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.

(Invited) Heterogeneous Integration of III-V Semiconductors on Si Photonics Platform

This paper reviews a technology to heterogeneously integrate III–V semiconductors on a Si platform for the laser diodes and high-efficiency Mach–Zehnder modulators (MZMs) of optical transceivers. Membrane III–V semiconductor films provide efficient optical coupling between the III–V semiconductor devices and widely developed thin Si waveguide circuits. The membrane laser diode shows a fiber coupled output power from the Si waveguide of 4.6 mW and single-mode lasing with a side-mode suppression ratio of 49 dB. In addition, the high-efficiency MZM with a 700 μm long membrane InGaAsP/Si metal-oxide-semiconductor capacitor phase shifter shows a V π L of 0.09 Vcm, insertion loss of around 2 dB, and an eye opening at for 32 Gbit s−1 nonreturn-to-zero signal with pre-emphasis signal inputs.

https://iopscience.iop.org/article/10.7567/1347-4065/ab0741/pdf

III–V/Si integration technology for laser diodes and Mach–Zehnder modulators

A photonic-crystal photodetector having a <1-fF junction capacitance, 1-A/W responsivity, and 40-Gbit/s eye opening was demonstrated. Its resistor-loaded configuration unveiled a light-to-voltage conversion with a kV/W efficiency without amplifiers, promising for an fJ/bit-energy on-chip receiver.

Sub-fF-capacitance photonic-crystal photodetector towards fJ/bit on-chip receiver

Sub-fF-Capacitance Photonic-Crystal Photodetector Towards fJ/bit On-Chip Receiver

Optical interconnects are expected to reduce the power consumption of ICT instruments. To realize chip-to-chip or chip-scale
optical interconnects, it is essential to fabricate semiconductor lasers with a smaller energy cost. In this context, we
are developing lambda-scale embedded active-region photonic-crystal (LEAP) lasers as light sources for chip-scale
optical interconnects.
We demonstrated the first continuous-wave (CW) operation of LEAP lasers in 2012 and reported a record low threshold
current and energy cost of 4.8 μA and 4.4 fJ/bit at 10 Gbit/s in 2013. We have also integrated photonic crystal
photodetectors on the same InP chip and demonstrated waveform transfer along 500-μm-long waveguides. Although
LEAP lasers exhibit excellent performance, they have to be integrated on Si wafers for use as light sources for chip-scale
optical interconnects.
In this paper, we give a brief overview of our LEAP lasers on InP and report our recent progress in fabricating them on
Si. We bonded the InP wafers with quantum-well gain layers directly on thermally oxidized Si wafers and performed all
process steps on the Si wafer, including high-temperature regrowth. After this process modification, we again achieved
CW operation and obtained a threshold current of 57 μA with a maximum output power of more than 3.5 μW at the
output waveguides. An output light was successfully guided through 500 × 250-nm InP waveguides.

Takuro Fujii

Papers

(Invited) Heterogeneous Integration of III-V Semiconductors on Si Photonics Platform

III–V/Si integration technology for laser diodes and Mach–Zehnder modulators

Sub-fF-capacitance photonic-crystal photodetector towards fJ/bit on-chip receiver

Sub-fF-Capacitance Photonic-Crystal Photodetector Towards fJ/bit On-Chip Receiver

Photonic-crystal lasers on silicon for chip-scale optical interconnects