We present high-efficiency silicon nitride grating couplers in the $\boldsymbol{C}$-band using an amorphous silicon overlay enhanced with subwavelength metamaterial. A uniform design is experimentally demonstrated with a coupling efficiency of 2.21 dB. The performance is further enhanced to -1.03 dB with an apodized-focalizing design.

Integrated photonics underpins a wide range of applications, including high-speed optical communications, advanced computing, and quantum technologies. A persistent challenge in these systems is efficient light coupling between standard single-mode optical fibers and sub-micrometer on-chip waveguides, which remains a major bottleneck for overall system performance. This work reports recent advances in compact surface grating couplers implemented on the silicon nitride (Si3 N4) photonic platform. Several CMOS-compatible native grating architectures are introduced and demonstrated, targeting improved mode matching and radiation efficiency in the short-wave-infrared regime. Experimental results demonstrate single-etch Si3 N4 surface grating couplers with coupling losses in the range of −6 dB to −3 dB.

Integrated photonics emerges as a foundational technology for a broad range of applications. This includes high-speed optical communications, ultrafast computing, and quantum information processing, among others. A persistent challenge across these domains is the efficient coupling of light between standard single-mode optical fibers and sub-micrometer planar photonic waveguides. Despite significant progress in on-chip integration, fiber-chip optical coupling remains a key bottleneck, constraining the power budget and limiting overall system performance. Direct fiber-chip interfacing is lossy, primarily due to mismatches in geometry, material properties, and mode field diameters, along with stringent spatial alignment requirements. In this work, we report on recent advancements in the development of compact and efficient surface grating couplers implemented on the silicon nitride (Si3N4) photonic platform. We introduce and compare several CMOS-compatible design strategies, including both native and hybrid surface grating architectures, tailored for improved mode matching and radiation efficiency at short-wave-infrared wavebands. Single-etch Si3N4 devices demonstrated coupling losses between -6 dB and -3 dB, while hybrid grating designs show great promise to reduce fiber-chip losses close to -1 dB level. This marks a significant step towards practical and low-loss interconnections for next-generation photonic integrated circuits.

Silicon nitride (Si3N4) is an attractive alternative to the silicon-on-insulator platform due to its broad spectral transparency window, low waveguide losses, and negligible twophoton absorption. However, the moderate refractive index contrast between the Si3N4 waveguide core and the cladding presents challenges, including limiting the efficiency and performance of surface grating fiber-chip coupling devices. Addressing this issue is crucial to fully leveraging the advantages offered by the silicon nitride platform. While various strategies have been developed to enhance the performance of surface grating couplers, they often come with increasingly complex fabrication requirements. In this work, we present a set of highefficiency silicon nitride grating couplers using standard singleetch fabrication processes. The devices are designed for three spectral regions: 950 nm, 1310 nm, and 1550 nm. Uniform grating couplers demonstrated experimental coupling efficiencies between -5.9 dB and -3.1 dB. Record-breaking performance was achieved using subwavelength metamaterial apodization and beam focalization, resulting in fiber-chip coupling losses as low as -2.5 dB, all achieved through a straight-forward single-etch fabrication process.

Advanced optical interconnects call upon innovative approaches to establish efficient photonic chip interfaces. Surface grating couplers are prime candidates for low-loss coupling between conventional optical fibers and photonic integrated circuits (PICs). Persistently, this remains a challenging task due to material, modal, and geometrical disparities. In this work, we report on recent advances in the design and fabrication of subwavelength grating (SWG) metamaterial-engineered silicon nitride (SiN) grating-coupled optical interfaces. The unique properties of SWG metamaterials offer an extra degree of freedom for controlling light in grating couplers, thus improving both diffraction and fiber-coupler field matching efficiencies. In turn, this enhances fiber-chip coupling performance, while maintaining cost-effective and foundry-compatible fabrication. Our SiN couplers, manufactured in open-access photonic prototyping facilities, have coupling losses as low as -3.1 dB at the 1310 nm wavelength. Moreover, hybrid SiN grating couplers with amorphous silicon (alpha-Si) overlays and SWG metamaterials achieve further reduced coupling losses of -2.4 dB and -1.3 dB for uniform and apodized layouts, respectively. These results underscore the potential of metamaterial-engineered couplers for low-loss fiber-chip interface integration within scalable SiN PICs for foundry-compatible optical and quantum systems.

