Press release

March 11, 2022
Keio University

High-Speed Silicon On-Chip Optical Switch Based on aGraphene Microheater

Applications anticipated in next-generation high-capacity optical communication
technologies, optical integrated circuits, and quantum chips.
By integrating a microheater using the atomically thin material, graphene1, with silicon
photonics2, Professor Hideyuki Maki and his colleagues at the Department of Applied Physics
and Physico-Informatics in the Faculty of Science and Technology at Keio University havefabricated a high-performance on-chip optical switch3. This technology is expected to haveapplications for new optical devices on silicon chips such as devices that require high-speed
path switching for optical communications, optical integrated circuits, and photonic chips.
An optical switch is a device that can re-route an optical signal without converting it into
an electrical signal. In the field of optical communications, they are utilized to select theoutput port of the optical path and are indispensable in supporting high-speed
communications. While at present there are a variety of optical switch types underdevelopment, for efficient next-generation compact switches, experts are looking to opticalswitches that have optical waveguides integrated onto silicon chips. Typical optical switcheswithin this category employ a mechanism which changes the refractive index of a materialusing the thermo-optic effect. However, current switches that use such metal heaters face
limitations in their performance related to speed and efficiency.
This research study focused on fabricating a new type of optical switch that uses graphene,a material known for its thermal properties, instead of conventional metals. The researchers
were able to form a graphene-based microheater directly on the silicon photonics device. As
a result, even while using the same basic structure of optical switches that employ
conventional metal-based heaters, the researchers were able to significantly improve theswitches’ performance, drastically shortening the rise and fall response times to a few mere
microseconds. This technology has demonstrated that graphene, a two-dimensional material,
is extremely promising for optical switches in silicon photonics. It is expected to be applied invarious on-chip integrated optical device technologies, including next-generation highcapacity optical communications chips, optical interconnects, optical integrated circuits, andquantum chips, etc.
This research was conducted in collaboration with Associate Professor Yasuaki Monnai(affiliated with the Faculty of Science and Technology at Keio University at the time of thisstudy) of the Research Center for Advanced Science and Technology (RCAST) at theUniversity of Tokyo.
The results of this research were published in the online edition of ACS Nano of theAmerican Chemical Society (ACS) on February 14, 2022.
1. Main Points of Research- A new type of on-chip optical switch was fabricated that replaces conventional metal heaters
with a microheater that uses graphene, a material known for its thermal properties, formed
directly on the silicon waveguide.
- The fabricated graphene-based microheater optical switch demonstrated rise and fall
response times of a mere few microseconds, a significant improvement over conventional
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metal heaters in optical switches that utilize the thermo-optical effect.
- This technology is expected to have wide applications across next-generation high-capacity
optical communication chips, optical integrated circuits, and quantum chips.
2. Background of Research
The rapid development of the information age has led to an era where various services andelectronics are operated via the internet and optical communications. An essential device insupporting the infrastructure of high-capacity communications is the optical switch. Opticalswitches take optical signals (light) and reroute them without needing to convert them toelectric signals. Within the optical communications infrastructure and optical communication
networks, they are an indispensable technology to the optical paths used when transferringdata. There are a variety of switch types used in modern technology such as mechanicalswitches, MEMS (micro-electro-mechanical systems) switches, and switches with waveguides
on glass substrates. However, these switches operate at relatively low speeds of around 1-10
milliseconds. For next-generation high-speed switches, researchers have turned theirattention to silicon photonics based optical switches that use on-chip waveguide structures,
reporting speeds in the order of tens of microseconds. Most optical switches of this variety are
made with a localized metal heater that uses the thermal modulation of the refractive index
of a material (the thermo-optical effect).
However, while research development has already made various discoveries related tooptical switches that rely on metal-based heaters, they cannot measure up to the even higherspeeds and efficiency that will be necessary for chips in next-generation high-capacity
communications, optical interconnects, optical integrated circuits, quantum chips, and othernew optical integrated devices. Instead, completely new technological innovations will need
to be introduced.
3. Content of Research and Results
This research study, focused on creating new technology for on-chip optical switches, wasthe first to replace conventional metal heaters with a graphene-based microheater, leading toan optical switch that can be formed directly on a silicon photonics device and operate atincredibly high speeds. Graphene is an atomically thin carbon material that is known for its
great thermal properties due to its monolayer thickness. This same group of researchers has
previously explored graphene’s capability of up to GHz order of temperature modulation.4
Based on those results, the team went on to make use of graphene’s thermal properties byincorporating it into a microheater, achieving vastly improved speeds to switches that rely on
conventional metal-based heaters. The team designed the optical switch’s structure with an
add/drop-type racetrack resonator and two output waveguides (the through and drop ports)
and placed the graphene microheater directly on the resonator (Figure 1). Applying a bias
voltage to the graphene device induces Joule heating in the resonator, modulating therefractive index of the resonator through the thermo-optical effect. This allows for electricalcontrol over light-coupling through the resonator and thereby creates a reliable optical switchthat can alternate between through and drop output ports (Figure 2). Furthermore, theresearchers measured that the graphene-based optical switch achieved real-time high-speed
operation at 100 kHz and verified modulation rise and fall response times of 1.2 and 3.6
microseconds respectively. This switching speed is up to tens of times faster than that of ametal-heater optical switch using the same mechanism, indicating that the properties of the2/5

