November 04, 2010

Meta-flex: Your new brand for invisibility clothing

Photograph of a Metaflex membrane placed on a disposable contact lens and illuminated with office light. The inset is an enlargement of the membrane.

Flexible smart materials that can manipulate light to shield objects from view have been much-theorised but now researchers in Scotland have made a practical breakthrough that brings the possibility of an invisibility cardigan – or any other item of invisibility clothing - one step closer.
Two challenges to the creation of smart flexible materials that can cloak from visible light are making meta-atoms small enough to interact with visible light, and the fabrication of metamaterials that can be detached from the hard surfaces they are developed on to be used in more flexible constructs.

Research published today, Thursday 4 November 2010, in New Journal of Physics (co-owned by the Institute of Physics and German Physical Society), details how Meta-flex, a new material designed by researchers from the University of St Andrews, overcomes both of these challenges.

New Journal of Physics - Flexible metamaterials at visible wavelengths

The response of MMs is determined by a tailored distribution of meta-atoms (typically periodically arranged and metallic), such as split ring resonators or, more recently, fishnet lattices. If the MM operates at optical wavelengths, the size of the meta-atoms has to be scaled down to a few tens of nanometers. For this reason, traditional fabrication techniques rely on approaches inherited from nanotechnology, e.g. electron beam lithography, which typically requires rigid substrates such as quartz or silicon.

The unique properties of MMs have led to the demonstration of exciting concepts such as superlensing and invisibility cloaking. Both these concepts, as well as other possible applications of MMs, are constrained by the limitation imposed by the fabrication constraints. For example, a 'real' cloaking device would have to be deformable and extend over a large area, rather than being fabricated on rigid substrates such as silicon.

The same is true for the superlens, where the flat geometry only allows for the formation of an image in the near field, while a magnifying superlens that pushes the focal length to more practical distances would require a curved realization. Such a curved realization has recently been demonstrated via a tour-de-force in lithography, again on a planar substrate, but it is not obvious how this method would scale up in size.

Metaflex can operate in the true optical regime by demonstrating plasmonic resonances down to a wavelength of 620 nm.

Metaflex can be fabricated using standard nanotechnology (e.g. electron beam lithography), but we have developed a fabrication technique to obtain supple and deformable substrates.

One of the most exciting applications of Metaflex is to fabricate three-dimensional flexible MMs in the optical range, which can be achieved by stacking several Metaflex membranes on top of one another.

While the experiment was realized on a flat membrane and for normal incidence, we believe that this sort of structure is the ideal candidate for addressing an advanced implementation of bulk MMs that consist of a multilayer stack.

Assembling such a stack requires an inter-layer distance of the order of a few hundreds of nanometers, which can be done using the Metaflex approach.

The critical factor then becomes the thickness of the membranes. SU8 can indeed be spun at thicknesses down to 100 nm, but there may be practical limitations in terms of the possible membrane area; a quantitative study of these limitations is currently in progress.

We have fabricated and characterized plasmonic nano-structures that were realized on flexible polymeric substrates. We studied both nanoantennas with varying geometrical parameters and fishnet structures, and demonstrated their operation in the NIR and the visible wavelength range, respectively. The experimental curves agreed well with the numerical calculations. These results confirm that it is possible to realize MMs on flexible substrates and operating in the visible regime, which we believe are ideal building blocks for future generations of three-dimensional flexible MMs at optical wavelengths.

Photograph of hand-held tweezers holding a typical Metaflex membrane

Although cloaks designed to shield objects from both Terahertz and Near Infrared waves have already been designed, a flexible material designed to cloak objects from visible light poses a greater challenge because of visible light’s smaller wavelength and the need to make the metamaterial’s constituent part – meta-atoms – small enough to interact with visible light.

These tiny meta-atoms have been designed but they have only traditionally been realized on flat, hard surfaces, making them rigid constructs impractical for use in clothing or other possible applications that would benefit from flexibility, such as super lenses.

The research team, led by EPSRC Career Acceleration Fellow Dr Andrea Di Falco, has developed an elaborate technique which frees the meta-atoms from the hard surface (‘substrate’) they are constructed on. The researchers predict that stacking them together can create an independent, flexible material, which can be adopted for use in a wide range of applications.

Di Falco says, “Metamaterials give us the ultimate handle on manipulating the behaviour of light. The impact of our new material Meta-flex is ubiquitous. It could be possible to use Meta-flex for creating smart fabrics and, in the paper, we show how easy it is to place Meta-flex on disposable contact lenses, showing how flexible superlenses could be used for visual prostheses.”

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