Subwavelength Grating Waveguide Research Articles

Compact adiabatic mode (de)multiplexer using a subwavelength grating waveguide in the silicon-on-insulator platform

Mode-division multiplexing technology holds great promise for addressing the ever-increasing demand for data transmission capacity. This paper proposes a compact adiabatic mode (de)multiplexer [(de)MUX] using a subwavelength grating (SWG) waveguide in the silicon-on-insulator platform, with optimization for the TE0-TE1 and TE0-TE2 mode (de)MUXs. By introducing an SWG waveguide, the coupling strength of waveguides can be enhanced, making it possible for the realization of more compact adiabatic mode (de)MUXs. The adiabatic evolution lengths for TE0-TE1 and TE0-TE2 mode (de)MUXs are only 10 µm and 17.5 µm, respectively. The numerical simulation shows that the operating bandwidth of the proposed device is beyond 200 nm. For the TE0-TE1 mode (de)MUX, the insertion loss and mode crosstalk of TE0-TE1 mode conversion within the whole wavelength range of 1450–1650 nm are less than 0.25 dB and −32.0dB, respectively. For the TE0-TE2 mode (de)MUX, the insertion loss and mode crosstalk of TE0-TE2 mode conversion within the entire bandwidth are less than 0.26 dB and −27.4dB, respectively. The TE0-TE1 and TE0-TE2 mode (de)MUXs can be cascaded to form a three-mode (de)MUX. For TE0-TE0, TE0-TE1, and TE0-TE2 processes within the entire bandwidth, the mode crosstalks are less than −33.5dB, −26.1dB, and −25.8dB, respectively. The device is scalable to the conversion of TE0 mode to higher-order mode.

Ultrasensitive Silicon Photonic Refractive Index Sensor Based on Hybrid Double Slot Subwavelength Grating Microring Resonator.

Silicon photonic-based refractive index sensors are of great value in the detection of gases, biological and chemical substances. Among them, microring resonators are the most promising due to their compact size and narrow Lorentzian-shaped spectrum. The electric field in a subwavelength grating waveguide (SWG) is essentially confined in the low-refractive index dielectric, favoring enhanced analyte-photon interactions, which represents higher sensitivity. However, it is very challenging to further significantly improve the sensitivity of SWG ring resonator refractive index sensors. Here, a hybrid waveguide blocks double slot subwavelength grating microring resonator (HDSSWG-MRR) refractive index sensor operating in a water refractive index environment is proposed. By designing a new waveguide structure, a sensitivity of up to 1005 nm/RIU has been achieved, which is 182 nm/RIU higher than the currently highest sensitivity silicon photonic micro ring refractive index sensor. Meanwhile, utilizing a unique waveguide structure, a Q of 22,429 was achieved and a low limit of detection of 6.86 × 10-5 RIU was calculated.

Subwavelength Grating Waveguide Supported by a Magnetic Dipole Mode

Subwavelength Grating Waveguide Supported by a Magnetic Dipole Mode

Artificial neural network assisted the design of subwavelength-grating waveguides for nanoparticles optical trapping.

In this work, we propose artificial neural networks (ANNs) to predict the optical forces on particles with a radius of 50 nm and inverse-design the subwavelength-grating (SWG) waveguides structure for trapping. The SWG waveguides are applied to particle trapping due to their superior bulk sensitivity and surface sensitivity, as well as longer working distance than conventional nanophotonic waveguides. To reduce the time consumption of the design, we train ANNs to predict the trapping forces and to inverse-design the geometric structure of SWG waveguides, and the low mean square errors (MSE) of the networks achieve 2.8 × 10-4. Based on the well-trained forward prediction and inverse-design network, an SWG waveguide with significant trapping performance is designed. The trapping forces in the y-direction achieve-40.39 pN when the center of the particle is placed 100 nm away from the side wall of the silicon segment, and the negative sign of the optical forces indicates the direction of the forces. The maximum trapping potential achieved to 838.16 kBT in the y-direction. The trapping performance in the x and z directions is also quite superior, and the neural network model has been further applied to design SWGs with a high trapping performance. The present work is of significance for further research on the application of artificial neural networks in other optical devices designed for particle trapping.

