Electro-optic modulators translate high-speed electronic signals into the optical domain and are critical components in modern telecommunication networks1,2 and microwave-photonic systems3,4. They are also expected to be building blocks for emerging applications such as quantum photonics5,6 and non-reciprocal optics7,8. All of these applications require chip-scale electro-optic modulators that operate at voltages compatible with complementary metal–oxide–semiconductor (CMOS) technology, have ultra-high electro-optic bandwidths and feature very low optical losses. Integrated modulator platforms based on materials such as silicon, indium phosphide or polymers have not yet been able to meet these requirements simultaneously because of the intrinsic limitations of the materials used. On the other hand, lithium niobate electro-optic modulators, the workhorse of the optoelectronic industry for decades9, have been challenging to integrate on-chip because of difficulties in microstructuring lithium niobate. The current generation of lithium niobate modulators are bulky, expensive, limited in bandwidth and require high drive voltages, and thus are unable to reach the full potential of the material. Here we overcome these limitations and demonstrate monolithically integrated lithium niobate electro-optic modulators that feature a CMOS-compatible drive voltage, support data rates up to 210 gigabits per second and show an on-chip optical loss of less than 0.5 decibels. We achieve this by engineering the microwave and photonic circuits to achieve high electro-optical efficiencies, ultra-low optical losses and group-velocity matching simultaneously. Our scalable modulator devices could provide cost-effective, low-power and ultra-high-speed solutions for next-generation optical communication networks and microwave photonic systems. Furthermore, our approach could lead to large-scale ultra-low-loss photonic circuits that are reconfigurable on a picosecond timescale, enabling a wide range of quantum and classical applications5,10,11 including feed-forward photonic quantum computation. Chip-scale lithium niobate electro-optic modulators that rapidly convert electrical to optical signals and use CMOS-compatible voltages could prove useful in optical communication networks, microwave photonic systems and photonic computation.

Integrated lithium niobate electro-optic modulators operating at CMOS-compatible voltages

Recent advances in photonic integration have propelled microwave photonic technologies to new heights. The ability to interface hybrid material platforms to enhance light–matter interactions has led to the development of ultra-small and high-bandwidth electro-optic modulators, low-noise frequency synthesizers and chip signal processors with orders-of-magnitude enhanced spectral resolution. On the other hand, the maturity of high-volume semiconductor processing has finally enabled the complete integration of light sources, modulators and detectors in a single microwave photonic processor chip and has ushered the creation of a complex signal processor with multifunctionality and reconfigurability similar to electronic devices. Here, we review these recent advances and discuss the impact of these new frontiers for short- and long-term applications in communications and information processing. We also take a look at the future perspectives at the intersection of integrated microwave photonics and other fields including quantum and neuromorphic photonics. This Review discusses recent advances of microwave photonic technologies and their applications in communications and information processing, as well as their potential implementations in quantum and neuromorphic photonics.

Integrated microwave photonics

Optics offers unique opportunities for reducing energy in information processing and communications while simultaneously resolving the problem of interconnect bandwidth density inside machines. Such energy dissipation overall is now at environmentally significant levels; the source of that dissipation is progressively shifting from logic operations to interconnect energies. Without the prospect of substantial reduction in energy per bit communicated, we cannot continue the exponential growth of our use of information. The physics of optics and optoelectronics fundamentally addresses both interconnect energy and bandwidth density, and optics may be the only scalable solution to such problems. Here we summarize the corresponding background, status, opportunities, and research directions for optoelectronic technology and novel optics, including subfemtojoule devices in waveguide and novel two-dimensional (2-D) array optical systems. We compare different approaches to low-energy optoelectronic output devices and their scaling, including lasers, modulators and LEDs, optical confinement approaches (such as resonators) to enhance effects, and the benefits of different material choices, including 2-D materials and other quantum-confined structures. With such optoelectronic energy reductions, and the elimination of line charging dissipation by the use optical connections, the next major interconnect dissipations are in the electronic circuits for receiver amplifiers, timing recovery, and multiplexing. We show we can address these through the integration of photodetectors to reduce or eliminate receiver circuit energies, free-space optics to eliminate the need for timing and multiplexing circuits (while also solving bandwidth density problems), and using optics generally to save power by running large synchronous systems. One target concept is interconnects from ∼1 cm to ∼10 m that have the same energy (∼10 fJ/bit) and simplicity as local electrical wires on chip.

