In a step that could lead to better communications and imaging technologies, engineers at Purdue University, West Lafayette IN, are claiming to be the first researchers to create a material that has a negative index of refraction in the wavelength of light used for telecommunications.
“This work represents a milestone because it demonstrates that 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,” Vladimir Shalaev, Robert and Anne Burnett Professor of Electrical and Computer Engineering at the university, says.
According to the university’s Emil Venere, the material consists of tiny parallel nanorods of gold that conduct clouds of electrons called plasmons at a wavelength in the near-infrared. The wavelength is 1.5 microns, the same used for fiber optic communications.
The nanorods are able to reverse refraction, which occurs as electromagnetic waves, including light, bend when passing from one material into another and is caused by a change in the speed of light as it passes from one medium into another. The amount of bending is measured by the index of refraction.
Each material has its own refraction index, which describes how much light will bend in that particular material and defines how much the speed of light slows down while passing through a material. All natural materials, such as glass, air and water, have positive refractive indices.
in the late 1960s, researchers hypothesized what would happen if a material had a negative refractive index, causing it to bend light in the opposite direction from ordinary materials.
It was theorized that slabs of such material might be used to create a superlens that would drastically improve the quality of medical diagnostic imaging and other technologies. Such lenses theoretically could compensate for the loss of a portion of the light transmitting an image as it passes through a lens.
Lenses and imaging systems could be improved if this lost evanescent light could be restored. An imaging system that used a combination of positive and negative refraction might do that.
Harnessing materials that have a negative index of refraction could make it possible to take optical images of objects that are smaller than the wavelength of visible light, including molecules such as DNA, for research and medical imaging; the development of photo-nanolithography, which would make it possible to etch smaller electronic devices and circuits, resulting in more powerful computers; new types of antennas, computer components and consumer electronics such as cell phones that use light instead of electricity for carrying signals and processing information, resulting in faster communications.
A major obstacle now hindering development of optoelectronic devices is that wavelengths of light are too large to fit into the tiny features needed for miniature circuits and components. Plasmonic nanomaterials, however, could make it possible to squeeze light waves into much smaller spaces, Shalaev says.
Various research groups have fabricated metamaterials made of tiny metal rings and rods, which have a negative index of refraction. No metamaterials have yet been created that have negative refraction indices for visible light, but the Purdue researchers have created the first metamaterial with a negative refractive index in the near-infrared portion of the spectrum.
This is just beyond the range of visible light, demonstrating the feasibility of applying the concept to communications and computers.
“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 says.
Image, taken with a field-emission scanning electron microscope, shows tiny parallel nanorods of gold that represent the first material that has a negative index of refraction in the wavelength of light used for telecommunications, a step that could lead to better communications and imaging technologies. The material, created by Purdue engineers, conducts clouds of electrons called plasmons with a frequency of light referred to as the near-infrared, the same wavelength used for fiberoptic communications. Each of the rods is about as 100nm wide.
