Manhasset, N.Y. Purdue researchers will detail a metamaterial with a negative refractive index in the near-infrared portion of the spectrum in a paper to appear in the journal Optics Letters next week. The finding demonstrates the feasibility of applying the concept to communications and computing.
The Purdue effort advances materials science toward a long-sought goal that's being pursued by researchers worldwide. A team from Britain's Manchester University and Russia's Chernogolovka Institute of Microelectronics Technologies recently reported fabrication of a material that has a negative permeability at visible wavelengths (see story, page 50).
Various research groups have fabricated metamaterials made of tiny metal rings and rods that have negative indices of refraction, but metamaterials that exhibit negative refraction indices for visible light have proved elusive. Obtaining optical images of objects that are smaller than the wavelength of visible light could advance research and medical imaging.
"It is possible to have a negative refractive index in the optical range, which increases the likelihood of harnessing this phenomenon for optics and communications," said Vladimir Shalaev, Robert and Anne Burnett professor of electrical and computer en
gineering at Purdue. "Although many researchers are skeptical about developing materials with a negative index of refraction in optical wavelengths and then using them in practical technologies, I think the challenges are mainly engineering problems that could eventually be overcome. There is no fundamental law of physics that would prevent this from happening."
Squeezing light waves
A major obstacle to development has been that wavelengths of light are too large to fit into the tiny features needed for miniature circuits and components. So-called plasmonic nanomaterials, however, could make it possible to squeeze light waves into much smaller spaces, according to Shalaev.
Purdue's metamaterial consists of tiny gold nanorods, arrayed in parallel, that conduct plasmons, or clouds of electrons, with near-infrared light at 1.5 microns, the wavelength commonly used for fiber-optic communications. "The challenge was to fabricate a structure that would have not only an electrical response but also a magnetic response in the near-infrared range," Shalaev said.
The gold nanorods conduct the clouds of electrons, which move in unison as if they were a single object. Light from a laser or other source is shone onto the nanorods, inducing an electro-optical current in the tiny circuit.
Each of the rods is about as wide as 100 nanometers and 700 nm long.
"These rods basically conduct current because they are a metal, producing an effect we call optical inductance, while a material between the rods produces an effect called optical capacitance," Shalaev said. "The result is the formation of a very small electromagnetic circuit, but this circuit works in higher frequencies than normal circuits, in a portion of the spectrum we call optical frequencies, which includes the near-infrared. So we have created a structure that works as kind of an optical circuit and interacts effectively with both of the field components of light: electrical and magnetic."
The Purdue findings will be detailed in a paper appearing Dec. 15 in Optics Letters, published by the Optical Society of America. The paper was written by Shalaev, his graduate research assistants Wenshan Cai and Uday K. Chettiar, doctoral student Hsiao-Kuan Yuan, senior research scientists Andrey K. Sarychev and Vladimir P. Drachev, and principal research scientist Alexander V. Kildishev.
The research has been funded by the U.S. Army Research Office and the National Science Foundation and is affiliated with Purdue's Birck Nanotechnology Center, the university's hub for interdisciplinary research.