Graphene Photonics: Low-voltage graphene electro-optic modulator operates in the near-IR

June 10, 2017
An extremely small graphene-based free-space electroabsorption modulator has a low operating voltage of 1 to 3 V and low power consumption -- and it's CMOS-compatible.
A free-space graphene-based near-IR electro-optical modulator with a modulation volume of about 5 μm3 consists of a quartz (SiO2) substrate, a copper (Cu) bottom electrode, a hafnium dioxide (HfO2) quarter-wave dielectric layer, and a small graphene flake with gold electrode contacts on the top; a gate voltage (Vg) is applied across the device to achieve optical modulation.
A free-space graphene-based near-IR electro-optical modulator with a modulation volume of about 5 μm3 consists of a quartz (SiO2) substrate, a copper (Cu) bottom electrode, a hafnium dioxide (HfO2) quarter-wave dielectric layer, and a small graphene flake with gold electrode contacts on the top; a gate voltage (Vg) is applied across the device to achieve optical modulation.

One essential component of many telecommunications-based optical systems is the electro-optical modulator, which allows the production of a stream of data using continuous-wave (CW) light as a source. Developers of leading-edge silicon (Si) photonic circuits wish to integrate these, along with compact and efficient lasers, photodetectors, and passive components, onto Si in a cost-effective way.

The complementary metal-oxide-semiconductor (CMOS) fabrication process, which is the standard in the massive Si electronics industry, allows inexpensive mass production of photonics on a Si substrate—at least if they're compatible with the CMOS process. As a result, any developer of a new photonics device who can say it's "CMOS-compatible" instantly has the ear of other developers.

Graphene is a relatively new material for photonics—it is being studied as an enabling material for various components because of its unique optical, electronic, thermal, and mechanical properties. Ideally, for cost reasons, a graphene photonic component should potentially be fabricable via the CMOS process.

A group of researchers at Manchester University (Manchester, England) has now developed an extremely small graphene-based free-space electroabsorption modulator (EAM) with a low operating voltage of 1 to 3 V and low power consumption—and it's CMOS-compatible.1

Very small modulation volume

The device, which contains one high-quality graphene monolayer, acts electronically as a parallel-plate capacitor—varying the applied electrical bias varies the optical transmission of the graphene. Underneath the graphene itself is a quarter-wave dielectric hafnium dioxide (HfO2) layer with a refractive index of about 2, beneath which sits a reflective copper thin-film mirror and a quartz substrate. Electrical contacts were made to the graphene using laser-written photolithography and electron beam deposition and lift-off, producing contacts consisting of a 2-nm-thick layer of chromium and a 30-nm-thick layer of gold.

The entire device has a modulation volume on the order of 5 μm3, which could potentially be made as small as λ3 /10, the researchers say (see figure). Its power consumption is only about 1 nW, and the device's insertion loss is <10%. Broadband modulation was observed down to a wavelength of 900 nm.

The researchers note that, while capacitor-based modulators with small operational voltages had previously been developed, their modulation effects were achieved using an ionic liquid electrolyte. Solid-state dielectrics are obviously more stable, but the researchers add that solid dielectrics conventionally require high voltages of around 50 to 150 for modulation at near-IR wavelengths. It is an electrochemical supercapacitor effect observed in the HfO2 dielectric, giving it an effective capacitive thickness 10 times smaller than its physical thickness, that led to low modulation voltages of 1 to 3 V in the new device.

REFERENCE

1. D. E. Aznakayeva et al., Opt. Express (2017); https://doi.org/10.1364/oe.25.010255.

About the Author

John Wallace | Senior Technical Editor (1998-2022)

John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.

Sponsored Recommendations

Working with Optical Density

Feb. 26, 2025
Optical Density, or OD, is a convenient tool used to describe the transmission of light through a highly blocking optical filter.

Custom-Engineered Optical Solutions for Your Application

Feb. 26, 2025
Explore the newest and most widely used applications of Semrock optical filters.

Linear Stages & Rotary Stages for High Precision Automation & Motion Control

Feb. 13, 2025
Motorized Linear Translation Stages & Rotary Precision Positioning Stages for High Performance Automation & Motion Control | PI USA

Motion Controllers for Precision Positioning and Automation

Feb. 13, 2025
PI manufactures a range of precision motion controllers and drivers for positioning systems, including stepper motors, brushless motors, and servo motors.

Voice your opinion!

To join the conversation, and become an exclusive member of Laser Focus World, create an account today!