Integrated optical filters play a key role in modern optical systems, finding extensive applications in quantum optics, biosensing, programmable photonics, and telecommunications. Among the most commonly used structures utilized for implementing integrated optical filters are ring resonators and Bragg gratings. Bragg gratings are characterized by a periodically perturbed refractive index profile along the propagation direction. By precisely engineering the strength of the perturbation along the grating length, filters with arbitrary spectral responses can be achieved.
In this work we summarize our recent contributions to integrated Bragg filters in Si-photonics, covering designs for applications ranging from telecommunications to quantum optics.
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.
Quantum photonics has emerged as a key driver for advancing applications, such as quantum communication and sensing, that harness the potential of quantum effects beyond classical capabilities. Photonic integrated circuits favor such implementations by providing low loss and interferometric stability. Nonetheless, to fully leverage these advantages, two major challenges need to be addressed: highly efficient fiber-to-chip light coupling and waveguide-integrated single-photon detection. In this work, we utilize subwavelength grating (SWG) metamaterials to design (i) novel off-chip couplers enabling sub-decibel coupling efficiency and (ii) a new superconducting nanowire single-photon detector (SNSPD) concept featuring enhanced photon absorption and fast detection.
In this talk we present our recent advances in SWG metamaterial engineering. We will show a 1D-optical phased array composed of 112 evanescent-coupled surface emitting antennas with a length of 1.5 mm and fed by a compact distributed Bragg deflector. The measurements demonstrate a wavelength-steerable collimated beam with a far-field angular divergence of 1.8o × 0.2o. Experimental results of a bricked SWG 2×2 MMI coupler are also shown, achieving polarization agnostic performance in the 1500nm to 1560nm wavelength range. Both devices were fabricated on a standard 220-nm SOI platform using a single full-etch step process, with a minimum feature size of 80 nm, and thus compatible with immersion deep-UV lithography.
We present two novel topologies of subwavelength grating (SWG) waveguides: the bricked-SWG and the evanescently-coupled-SWG. The bricked topology enables accurate control of waveguide anisotropy while maintaining the index and dispersion engineering advantage intrinsic to SWG waveguides. The evanescently-coupled-SWG allows unprecedented control of the strength of the modal perturbation in waveguide Bragg gratings and nanophotonic antennas. Both topologies leverage a Manhattan-like pattern, with pixel sizes compatible with deep-uv lithography. Our recent results will be discussed, focusing on polarization-independent multimode interference couplers for the O and C bands and a millimeter-long narrow-beam steerable optical antenna array with angular divergence of only 1.8o×0.2o.
Subwavelength grating (SWG) structures are extremely versatile optical metamaterials that have become a fundamental design tool for the optimization of photonic devices. The importance of SWG structures arises from their capability to synthesize artificial materials with tailorable optical properties, including refractive index, dispersion, or anisotropy. In this invited talk, our advances in SWG topologies to further expand the design space of silicon photonic devices are discussed. The proposed topologies enable enhanced performance such as bandwidth broadening, polarization independency, or increased feature sizes, paving the way for the next generation of SWG-optimized devices.
Optical phased arrays in silicon photonics are an emerging technology for free-space communications and light detection and ranging (LIDAR). While traditional LIDARs with discrete components and mechanical beam steering are difficult to integrate and scale, silicon-based arrays have taken a massive leap forward in developing beam steering systems with compact footprint and high performance on a single chip. Here, we report our results in the development of chip-scale circular phased arrays. Arrays formed in a grid of concentric rings are shown to suppress the sidelobes, expand the steering range and obtain narrower beamwidths, with large spacing between optical elements.
Subwavelength grating metamaterials have become an integral design tool in silicon photonics. The lithographic segmentation of integrated waveguides at the subwavelength scale allows us to control optical properties such as mode delocalization, wavelength dispersion, and birefringence. So far, a range of subwavelength-based devices with unprecedented performance has been demonstrated, such as couplers, polarization-handling structures, filters, and input/output chip interfaces. In this invited talk, we will review the anisotropic foundations of subwavelength-grating metamaterial design and will provide an overview of our latest advances in subwavelength-enhanced silicon photonics devices, including optical antennas for beam steering and multi-line Bragg filters for spectral shaping.
Silicon photonic waveguides patterned at the subwavelength level behave as metamaterials whose optical properties, including refractive index, dispersion and anisotropy can be tuned by judiciously designing the subwavelength geometry. Over the past years, the added design freedom afforded by these structures has enabled a wide variety of novel high performance devices, ranging from high efficiency fibre-to-chip couplers, to on-chip polarization and mode management, and ultra-broadband waveguide couplers covering several optical communication bands. In this invited keynote talk we will revisit the physical foundations of these structures, explore some of the latest advances in the field with applications in both telecommunications and sensing, and discuss some of the outstanding challenges to move these structures from research labs to large-scale commercialisation.
