Segmenting silicon waveguides at the subwavelength scale produce an equivalent homogenous material. The geometry of the waveguide segments provides precise control over modal confinement, effective index, dispersion and birefringence, thereby opening up new approaches to design devices with unprecedented performance. Indeed, with ever-improving lithographic technologies offering sub-100-nm patterning resolution in the silicon photonics platform, many practical devices based on subwavelength structures have been demonstrated in recent years. Subwavelength engineering has thus become an integral design tool in silicon photonics, and both fundamental understanding and novel applications are advancing rapidly. Here, we provide a comprehensive review of the state of the art in this field. We first cover the basics of subwavelength structures, and discuss substrate leakage, fabrication jitter, reduced backscatter, and engineering of material anisotropy. We then review recent applications including broadband waveguide couplers, high-sensitivity evanescent field sensors, low-loss devices for mid-infrared photonics, polarization management structures, spectral filters, and highly efficient fiber-to-chip couplers. We finally discuss the future prospects for subwavelength silicon structures and their impact on advanced device design.
Bragg filters stand as a key building blocks of the silicon-on-insulator (SOI) photonics platform, allowing the implementation of advanced on-chip signal manipulation. However, achieving narrowband Bragg filters with large rejection levels is often hindered by fabrication constraints and imperfections. Here, we present a new generation of high-performance Bragg filters that exploit subwavelength and symmetry engineering to overcome bandwidth-rejection trade-off in state-of-the-art implementations. We experimentally show flexible control over the width and depth of the Bragg resonance. These results pave the way for the implementation of high-performance on-chip pump-rejection filters with a great potential for Si-based quantum photonic circuits.
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.
Periodically patterning silicon with a subwavelength period enables flexible control of the propagation of light and sound in silicon photonic circuits. In this invited presentation, we will show our most recent demonstration of supercontinuum generation in the near-IR and mid-IR using suspended silicon waveguides. We will also discuss our recent results on subwavelength engineering of photons and phonons in suspended and non-suspended silicon optomechanical cavities
Silicon nitride (SiN) has emerged as an important waveguide platform to implement integrated photonic circuits for a wide range of applications, including telecommunications, nonlinear optics, and quantum information. The SiN platform is compatible with a CMOS fabrication and provides attractive properties such as low propagation loss and increased tolerance to fabrication errors. However, a comparatively low index contrast is a challenge for coupling light off-chip using surface grating couplers. Compared to silicon waveguides, reduced grating scattering strength limits attainable coupling efficiency since the size of the radiating beam is significantly larger compared to near-Gaussian optical mode of standard SMF-28 optical fibers. In this work, we present, both theoretically and experimentally, a set of robust uniform and apodized grating couplers implemented in 400 nm SiN platform. Grating couplers operate with TE polarization at telecom C-band, with measured coupling losses between -4 dB to -3 dB near 1550 nm wavelength. Prospectively, our designs can be further optimized by using sub-wavelength grating metamaterial engineering and self-focusing topology, with simulated fiber-chip coupling loss as low as -1.6 dB. Our results pave the way towards development of highly efficient and robust off-chip coupling interfaces in SiN platform.
The large transparency window of silicon (1.1 - 8 µm wavelength range) makes it a promising material for the implementation of a wide range of applications, including datacom, nonlinear and quantum optics, or sensing in the near- and mid-infrared wavelength ranges. However, the implementation of the silicon-on-insulator (SOI) platform in the mid-infrared is restricted by the absorption of buried oxide layer for wavelengths above 4 µm. A promising solution is to combine silicon membranes and subwavelength nanostructuration to locally remove the buried oxide layer, thus allowing access to the full transparency window of silicon. Additionally, structuring silicon with features smaller than half of the wavelength releases new degrees of freedom to tailor material properties, allowing the realization of innovative high-performance Si devices. Implementing Si membrane waveguides providing simultaneous single-mode operation at both near-infrared and mid-infrared wavelengths is cumbersome. Due to the high index contrast between Si and air cladding, conventional strip waveguides with cross-sections large enough to guide a mode in the mid-infrared are multi-mode in the near-infrared. Here, we exploit periodic corrugation to engineer light propagation properties of Si membrane waveguides allowing effective single-mode operation in near- and mid-IR. Single-mode propagation in the mid-IR is allowed by choosing a 500-nm-thick and 1100-nm-wide silicon waveguide. A novel waveguide corrugation approach radiates out the higher order modes in the near-IR, resulting in an effectively single-mode operation in near-IR. Based on this concept, we demonstrated Bragg filters with 4 nm bandwidth and 40 dB rejection.
Grating couplers enable position-friendly interfacing of silicon chips by optical fibers. The conventional coupler designs call upon comparatively complex architectures to afford efficient light coupling to sub-micron silicon-on-insulator (SOI) waveguides. Conversely, the blazing effect in double-etched gratings provides high coupling efficiency with reduced fabrication intricacy. In this Letter, we demonstrate for the first time, to the best of our knowledge, the realization of an ultra-directional L-shaped grating coupler, seamlessly fabricated by using 193 nm deep-ultraviolet (deep-UV) lithography. We also include a subwavelength index engineered waveguide-to-grating transition that provides an eight-fold reduction of the grating reflectivity, down to 1% (−20 dB). A measured coupling efficiency of −2.7 dB (54%) is achieved, with a bandwidth of 62 nm. These results open promising prospects for the implementation of efficient, robust, and cost-effective coupling interfaces for sub-micrometric SOI waveguides, as desired for large-volume applications in silicon photonics.
