Fabrication Tolerances Research Articles

Measuring dielectric and electro-optic responses of thin films using plasmonic devices.

This paper introduces a simple method for the measurement of the relative permittivity and the Pockels coefficient of electro-optic (EO) materials in a waveguide up to sub-THz frequencies. By miniaturizing the device and making use of plasmonics, the complexities of traditional methods are mitigated. This work elaborates the fabrication tolerance and simplicity of the method, and highlights its applicability to various materials, substrates and configurations. The method is showcased using drop-casted perovskite barium titanate (BaTiO3, BTO) nano-particle thin-films and it has previously been used to measure epitaxial thin film BTO. In this work we show the effective relative permittivity of drop casted BTO to be εeff ∼ 30 at 200 MHz, dropping to ∼ 18 at 67 GHz and similarly, the effective Pockels coefficient was found to be reff ∼ 16 at 350 MHz and ∼ 8 at 70 GHz. These values are a factor > 50 below the values found for thin film BTO. Yet, the fact that the method can be applied to such different samples and Pockels strengths gives testimony to its versatility and sensitivity.

Plasmonic slanted slit gratings for efficient through-substrate light-plasmon coupling and sensing

We present an experimental study of plasmonic slanted slit gratings (PSSGs) designed to achieve directional coupling between an incident light beam and surface plasmon polaritons (SPPs) propagating along the surface of the structure. We also investigate mirrored PSSG pairs interconnected by a plasmonic slab waveguide. The structures are fabricated using direct milling by a gallium focused ion beam (FIB). In a mirrored pair arrangement, the first PSSG couples a perpendicularly-incident light beam to SPPs propagating in one direction along the waveguide, while the second PSSG decouples SPPs to perpendicularly-emerging light. This configuration shows promise for sensing applications due to the high sensitivity of the excited SPPs to changes in the refractive index of the bounding medium, and the separation of the optics from the fluidics by the substrate. The design also exhibits robustness to fabrication tolerances. The optical characteristics and sensing potential are investigated theoretically and experimentally, highlighting its potential for a wide range of applications.

Low-loss hollow-core fiber with stadium-shaped nested tubes for near-infrared communications

We propose a hollow-core fiber structure consisting of multilayer nested stadium-shaped tubes (HC-SSF), designed to reduce attenuation in the near-infrared bands for communications. The HC-SSF exhibits low loss characteristics, resulting in a four-order-of-magnitude attenuation reduction in confinement loss. The proposed HC-SSF achieves a confinement loss of 3.42×10−9 dB/m at 1.30 μm, while maintaining favorable single-mode characteristics and excellent bend resistance. At a bend radius of 8 cm, the HC-SSF demonstrates a bend-introduced loss of 1.72×10−7 dB/m at 1.30 μm. The total loss of HC-SSF remains below 0.1 dB/km within the 1.15 μm to 2.0 μm range. Due to its high fabrication tolerance and simple structure, the HC-SSF offers valuable reference for the extensive application of hollow-core fiber in the field of communication. It has a wide range of promising applications in gas and liquid sensing, and optical communications.

A systematic approach for the development of wide band band pass filters using resonators

In this paper, for the first time a novel and systematic approach is presented to construct a wideband Band Pass Filters (BPFs) with high selectivity and stopband response. Two BPFs (Filter I and Filter II) were built on a Taconic (TLX-9) substrate using semi-circular tapered coupled lines, Defected Microstrip Structures (DMSs), Defected Ground Structures (DGS), and Stepped Impedance Resonators (SIRs). The precision of the design procedure is confirmed through 3-D full wave Electro-Magnetic (EM) simulation software where two wideband filters were designed for L, S, and C-bands. Additionally, parametric analysis has been done to evaluate the performance of both wideband filters with key features such as return loss, insertion loss, Band Width (BW), selectivity, and stopband response. Even-odd mode analysis has been implemented using the ABCD matrix that acts as a source for mathematical analysis of Filter I. Eventually, two wideband filters have been fabricated and tested for their effectiveness, and the measured results of two wideband filters are in line with the simulated ones with slight deviations due to fabrication tolerances and connector losses. The prototyped Filter I functions as a bandpass in the frequency range of 2.4 GHz to 4.8 GHz with a maximum insertion loss of 0.85 dB and return loss of 19.5 dB thereby providing 64% Fractional Band Width (FBW). Near 19 dB rejection is observed in the lower and upper stopband regions with BW (stopbands) greater than 2 GHz. Moreover, Filter II is also fabricated and tested. The experimental return loss of 20 dB and insertion loss (maximum) of 0.73 dB over the passband (1.9 GHz to 3.4 GHz) were found with 0.7 Selectivity Factor (SF). More than 20 dB attenuation of stopbands (lower and upper) was achieved. Thus, the fabricated wideband BPFs operate in L, S, and C-bands for radars, satellite and long-distance radio communications, and military telemetry applications.

