Academic literature on the topic 'Subwavelength grating metamaterials'

Abstract Silicon photonics is playing a key role in areas as diverse as high-speed optical communications, neural networks, supercomputing, quantum photonics, and sensing, which demand the development of highly efficient and compact light-processing devices. The lithographic segmentation of silicon waveguides at the subwavelength scale enables the synthesis of artificial materials that significantly expand the design space in silicon photonics. The optical properties of these metamaterials can be controlled by a judicious design of the subwavelength grating geometry, enhancing the performance of nanostructured devices without jeopardizing ease of fabrication and dense integration. Recently, the anisotropic nature of subwavelength gratings has begun to be exploited, yielding unprecedented capabilities and performance such as ultrabroadband behavior, engineered modal confinement, and sophisticated polarization management. Here we provide a comprehensive review of the field of subwavelength metamaterials and their applications in silicon photonics. We first provide an in-depth analysis of how the subwavelength geometry synthesizes the metamaterial and give insight into how properties like refractive index or anisotropy can be tailored. The latest applications are then reviewed in detail, with a clear focus on how subwavelength structures improve device performance. Finally, we illustrate the design of two ground-breaking devices in more detail and discuss the prospects of subwavelength gratings as a tool for the advancement of silicon photonics.

The use of subwavelength grating structures in silicon waveguides have fuelled the development of integrated optical components with superior performance. By a judicious lithographic patterning of the grating, the optical properties of the synthesized metamaterial can be accurately tailored. In this work, we review our latest advances in subwavelength-grating-engineered silicon and germanium planar devices.

The silicon-on-insulator (SOI) platform enables high integration density in photonic integrated circuits while maintaining compatibility with CMOS fabrication processes. Nevertheless, its inherently high modal birefringence hinders the development of polarization-insensitive devices. The dispersion and anisotropy engineering leveraging subwavelength grating (SWG) metamaterials makes possible the development of polarization agnostic waveguide components. In this work we build upon the bricked SWG metamaterial nanostructures to design a polarization independent 2×2 multimode interference (MMI) coupler for the 220 nm SOI platform, operating in the telecom O-band. The designed device exhibits a 160 nm bandwidth with excess loss, polarization dependent loss and imbalance below 1 dB and phase error lower than 5°.

Subwavelength grating (SWG) metamaterials have garnered a great interest for their singular capability to shape the material properties and the propagation of light, allowing the realization of devices with unprecedented performance. However, practical SWG implementations are limited by fabrication constraints, such as minimum feature size, that restrict the available design space or compromise compatibility with high-volume fabrication technologies. Indeed, most successful SWG realizations so far relied on electron-beam lithographic techniques, compromising the scalability of the approach. Here, we report the experimental demonstration of an SWG metamaterial engineered beam splitter fabricated with deep-ultraviolet immersion lithography in a 300-mm silicon-on-insulator technology. The metamaterial beam splitter exhibits high performance over a measured bandwidth exceeding 186 nm centered at 1550 nm. These results open a new route for the development of scalable silicon photonic circuits exploiting flexible metamaterial engineering.

Terahertz surface plasmon resonance (SPR) sensors have been regarded as a promising technology in biomedicine due to their real-time, label-free, and ultrasensitive monitoring features. Different authors have suggested a lot of SPR sensors, including those based on 2D and 3D metamaterials, subwavelength gratings, graphene, and graphene nanotube, as well as others. However, one of the traditional approaches to realize high sensitivity SPR sensors based on metal diffraction gratings has been studied poorly in the terahertz frequency range. In this article, a linear metal rectangular diffraction grating with high aspect ratio is studied. The influence of the grating structure parameters on the sensor sensitivity is simulated. Effects arising from different ratios of depth and width were discovered and explained. The results show that the sensitivity can be increased to 2.26 THz/RIU when the refractive index range of the gas to measure is between 1 and 1.002 with the resolution 5×10−5 RIU.