This feature issue highlights the rapid progress and expanding frontiers of the field of integrated photonics. The collected papers cover novel materials, device architectures, modeling techniques, and emerging applications in communications, sensing, and quantum technologies. Together, they showcase the growing maturity of integrated photonics as a cornerstone for next-generation optical systems and emphasize the synergy between theory, design, and experiment driving ongoing innovation. (c) 2025 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.

Silicon nitride offers a broad transparency window and low losses that make it a complementary alternative to silicon-on-insulator. The moderate refractive index contrast of the silicon nitride platform reduces the efficiency of fibre-chip coupling devices. This study presents high-efficiency grating couplers designed for the 1310 nm wavelength regions. Our devices are compatible with a single full-etch step fabrication process. Enhanced performance is predicted through subwavelength metamaterial apodization and beam focalization.

Integrated photonics has become a mainstream technology driven by advances in optical communications and leveraging mature processing infrastructure of silicon microelectronics. In practical applications of photonic integrated circuits (PICs), the presence of low-loss off-chip optical coupling interfaces is of key importance. Photonic chips require optical connection to the external world, facilitating both multi-channel fiber connections and free-space multi-port optics. Optical coupling in and out of the planar waveguide circuits is still a critical challenge in integrated photonics due to limitations caused by geometrical, material, and modal mismatches. The utilization of subwavelength grating (SWG) metamaterials, i.e. nano-structured waveguide segments with structural periodicity smaller than the wavelength of the propagating light, is often harnessed as an effective design tool to improve the performance of fiber-chip optical couplers, without compromising the fabrication simplicity. In this work, we present recent advances in the development of low-loss photonic chip interfaces based on surface grating couplers with SWG metamaterials. In particular, we report on advanced design solutions of surface gratings realized on silicon and silicon nitride waveguide platforms, facilitating effective control of polarization and enhanced fiber-chip coupling performance with losses down to -1 dB.

Optical antennas are key components of an optical phased array system, enabling light coupling between the chip and the free space. In such systems, surface gratings are commonly used as antenna elements, which however suffer from a strong polarization sensitivity of their scattering angle and efficiency. Here, we propose a versatile approach to realize micro antennas based on surface gratings with a polarization insensitive behavior exploiting a subwavelength metamaterial in the silicon-on-insulator platform. In the experimental demonstration, the antenna successfully achieves the same diffraction angle of 10° for both TE and TM polarizations and an estimated scattering efficiency of -4 dB despite a very compact footprint of 6.4 $$\mu m$$ x 2.9 $$\mu m$$ . The difference in diffraction efficiency between the two polarizations remains smaller than 1 dB over a bandwidth of 31 nm.

High-performance photonic interconnects call upon novel solutions for efficient input/output optical interfaces. However, obtaining low-loss coupling between conventional optical fibers and photonic integrated circuits (PICs) remains a critical challenge due to material, modal, and geometrical discrepancies. In this work, we report advanced design approaches, leveraging subwavelength grating (SWG) metamaterials in silicon nitride (Si3N4) substrates. Harnessing unique properties of metamaterials within moderate index contrast SiN platform brings an extra degree of design freedom to seamlessly tailor light flow in grating couplers, improving both radiation efficiency and grating-to-fiber field matching. In turn, this enhances fiber-chip coupling performance, while maintaining single-etch step manufacturing. SWG-engineered uniform Si3N4 couplers realized on low-pressure chemical vapor deposited (LPCVD) wafers showed experimental efficiency of about -4.5 dB at 1550 nm wavelength. Moreover, hybrid Si3N4 grating coupler designs with SWG metamaterials, operated at datacom range around 1310 nm, boost the fiber-chip coupling efficiency to -2.4 dB and -1.3 dB levels for uniform and apodized structures, respectively. These results highlight the promising potential of metamaterial-engineered off-chip waveguide couplers in future innovations of complex SiN PICs, demanding low-loss interconnection with the outside world and cost-effective foundry-compatible fabrication.