Highly Sensitive Refractive Index Sensor based on Silicon Nitride Strip Waveguide Directional Coupler

We propose a refractive index sensor, based on grating-assisted directional coupler, by comprising two coupled asymmetric strip waveguides with a top-loaded grating structure. We have studied variation of the sensitivity with the waveguide parameters to optimize the design of the sensor. The proposed sensor is an on-chip device with a high refractive index sensitivity of 104nm/RIU with $ \pm $ 140 nm fabrication tolerance, which is more than two times higher than the reported work, and the sensor can also achieve the ultrahigh sensitivity 105nm/RIU with the fabrication tolerance $ \pm $ 13nm. The sensor has large dynamic range from refractive index 1.0 to 1.55. We have also studied the propagation loss due to side wall roughness of the strip waveguide and found that it does not significantly affect sensor performance.

Ultra-wideband thin frequency-selective surface absorber against sheet resistance fluctuation

The design of thin frequency selective surface (FSS) absorber based on resistive film that meets the requirements of broadband, polarization independence, incident angle stability, and strong absorption is a challenging task. Fabrication tolerance of resistive film can result in fluctuations in sheet resistance, which negatively affects the absorber performance. To tackle these problems, this work firstly investigates how sheet resistance fluctuations affect the absorbing performance of resistive film FSS absorber. The analysis of simulated surface current density distribution and impedance reveals that the diversity of current paths provides an effective way to mitigate the influence of sheet resistance fluctuation. This is achieved by enabling flexible variation of surface current in response to sheet resistance fluctuations. Consequently, the variation of input impedance of the FSS absorber due to the fluctuation of sheet resistance is suppressed within a small range. Then, a method of extending bandwidth is proposed by employing the complementary variation of FSS impedance with frequency at different layers. By combining this approach with a miniaturization design, a thin and light FSS absorber is developed that exhibits ultra-wide bandwidth, polarization independence and angle stability while mitigating the effects of sheet resistance perturbation. The proposed FSS absorber achieves a 90% absorption bandwidth from 1.50 GHz to 20.50 GHz, covering Ku, X, C, S bands and part of the L and K bands, with a relative bandwidth reaching 173%. The absorber has a thickness of 0.093<i>λ</i><sub>L</sub> for both transverse electric (TE) polarization and transverse magnetic (TM) polarization, yielding a figure of merit (FoM, the ratio of the theoretical minimum thickness to the actual thickness) of 0.95, indicating that the thickness is close to the theoretical limit. The absorber maintains over 90% absorption rate for TM polarization at an incidence angle of up to 70°, and 80% absorption for TE polarization at 45°. Furthermore, the 90% absorbance bandwidth of the absorber remains at 167.0% when the sheet resistance of any FSS layer fluctuates within a range from 12 to 30 Ω/sq. A prototype of the proposed FSS absorber is fabricated and measured, and the experimental results are in good agreement with the simulation results, thus validating the effectiveness of the proposed method.