Silicon nitride (Si3N4) is an ideal candidate for the development of low-loss photonic integrated circuits. However, efficient light coupling between standard optical fibers and Si3N4 chips remains a significant challenge. For vertical grating couplers, the lower index contrast yields a weak grating strength, which translates to long diffractive structures, limiting the coupling performance. In response to the rise of hybrid photonic platforms, the adoption of multi-layer grating arrangements has emerged as a promising strategy to enhance the performance of Si3N4 couplers. In this work, we present the design of high-efficiency surface grating couplers for the Si3N4 platform with an amorphous silicon (α-Si) overlay. The surface grating, fully formed in an α-Si waveguide layer, utilizes subwavelength grating (SWG)-engineered metamaterials, enabling simple realization through single-step patterning. This not only provides an extra degree of freedom for controlling the fiber–chip coupling but also facilitates portability to existing foundry fabrication processes. Using rigorous three-dimensional (3D) finite-difference time-domain (FDTD) simulations, a metamaterial-engineered grating coupler is designed with a coupling efficiency of −1.7 dB at an operating wavelength of 1.31 µm, with a 1 dB bandwidth of 31 nm. Our proposed design presents a novel approach to developing high-efficiency fiber–chip interfaces for the silicon nitride integration platform for a wide range of applications, including datacom and quantum photonics.

It is challenging to simultaneously consider device dimension, polarization extinction ratio (PER), insertion loss (IL), and operable bandwidth (BW) to design a polarization beam splitter (PBS) that is extensively used in photonic integrated circuits. The function of a PBS is to separate polarizations of light, doubling the transmission bandwidth in optical communication systems. In this work, we report a high-performance PBS comprising two-dimensional subwavelength grating metamaterials (2D SWGMs) between slot waveguides. The 2D SWGMs exhibited biaxial permittivity by tailoring the material anisotropy. The proposed PBS showed PERs of 26.8 and 26.4 dB for TE and TM modes, respectively, and ILs of ~0.25 dB for both modes, with an unprecedented small footprint of 1.35 μm × 2.75 μm working at the wavelength λ = 1550 nm. Moreover, the present structure attained satisfactory PERs of >20 dB and ILs of <0.5 dB within an ultrabroad BW of 200 nm.

