiral. Experimental verification of theoretical predictions is made in the microwave range using 3D-printed negatively anisotropic water-dielectric metamaterial slabs. The existence of Dyakonov surface waves in negative crystals prompts a reassessment of the list of materials suitable for practical realization of these waves in the visible and infrared ranges. Due to the ability of the considered modes to transmit chiral light, they have potential in the sensing of chiral organic molecules.

Nanophotonic chalcogenide waveguides for supercontinuum generation pumped at 1550 nm

Nanophotonic chalcogenide waveguides (ChGs) fabricated from Ge28Sb12Se60 (GeSbSe) glass were designed and fabricated for efficient supercontinuum (SC) generation. Dispersion engineering and nonlinear optical simulations were used to optimize the cross-sectional dimensions of the strip waveguides for pumping at 1550 nm. The waveguides were fabricated using e-beam lithography and plasma etching to achieve submicron confinement and high optical quality. By pumping anomalous dispersion engineered waveguides with femtosecond pulses at 1550 nm wavelength, supercontinuum generation spanning over 750 nm was demonstrated on-chip. The findings provide experimental evidence of the potential applications of GeSbSe waveguides in nonlinear photonics, highlighting their advantages in terms of performance and spatial efficiency.

Generation of entangled waveguided photon pairs by free electrons.

Entangled photons are a key resource in quantum technologies. While intense laser light propagating in nonlinear crystals is conventionally used to generate entangled photons, such schemes have low efficiency due to the weak nonlinear response of known materials and losses associated with in/out photon coupling. Here, we show how to generate entangled polariton pairs directly within optical waveguides using free electrons. The measured energy loss of undeflected electrons heralds the production of counter-propagating polariton pairs entangled in energy and emission direction. For illustration, we study the excitation of plasmon polaritons in metal strip waveguides that strongly enhance light-matter interactions, rendering two-plasmon generation dominant over single-plasmon excitation. We demonstrate that electron energy losses detected within optimal frequency ranges can reliably signal the generation of plasmon pairs entangled in energy and momentum. Our proposed scheme is directly applicable to other types of optical waveguides for in situ generation of entangled photon pairs.

Design of ultrahigh-Q silicon microring resonators based on free-form curves.

A design method for ultrahigh-Q microring resonators (MRRs) based on Bezier free-form curves was proposed and demonstrated. An MRR consisting of a specially designed 180° waveguide bend, a directional coupler, and two low-loss multi-mode strip waveguides was designed. The free-form curves were used to increase the degree of freedom in the design, shaping the waveguide bend with a gradient width and curvature. This design effectively reduced the propagation loss caused by the roughness of waveguide sidewalls and the mode mismatch loss caused by the excitation of high order modes. The small effective radius of only 20µm enabled the MRR to have a large free spectral range (FSR) and a compact and flexible structure. The MRR was manufactured using a standard process provided by foundry and measured to have an ultrahigh loaded Q factor of 1.86 × 106 and a FSR of about 1 nm.

Non-uniform distributed silicon optical phased array for high directionality and a wide steering range.

A non-uniform distributed silicon optical phased array (OPA) is proposed and numerically demonstrated to realize high directionality and a wide range for beam steering. The OPA is composed of grating antennas with dual-layer corrugations along silicon strip waveguides, which can achieve a high directionality of 0.96 and a small divergence angle of 0.084°. To reduce the crosstalk between adjacent antennas and realize a wide steering range, the genetic algorithm is improved and utilized to arrange the locations of grating antennas. As a proof of concept, a 32-channel non-uniform distributed OPA is designed and thoroughly optimized. The simulation results successfully demonstrate a two-dimensional wide steering range of 70∘×18.7∘ with a side-mode suppression ratio (SMSR) over 10dB.

A new SOI sensor design for detecting cancer using hybrid waveguide with higher sensitivity than both strip and slot waveguides

A new highly accurate optical biomedical sensor is proposed for cancer detection characterized by high sensitivity, small footprint, low cost, and low limit of detection. The sensor is based on double-ring resonators made of silicon on insulator. The type of the waveguide is critical in determining the sensor performance. To compromise the advantages and disadvantages of strip and slot waveguides, a mixed design of both has been introduced in literature at the expense of increased footprint compared to traditional sensors. Indeed, almost 27-fold footprint increase is required to improve the sensitivity by only one third of that of slot waveguide’s sensitivity. In this paper, we introduce a new design that keeps the same footprint of traditional sensors, while achieving high sensitivity. This sensitivity depends on the resonance wavelength shift due to different refractive indices of the biosample. It has the value 109.8 nm/RIU compared to 55.57 nm/RIU and 129.621 nm/RIU for strip and slot waveguides, respectively. The hybrid waveguide quality factor is 537.7 while the quality factors of the strip and the slot waveguides are 627.99 and 380.76, respectively. In addition, the new design achieves the minimum limit of detection (0.0255) when compared to that of traditional designs. Furthermore, a new method of detection is proposed with the same design, providing a higher sensitivity over both traditional waveguide types with a value of 15.3, compared to 13.2 and 11.5 for strip and slot waveguides, respectively. In this method, the sensitivity relies on various values of output transmitted light at the same wavelength due to altering the biocell refractive index. The biosensor output equation is developed. In addition, the relationship between the supermodes and the sensitivity is determined at variance conditions. It is found that there is an inverse relation between them.

Machine learning assisted strip waveguide Bragg gratings design for refractive index-based biosensing applications

Waveguide Bragg grating (WBG) biosensors have attracted significant interest because of their high sensitivity, immunity to electromagnetic signals, and capability for lab-on-chip applications. Designing a WBG biosensor requires solving Maxwell’s equations for the optical waveguide geometries numerically. This process is resource-intensive and hinders quick optimization of the waveguides for sensing. In this work, we explore the design of a silicon strip WBG structure for biosensing using machine learning. We develop an artificial neural network (ANN) classification model for mode classification and a regression model for predicting the effective refractive index (neff) for a given strip waveguide. We utilize results from finite element method and Transfer Matrix method for strip WBG structures to train a multi-output ANN regression model for quickly predicting sensing parameters. The mode classification model provides an accuracy of over 99%, and the regression model predicts the neff with a mean absolute error (MAE) of 0.5%. The multi-output regression model predicts the sensitivity with 0.5% MAE and quality factor and maximum reflectivity with less than 3% MAE. ANN model training requires a few minutes of computational time and reduces the dependency on computationally expensive resources for optimization of the sensor design, thereby fast-tracking the biosensor design cycle.