Attojoule Optoelectronics for Low-Energy Information Processing and Communications

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.

We have successfully demonstrated epitaxial growth of an InP layer to fabricate an InGaAsP/InP buried heterostructure (BH) on Si substrate, in which a thin InGaAsP-based active layer is directly bonded to SiO2/Si substrate before fabricating the BH. An O2 plasma-assisted wafer bonding is used to combine a 2-inch Si substrate with 1-μm thick thermal oxide and a 2-inch InP substrate with the active layer. We confirm that the crystal quality is not significantly degraded compared with that on InP substrate. These results indicate that the proposed fabrication method is suitable for integrating III-V materials on Si substrate.

InGaAsP/InP buried heterostruture on SiO 2 /Si substrate using epitaxial growth after direct bonding of thin active layer

A high-efficiency and low-loss Mach-Zehnder modulator on a Si platform is a key component for meeting the demand for high-capacity, low-cost and low-power optical transceivers in future optical fiber links. We report a III-V/Si MOS capacitor Mach-Zehnder modulator with an ultrahigh-efficiency phase shifter, which consists of n-type InGaAsP and ptype Si. The main advantage of this structure is a large electron-induced refractive index change and low free-carrier absorption loss of the n-type InGaAsP. The heterogeneously integrated InGaAsP/Si MOS capacitor structure is fabricated by using the oxygen plasma assisted bonding method. The fabricated device shows VπL of 0.09 Vcm, a value over three-times smaller than that of the conventional Si MOS capacitor Mach-Zehnder modulator, without an increase in the insertion loss. This clearly indicates that the proposed III-V/Si MOS capacitor Mach-Zehnder modulator overcomes the performance limit of the Si Mach-Zehnder modulator.

Ultrahigh efficiency III-V on Si MOS capacitor optical modulator

Based on the combination of cascaded MZIs and an EAM-gate array, we have developed an InP monolithically integrated very-small-footprint 4×4 optical switch that exhibits high-extinction ratio of 40 dB, low-loss of 21 dB, and high-switching-speed of 2 ns.

Monolithically Integrated 4×4 Optical Switch with Cascaded MZIs and EAM-Gate Array

We have developed energy-efficient 1.3-μm membrane lasers on Si for datacom applications. We employ InGaAlAs-based MQWs as an active material to improve the temperature characteristics and modulation speed. We achieved lasing at up to 95°C and direct modulation up to 40-Gbit/s at 25°C.

Temperature Characteristics of 1.3-μm Membrane Lasers on InP-on-Insulator Substrate

A Si-waveguide-coupled membrane photodetector (PD) with an InGaAsP multiple-quantum-well absorption layer shows a fiber-to-PD responsivity of 0.4 A/W and 3-dB bandwidth over 20 GHz at a fiber input power up to +5 dBm.

Takuro Fujii

Papers

InGaAsP/InP buried heterostruture on SiO 2 /Si substrate using epitaxial growth after direct bonding of thin active layer

Ultrahigh efficiency III-V on Si MOS capacitor optical modulator

Monolithically Integrated 4×4 Optical Switch with Cascaded MZIs and EAM-Gate Array

Temperature Characteristics of 1.3-μm Membrane Lasers on InP-on-Insulator Substrate

Si-waveguide-coupled Membrane InGaAsP-multiple-quantum-well Photodetector with Large Bandwidth at High Optical Input Power