La technologie photonique sur silicium s'appuie sur les procédés matures de fabrication de l'industrie du semi-conducteur pour produire des composants opto-électroniques à échelle industrielle. Les métamatériaux à base de réseaux sub-longueur d'onde permettent de contrôler le confinement du mode et la dispersion, et ont ainsi été implémentés pour démontrer des performances de pointe de composants photoniques intégrés. Les effets de diffraction et de réflexions sont supprimés dans les matériaux sub-longueur d'onde. Leurs dimensions sont petites et sont environ de 100 nm. Jusqu'à présent, la majorité des composants sub-longueur d'onde ont été fabriqués par lithographie électronique. Or, cette technique n'est pas compatible avec une production à large échelle. Aujourd'hui, la lithographie à immersion se déploie dans les fonderies photoniques sur silicium. Elle permet de définir des dimensions aussi petites que 70 nm, avec un modèle de correction d'effets optiques de proximité. Le but principal de cette thèse est d'étudier la faisabilité de l'utilisation de la lithographie à immersion avec la correction d'effets optiques de proximité pour la fabrication de composants photoniques sub-longueur d'onde de pointe. Ces composants ont été développés sur des plaques de 300 mm de diamètre au CEA-Leti. Trois composants ont été étudiés, chacun avec une spécificité technologique : i) un diviseur de puissance avec une seule étape de gravure complète, ii) un réseau de couplage puce-fibre alternant des gravures partielles et complètes, et iii) une matrice d'antennes optiques, couvrant une large surface, avec une étape de gravure partielle. Le diviseur de puissance est constitué d'un coupleur par interférométrie multi-mode (MMI) avec des réseaux sub longueur d'onde pour contrôler la dispersion des modes optiques et ainsi pour obtenirune très large bande passante spectrale, qui a été mesurée expérimentalement à 350 nm, et qui en bon accord avec les simulations. La bande passante d'un MMI conventionnel sans structures sub longueur d'onde n'est que de 100 nm environ. Le réseau de couplage puce-fibre s'appuie sur une géométrie en forme de « L », avec des structures sub-longueur d'onde gravés partiellement et complètement, pour augmenter l'efficacité de couplage. Celle-ci a été mesurée à -1.70 dB (68 %) à une longueur d'onde de 1550 nm et représente la meilleure performance pour une telle structure complexe, utilisant une technologie autre que la lithographie électronique. Néanmoins, la valeur mesurée est inférieure à la valeur simulée de 0.80 dB (83 %). Une des raisons principales de cette performance limitée est la sensibilité de cette structure aux erreurs d'alignement entre les deux étapes de gravure pendant la fabrication. L'antenne optique est constituée de structures sub longueur d'onde partiellement gravées pour obtenir une grande surface d'émission de 48 µm×48 µm, réduisant ainsi la divergence du faisceau. Cette antenne a été implémentée comme antenne unitaire dans une matrice 4×4 à réseau phasé avec un pas de 90 µm×90 µm. A une longueur d'onde de 1550 nm, le faisceau émis par l'antenne unitaire a une divergence à mi-hauteur mesurée de 1.40° et celui émis par la matrice d'antennes a une divergence à mi hauteur de 0.25°. Ces valeurs sont en accord avec les valeurs simulées. Ces résultats servent comme preuve de concept de l'implémentation d'une telle antenne dans une matrice à réseau phasé. En résumé, les résultats de cette thèse illustrent le grand potentiel de la lithographie à immersion avec la correction d'effets optiques de proximité pour la fabrication de composants photoniques sub- longueur d'onde, ouvrant ainsi la voie pour la commercialisation de ces derniers<br>Silicon photonics technology leverages the mature fabrication processes of the semi-conductor industry for the large volume production of opto-electronic devices. Subwavelength grating (SWG) metamaterials enable advanced engineering of mode confinement and dispersion, that have been used to demonstrate state-of-the-art performance of integrated photonic devices. SWGs generally require minimum feature sizes as small as a 100 nm to suppress reflection and diffraction effects. Hitherto, most reported SWG-based devices have been fabricated using electron-beam lithography. However, this technique is not compatible with large volume fabrication, hampering the commercial adoption of SWG-based photonic devices. Currently, immersion lithography is being deployed in silicon photonic foundries, enabling the patterning of features of 70 nm, when used in conjunction with optical proximity correction (OPC) models. The main goal of this PhD is to study the feasibility of immersion lithography and OPC for the realization of high-performance SWG devices. The SWG devices developed here have been fabricated using the OPC models and 300 mm SOI wafer technology at CEA-Leti. Three devices have been considered as case studies, each with a specific technological challenge: i) a power splitter requiring a single full etch step, ii) a fiber-chip grating coupler interleaving full and shallow etch steps, and iii) an optical antenna array covering a large surface area with a shallow etch step. The power splitter is implemented using a SWG-engineered multi-mode interferometer (MMI) coupler. The SWG is used to control the dispersion of the optical modes to achieve an ultrawide operating spectral bandwidth. This device experimentally showed state-of-the-art bandwidth of 350 nm, in good agreement with simulations. Note that the bandwidth of a conventional MMI without SWG is around 100 nm. The fiber-chip coupler relies on an L-shaped geometry with SWG in full and shallow etch steps to maximize the field radiated towards the fiber. The measured coupling efficiency, of - 1.70 dB (68 %) at a wavelength of 1550 nm, is the highest value reported for an L-shaped coupler fabricated without electron-beam lithography. Still, this value differs from the calculated efficiency of 0.80 dB (83 %), and compares to experimental values achieved with fiber-chip grating couplers without SWG (~ -1.50 dB). One of the main reasons for the limited experimental performance is the strong sensitivity of the structure to errors in the alignment between the full and shallow etch steps. The optical antenna uses shallowly etched SWG teeth to minimize the grating strength, allowing the implementation of a large area emission aperture, of 48 × 48 µm, which is required to minimize the beam divergence. A two-dimensional (2D) optical phased array (OPA) with an antenna pitch of 90 µm × 90 µm, comprising 16 antennas was designed and fabricated. The SWG-based unitary antenna has a measured full width at half maximum divergence of 1.40° at a wavelength of 1550 nm, while the beam emitted from the phased array has a divergence of 0.25°, both in very good agreement with expected values. These results serve as a good proof-of-concept demonstration of this novel antenna architecture. In summary, the results shown in this PhD illustrate the great potential of immersion lithography and OPC for harnessing SWG-engineering, paving the way for their commercial adoption. Devices with full or shallow etch steps exhibited excellent performance close to that predicted by simulations. The fiber-chip grating couplers deviated from expected results, probably due to the tight fabrication tolerances associated with the combination of full and shallow etch steps

