Imaging through a flat lens
RESEARCHERS AT the Northeastern University recently announced that they have been able to demonstrate the unique feature of imaging through a flat lens. Using the phenomenon of negative refraction through a novel photonic crystal, the researchers observed that a flat slab of such material behaves as a lens and focuses electromagnetic waves at microwave frequencies to produce a real image.
The research, published in the journal Nature, represents an important advance in the field of imaging. The lead author of the article is Srinivas Sridhar, from the Department of Physics and the Electronic Materials Institute at Northeastern. Contributors also include NU researchers Patanjali Parimi, Wentao Lu, and Plarenta Vodo.
"The significance of this research is that, for the first time, we have been able to image using a flat surface by employing a special material fabricated from a photonic crystal, which possesses a negative index of refraction," said Sridhar.
"Conventional materials, like glass or teflon, possess positive indices of refraction and, in order to focus light or microwaves with them, you need to have a curved surface. When the concept of negative refraction emerged about 30 years ago, its most striking proposal was the notion that you could form an image using flat rather than curved surfaces.
This research not only demonstrated this to be true but is a significant achievement toward the realisation of several applications in imaging such as the concept of a `superlens' with vastly improved power of resolution."
In the latest study, the researchers sought to expand upon a recent observation that in some of the composite metamaterials, electromagnetic waves bend negatively.
The key advance of the new research lies in the design of a new photonic crystal which is an artificial structure usually made out of dielectric or metal designed to control photons in a manner similar to the way a solid crystal controls the electrons.
Using a photonic crystal with suitable dispersion characteristics, in this case, an array of alumina rods, the researchers were able to achieve negative refraction at microwave frequencies.
This ability to demonstrate negative bending is significant because it allows considerable control over electromagnetic wave propagation, which could lead to new approaches to a variety of applications from microwave and optical frequencies. Some of the immediate applications of these negative index materials (also known as left-handed materials) are: sub-wavelength imaging by flat lenses, scanning photon tunnelling microscopy, ultra high sensitive phase shifters, leaky wave antennas and optical switches based on negative refraction. Negative index materials could eventually be used to build new components for optical and microwave telecommunications equipment.
"The advantages of focusing by flat lenses are many," said Sridhar. "Conventional optical systems have a single optical axis and limited aperture, both due to curved surfaces, and cannot focus light onto an area smaller than a square wavelength.
In contrast, the present flat lens does not have a unique optical axis and is not restricted by the aperture size. One of the challenges of this knowledge will be trying to apply it to optical frequencies, which will require fabricating material using nanotechnology. We could potentially see a more immediate impact in the development of microwave antennas and other communications devices."
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