In the late nineteenth century, Heinrich Hertz demonstrated that the electromagnetic properties of materials are intimately related to their structure at the subwavelength scale by using wire grids with centimetre spacing to manipulate metre-long radio waves. More recently, the availability of nanometre-scale fabrication techniques has inspired scientists to investigate subwavelength-structured metamaterials with engineered optical properties at much shorter wavelengths, in the infrared and visible regions of the spectrum. Here we review how optical metamaterials are expected to enhance the performance of the next generation of integrated photonic devices, and explore some of the challenges encountered in the transition from concept demonstration to viable technology.

Subwavelength integrated photonics.

Optical modulators are at the heart of optical communication links. Ideally, they should feature low loss, low drive voltage, large bandwidth, high linearity, compact footprint and low manufacturing cost. Unfortunately, these criteria have been achieved only on separate occasions. Based on a silicon and lithium niobate hybrid integration platform, we demonstrate Mach–Zehnder modulators that simultaneously fulfil these criteria. The presented device exhibits an insertion loss of 2.5 dB, voltage–length product of 2.2 V cm in single-drive push–pull operation, high linearity, electro-optic bandwidth of at least 70 GHz and modulation rates up to 112 Gbit s−1. The high-performance modulator is realized by seamless integration of a high-contrast waveguide based on lithium niobate—a popular modulator material—with compact, low-loss silicon circuitry. The hybrid platform demonstrated here allows for the combination of ‘best-in-breed’ active and passive components, opening up new avenues for future high-speed, energy-efficient and cost-effective optical communication networks. Low-loss, high-speed and efficient optical modulators on a silicon platform are demonstrated.

High-performance hybrid silicon and lithium niobate Mach–Zehnder modulators for 100 Gbit s −1 and beyond

Research in photonic computing has flourished due to the proliferation of optoelectronic components on photonic integration platforms. Photonic integrated circuits have enabled ultrafast artificial neural networks, providing a framework for a new class of information processing machines. Algorithms running on such hardware have the potential to address the growing demand for machine learning and artificial intelligence in areas such as medical diagnosis, telecommunications, and high-performance and scientific computing. In parallel, the development of neuromorphic electronics has highlighted challenges in that domain, particularly related to processor latency. Neuromorphic photonics offers sub-nanosecond latencies, providing a complementary opportunity to extend the domain of artificial intelligence. Here, we review recent advances in integrated photonic neuromorphic systems, discuss current and future challenges, and outline the advances in science and technology needed to meet those challenges. Photonics offers an attractive platform for implementing neuromorphic computing due to its low latency, multiplexing capabilities and integrated on-chip technology.

Photonics for artificial intelligence and neuromorphic computing

Research in photonic computing has flourished due to the proliferation of optoelectronic components on photonic integration platforms. Photonic integrated circuits have enabled ultrafast artificial neural networks, providing a framework for a new class of information processing machines. Algorithms running on such hardware have the potential to address the growing demand for machine learning and artificial intelligence, in areas such as medical diagnosis, telecommunications, and high-performance and scientific computing. In parallel, the development of neuromorphic electronics has highlighted challenges in that domain, in particular, related to processor latency. Neuromorphic photonics offers sub-nanosecond latencies, providing a complementary opportunity to extend the domain of artificial intelligence. Here, we review recent advances in integrated photonic neuromorphic systems, discuss current and future challenges, and outline the advances in science and technology needed to meet those challenges.

Optical modulators are at the heart of optical communication links Ideally, they should feature low insertion loss, low drive voltage, large modulation bandwidth, high linearity, compact footprint and low manufacturing cost Unfortunately, these criteria have only been achieved on separate occasionsBased on a Silicon and Lithium Niobate hybrid integration platform, we demonstrate Mach-Zehnder modulators that simultaneously fulfill these criteria The presented device exhibits an insertion loss of 25 dB, voltage-length product of 22 Vcm, high linearity, electro-optic bandwidth of at least 70 GHz and modulation rates up to 112 Gbit/s The high-performance modulator is realized by seamless integration of high-contrast waveguide based on Lithium Niobate - the most mature modulator material - with compact, low-loss silicon circuits The hybrid platform demonstrated here allows for the combination of 'best-in-breed' active and passive components, opening up new avenues for enabling future high-speed, energy efficient and cost-effective optical communication networks

High-Performance Hybrid Silicon and Lithium Niobate Mach-Zehnder Modulators for 100 Gbit/s and Beyond

Experimental results for photonic crystal ring resonators (PhCRR) fabricated on the silicon-on-insulator (SOI) platform are presented. Spectral features of the PhCRR's transmission spectrum are discussed and quality factors in excess of 30,000 are reported.

Slow light enhancement of Q-factors in fabricated photonic crystal ring resonators

The cause of a measured −1.4-dB dip at 80 GHz in a silicon capacitor was investigated and determined to be the primary resonance of the integrated metal seal ring (SR) enclosing the component. Specifically, the proximity of a capacitor electrode to the SR was found to activate two parasitic, parallel, and open-circuited coplanar strip transmission lines (TLs), each having their ${\lambda /2}$ resonance at this frequency. This model was confirmed with 3-D finite-element method electromagnetic simulations and measurements of modified SRs. Building on the proposed TL interpretation of the problem, an ultra-broadband equivalent circuit model was developed for such capacitors, successfully replicating the measured SR resonance. Then, the fundamental resonance frequency and related quality factor of various-sized integrated metal SRs were accurately predicted. Finally, potential solutions to damp or cancel such parasitic resonances were investigated. Although cutting the SR at selected locations to disable the parasitic lines was experimentally proven to be successful, damping the parasitic signal with highly doped silicon is found to be the best option in terms of effectiveness and practical implementation. Novel solutions, analogous to the use of radio frequency terminations, were also tested. This paper, to the best of our knowledge, is the first extensive characterization of parasitic resonance afflicting electronics enclosed by an integrated metal SR. The analysis, equivalent circuit model, and solutions reported here can be directly applied to arrays of components or circuits, and are scalable to any device size and operating frequency.

Analysis, Modeling, and Mitigation of Parasitic Resonances in Integrated Metallic Seal Rings

We present an O-band dual-polarization silicon photonic intensity-modulator for short reach direct-detection applications. We demonstrate 112 Gb/s DP-OOK transmission over 10 km at a BER of 6.6 ×10−6 using a Stokes vector direct-detection receiver.

Dual polarization O-band silicon photonic intensity modulator for stokes vector direct detection systems

We demonstrate a compact yet fabrication tolerant polarization beam splitter. Customized angles at each port facilitate a broadband response over 100 nm, including 3 dB insertion loss and 11.2 dB extinction ratio in the C-band.

Extremely Compact and Broadband Polarization Beam Splitter Enabled by Customized Port Angles

We experimentally demonstrate a compact optical add-drop multiplexer based on misaligned sidewall Bragg gratings in a MZI. We achieved 51dB transmission isolation and 5nm 3-dB bandwidth with a footprint of 400 μm × 125 μm.

David Patel

Papers

Slow light enhancement of Q-factors in fabricated photonic crystal ring resonators

Analysis, Modeling, and Mitigation of Parasitic Resonances in Integrated Metallic Seal Rings

Dual polarization O-band silicon photonic intensity modulator for stokes vector direct detection systems

Extremely Compact and Broadband Polarization Beam Splitter Enabled by Customized Port Angles

A compact silicon photonic add-drop multiplexer with misaligned sidewall Bragg gratings in a MZI