A Bendable, Light-Bending Material
Researchers at the University of St. Andrews have created sheets of a flexible metamaterial that can manipulate visible light. “It’s a pretty significant step forward,” says Steven Cummer, professor of electrical and computer engineering at Duke University and the inventor of the first metamaterial-based invisibility cloak. “At radio frequencies we know how to make a lot of these things. But at optical wavelengths, things have been very fabrication-limited.”
Metamaterials allow researchers to manipulate electromagnetic waves beyond the boundaries of what physics allows in natural materials. As well as promising better solar cells and high-resolution microscope lenses, metamaterials have also been used to create so-called invisibility cloaks, in which electromagnetic waves are bent around an object as if it simply weren’t there.
However, metamaterials must be constructed out of elements smaller than the wavelength of the electromagnetic radiation being manipulated. This means that invisibility cloaks (and most metamaterial devices in general) only work with wavelengths longer than those found in visible light, such as radio and microwave frequencies. Metamaterials designed to work with optical wavelengths are built on rigid and fragile substrates, and as a result they’ve been confined to the lab.
The new metamaterial, dubbed “Metaflex” by its creators, is manufactured on top of a rigid substrate. An initial, sacrificial layer of the material is deposited on this substrate to stop the subsequent layers from sticking to this substrate. A sheet of a flexible, transparent, plastic polymer is then laid down. Next, a lithographic process, similar to that used to make silicon chips, creates a lattice of gold bars, each 100 to 200 nanometers long and 40 nanometers thick, on top of the polymer. (These bars act as “nanoantennas” that interact with incoming electromagnetic waves.) The Metaflex material is then bathed in a chemical that releases the polymer from the layer below and from the rigid substrate.
By varying the length and spacing of the nanoantennas, Metaflex can be tuned to interact with different wavelengths of light. The simple sheets tested by the researchers simply blocked a portion of an incoming beam of light at specific wavelengths, but this is enough to demonstrate that Metaflex is a working metamaterial. The St. Andrew’s researchers tested wavelengths as short as 620 nanometers (corresponding to a red color).
So far, the researchers have produced flexible sheets as large as five by eight millimeters and as thin as four micrometers. While a fingernail-sized sample may seem small, it’s a big step up from the microscopic dimensions of other optical metamaterials. The St. Andrew’s scientists are confident that Metaflex can be produced in even larger sizes and at high volumes. “It’s absolutely scalable to industrial levels,” says Andrea Di Falco, the lead author of a paper published in the New Journal of Physics yesterday that describes the material.
Even at small sizes, the flexibility of the material is likely to confer some big advantages. “You really would like to be able to shape optical metamaterials into cylinders or spherical sections.” This could allow, for example, the creation of curved superlenses that could magnify objects so small that they currently can’t be seen with optical lenses due to diffraction effects. “On rigid substrates, it’s just next to impossible to fabricate that kind of thing,” says Duke University’s Cummer, but with a flexible material, “you could fabricate flat and easily bend it into shape.”
Di Falco believes it should be possible to stack sheets of Metaflex together to create thick layers and blocks of the material, creating the first optical metamaterial with a significant three-dimensional bulk. Such a development would open the door to new properties, including, perhaps, the ability to work with more than a single wavelength at a time. Other researchers have been able to create metamaterials that can be tuned to respond to different single wavelengths after fabrication, but ideally, they’d like a material that can work across a wide band of wavelengths simultaneously. This might be achieved through stacking sheets of MetaFlex, each tuned to a different wavelength.
The researchers’ next step is to create these stacks and study how the properties of Metaflex change when sheets are twisted, stretched, or bent.
Ultimately, Di Falco says, Metaflex could have applications such as manipulating light from an LED built into a contact lens for augmented reality, so that computer-generated images are projected onto the wearer’s retina. And of course, there’s invisibility. “If you have something flexible, you could embed it into a fabric. Then you could think of tuning the properties of each individual layer to change the response of the fabric, giving something similar to camouflage. So, yes—there’s some grounds for [an invisibility cloak]. Not tomorrow. But that’s what I’ll be working on,” says Di Falco.
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