Scientists at the University of California, Berkeley, have fabricated negative refractive index materials for visible and near-infrared light.
“Other research teams have previously developed metamaterials that function at optical frequencies, but those 2-D materials have been limited to a single monolayer of artificial atoms whose light-bending properties cannot be defined,” said the University.
“Thicker, 3-D metamaterials with negative refraction have only been reported at longer microwave wavelengths.”
For a metamaterial to achieve negative refraction, its structural array must be smaller than the electromagnetic wavelength being used.
“What we have done is take two approaches to creating bulk metamaterials that can exhibit negative refraction in optical frequencies,” said Professor Xiang Zhang.
In one, researchers stacked together alternating layers of silver and non-conducting magnesium fluoride, and cut nanoscale-sized fishnet patterns into the layers to create a bulk optical metamaterial.
“At wavelengths as short as 1.5µmeters, the near-infrared light range, researchers measured a negative index of refraction,” said the University.
According to Berkley, each pair of conducting and non-conducting layers forms a current loop. Stacking the alternating layers together creates a series of circuits that respond together in opposition to that of the magnetic field from the incoming light.
Another metamaterial from the Berkley, this time the first 3D ‘fishnet’ that can achieve a negative refractive index at optical frequencies.
“Natural materials do not respond to the magnetic field of light, but the metamaterial we created here does,” said student Jason Valentine.
“It is the first bulk material that can be described as having optical magnetism, so both the electrical and magnetic fields in a light wave move backward in the material.”
The second material is composed of silver nanowires grown inside porous aluminium oxide.
“Although the structure is about 10 times thinner than a piece of paper, it is considered a bulk metamaterial because it is more than 10 times the size of a wavelength of light. It is the first demonstration of bulk media bending visible light backwards,” claimed researchers, who observed negative refraction from red light wavelengths as short as 660nm.
“The geometry of the vertical nanowires, which were equidistant and parallel to each other, were designed to only respond to the electrical field in light waves,” said student Jie Yao.
“The magnetic field, which oscillates at a perpendicular angle to the electrical field in a light wave, is essentially blind to the upright nanowires, a feature which significantly reduces energy loss.”
According to the university, for there to be a negative index of refraction in a metamaterial, its values for permittivity and permeability must both be negative.
The innovation of this nanowire material, researchers said, is that it finds a way to bend light backwards without technically achieving a negative index of refraction.
True negative refractive index materials like the fishnet can improve the performance of antennas by reducing interference, and are also able to reverse the Doppler effect, said researchers. But for optical imaging or cloaking both the nanowire and fishnet metamaterials could be of value.
“What makes both these materials stand out is that they are able to function in a broad spectrum of optical wavelengths with lower energy loss,” said Zhang.
“We’ve also opened up a new approach to developing metamaterials by moving away from previous designs that were based upon the physics of resonance. Previous metamaterials in the optical range would need to vibrate at certain frequencies to achieve negative refraction, leading to strong energy absorption. Resonance is not a factor in both the nanowire and fishnet metamaterials.”
Top and side views of a metamaterial developed by the University of California, Berkeley researchers. It is composed of parallel nanowires embedded in porous aluminium oxide. Exhibiting negative refractive index, visible light passing through the material is bent backwards.