Broadband mode-division (de)multiplexer using nanorod-assisted multimode subwavelength gratings

—Subwavelength grating (SWG) allows flexible tuning of the dispersion of waveguides, enabling wider bandwidths for directional coupler-based devices. However, its light field constraint capability for high-order modes is so weak that the mode scattering loss from sidewalls is high, which impedes low-loss transmission and efficient energy coupling from adjacent waveguides. In this paper, we propose nanorod-assisted multimode subwavelength grating (NSWG) waveguides that support stable high-order modes transmission like strip waveguides, and maintain refractive index tunability similar to SWGs. We fully analyze the mode transmission and coupling characteristics of NSWG waveguides for modes TE1, TE2, and TE3. Results show that such waveguides can hold high transmission fidelity and low losses for high-order modes (<0.33 dB/cm, 0.58 dB/cm, and 1.0 dB/cm for TE1, TE2, and TE3). Arbitrary high-order modes among NSWGs can also be excited efficiently by evanescent field coupling. As proofs of concept, we also demonstrate a three-channel mode-division multiplexer with broad bandwidth (136 nm/135 nm/100 nm), low insertion loss (0.29 dB/0.85 dB/0.62 dB), and crosstalk (-21.4 dB/-20.4 dB/-18.0 dB) for TE0/TE1/TE2. And the device has a compact footprint of 5 × 61 μm. At 1.55 μm, all three modes satisfy IL < 0.19 dB and CT < -20.0 dB. The proposed waveguide structures may provide an alternative building block for broadband and compact multimode on-chip directional coupler-based devices.

Silicon Subwavelength Grating Slot Waveguide based Optical Sensor for Label Free Detection of Fluoride Ion in Water

Recently, Subwavelength Grating Waveguides (SWG WG) attract a lot of attention for sensing applications because it offers better sensitivity than conventional WGs. The sensitivity of SWG WG is greatly improved due to the interaction between the analyte and guiding mode in the SWG segments. In this study, a highly sensitive sensor based on a silicon SWG slot waveguide (SWGSW) is proposed for detecting low-concentration fluoride contaminations in water. The gap called as “slot” between the silicon slab along with cladding and the spaces between the segmented silicon rail waveguide is filled with fluoride ion-contaminated water samples as analyte for strong light–matter interaction between guiding modes of the SWG structure, resulting in greater sensitivity of the proposed sensor. The sensitivity of the proposed structure is estimated by numerical simulations as S = 615.38 nm/RIU which is comparable to the existing reported optical sensor structures based on optical waveguide and optical fibre with a smaller footprint. Additionally, the effect of variation of geometrical parameters on the transmission resonances of the proposed structure is also analyzed for the optimization of a SWG WG structure. The obtained results will be helpful to realize and design highly sensitive sensors based on the SOI platform.

Subwavelength grating-based silicon photonic TE mode division multiplexer for C + L band operation

This paper reports a subwavelength grating (SWG) based multiplexer (MUX) on a silicon photonics platform capable of multiplexing three transverse electric modes. The designed MUX is simulated using a commercial 3D finite-difference time-domain solver and shows broadband operation over the whole C and L optical telecom bands from 1530 nm to 1625 nm wavelength range. The effective indices of the Bloch modes in the SWG waveguides are extracted from the band structure plot. The designed MUX consists of two co-directional coupling regions for fundamental to higher-order mode coupling, with each coupling stage consisting of single-mode and multimode SWG waveguides. The transmission characteristics, viz. transmittance, insertion loss, and return loss, are presented and discussed. The coupling lengths without the tapering regions for TE0–TE1 and TE0–TE2 mode couplings are 14μm and 1.48μm, respectively. The transmittance is >78% with the highest insertion loss and return loss of 1.1 dB and –15 dB, respectively. At 1550 nm, the transmission is >88%, insertion loss is <0.6 dB, and return loss is <−15 dB. A uniform under-etch and over-etch of 5 nm are taken for the fabrication tolerance study, which shows a maximum variation of 0.58 dB for the insertion loss with return loss <−14.6 dB at 1550 nm. Over the whole simulated range, the insertion loss is <1.4 dB, and return loss is <−14.6 dB with ±10 nm change in device dimension. A temperature tolerance study with 50 °C and 100 °C rise in temperature has been done, and the device retains its broadband operation over the simulated range. The maximum increase in insertion loss is 0.1 dB for the TE0–TE2 coupling, while the overall return loss of the device decreases to <−20 dB for the TE0–TE1 coupling.

