Light waves guided with nanoscale precision inside the circuits of an electronic chip can bring about applications in spectroscopy, sensing and bioimaging, and hasten the advent of faster all-optical telecommunication networks.
To enable light to store and transmit data with optimal efficiency, engineers must first learn to slow or stop light waves across the various regions of the spectrum.
Qiaoqiang Gan, a Ph.D. candidate, and Filbert J. Bartoli, department chair of electrical and computer engineering, have developed a graded metal grating structure that arrests light waves in the terahertz (THz) and telecommunications portions of the spectrum.
The achievement, says Bartoli, “opens a door to the control of light waves on a chip.” It could reduce the size of optical structures, enabling them to be integrated at the nanoscale with electronic devices.
Bartoli and Gan reported their achievement in June 2008 and February 2009 in Physical Review Letters. The articles were coauthored with two other researchers in Lehigh’s Center for Optical Technologies.
THz waves measure several hundred microns in length and are suitable for security applications. Telecom wavelengths measure 1330 to 1550 nm and are suitable for optical communications.
The researchers describe their structure as a “metallic grating structure with graded depths, whose dispersion curves and cutoff frequencies are different at different locations.” The grating looks like a pipe organ whose pipes decrease gradually in length from one end of the assembly to the other. The degree of grade in the grating can be tuned by altering the temperature and modifying the physical features on the surface of the structure.
The structure arrests the progress of light waves at multiple locations on the surface and at different frequencies. Previous researchers, Gan says, had been able “to slow down one single wavelength within a narrow bandwidth, but not many wavelengths over a wide spectrum.”
To release trapped light waves from the grating structure, and enable their use in telecommunications, the group covers the structure with a dielectric material. “By tuning the temperature of the dielectric,” says Gan, “we can change the optical properties of the metal grating structure. This in turn enables the trapped light waves to be released.”
Most of the group’s initial work has involved mathematical equations and computer simulation. The group plans next to use focused ion beam milling to fabricate structures and near-field scanning optical microscopy to characterize them.
“Our goal,” says Gan, “is to achieve a grade of grating depths that range from very shallow to as much as 50 nm on a 200-nm substrate.”
The group’s results, says New Scientist, “suggest that one day we might be able to slow down light long enough to store it as a ‘rainbow’ of colors - an advance that would revolutionize computing and telecom networks.”