On the left is a schematic of the first 3-D "fishnet" metamaterial that can achieve a negative index of refraction at optical frequencies. On the right is a scanning electron microscope image of the fabricated structure, developed by UC Berkeley researchers. The alternating layers form small circuits that can bend light backwards. Image by Jason Valentine, UC Berkeley
Three-dimensional optical metamaterial with a negative refractive index brings invisibility and superlenses closer to reality. Superlenses will mean better microscopes, superior capability to see and work at the nanoscale and better computer lithography which will make faster computers.
Metamaterials are artificially engineered structures that have properties, such as a negative refractive index not attainable with naturally occurring materials. Negative-index metamaterials (NIMs) were first demonstrated for microwave frequencies but it has been challenging to design NIMs for optical frequencies and they have so far been limited to optically thin samples because of significant fabrication challenges and strong energy dissipation in metals. Such thin structures are analogous to a monolayer of atoms, making it difficult to assign bulk properties such as the index of refraction. Negative refraction of surface plasmons was recently demonstrated but was confined to a two-dimensional waveguide. Three-dimensional (3D) optical metamaterials have come into focus recently, including the realization of negative refraction by using layered semiconductor metamaterials and a 3D magnetic metamaterial in the infrared frequencies; however, neither of these had a negative index of refraction. Here we report a 3D optical metamaterial having negative refractive index with a very high figure of merit of 3.5 (that is, low loss). This metamaterial is made of cascaded 'fishnet' structures, with a negative index existing over a broad spectral range. Moreover, it can readily be probed from free space, making it functional for optical devices. We construct a prism made of this optical NIM to demonstrate negative refractive index at optical frequencies, resulting unambiguously from the negative phase evolution of the wave propagating inside the metamaterial. Bulk optical metamaterials open up prospects for studies of 3D optical effects and applications associated with NIMs and zero-index materials such as reversed Doppler effect, superlenses, optical tunnelling devices compact resonators and highly directional sources.
In the Nature paper, the UC Berkeley 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 1500 nanometers, the near-infrared light range, researchers measured a negative index of refraction.
Jason Valentine, UC Berkeley graduate student and co-lead author of the Nature paper, explained that each pair of conducting and non-conducting layers forms a circuit, or 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.
The metamaterial described in the Science paper takes another approach to the goal of bending light backwards. It is composed of silver nanowires grown inside porous aluminum oxide. Although the structure is about 10 times thinner than a piece of paper - a wayward sneeze could blow it away - it is considered a bulk metamaterial because it is more than 10 times the size of a wavelength of light.
The authors of the Science paper observed negative refraction from red light wavelengths as short as 660 nanometers. It is the first demonstration of bulk media bending visible light backwards.
The innovation of this nanowire material, researchers said, is that it finds a new way to bend light backwards without technically achieving a negative index of refraction. For there to be a negative index of refraction in a metamaterial, its values for permittivity - the ability to transmit an electric field - and permeability - how it responds to a magnetic field - must both be negative.
The benefits of having a true negative index of refraction, such as the one achieved by the fishnet metamaterial in the Nature paper, is that it can dramatically improve the performance of antennas by reducing interference. Negative index materials are also able to reverse the Doppler effect - the phenomenon used in police radar guns to monitor the speed of passing vehicles - so that the frequency of waves decreases instead of increases upon approach.
But for most of the applications touted for metamaterials, such as nanoscale optical imaging or cloaking devices, both the nanowire and fishnet metamaterials can potentially play a key role, the researchers said.
"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."