18 Oct 2007
US researchers have found a simpler, cheaper and more reliable way to make metamaterials with a negative index of refraction.
Negative-index metamaterials have hit the headlines this year because of their unusual ability to bend light the "wrong" way, but existing versions based on intricate metallic structures are difficult and expensive to fabricate. Now, Claire Gmachl and colleagues at Princeton University have shown that a negative-index metamaterial can be made from thin layers of semiconductors.
"It doesn't need any complicated fabrication process," team member Anthony Hoffman told optics.org. "We built our substrate in a single growth step using a standard molecular beam epitaxy process."
What's more, the Princeton invention is a 3D structure, rather than the 2D arrangements of metals used in previous metamaterials, and also loses less light than metallic structures. "Metamaterials must be three-dimensional to be useful in a variety of devices," added Gmachl. "Furthermore, this is made from semiconductors, from which true applications are made."
The latest result validates a theoretical prediction made by Princeton's Viktor Podolskiy and Evgenii Narimanov in 2005, who asserted that a strongly anisotropic waveguide would under certain conditions behave as a negative-index material. While most metamaterials have so far been artificially engineered to have negative electric and magnetic permeabilities, this anisotropic structure requires only one of these characteristics to be negative.
The new metamaterial is made from alternating layers of highly doped indium gallium arsenide (InGaAs) and undoped aluminium indium arsenide (AlInAs). Each layer is about 80 nm thick, much smaller than the wavelength of light, which means an incoming light beam encounters multiple layers at once.
"The result is that while the wavefront continues moving roughly in the same direction, the energy vector gets bent in the other direction; effectively giving us negative refraction," explained Hoffman.
The two semiconductors used to make the layers are both isotropic, but arranging them in thin patterns next each other causes the material as a whole to become anisotropic. According to Hoffman, this characteristic feature was the key to making a metamaterial with a single negative resonance. "We adjusted the anisotropy in such a way that the wave vector of the light bends normally, but the energy vector gets bent in the other direction," he said.
Another key benefit of the Princeton metamaterial is that it offers negative refraction in the mid-infrared, which is important for biosensing and communications applications. The team is now attempting to make a high-resolution "super" lens that exploits the negative-index material to overcome the diffraction limit.
"Currently, the infrared lens is a massive object," said Hoffman. "This new material may enable more compact mid-infrared optics because we have an entirely new set of optical parameters in our toolkit."
Such super lenses were first suggested in 2000 by John Pendry, a theorist at Imperial College London, who gave optics.org his views on the new work. "They have found a way to make anisotropic materials to a high degree of perfection, and that's quite remarkable." he said. But he added that this material cannot be termed a negative-index metamaterial in the strictest sense. "Since it is not completely isotropic, it's properties and hence the applications may be quite different from the metamaterials that have been built by other teams."
Now, the Princeton team plans to incorporate its metamaterial into lasers. "We are quite excited about using this lens in quantum cascade lasers, particularly those operating in the mid-infrared band," said Hoffman.
The work was published this week in Nature Materials.