All-silicon TM polarizer covering the 1260-1675 nm bandwidth using a band engineered subwavelength grating waveguide.

A TM polarizer working for whole optical communication bands with high performance is proposed on a 220-nm-thick silicon-on-insulator (SOI) platform. The device is based on polarization-dependent band engineering in a subwavelength grating waveguide (SWGW). By utilizing an SWGW with a relatively larger lateral width, an ultra-broad bandgap of ∼476 nm (1238 nm-1714nm) is obtained for the TE mode, while the TM mode is well supported in this range. Then, a novel tapered and chirped grating design is adopted for efficient mode conversion, which results in a polarizer with a compact footprint (3.0 µm × 18 µm), low insertion loss (IL < 1.15 dB) and high polarization extinction ratio (PER > 21 dB) covering O-U bands (1260 nm-1675 nm). Experimental results show that the fabricated device has an IL < 1.0 dB and PER > 22 dB over a 300- nm bandwidth, which is limited by our measurement setup. To the best of our knowledge, no TM polarizer on the 220-nm SOI platform with comparable performance covering O-U bands has ever been reported.

Anisotropic leaky-like perturbation with subwavelength gratings enables zero crosstalk

Electromagnetic coupling via an evanescent field or radiative wave is a primary characteristic of light, allowing optical signal/power transfer in a photonic circuit but limiting integration density. A leaky mode, which combines both evanescent field and radiative wave, causes stronger coupling and is thus considered not ideal for dense integration. Here we show that a leaky oscillation with anisotropic perturbation rather can achieve completely zero crosstalk realized by subwavelength grating (SWG) metamaterials. The oscillating fields in the SWGs enable coupling coefficients in each direction to counteract each other, resulting in completely zero crosstalk. We experimentally demonstrate such an extraordinarily low coupling between closely spaced identical leaky SWG waveguides, suppressing the crosstalk by ≈40 dB compared to conventional strip waveguides, corresponding to ≈100 times longer coupling length. This leaky-SWG suppresses the crosstalk of transverse–magnetic (TM) mode, which is challenging due to its low confinement, and marks a novel approach in electromagnetic coupling applicable to other spectral regimes and generic devices.

Polarization independent Bragg gratings using tilted subwavelength grating waveguide Bragg gratings.

We propose and experimentally demonstrate a polarization independent subwavelength grating (SWG) waveguide Bragg grating (WBG) by using an SWG waveguide with tilted segments. By optimizing the tilting angle and other geometry parameters, such as the width and the length of the loading segments used to create the BG, we can obtain a zero birefringence tilted SWG waveguide and consequently, a polarization independent SWG WBG. In our simulations, the optimal tilting angle is ∼ 58°, whereas the optimal angle obtained in fabrication is ∼ 46°. This deviation is mainly due to fabrication errors, e.g., on the sidewall angle of the silicon segments. For the optimal tilting angle of 46°, the characterized Bragg wavelengths of the TE and TM modes are both ∼ 1517 nm. We believe that the proposed device can have applications in optical communications and interconnections.

Integrated Subwavelength Grating Waveguide Bragg Gratings for Microwave Photonics

Bragg gratings (BGs) have numerous applications in microwave photonic (MWP) signal processing and MWP systems due to the ability to obtain custom-designed amplitude and phase responses. Moreover, waveguide BGs (WBGs) are compact and can be readily integrated with other passive and active components to realize MWP building blocks and processing systems. Subwavelength grating (SWG) waveguides and passive devices in silicon-on-insulator (SOI) have emerged as an enabler for developing break-through devices based on anisotropic metamaterials. In this paper, we review recent advances on developing SWG WBGs in SOI, including chirped, phase-shifted, sampled, and polarization (in)dependent structures. Applications of these SWG WBGs in MWP are described.

Polarization Insensitive Edge Coupler Assisted by Subwavelength Grating and Suspended Structure

Based on the Si-Si3N4 hybrid photonic platform, an integrated edge coupler embedded with both sub-wavelength grating (SWG) waveguide and suspended structure was demonstrated. The proposed device structure and geometrical parameters were fully optimized to realize efficient optical interconnections with standard single-mode fiber (SMF-28). The coupling loss of 0.86 dB/port for TE mode and 0.94 dB/port for TM mode was achieved with a remarkable polarization dependent loss (PDL) less than 0.1 dB at the input wavelength of 1600 nm. It revealed that the integrated edge coupler assisted by sub-wavelength grating can further reduce the PDL. Moreover, the device also exhibited the advantages of large alignment tolerance. The alignment tolerances for 1-dB excess loss are ±1.8 and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pm 2.5~\mu \text{m}$ </tex-math></inline-formula> in horizontal and vertical direction, respectively.

