An overview is presented of the current state-of-the-art in silicon nanophotonic ring resonators. Basic theory of ring resonators is discussed, and applied to the peculiarities of submicron silicon photonic wire waveguides: the small dimensions and tight bend radii, sensitivity to perturbations and the boundary conditions of the fabrication processes. Theory is compared to quantitative measurements. Finally, several of the more promising applications of silicon ring resonators are discussed: filters and optical delay lines, label-free biosensors, and active rings for efficient modulators and even light sources.

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Silicon microring resonators

Nonlinear photonic chips can generate and process signals all-optically with far superior performance to that possible electronically — particularly with respect to speed. Although silicon-on-insulator has been the leading platform for nonlinear optics, its high two-photon absorption at telecommunication wavelengths poses a fundamental limitation. We review recent progress in non-silicon CMOS-compatible platforms for nonlinear optics, with a focus on Si3N4 and Hydex®. These material systems have opened up many new capabilities such as on-chip optical frequency comb generation and ultrafast optical pulse generation and measurement. We highlight their potential future impact as well as the challenges to achieving practical solutions for many key applications. This article reviews recent progress in the use of silicon nitride and Hydex as non-silicon-based CMOS-compatible platforms for nonlinear optics. New capabilities such as on-chip optical frequency comb generation, ultrafast optical pulse generation and measurement using these materials, and their potential future impact and challenges are covered.

New CMOS-compatible platforms based on silicon nitride and hydex for nonlinear optics

An on-chip integrated wavelength demultiplexer designed using an inverse computational algorithm is experimentally demonstrated. 1,300 and 1,550 nm wavelength light is sorted in a device area of just 2.8 × 2.8 μm2.

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Inverse design and demonstration of a compact and broadband on-chip wavelength demultiplexer

We present a comprehensive overview of sensor technology exploiting optical whispering gallery mode (WGM) resonances. After a short introduction we begin by detailing the fundamental principles and theory of WGMs in optical microcavities and the transduction mechanisms frequently employed for sensing purposes. Key recent theoretical contributions to the modeling and analysis of WGM systems are highlighted. Subsequently we review the state of the art of WGM sensors by outlining efforts made to date to improve current detection limits. Proposals in this vein are numerous and range, for example, from plasmonic enhancements and active cavities to hybrid optomechanical sensors, which are already working in the shot noise limited regime. In parallel to furthering WGM sensitivity, efforts to improve the time resolution are beginning to emerge. We therefore summarize the techniques being pursued in this vein. Ultimately WGM sensors aim for real-world applications, such as measurements of force and temperature, or alternatively gas and biosensing. Each such application is thus reviewed in turn, and important achievements are discussed. Finally, we adopt a more forward-looking perspective and discuss the outlook of WGM sensors within both a physical and biological context and consider how they may yet push the detection envelope further.

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Whispering gallery mode sensors

The application of portable, easy-to-use and highly sensitive lab-on-a-chip biosensing devices for real-time diagnosis could offer significant advantages over current analytical methods. Integrated optics-based biosensors have become the most suitable technology for lab-on-chip integration due to their ability for miniaturization, their extreme sensitivity, robustness, reliability, and their potential for multiplexing and mass production at low cost. This review provides an extended overview of the state-of-the-art in integrated photonic biosensors technology including interferometers, grating couplers, microring resonators, photonic crystals and other novel nanophotonic transducers. Particular emphasis has been placed on describing their real biosensing applications and wherever possible a comparison of the sensing performances between each type of device is included. The way towards achieving operative lab-on-a-chip platform incorporating the photonic biosensors is also reviewed. Concluding remarks regarding the future prospects and potential impact of this technology are also provided.

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Integrated optical devices for lab-on-a-chip biosensing applications

We give an overview of recent progress in passive spectral filters and demultiplexers based on silicon-on-insulator photonic wire waveguides: ring resonators, interferometers, arrayed waveguide gratings, and echelle diffraction gratings, all benefit from the high-index contrast possible with silicon photonics. We show how the current generation of devices has improved crosstalk levels, insertion loss, and uniformity due to an improved fabrication process based on 193 nm lithography.

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Silicon-on-Insulator Spectral Filters Fabricated With CMOS Technology

We present a slot-waveguide-based ring resonator in silicon on insulator (SOI) with a footprint of only 13 mum times 10 mum, fabricated with optical lithography. Experiments show that it has 298 nm/RIU sensitivity and a detection limit of 4.2middot10-5 RIU for changes in the refractive index of the top cladding. We prove for the first time that surface chemistry for selective label-free sensing of proteins can be applied inside a 100 nm-wide slot region and demonstrate that the application of a slot waveguide instead of a normal waveguide increases the sensitivity of an SOI ring resonator with a factor 3.5 for the detection of proteins.

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Label-Free Biosensing With a Slot-Waveguide-Based Ring Resonator in Silicon on Insulator

Optical cavities are considered promising devices for biosensing to satisfy the high demands of the growing proteomics market. We present a prototype sensor for multiplexed label-free monitoring of biomolecular interactions based on an array of three-by-four silicon-on-insulator microring resonators fabricated by standard complementary metal-oxide semiconductor (CMOS) technology. Parallel readout was performed with an infrared camera. We have demonstrated the simultaneous detection of several model antibodies in a highly specific way. For that, we integrated the optical chip with low-temperature polydimethylsiloxane (PDMS) packaging, enabling excellent compatibility with the receptor molecules. We obtain a very selective biomolecular interaction by grafting a 2.5-nm poly(ethylene glycol) layer to the silicon. Using silicon on insulator as the material platform offers high sensitivity through miniaturization and low-cost mass fabrication through standard CMOS technology steps. The sensors in this interrogation method are able to detect 3.4 pg/mm2 of a protein overlayer or a theoretical total mass of about 74 ag. For this novel lab on a chip, the fabrication, chemistry, readout, and packaging are developed to be scalable to detect hundreds of affinity interactions within several minutes.

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Multiplexed Antibody Detection With an Array of Silicon-on-Insulator Microring Resonators

In recent years, portable, sensitive, and cheap sensors have gained interest in fields such as medical diagnostics, food quality control, environmental monitoring, and drug development. Especially optical sensors in silicon-on-insulator (SOI) have proven to be very promising, because they have a low limit of detection, are very compact, and can be made with cheap mass fabrication techniques. While a lot of progress has been made to implement such sensors in small and easy-to-use cartridges, many applications require a sensor probe that can perform measurements at locations that are hard to reach. We introduce a method to integrate an SOI photonic sensor on the facet of a standard single-mode optical fiber, and apply this method to a well-known ring resonator refractive index sensor without deteriorating its sensitivity. We thus combined the good performance of SOI sensors with the high mobility of optical fibers.

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K. De Vos

Papers

Silicon microring resonators

Silicon-on-Insulator Spectral Filters Fabricated With CMOS Technology

Label-Free Biosensing With a Slot-Waveguide-Based Ring Resonator in Silicon on Insulator

Multiplexed Antibody Detection With an Array of Silicon-on-Insulator Microring Resonators

Silicon-on-Insulator Microring Resonator Sensor Integrated on an Optical Fiber Facet