High-efficiency edge couplers enabled by vertically tapering on lithium-niobate photonic chips

In the past decade, photonic integrated circuits (PICs) based on thin-film lithium niobate (TFLN) have made substantial progress in various fields, including optical communication, nonlinear photonics, and quantum optics. A critical component is an efficient edge coupler facilitating the connection between PICs and light sources or detectors. Here, we propose an innovative edge coupler design with a wedge-shaped TFLN waveguide and a silicon oxynitride cladding. Experimental results show a low coupling loss between the TFLN PIC and a 3-μm mode field diameter (MFD) lensed fiber, measuring at 1.52 dB/facet, with theoretical potential for improvement to 0.43 dB/facet. Additionally, the coupling loss between the edge coupler and a UHNA7 fiber with an MFD of 3.2 μm is reduced to 0.92 dB/facet. This design exhibits robust fabrication and alignment tolerances. Notably, the minimum linewidth of the TFLN waveguide of the coupler (600 nm) can be readily achieved using commercially available i-line stepper lithography. This work benefits the development of TFLN integrated devices, such as on-chip electro-optic modulators, frequency combs, and lasers.

Compact polarization beam splitter based on metal-capped thin-film LiNbO3-on-insulator plasmonic waveguide

A compact polarization beam splitter based on a triple-waveguide plasmonic directional coupler is proposed and modeled. The directional coupler consists of two usual lithium-niobate-on-insulator (LNOI) ridge waveguides and a metal-capped plasmonic LNOI waveguide located between them. It utilizes stronger polarization dependence of modal birefringence of a plasmonic waveguide. By appropriately designing the plasmonic waveguide geometry, the TM component wave meets the resonant coupling condition and outputs at cross port. Instead, the TE component wave does not meet the resonant coupling condition, cannot be resonant in the triple-waveguide directional coupler and outputs at through port directly. Excellent performance is expected for an optimal device. First, the device shows an ultra-high polarization extinction ratio(PER) of 28.3 dB and an ultra-low insertion loss(IL) of 0.06 dB for the TE-polarized wave, and of 31.7 dB and of 0.31 dB in order for the TM-polarized wave. Second, the working bandwidth with PER > 25 dB and IL < 0.08 dB is 100 nm (1500–1600 nm) for the TE-polarized wave, and that with PER > 20 dB and IL < 0.4 dB is 30 nm (1534–1564 nm) for the TM-polarized wave. Third, the device has a compact structure of 33 μm in length and exhibits definite fabrication tolerance. Overall, the proposed device exhibits ultra-high PER and ultra-low IL, and has a compact size suitable for on-chip use. A comparison shows that present device exhibits advantage in IL and/or size over that based on other LNOI structures or the SOI platform.

High-responsivity on-chip waveguide coupled germanium photodetector for 2 μm waveband

Recently, the emerging 2 μm waveband has gained increasing interest due to its great potential for a wide scope of applications. Compared with the existing optical communication windows at shorter wavelengths, it also offers distinct advantages of lower nonlinear absorption, better fabrication tolerance, and larger free carrier plasma effects for silicon photonics, which has been a proven device technology. While much progress has been witnessed for silicon photonics at the 2 μm waveband, the primary challenge still exists for on-chip detectors. Despite the maturity and compatibility of the waveguide coupled photodetectors made of germanium, the 2 μm regime is far beyond its cutoff wavelength. In this work, we demonstrate an efficient and high-speed on-chip waveguide-coupled germanium photodetector operating at the 2 μm waveband. The weak sub-bandgap absorption of epitaxial germanium is greatly enhanced by a lateral separation absorption charge multiplication structure. The detector is fabricated by the standard process offered by a commercial foundry. The device has a benchmark performance with responsivity of 1.05 A/W and 3 dB bandwidth of 7.12 GHz, which is able to receive high-speed signals with up to 20 Gbit/s data rate. The availability of such an efficient and fast on-chip detector circumvents the barriers between silicon photonic integrated circuits and the potential applications at the 2 μm waveband.