Integrated Subwavelength Gratings on a Lithium Niobate on Insulator Platform for Mode and Polarization Manipulation (Laser Photonics Rev. 16(7)/2022)

Subwavelength Gratings In article number 2200130, Yonghui Tian and colleagues experimentally demonstrate compact subwavelength grating (SWG) waveguides on a LNOI platform for on-chip mode and polarization manipulation. To overcome the limitation of direct etching waveguide fabrication, a silicon nitride thin film was deposited onto the surface of LNOI chip, where the SWGs were formed using CMOS-compatible etching processes developed by microelectronics industry.

Subwavelength Grating Waveguide Structures Proposed on the Low-Cost Silica-Titania Platform for Optical Filtering and Refractive Index Sensing Applications.

The sol–gel dip-coating method is a cost-efficient way for the realization of thin films on a planar substrate. In this work, high-quality, low-loss, and low-surface roughness silica–titania thin films are deposited on a glass substrate with the sol–gel dip-coating method. This platform works in the visible to near-IR wavelength ranges and can be useful for several eye-catching photonic components. The paper is comprised of two parts: the first part deals with the development of a low-cost silica–titania waveguide system, whereas the second part provides detail on the numerical modeling of the SWG waveguide filter and SWG waveguide FP-sensor design. The SWG waveguide NIR-stopband filter can achieve an ER of >40 dB and 3-dB bandwidth of 110 nm designed at optimized parameters. The SWG waveguide-FP structure proposed in this work act as a refractive index sensor where the sensitivity is ~120 nm/RIU by reducing the width of the waveguide. This sensitivity can be further enhanced by reducing the waveguide height. We believe that this work is quite important for the realization of low-cost integrated photonic devices based on the silica–titania platform developed via the sol–gel dip-coating method.

Integrated Subwavelength Gratings on a Lithium Niobate on Insulator Platform for Mode and Polarization Manipulation

AbstractLithium niobate on insulator (LNOI) has emerged as a promising platform for photonic integrated circuits, with a fast‐growing toolbox of components. This paper proposes, designs, and experimentally demonstrates compact subwavelength grating (SWG) waveguides on an LNOI platform for on‐chip mode and polarization manipulation. To overcome the limitation of waveguide fabrication, the SWGs are designed and formed on a silicon nitride thin film deposited onto the surface of LNOI chip. As proof‐of‐concept devices, the SWG‐based spatial mode filters (including a TE1‐mode‐pass filter and a TE2‐mode‐pass filter) and a TM‐pass polarizer are fabricated successfully on the same chip, with the device lengths of only ≈50 μm. The measured insertion losses for the devices are lower than 3.1 dB, with high extinction ratio larger than 30 dB, at a wavelength of 1550 nm. The proposed and demonstrated SWGs can serve as important building blocks in a series of mode and polarization handling devices for LNOI integrated photonics.

Highly Sensitive Refractive Index-Based Sensor for DNA Hybridization Using Subwavelength Grating Waveguide

A biosensor for detecting DNA hybridization via refractive index sensing has been demonstrated using a subwavelength grating (SWG) waveguide. Waveguide dimensions are selected by parametric optimizations to increase the mode-analyte overlap with considerations of typical silicon-on-insulator fabrications technology. DNA layer is added with a linker layer on silicon pillars of the SWG waveguide to optimize and detect the DNA hybridization. Some essential characteristics such as mode overlap factor, change in effective refractive index, waveguide sensitivity, and shift in resonance wavelength are computed for the different dimensional parameters of the SWG waveguide in DNA hybridization along with mode field intensity and normalized power using the finite element method. The DNA hybridization is shown by variation in normalized power of interacting light in cladding region of the waveguide for 0% to 100% fractional change of dsDNA and ssDNA. Waveguide sensitivity, shift in resonance wavelength, device sensitivity, and intrinsic limit of detection are obtained to ∼ 0.157, 7.01, 605 nm/RIU, and 1.30 × 10−4 RIU, respectively, for the optimized structure of SWG waveguide in sensing of DNA hybridization, which could be suitable aspects for detecting DNA hybridization.

