Arrested Rainbow

Working on the nanoscale, researchers at Lehigh University, have found a way to control the rate at which light from across the spectrum moves through optical circuits. The challenge for this group of researchers, led by Qiaoqiang Gan, Ph.D. candidate in electrical engineering at the Lehigh University, was integrating optical structures with electrical devices.

Light waves transmit data with greater speed and control than electrical signals, which are typically hindered by the mobility of the electrons in semiconducting materials. On the other hand, light is more difficult to control at the nanoscale because of natural limits on its diffraction.

Filbert J. Bartoli and Qiaoqiang Gan at Lehigh Universtity. Photos courtesy of Qiaoqiang Gan.

There is a mismatch between nanoelectronics and nanophotonics," says Gan. "Because of the diffraction limit of light, optical circuits are now much larger than their electronic counterparts. This poses an obstacle to the integration of optical structures with electrical devices. For that reason, the dream now among photonics researchers is to make optical structures as small as possible and integrate them with electrical devices."

Gan and his colleagues have made a major contribution towards this effort by developing a relatively simple structure that can slow down or even stop light waves over a wide portion of the light spectrum. The structure developed by his team, says Gan, has the unique ability to arrest the progress of terahertz (THz) light waves at multiple locations on the structure's surface and also at different frequencies.

The new Lehigh grating structure arrests terahertz waves of various frequencies at different positions on the structure’s surface.

"Previous researchers have reported the ability to slow down one single wavelength at one narrow bandwidth," says Gan. "We've succeeded in actually stopping THz waves at different positions for different frequencies. Our next goal is to develop structures that extend this capability to the near infrared and visible ranges of the spectrum, where optical communications signals are transferred."

According to the Lehigh researchers, the key innovation is the metallic grating structure with graded depths, whose dispersion curves and cutoff frequencies are different at different locations.

In appearance, this grate resembles the pipes of a pipe organ arranged side by side and decreasing gradually in length from one end of the assembly to the other.

The degree of grade in the metal grate can be "tuned," says Gan, by altering the temperature and modifying the physical features on the surface of the structure.

Likewise, he says, temperature and surface structure can also be adjusted to trigger the release of the light signals after they have been slowed or trapped.

According to co-author Filbert J. Bartoli, by controlling light waves on a chip, the new grating structure could help scientists and engineers reduce the size of optical structures so they can be integrated at the nanoscale with electronic devices.

"Our grating structure can also be scaled to telecommunications frequencies for future possible applications in integrated optical and nano-photonic circuits," says Gans. "This might even help us realize such novel applications as a spectrometer integrated on a chip for chemical diagnostics, spectroscopy and signal processing applications."

The article, titled “Ultrawide-Bandwidth Slow-Light System Based on THz Plasmonic Graded Metallic Grating Structures,” describing the team’s progress was published in the June 27 issue of Physical Review Letters. The article is coauthored by Gan; Zhan Fu, PhD and candidate in electrical engineering; Yujie Ding, professor of electrical and computer engineering; and Filbert Bartoli.

For more information, visit: www.lehigh.edu