PORTLAND, Ore.—The world's smallest graphene modulator was unveiled recently by researchers at the National Science Foundation (NSF) Nanoscale Science and Engineering Center at the University of California-Berkeley. The research team, led by professor Xiang Zhang, claimed its breakthrough will someday allow smartphones to download entire movies in a matter of seconds.
Today, optical modulators are used to speed communications by using electrical signals to switch a laser on and off for long-haul communications between systems. However, high-speed optical communications is migrating to short-haul communications and someday may even be used by mobile devices to quicken the transfer of large files.
Unfortunately, today's optical devices are discrete, bulky and require III-V materials like gallium indium arseide. By fabricating modulators in graphene—pure sheets of carbon—high-speed optical communications will become small enough and cheap enough to be integrated onto mobile device chips, according to Zhang.
The team demonstrated a graphene-based optical modulator 100-times smaller than the typical III-V device, measuring just 25 square microns. The device worked by applying an alternating electrical field to an optical waveguide fabricated in graphene. When the electrical signal was at its peak and its trough, the electrical, the electrical field caused the graphene to become opaque, thus turning off the laser driving the waveguide. However, near the center of the modulation range, the graphene became clear thus switching on the laser.
The world's smallest graphene modulator uses electrical signals to switch an laser on and off for faster, smaller, cheaper optical communications. Source: UC Berkeley.
In characterizing their device, the researchers discovered that graphene can also operate over a 100-times wider bandwidth, ranging over thousands of nanometers—from ultraviolet to infrared—compared to the narrow (10 nanometer) bandwidth of typical II-V modulators today. Their demonstration device ran at one gigahertz (GHz), but could can be extended to as high as 500 GHz, according to Zhang.
"Graphene enables us to make modulators that are incredibly compact and that potentially perform at speeds up to 10 times faster than current technology," according to Zhang, who performed the work with fellow professor Feng Wang and post-doctoral researchers Ming Liu, Thomas Zentgraf, visiting scholar Baisong Geng, and doctoral candidates Erick Ulin-Avila and Long Ju.
Funding was for the project was provided by the NSF's Center for Scalable and Integrated Nano-Manufacturing, and the Department of Energy's Basic Energy Science program at Lawrence Berkeley National Laboratory.