Proposal and analysis of ultra-high amplitude-sensitive refractive index sensor by thick silicon multi-slot sub-wavelength Bragg grating waveguide

In this paper, we proposed an ultra-high amplitude-sensitive multi-slot sub-wavelength Bragg grating (MS-SW BG) waveguide sensor on silicon on insulator (SOI) platform with a thick silicon waveguide (WG) and analyzed the design and performances. The MS-SW BG sensor with a thick waveguide supports multi-transverse modes for the first and second modes, whose stopbands (SBs) have different center wavelengths and bandwidths to form a mid-transmission band (MTB) between these SBs. By adjusting sensor parameters, the MTB can be made sharp and steep, and these spectral profiles shift in response to a refractive index change in the surrounding medium. Due to the sharp spectral feature of the MTB, ultra-high amplitude-sensitive performance can be realized. The sensor length is as short as about 10 μm for a grating period count of 21. With an additional increase of the period count the transmission peak count in the MTB region increases, and the shortest wavelength peak among them exhibits steeper slope which is suitable for higher sensitivity. The appearance of the MTB is explained by coupled mode theory and band structure approach. The structural design is investigated from viewpoints of WG thickness, WG width, duty ratio of the grating and so on to demonstrate an ultra-high amplitude sensitivity of 5000 RIU−1 with transmission loss limit up to 3 dB.

Design of highly sensitive interferometric sensors based on subwavelength grating waveguides operating at the dispersion turning point

In this paper, a dual-mode interferometric sensor based on a subwavelength grating (SWG) waveguide is proposed. The design operates at the dispersion turning point (DTP) of the two dispersive TE modes in the periodic structure, exhibiting a group velocity difference of almost zero, to improve the performance of refractive index (RI) sensors. The necessary conditions for realizing DTP in SWG are analyzed. Moreover, the interference spectrum characteristics and the influence of structural parameters on gas RI sensing performance are further studied. The results show that the sensitivity value of the proposed sensor can reach ∼ 38 , 000 n m / R I U . The proposed structure can also achieve a gas pressure sensitivity of ∼ 105 n m / M P a . The proposed sensors provide a potential platform for highly sensitive RI sensors.

Millimeter-long metamaterial surface-emitting antenna in the silicon photonics platform.

Integrated optical antennas are key components for on-chip light detection and ranging technology (LIDAR). In order to achieve a highly collimated far field with reduced beam divergence, antenna lengths on the order of several millimeters are required. In the high-index contrast silicon photonics platform, achieving such long antennas typically demands weakly modulated gratings with lithographic minimum feature sizes below 10 nm. Here, we experimentally demonstrate a new, to the best of our knowledge, strategy to make long antennas in silicon waveguides using a metamaterial subwavelength grating (SWG) waveguide core loaded with a lateral periodic array of radiative elements. The mode field confinement is controlled by the SWG duty cycle, and the delocalized propagating mode overlaps with the periodic perturbations. With this arrangement, weak antenna radiation strength can be achieved while maintaining a minimum feature size as large as 80 nm. Using this strategy, we experimentally demonstrate a 2-millimeter-long, single-etched subwavelength-engineered optical antenna on a conventional 220 nm SOI platform, presenting a measured far-field beam divergence of 0.1° and a wavelength scanning sensitivity of 0.13°/nm.

Reconfigurable bandpass optical filters based on subwavelength grating waveguides with a Ge2Sb2 Te5 cavity

We demonstrate a reconfigurable optical bandpass filter based on subwavelength grating (SWG) waveguide operating in the Bragg reflection regime and integrated with a cavity of the phase-change material G e 2 S b 2 T e 5 (GST). Partial crystallization of GST provides us with an efficient tool to modify the effective optical properties of the GST governed by the effective medium theory. Consequently, the resonance wavelength as well as the transmission peak can be tuned in the designed filters. Numerical simulations indicate that the presented SWG waveguide with a single GST cavity offers up to 8.8 nm redshift while the transmission amplitude can be modulated from 0.544 to 0.007. The presented Fabry–Perot structure can also be used as a nonvolatile optical switch with a high extinction ratio of about 24 dB at the wavelength of 1548.3 nm.