‘Sawfish’ Photonic Crystal Cavity for Near‐Unity Emitter‐to‐Fiber Interfacing in Quantum Network Applications

AbstractPhoton loss is one of the key challenges to overcome in complex photonic quantum applications. Photon collection efficiencies directly impact the amount of resources required for measurement‐based quantum computation and communication networks. Promising resources include solid‐state quantum light sources. However, efficiently coupling light from a single quantum emitter to a guided mode remains demanding. In this work, photon losses are eliminated by maximizing coupling efficiencies in an emitter‐to‐fiber interface. A waveguide‐integrated ‘Sawfish’ photonic crystal cavity is developed and finite element (FEM) simulations are employed to demonstrate that such an emitter‐to‐fiber interface transfers, with 97.4 % efficiency, the zero‐phonon line (ZPL) emission of a negatively‐charged tin vacancy center in diamond (SnV−) adiabatically to a single‐mode fiber. A surrogate model trained by machine learning provides quantitative estimates of sensitivities to fabrication tolerances. The corrugation‐based Sawfish design proves robust under state‐of‐the‐art nanofabrication parameters, maintaining an emitter‐to‐fiber coupling efficiency of 88.6 %. Applying the Sawfish cavity to a recent one‐way quantum repeater protocol substantiates its potential in reducing resource requirements in quantum communication.

A sodium-based hybrid plasmonic waveguide with ultra-deep subwavelength confinement and low cross-talk

Surface plasmon photonics provides a feasible method for achieving high-density photonic integration and low-loss optical field transmission. Here we propose a sodium-based hybrid plasmonic waveguide consisting of a cylindrical sodium nanowire and a Si nano-rib waveguide located on the silica substrate at the wavelength of 1550 nm and investigate the mode properties by using the finite element method. The results indicate that ultra-deep subwavelength modal fields with the mode areas of 9.64 × 10−6A0 ∼ 9.507 × 10−4A0 (A0 = λ2/4) and propagation lengths of 8 ∼ 46.5 μm could be achieved when the thickness of the spacer is less than 4 nm. Moreover, investigations on fabrication tolerance and crosstalk show that the proposed waveguide exhibits a good fabrication error tolerance and very low cross-talk between adjacent structures with a minimum separation distance of around 300 nm, which makes it quite suitable for applications in photonic integration. These findings may have potential applications in the field of photonic devices, such as nanowaveguides, lasers, and sensors.

Design and analysis of a flexible Ruddlesden–Popper 2D perovskite metastructure based on symmetry-protected THz-bound states in the continuum

A Ruddlesden–Popper 2D perovskite PEA2PbX4 (X = I, Br, and Cl) is proposed for metasurface applications. Density functional theory is used to analyze the optical, electrical, mechanical properties, moisture and thermodynamic stability of PEA2PbX4. The refractive index of PEA2PbX4 varies with the halides, resulting in 2.131, 1.901, and 1.842 for X = I, Br, and Cl, respectively. Mechanical properties with Voigt-Reuss-Hill approximations indicate that all three materials are flexible and ductile. Based on the calculations of formation energy and adsorption of water molecules, PEA2PbI4 has superior thermodynamic and moisture stability. We present a novel metasurface based on 2D-PEA2PbI4 and analyze symmetry protected-bound states in the continuum (sp-BIC) excitation. The proposed structure can excite multiple Fano quasi-BICs (q-BICs) with exceptionally high Q-factors. We verify the group theoretical analysis and explore the near-field distribution and far-field scattering of q-BICs. The findings indicate that x-polarized incident waves can excite magnetic toroidal dipole-electromagnetic-induced transparency-BIC and magnetic quadrupole-BIC, while y-polarized incident waves can excite electric toroidal dipole-BIC and electric quadrupole-BIC. The influence of meta-atom and substrate losses, array size limitations, and fabrication tolerances are also discussed. The proposed structure can be employed for applications in the THz region, such as polarization-dependent filters, bidirectional optical switches, and wearable photonic devices.

FDTD analysis of direct optical integration between SiNx waveguides and Ge/SiGe multiple quantum wells on graded buffer for low-voltage optical modulators around the O-band wavelength

We focus on investigating the possibility to achieve a direct butt coupling integration of SiNx passive waveguides with germanium/silicon–germanium (Ge/SiGe) multiple quantum wells (MQWs) on a thick graded buffer platform using 3D finite-difference time-domain simulation. Despite the promising potential of a thick graded buffer platform to provide superior crystal quality, its integration with SiNx waveguides has not been discussed so far. We find that in spite of a relatively-low refractive index contrast of the graded buffer platform, it is still possible to attain low-voltage SiNx waveguide-integrated Ge/SiGe MQWs optical modulators around the telecommunication wavelength of 1.31 µm with competitive integrated extinction ratio and insertion loss performance with a compact footprint. Moreover, the fabrication tolerance and the reflection from the butt coupling scheme between the SiNx waveguides and the Ge/SiGe MQWs are also promisingly considered.

