Ultrafast, Energy-Efficient All-Optical Switching with Plasmonic Waveguides

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TOKYO, Dec. 20, 2019 — NTT and the Tokyo Institute of Technology (Tokyo Tech) have developed an all-optical switch that operates in the ultrafast regime with a response time in the femtosecond (fs) range and with energy consumption in the femtojoule (fJ) range. To achieve both speed and energy efficiency in the switching, the researchers combined a nanosize optical waveguide based on plasmonics with graphene.

All-optical switches are expected to operate faster than switches that are electrically controlled. However, the switching energies of all-optical switches are relatively high. For ultrafast information processing, optical components need to be densely integrated in photonic circuits, and the energy consumption of each device must be low.
The researchers used graphene because it has an ultrafast nonlinear optical response and a large absorption coefficient over a broadband wavelength range.

Monolayer graphene absorbs 2.3% of light over a wavelength range from the visible to infrared, giving it a much larger absorption coefficient than conventional semiconductors. In addition, graphene shows saturable absorption and a nonlinear optical effect, and its response time can be less than 100 fs. This ultrafast response originates from the very short relaxation times of graphene carriers, the researchers said.

In the new all-optical switch, a transmittance change induced by saturable absorption switches the on/off state, and the absorption saturates as a result of photo-excited carriers.

However, graphene is only 1 atom thick — too thin for optical devices, according to the team. Further, the interaction between graphene and light is weak when a conventional waveguide made of silicon is used, which could increase energy consumption. The research team overcame this problem by strongly confining light within a nanoscale plasmonic waveguide. With a core width of only 30 nm and a height of only 20 nm, the core area of the waveguide was as small as λ2/4000 (wavelength λ; in this case the wavelength was 1550 nm).

The absorption coefficient for a plasmonic waveguide with a core size of 30 × 20 nm is estimated to be much higher than for a graphene-loaded silicon waveguide with a core measuring 400 × 200 nm. This enabled the researchers to decrease the device length. Further, light intensity at the position of the graphene in a plasmonic waveguide is 310× larger than it would be in a silicon waveguide, because the plasmonic waveguide core is much smaller. This enhancement significantly reduced the energy required for switching, the team said.

This is a comparison and performance of different optical switching architectures. Courtesy of NTT, Tokyo Tech.

A comparison and performance of different optical switching architectures. Courtesy of NTT, Tokyo Tech.

The researchers loaded graphene on the plasmonic waveguides using nanofabrication technologies developed by NTT. The cross-sectional area of these plasmonic waveguides was about 1/100th that of what would be considered compact silicon waveguides, and about 1/105th that of single-mode optical fibers.

The researchers' propose a switch that consists of graphene-loaded MIM-WGs equipped with plasmonic mode converters connected to Si-WGs. Courtesy of NTT, Tokyo Tech.

The researchers propose an all-optical switch that consists of graphene-loaded MIM-WGs equipped with plasmonic mode converters connected to Si-WGs. Courtesy of NTT, Tokyo Tech.

The experimentally obtained absorption coefficient for the graphene-loaded plasmonic waveguide was 1.7 dB/μm. The obtained saturation energy was 12 fJ, which is four orders of magnitude smaller than a graphene-loaded silicon waveguide. Because an increase in light intensity can be regarded as a reduction of the saturation energy in saturable absorption, this result means that the light intensity was enhanced by four orders of magnitude. The new all-optical switch demonstrated a switching time of 260 fs, which was achieved with a switching energy of 35 fJ. According to the researchers, the switching energy is the smallest value ever reported (1/100th of that previously reported) for any type of all-optical switch operating at less than one picosecond.

The signal light is switched on/off with control light in all-optical switches. In this switch, the control light induces a nonlinear optical effect of graphene; i.e., it changes the degree of absorption by graphene. Courtesy of NTT, Tokyo Tech.

The signal light is switched on/off with a control light. In the TokyoTech/NTT switch, the control light induces a nonlinear optical effect of graphene; that is, it changes the degree of absorption by graphene. A switching time of 260 fs can be achieved with a switching energy of 35 fJ. Courtesy of NTT, Tokyo Tech.

The researchers hope that their all-optical switch will be used in future photonic integrated circuits for ultrafast information processing. The nanoscale waveguide of the switch could also serve as a platform for developing nanophotonic information processing devices incorporating nanowires and other 2D materials. Further, the all-optical switching device could be used as a nonlinear activation function in optical neural networks. In the future, the researchers plan to increase the performance of the all-optical switch, apply the technology to other photonic devices such as detectors, and explore the use of other nanomaterials for building the device.

The research was published in Nature Photonics ( 

Published: December 2019
Optoelectronics is a branch of electronics that focuses on the study and application of devices and systems that use light and its interactions with different materials. The term "optoelectronics" is a combination of "optics" and "electronics," reflecting the interdisciplinary nature of this field. Optoelectronic devices convert electrical signals into optical signals or vice versa, making them crucial in various technologies. Some key components and applications of optoelectronics include: ...
Graphene is a two-dimensional allotrope of carbon consisting of a single layer of carbon atoms arranged in a hexagonal lattice pattern. It is the basic building block of other carbon-based materials such as graphite, carbon nanotubes, and fullerenes (e.g., buckyballs). Graphene has garnered significant attention due to its remarkable properties, making it one of the most studied materials in the field of nanotechnology. Key properties of graphene include: Two-dimensional structure:...
Plasmonics is a field of science and technology that focuses on the interaction between electromagnetic radiation and free electrons in a metal or semiconductor at the nanoscale. Specifically, plasmonics deals with the collective oscillations of these free electrons, known as surface plasmons, which can confine and manipulate light on the nanometer scale. Surface plasmons are formed when incident photons couple with the conduction electrons at the interface between a metal or semiconductor...
integrated photonics
Integrated photonics is a field of study and technology that involves the integration of optical components, such as lasers, modulators, detectors, and waveguides, on a single chip or substrate. The goal of integrated photonics is to miniaturize and consolidate optical elements in a manner similar to the integration of electronic components on a microchip in traditional integrated circuits. Key aspects of integrated photonics include: Miniaturization: Integrated photonics aims to...
An SI prefix meaning one billionth (10-9). Nano can also be used to indicate the study of atoms, molecules and other structures and particles on the nanometer scale. Nano-optics (also referred to as nanophotonics), for example, is the study of how light and light-matter interactions behave on the nanometer scale. See nanophotonics.
Nanophotonics is a branch of science and technology that explores the behavior of light on the nanometer scale, typically at dimensions smaller than the wavelength of light. It involves the study and manipulation of light using nanoscale structures and materials, often at dimensions comparable to or smaller than the wavelength of the light being manipulated. Aspects and applications of nanophotonics include: Nanoscale optical components: Nanophotonics involves the design and fabrication of...
nonlinear optics
Nonlinear optics is a branch of optics that studies the optical phenomena that occur when intense light interacts with a material and induces nonlinear responses. In contrast to linear optics, where the response of a material is directly proportional to the intensity of the incident light, nonlinear optics involves optical effects that are not linearly dependent on the input light intensity. These nonlinear effects become significant at high light intensities, such as those produced by...
Research & TechnologyeducationAsia-PacificNTTTokyo Institute of TechnologyBusinessLight SourcesMaterialsOpticsoptoelectronics2D materialsgrapheneplasmonicsplasmonic waveguidesmicroelectronicsintegrated photonicsnanonanophotonicsnonlinear opticsultrafast photonicsCommunicationsConsumeroptical switchesall-optical switchesphotonic integrated circuits

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