optical switch can be significantly improved simply by replacing the metal with graphene.
Finally, the research team theoretically investigated the mechanism of this high-speed
operation by using a heat conduction simulation and theoretically demonstrated thatgraphene’s thermal properties were what enabled such high-speed modulation.
4. Future Developments
Integrated device technology used to undergird the infrastructure of the information age isreaching the uttermost limits of simple electronic circuits in terms of speed, limited power
consumption, and compactness. As a result, integrated optical devices such as silicon photonic
options are attracting attention as a way to break through these limitations and form thebackbone of next-generation device technology. The graphene-based high-speed optical switch
technology demonstrated in this study enables high-speed modulation of optical paths, andthus can be applied in chips for next-generation high-capacity optical communications. Silicon
photonics technology also has likely applications outside of long-distance opticalcommunications such as optical interconnects and optical integrated circuits that can replace
traditional wiring between and within machinery. Likewise, the research findings of thisstudy have potential for quantum computing chips, quantum cryptography chips, and various
other up-and-coming optical technology. Nanocarbon materials such as graphene and carbon
nanotubes have been reported to be used not only for optical switches as demonstrated in thisstudy, but also for new on-chip optical device applications such as light sources on siliconchips or even quantum light sources.5 They are also expected to aid in the development of
nanocarbon integrated optical devices.
This work was partially supported by Grants-in-Aid for Scientific Research (KAKENHI),(16H04355, 23686055, 18K19025, 20H02210); the Japan Science and Technology Agency(JST) Adaptable and Seamless Technology Transfer Program through Target-driven R&D (ASTEP) Industry-Academia Collaboration, (JPMJTR20R4); the Kanagawa Institute ofIndustrial Science and Technology (KISTEC) Strategic Research Seeds Development Project;
the Spintronics Academic Research Base and Collaboration Network; and the Ministry ofEducation, Culture, Sports, Science and Technology (MEXT) Nanotechnology Platform
Project (Nanofabrication).

Figure 1 Left: Schematic of the graphene optical switch.
Right: Scanning electron microscope image of the area surrounding the resonatorof the fabricated optical switch.

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Figure 2 Above: The Optical Switch
Nonresonance condition (left) Resonance condition (right)
Below: Optical Switch Device Operation
Because the refractive index of the resonator changes based on whether a bias
voltage is introduced to the graphene, it is possible to control light coupling in theresonator. This means that it is possible to make the device switch outputs
between through (left) and drop output (right) ports.
Details of Journal Article :“High-Speed and On-Chip Optical Switch Based on a Graphene Microheater”
Shoma Nakamura, Kota Sekiya, Shinichiro Matano, Yui Shimura, Yuuki Nakade, Kenta
Nakagawa, Yasuaki Monnai, Hideyuki Maki
ACS Nano, https://doi.org/10.1021/acsnano.1c09526
Glossary and References
1: Graphene
A nanomaterial. When monolayers of graphene are stacked together, they form graphite, amaterial used in pencil lead.
2: Silicon Photonics
The study of optical devices that use microfabrication on silicon chips to fabricate new highly
integrated optical technology. Conventional integrated circuits use electronic devices integrated
on silicon semiconductor chips to send/receive signals and perform operations, but there are limits
to improving both their speed and power consumption. As such, experts are trying to convert tooptical technology for parts of next-generation hybrid devices.
3: Optical Switch

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In optical communications, an optical switch is a device that can directly re-route an optical signal
without converting it into an electrical signal. This is an indispensable device in high-speed
communication because it maintains the signal speed.
4: Press Release*(Released March 30, 2018)“Development of Graphene High-Speed Optical Devices for use on Silicon Chips: a new path to onchip integrated optical devices” (Japanese language only)
https://www.keio.ac.jp/ja/press-releases/2018/3/30/28-43298/
5: Press Releases*(English Version Released January 28, 2020)“Pure and Efficient Single-photon Source Based on Carbon Nanotubes for Quantum Cryptography”
https://www.keio.ac.jp/en/press-releases/2020/Jan/28/49-67070/*(Released August 13, 2020)"Highly Efficient and Linewidth Carbon Nanotube Optical Devices Integrated into a Silicon
Optical Circuit: Driving communication wavelengths opens doors for applications in optical
integrated circuits and quantum cryptography chips” (Japanese language only)
https://www.keio.ac.jp/ja/press-releases/2020/8/13/28-74127/*Please direct any requests or inquiries on press coverage to the contact information provided
below in advance.
Inquiries about research:
Professor Hideyuki Maki
Department of Applied Physics and Physico-Informatics, Faculty of Science and Technology
Tel.: +81 45-566-1643
Email: maki@appi.keio.ac.jp
http://www.az.appi.keio.ac.jp/maki/
Inquiries about press release:
Keio University Office of Communications and Public Relations (Mr. Sawano)
Tel.: +81 3-5427-1541
Fax: +81 3-5441-7640
Email: m-pr@adst.keio.ac.jp
https://www.keio.ac.jp/en/

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