The widespread adoption of photonic circuits requires the economics of volume manufacturing offered by integration technology. A Complementary Metal-Oxide Semiconductor compatible silicon material platform is particularly attractive because it leverages the huge investment that has been made in silicon electronics and its high index contrast enables tight confinement of light which decreases component footprint and energy consumption. Nevertheless, there remain challenges to the development of photonic integrated circuits. Although the density of integration is advancing steady and the integration of the principal components – waveguides, optical sources and amplifiers, modulators, and photodetectors – have all been demonstrated, the integration density is low and the device library far from complete. The integration density is low primarily because of the difficulty of confining light in structures small compared to the wavelength which measured in micrometers. The device library is incomplete because of the immaturity of hybridisation on silicon of other materials required by active devices such as III-V semiconductor alloys and ferroelectric oxides and the difficulty of controlling the coupling of light between disparate material platforms. Metamaterials are nanocomposite materials which have optical properties not readily found in Nature that are defined as much by their geometry as their constituent materials. This offers the prospect of the engineering of materials to achieve integrated components with enhanced functionality. Metamaterials are a class of photonic crystals includes subwavelength grating waveguides, which have already provided breakthroughs in component performance yet require a simpler fabrication process compatible with current minimum feature size limitations. The research reported in this PhD thesis advances our understanding of the structure-property relations of key planar light circuit components and the metamaterial engineering of these properties. The analysis and simulation of components featuring structures that are only just subwavelength is complicated and consumes large computer resources especially when a three dimensional analysis of components structured over a scale larger than the wavelength is desired. This obstructs the iterative design-simulate cycle. An abstraction is required that summarises the properties of the metamaterial pertinent to the larger scale while neglecting the microscopic detail. That abstraction is known as homogenisation. It is possible to extend homogenisation from the long-wavelength limit up to the Bragg resonance (band edge). It is found that a metamaterial waveguide is accurately modeled as a continuous medium waveguide provided proper account is taken of the emergent properties of the homogenised metamaterial. A homogenised subwavelength grating waveguide structure behaves as a strongly anisotropic and spatially dispersive material with a c-axis normal to the layers of a one dimensional multi-layer structure (Kronig-Penney) or along the axis of uniformity for a two dimensional photonic crystal in three dimensional structure. Issues with boundary effects in the near Bragg resonance subwavelength are avoided either by ensuring the averaging is over an extensive path parallel to boundary or the sharp boundary is removed by graded structures. A procedure is described that enables the local homogenised index of a graded structure to be determined. These finding are confirmed by simulations and experiments on test circuits composed of Mach-Zehnder interferometers and individual components composed of regular nanostructured waveguide segments with different lengths and widths; and graded adiabatic waveguide tapers. The test chip included Lüneburg micro-lenses, which have application to Fourier optics on a chip. The measured loss of each lens is 0.72 dB. Photonic integrated circuits featuring a network of waveguides, modulators and couplers are important to applications in RF photonics, optical communications and quantum optics. Modal phase error is one of the significant limitations to the scaling of multimode interference coupler port dimension. Multimode interference couplers rely on the Talbot effect and offer the best in-class performance. Anisotropy helps reduce the Talbot length but temporal and spatial dispersion is necessary to control the modal phase error and wavelength dependence of the Talbot length. The Talbot effect in a Kronig-Penny metamaterial is analysed. It is shown that the metamaterial may be engineered to provide a close approximation to the parabolic dispersion relation required by the Talbot effect for perfect imaging. These findings are then applied to the multimode region and access waveguide tapers of a multi-slotted waveguide multimode interference coupler with slots either in the transverse direction or longitudinal direction. A novel polarisation beam splitter exploiting the anisotropy provided by a longitudinally slotted structure is demonstrated by simulation. The thesis describes the design, verification by simulation and layout of a photonic integrated circuit containing metamaterial waveguide test structures. The test and measurement of the fabricated chip and the analysis of the data is described in detail. The experimental results show good agreement with the theory, with the expected errors due to fabrication process limitations. From the Scanning Electron Microscope images and the measurements, it is clear that at the boundary of the minimum feature size limit, the error increases but still the devices can function.

We demonstrate subwavelength grating metamaterial ring resonators on a silicon nitride platform. We report an internal Q factor of 2.11˖105 and 1.48 dB/cm propagation loss at 1540 nm wavelength. We discuss the potential of these novel devices for applications in evanescent field sensing and light amplification.

We present a silicon photonic polarization splitter-rotator (PSR) with subwavelength gratings (SWGs). SWGs enhance the local coupling coefficient during the modal transition and make the PSR more compact than a strip waveguide-based PSR.

We exploit the new degrees of freedom offered by subwavelength grating metamaterials to demonstrate two novel Y–junction power-splitting architectures for the silicon-on-insulator platform, achieving ultra-broadband and fabrication-tolerant performance.

We show that a TM leaky mode realized by subwavelength gratings (SWGs) can eliminate crosstalk between proximate waveguides. Oscillating fields i n t he SWGs allow coupling coefficient components to counteract each other, resulting in zero crosstalk.