Periodic silicon waveguides with a pitch that is below half the effective wavelength of light support diffraction-less Bloch modes. These modes propagate as through a homogeneous, artificial-core metamaterial waveguide whose optical characteristics can be engineered by lithographic patterning. Subwavelength gratings (SWGs) provide designers with unique tools to control the refractive index, dispersion and birefringence of the equivalent metamaterial, yielding improved device performance. Based on this approach many high-performance optical devices have been designed and experimentally demonstrated in the last years. In this paper we will review the fundamentals of SWG engineering and present some of our latest findings.
The subwavelength patterning of planar structures is now widely used in silicon photonics, enabling the synthesis of metamaterials with engineered optical properties, including refractive index, dispersion, and anisotropy. A wide range of integrated devices based on subwavelength grating (SWG) metamaterials have been demonstrated at telecom wavelengths, some with unprecedented performance. The benefits of SWG metamaterials can be leveraged not only in the typical telecom near-infrared bands, but also at the longer mid-infrared wavelengths. In this invited presentation, we will review our latest developments in SWG-based silicon and germanium photonic devices for the near- and the midinfrared.
Presentation: "Suspended silicon waveguide devices for mid-infrared sensing undefined" for the Emerging Applications in Silicon Photonics conference of the SPIE Photonex and Vacuum Expo 2020
Enabled by technological improvements, photonic devices and circuits are becoming increasingly more complex. Non-trivial geometries are designed to reduce device footprint, improve performance, and introduce novel functionalities. However, the number of design variables required to properly represent these geometries quickly grows, limiting the effectiveness of classical design approaches. Moreover, parameters are often strongly interdependent, restricting the use of sequential optimizations or independent parameter sweeps. Although several optimization techniques can be effective for multi-parameter design, they commonly allow to optimize for a single or a handful designs and the optimization process needs to be repeated if new performance criteria are introduced. In contrast to classical design approaches, the in uences of the design parameters remain hidden as well as the general behavior of the design space. In this paper we present an extension of our recent work on the application of machine learning pattern recognition to the design of multi-parameter photonic devices. In particular, we propose using a combination of local optimization based on the adjoint method and the use of dimensionality reduction. Adjoint optimization is used multiple times to generate a small set of different designs with high performance. Dimensionality reduction is applied to analyze the relationship between these degenerate designs and identify a lower-dimensional design sub-space that includes all alternative good designs. This sub-space can be mapped for any performance criteria thus enabling informed decisions based on the relative priorities of all relevant performance specifications. As a proof of concept, we demonstrate a ten-parameter design of an integrated photonic power splitter using silicon-on-insulator technology. We identify a region of possible high performance design solutions and select two design candidates either maximizing the splitter efficiency or minimizing back-reflection.
Integrated photonics devices, based in subwavelength grating (SWG) metamaterials, have shown unprecedented performance in a wide variety of situations. Since their proposal and first experimental demonstration in 2010 designers have made use of the new degrees of freedom provided by these structures to design advanced devices with improved capabilities. The extended design space provided by SWG structures has been successfully used to engineer the refractive index, the dispersion and, more recently, the waveguide birefringence, thus allowing novel advanced device design. In this invited talk we will review some of the advances made by our group in the field
Silicon photonics has emerged as an intense field of research due to its unique capabilities to integrate photonics and electronics into the same platform using standard semiconductor fabrication facilities. Subwavelength grating (SWG) structures, i.e. periodic nanostructured waveguides with a pitch below half the wavelength of light, allow the lossless propagation of Bloch-Floquet modes which closely resemble propagation through a homogenous waveguide with optical properties (refractive index, dispersion, birefringence) that can be tailored to fulfill specific design goals. SWG engineering is now routinely used for novel and advanced device design. Fiber-chip couplers, polarization and mode multiplexers, multimode interference couplers (MMIs), lenses, and bragg filters have been successfully designed in our group based in these concepts. In this invited talk we will review some of our last advances in the field.
Silicon photonics is one of the most promising candidates to achieve lab-on-a-chip systems. Making use of the evanescent-field sensing principle, it is possible to determine the presence and concentration of substances by simply measuring the variation produced by the light-matter interaction in the real part of the mode effective index (in the near-infrared band), or in its imaginary part in a specific range of wavelengths (in the mid-infrared band).
Regardless of which is the operating wavelength range, it is essential to select the proper sensing waveguide in order to maximize the device sensitivity. In this work we will review the potential of diffractionless subwavelength grating waveguides (SWG) for sensing applications by demonstrating their powerful capability to engineer the spatial distribution of the mode profile, and thereby to maximize the light-matter interaction. Among other things, we will demonstrate that the SWG waveguide dimensions used until now in the near-infrared are not optimal for sensing applications.
In the mid-infrared band, due to the unacceptable losses of silicon dioxide for wavelengths longer than 4 μm, an additional effort is required to provide a more convenient platform for the development of future applications. In this regard, we will also show our recent progress in the development of a new platform, the suspended silicon waveguide with subwavelength metamaterial cladding. A complete set of elemental building blocks capable of covering the full transparency window of silicon (λ < ∼8.5 μm) will be discussed.