Fiber-chip grating couplers providing high-efficiency, robustness and cost-effectivity are recognized as a key building block for large-volume photonic applications. However, the efficiency of silicon-on-insulator (SOI) grating couplers is limited by the mismatch between the beam diffracted by the grating and the fiber mode, back-reflections at the grating-to-waveguide interface, and the power radiated towards the substrate. While the first two limitations can be overcome by grating apodization, the limited diffraction efficiency (directionality) towards the fiber remains a challenge. Typically, grating directionality is optimized by backside metallization, distributed Bragg mirrors, multi-level grating architectures or non-standard etching depths. However, these approaches yield comparatively complex structures, which in turn, come with the expense of extra fabrication costs, hindering the mass-scale development. Alternatively, the blazing effect has been exploited to provide remarkably high directionalities, relying on standard deep and shallow etch depths. Here, we report on the first experimental demonstration of an ultra-directional L-shaped fiber-chip grating coupler fabricated on 300 mm SOI wafer using 193-nm deep-ultraviolet lithography. The grating coupler is realized on a 300-nm-thick Si layer, combining standard full (300 nm) and shallow (150 nm) etch steps in an L-shaped arrangement. This approach yields a remarkably high grating directionality up to 98%. A single-step subwavelength-engineered transition provides an eight-fold reduction of the reflectivity, from ~8% to ~1%. We experimentally demonstrate a coupling efficiency of -2.7 dB, with a 3-dB bandwidth of 62 nm. These results open a new route towards exploiting the blazing effect for the large-volume realization of high-efficiency fiber-chip grating couplers in the low-cost 300 mm SOI photonic platform.
Subwavelength engineering in silicon photonic integrated circuits is a powerful design tool that allows one to synthesize an effective photonic medium with adjustable refractive index. This creates a new degree of freedom in photonic circuit design. We present an overview of the fundamental concept and its application to address several important practical challenges in the implementation of silicon photonics as a next generation photonics platform for telecom, datacom and sensing. In particular, we report our results in developing highly efficient and broadband fiber-chip couplers for silicon photonic wire waveguides using subwavelength engineered edge coupling structures. We experimentally demonstrate a coupling efficiency of −0.4 dB and polarization independent operation for a broad spectral range exceeding 100 nm for optical fiber with a core diameter of 3.2 µm. For coupling to standard SMF-28 fiber with 10.4 µm mode field diameter we numerically demonstrate a subwavelength engineered overlayer structure composed of SiO 2 and Si 3 N 4 which exhibits an overall coupling efficiency exceeding 90%. We have also used our subwavelength structure for coupling experiments with a conventional In- GaAsP/InP buried heterostructure laser at λ = 1.3 µm with a measured near field mode size of 2.1 µm×2.8 µm. Peak coupling efficiency is 1.5 dB with 1-dB alignment tolerance of approximately ±1.2 µm horizontally and ±0.8 µm vertically. We further present subwavelength engineered grating couplers fabricated in a single-etch step for the telecom (1.55 µm) and datacom (1.3 µm) wavelengths with efficiencies exceeding −0.5 dB. Further applications that will be discussed include waveguide crossings, microspectrometers, ultra-fast optical switches, athermal waveguides, evanescent field sensors, polarization rotators and colorless interference couplers.
The Si transparency (1.1 μm – 8 μm wavelength) contains the strongest absorption features of a wide range of chemical and biological substances. However, the use of SOI in the mid-IR is hampered by the large absorption of the buried oxide (BOX) for wavelengths above 4 μm. Silicon membranes have garnered great interest for their unique capability to overcome the BOX limitation while leveraging the advantages of Si photonics. On the other hand, silicon is uniquely poised for the implementation of wideband mid-IR sources based on nonlinear frequency generation. Promising supercontinuum and frequency comb generation have already been demonstrated in Si. Still, current implementations have a limited flexibility in the engineering of phase-matching conditions and dispersion, which complicates the shaping of the nonlinear spectrum. Patterning Si with features smaller than half of the wavelength (well within the capabilities of standard large-volume fabrication processes) has proven to be a simple and powerful tool to implement metamaterials with optimally engineered properties. Here, we present the design of nanostructured silicon membrane waveguides with ultra-wideband flat anomalous dispersion in a wavelength span exceeding 5 µm. Our three-dimensional finite difference time domain (FDTD) calculations predict flat anomalous dispersion near 50 ps/km⋅nm between 2.5 µm and 8 µm wavelength. These results illustrate the potential of subwavelength metamaterial engineering to control chromatic dispersion in Si membrane waveguides. This is a promising step towards the implementation of wideband nonlinear sources in the mid-IR for silicon photonics.