Efficient and robust second-harmonic generation in thin-film lithium niobate using modal phase matching

A double-ridge waveguide is designed for efficient and robust second-harmonic generation (SHG) using the thin-film lithium-niobate-on-insulator (LNOI) platform. Perfect phase matching (PhM) is achieved between the fundamental waveguide mode at 1,550 nm and a higher-order mode at the second harmonic. The fabrication tolerances of the PhM condition are simulated using a finite-difference method mode solver, and conversion efficiencies as high as 3.92 W−1 are obtained for a 1-cm long waveguide. This design allows access to the largest element of the second-order nonlinear susceptibility tensor, and represents a scalable alternative to waveguides based on periodically-poled lithium niobate (PPLN). The design has the potential for generating pairs of entangled photons in the infrared C-band by spontaneous parametric down-conversion (SPDC).

Gain equalization for a few-mode erbium-doped fiber amplifier supporting eight spatial modes

A trench-assisted ring few-mode erbium-doped fiber amplifier (FM-EDFA) supporting eight spatial modes is designed and proposed in this work. The gain equalization for the FM-EDFA is achieved by selecting the appropriate doping radius and concentration using a particle swarm optimization (PSO) algorithm when only the pump in the fundamental mode (LP01) is applied. When the signals in the eight spatial modes are simultaneously amplified, the average modal gain is about 20 dB, and the DMG is less than 0.3 dB for a signal at 1550 nm. Considering the gain competition of six wavelength signals, the modal gain and DMG are more than 20 and 1 dB, respectively. In addition, the tolerance analysis for manufacturing with this design is also discussed. For a fluctuation in the refractive index, the average modal gain is about 19.5 dB, and the DMG is 0.77 dB, indicating that the structure has good fabrication tolerance.

Silicon nano-bridge waveguide assisted polarizing beam splitter

A low-loss and wideband silicon polarizing beam splitter is demonstrated with the assistance of a nano-bridge waveguide. Transverse magnetic light can be coupled to the cross port through the nano-bridge waveguide, while the transverse electric light comes out mainly from the through port. The designed device has a coupling length of 19.6 µm, which can realize an extinction ratio of 32.43 dB for TM mode or 34.23 dB for TE mode at a 1565-nm wavelength. Compared with the conventional three-waveguide coupler structure, the proposed device is based on a resonant tunneling principle, which can help to effectively improve the fabrication tolerance. The device is fabricated with a commercial CMOS processing facility, which can achieve an extinction ratio of 23.07 dB for TM or 23.46 dB for TE mode with a low excess loss, and the extinction ratio of more than 10 dB can be realized in the wavelength range from 1525 to 1610 nm for both modes. The device performance can be further improved, which would facilitate its practical applications in commercial integrated optical circuits.

Foundry-Processed Compact and Broadband Adiabatic Optical Power Splitters with Strong Fabrication Tolerance

Optical power splitters play a crucial role as the fundamental building blocks for many integrated optical devices. They should have low losses, a broad bandwidth, and a high tolerance to fabrication errors. Adiabatic optical power splitters inherently possess these qualities while being compatible with foundry processes well suited for mass production. The long device lengths of adiabatic power splitters, however, are a limiting factor to achieve compact device sizes, which must be reduced. Here, we used a polynomial taper profile optimization algorithm to design 1 × 2 and 2 × 2 adiabatic power splitters with significantly shorter lengths than their adiabatic counterparts. The best-performing 1 × 2 and 2 × 2 power splitters had 20 μm and 16 μm coupling lengths, respectively. Our designs had minimum feature sizes ranging from 140 nm to 200 nm, and our measurements averaged across nine different chips showed excellent consistency in performance for devices with 180 nm and 200 nm minimum features. Both the 1 × 2 and 2 × 2 adiabatic optical power splitters had excess losses less than 0.7 dB over a 100 nm bandwidth, with a standard deviation lower than 0.3 dB. Furthermore, our measurements showed splitting ratios within 50 ± 3% over a 130 nm bandwidth. We also demonstrated the design of 1 × 2 power splitters with arbitrary splitting ratios, where splitting ratios ranging from 50:50 to 94:6 were achieved with standard deviations between 2% and 6%.