Grating couplers are essential building blocks to form low-loss photonic chip interfaces. Optical coupling between sub-micrometric waveguides and standard optical fibers remains a practical hurdle, because of geometrical, material, and modal mismatches. Grating couplers on low-index contrast platforms like silicon nitride (Si3N4) suffer from low directionality and poor fiber-to-grating field overlap, thus overall efficiency is limited. To address this, Si3N4 couplers with high-index overlays are appealing solution to improve coupling performance, yet with cost-effective fabrication. However, adding extra layers increases the design complexity, resulting in unintuitive operation and time-consuming manual device optimization. To overcome this, machine learning (ML) optimization emerges as an attractive lever to design integrated photonic devices. In this work, we present a comparative study of three metaheuristic optimization techniques - particle swarm optimization (PSO), gray wolf optimization (GWO), and genetic algorithm (GA) - to enhance the coupling performance in hybrid alpha-Si/SiN grating couplers. The devices are studied for transverse electric (TE) polarization at telecom waveband, centered around 1550 nm wavelength. Employing metaheuristic optimization with Ansys Lumerical finite difference time domain (FDTD) solver, we obtained sub-decibel fiber-chip coupling loss (<1 dB). Proposed strategy presents an effective design approach to develop low-loss and scalable grating couplers on hybrid Si3N4 platform for telecom applications.

Silicon nitride (Si3N4) has emerged as an attractive photonic platform for implementing passive optical components due to its ultra-low propagation losses and broad transparency window. However, its low refractive index contrast presents persistent design challenges for low-loss surface grating couplers, resulting in poor radiation and sub-optimal grating-to-fiber mode matching. High-index overlays situated atop the Si3N4 structure are a promising strategy to tackle these limitations. Although this hybrid configuration offers enhanced design flexibility, it also introduces additional complexity into the design space, rendering conventional optimization methods inefficient. To address this, in this work, we investigate inverse-designed grating couplers employing metaheuristic optimization techniques tailored for the hybrid alpha -Si/Si3N4 platform. In particular, we apply three distinct meta-heuristic algorithms-particle swarm optimization (PSO), grey wolf optimization (GWO), and genetic algorithm (GA)-in conjunction with finite-difference time-domain (FDTD) simulations. Our results demonstrate that all three methods can effectively explore the multi-dimensional design space and achieve low fiber-chip coupling losses at the telecom waveband. This work establishes a scalable inverse design framework for the development of high-performance grating couplers on hybrid Si3N4 platforms, demonstrating the practical viability of meta-heuristic optimization for low-loss input/output optical interfaces. The proposed approach not only advances current coupling performance but also paves the way for future innovations in the automated design of photonic chip interfaces for next-generation on-chip systems. (c) 2025 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI)training, and similar technologies, are reserved.

Efficient fiber-chip coupling interfaces are critically important for integrated photonics. Since surface gratings diffract optical signals vertically out of the chip, these couplers can be placed anywhere in the circuit allowing for wafer-scale testing. While state-of-the-art grating couplers have been developed for silicon-on-insulator (SOI) waveguides, the moderate index contrast of silicon nitride (SiN) presents an outstanding challenge for implementing efficient surface grating couplers on this platform. Due to the reduced grating strength, a longer structure is required to radiate the light from the chip which produces a diffracted field that is too wide to couple into the fiber. In this work, we present a novel grating coupler architecture for silicon nitride photonic integrated circuits that utilizes an amorphous silicon (α-Si) overlay. The high refractive index of the α-Si overlay breaks the coupler’s vertical symmetry which increases the directionality. We implement subwavelength metamaterial apodization to optimize the overlap of the diffracted field with the optical fiber Gaussian mode profile. Furthermore, the phase of the diffracted beam is engineered to focalize the field into an SMF-28 optical fiber placed 55 µm above the surface of the chip. The coupler was designed using rigorous three-dimensional (3D) finite-difference time-domain (FDTD) simulations supported by genetic algorithm optimization. Our grating coupler has a footprint of 26.8 × 32.7 µm2 and operates in the O-band centered at 1.31 μm. It achieves a high directionality of 85% and a field overlap of 90% with a target fiber mode size of 9.2 µm at the focal plane. Our simulations predict a peak coupling efficiency of − 1.3 dB with a 1-dB bandwidth of 31 nm. The α-Si/SiN grating architecture presented in this work enables the development of compact and efficient optical interfaces for SiN integrated photonics circuits with applications including optical communications, sensing, and quantum photonics.

Over the past 15 years since their first demonstration, subwavelength grating metamaterials in silicon photonic devices have become widely used and attracted rapidly growing research interest while also breaking into commercial applications. We will discuss recent advances in this research field, with a focus on novel components and circuits for beam steering applications, on-chip filtering and quantum optics. On-chip optical waveguides comprised of Mie resonant particle chains have only recently been demonstrated and promise to be the foundation of a new and exciting branch of integrated metamaterials research. We will review the early work in this area.