Integrated nanophotonic component design processes are often constrained by computational resources. Advances in simulation and optimization tools have allowed more efficient exploration of larger design spaces. These developments reduce the time-consuming and intuition-limited effort of encoding physical insights into the design structure. However, we argue that efficient optimization is only part of the solution to tackle larger multi-parameter design spaces. Finding patterns in such a space can be more valuable than identifying the individual optima alone. This is particularly true when transitioning from simulation to real device fabrication, where considerations such as tolerance to fabrication imperfections, bandwidth, etc. take an important role but are ignored at the optimization stage. The elucidation of patterns in a complex design space enables efficient identification of designs addressing these additional considerations. As an example, in this presentation we demonstrate how limited data collected from the optimization process of a multisegment vertical grating coupler can be used to identify such patterns through the application of machine learning techniques. The identified patterns, some more interpretable than others, can be used in multiple ways: from speeding up the remaining optimization process itself to gaining insight into the properties of an interesting subset of designs. Together those insights offer a significantly clearer picture of the design space and form the basis for making much more informed decisions on the final designs to be fabricated.
Subwavelength gratings (SWGs) are periodic structures with a pitch (Λ) smaller than the wavelength of the propagating wave (λ), so that diffraction effects are suppressed. These structures thus behave as artificial metamaterials where the refractive index and the dispersion profile can be controlled with a proper design of the geometry of the structure. SWG waveguides have found extensive applications in the field of integrated optics, such as efficient fiber-chip couplers, broadband multimode interference (MMI) couplers, polarization beam splitters or evanescent field sensors, among others. From the point of view of nano-fabrication, the subwavelength condition (Λ << λ) is much easier to meet for long, mid-infrared wavelengths than for the comparatively short near-infrared wavelengths. Since most of the integrated devices based on SWGs have been proposed for the near-infrared, the true potential of subwavelength structures has not yet been completely exploited. In this talk we summarize some valuable guidelines for the design of high performance SWG integrated devices. We will start describing some practical aspects of the design, such as the range of application of semi-analytical methods, the rigorous electromagnetic simulation of Floquet modes, the relevance of substrate leakage losses and the effects of the random jitter, inherent to any fabrication process, on the performance of SWG structures. Finally, we will show the possibilities of the design of SWG structures with two different state-of-the-art applications: i) ultra-broadband MMI beam splitters with an operation bandwidth greater than 300nm for telecom wavelengths and ii) a set of suspended waveguides with SWG lateral cladding for mid-infrared applications, including low loss waveguides, MMI couplers and Mach-Zehnder interferometers.
Efficient coupling of light from a chip into an optical fiber is a major issue in silicon photonics, as the dimensions of high-index-contrast photonic integrated waveguides are much smaller than conventional fiber diameters. Surface grating couplers address the coupling problem by radiating the optical power from a waveguide through the surface of the chip to the optical fiber, or vice versa. However, since the grating radiation angle substantially varies with the wavelength, conventional surface grating couplers cannot offer high coupling efficiency and broad bandwidth simultaneously. To overcome this limitation, for the near-infrared band we have recently proposed SOI-based zero-order grating couplers, which, making use of a subwavelength-engineered waveguide and a high-index prism, suppress the explicit dependence between the radiation angle and the wavelength, achieving a 1-dB bandwidth of 126 nm at λ = 1.55 μm. However, in the near-infrared, the bandwidth enhancement of zero-order grating couplers is limited by the effective index wavelength dispersion of the grating. In the mid-infrared spectral region, the waveguide dispersion is lower, alleviating the bandwidth limitation. Here we demonstrate numerically our zero-order grating coupler concept in the mid-infrared at λ = 3.8 μm. Several couplers for the silicon-on-insulator and the germanium-on-silicon nitride platforms are designed and compared, with subdecibel coupling efficiencies and 1-dB bandwidths up to ~680 nm.
Waveguides structured at the subwavelength scale frustrate diffraction and behave as optical metamaterials with controllable refractive index. These structures have found widespread applications in silicon photonics, ranging from sub-decibel efficiency fibre-chip couplers to spectrometers and polarization rotators. Here, we briey describe the design foundations for sub-wavelength waveguide devices, both in terms of analytic effective medium approximations, as well as through rigorous Floch-Bloquet mode simulation. We then focus on two novel structures that exemplify the use of subwavelength waveguides: mid-infrared waveguides and ultra-broadband beamsplitters.
The mid-infrared is attracting increasing attention since many molecules, including potentially hazardous gases such as methane and carbon dioxide, exhibit very specific absorption spectra in this wavelength region. Integrated silicon photonics circuits are envisioned to enable compact and low-cost measurement solutions for these molecules. Multimode interference couplers (MMIs) are basic building blocks for photonic circuits and a broad operational bandwidth is key if flexible operation is to be achieved, e.g. to detect different gases. Here we overcome the bandwidth limitations found in classical MMIs by segmenting the multimode region at a sub-wavelength pitch to engineer its refractive index and dispersion. We achieve less than 0:5 dB imbalance and excess loss in the complete 3 ̶ 4 µm wavelength range. The sub-wavelength MMI not only exhibits nearly threefold improvement in bandwidth, but is also about three times shorter than the conventional device.
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