Low-power and wide-band 1 × 8 silica waveguide optical switch

A silica waveguide 1 × 8 thermo-optic switch consisting of cascaded three-stage Mach-Zehnder interferometers (MZIs) is demonstrated. The multimode interference (MMI) coupler is adopted in the MMI-MZI 1 × 2 switch unit due to its insensitivity to wavelength and favorable fabrication tolerance. Beam propagation method (BPM) is used in the switch design and optimization. Standard CMOS technologies have been adopted in the preparation of switch fabric. At 1550 nm, the measurement results of 1 × 8 switch show that the fiber-to-fiber insertion loss ranges from 3.4 to 3.6 dB for all routing states. Meanwhile, the extinction ratio is no less than 15.63 dB, and the crosstalk is restrained to be lower than −15.65 dB. Due the adoption of MZI structure, the average driving power for the channel switching is 315.8 mW. Within the wavelength range from 1500 to 1620 nm, the extinction ratio is larger than 15.06 dB, the insertion loss is less than 5.8 dB, and PDL is smaller than 1.18 dB for all routing states. The experimental results fit the simulations well, which proves the proposed switch good potentials in wideband applications.

Wide angle metamaterial absorber for S, C and X band application

Abstract In this article, an angle independent Metamaterial Microwave Absorbers (MMA) is designed and demonstrated. The design consists of two concentric hexagon connected through consolidated resistors. The absorber is imprinted on a metal-backed low cost FR4 dielectric substrate with a thickness of 3.2 mm (λ/0.07) and a dielectric constant of 4.3. The proposed structure exhibits a wide absorption bandwidth ranging from 2.8 to 10.42 GHz with absorptivity above 90 % covering S, C and X Band. In the area of interest, the current and electric field distribution has been examined, for two maximum peaks at a frequency of 3.66 and 9.54 GHz, with maximum absorptivity of 99.99 % and 99.44 % respectively. The presented absorber is examined under different polarization angles for phi and theta variation. The fabricated sheet consists of an array of a unit cell, which is examined inside the anechoic chamber with the help of two horn antennas and VNA. The tested and simulated results are compared and it was found that they are almost similar to each other with little variation due to fabrication tolerance. The presented absorber can be practically used for defence applications for Radar Cross Sections (RCS) reduction.

Impacts of PTL coating gaps on cell performance for PEM water electrolyzer

The anode porous transport layer (PTL) is a critical component of polymer electrolyte membrane (PEM) water electrolyzers, providing electrical conduction and water/oxygen transport to the anode catalyst layer. Platinum group metal (PGM) coatings are applied to titanium PTLs to prevent oxidation and preserve conductivity. To implement PEM water electrolysis on a large scale, it is essential to minimize the cost of mass-manufactured cell components while ensuring that defects in materials and components do not adversely affect the performance of the cell. In this study, we created gaps in the Pt coatings that are typically applied to PTLs to understand their effects on cell performance and inform on fabrication tolerances. The PTL coatings were fabricated by sputter deposition of Pt, intentionally leaving uncoated regions in either large patches or thin strips. For either geometry, the cell performance decreased with the increasing fraction of the uncoated regions. However, PTLs containing multiple thin uncoated strips caused a less severe performance effect than PTLs with continuous uncoated regions of the same relative total area. Through analysis of cell performance, impedance, and 3D charge transport modelling, we demonstrate that both the anode catalyst layer and membrane play a role in distributing current to mitigate the impact of coating defects at a short length scale. These results allow to draw conclusions about the tolerance requirements with regards to PTL coating uniformity in terms of both the size and total area of defects.