Making metamaterials ‘digital’ could simplify invisibility cloaks

New way to make materials that manipulate light inspired by the 1s and 0s of computers

Invisibility cloaks and superlenses could be improved by taking a page from the digital playbook, two scientists contend September 14 in Nature Materials. Their method, inspired by the 1s and 0s of digital electronics, involves arranging nano-sized blocks of just two materials to bend and focus light.

It’s a new approach to making metamaterials — intricately engineered combinations of metal, plastic and other components with structural features small enough to manipulate waves of light. By developing metamaterials that bend those waves in just the right way, scientists have built rudimentary invisibility cloaks (SN: 7/15/06, p. 42) as well as superlenses that focus light.

Building those metamaterial devices can get complicated. Their components need to be smaller than the wavelength of visible light, in the range of only hundreds of nanometers. And it’s hard to find the right mix of materials to interact with light in the desired way.

Nader Engheta, an electrical engineer at the University of Pennsylvania, and Penn colleague Cristian Della Giovampaola set out to simplify metamaterial construction, turning to electronics for inspiration. Computers and smartphones are complex devices that can perform a variety of tasks, yet they work by simplifying electric voltages into collections of 1s and 0s, or bits.

Every material has a property called permittivity, which determines how it interacts with light. Engheta and Della Giovampaola realized that instead of seeking the perfect mix of multiple materials to make a cloak or lens, scientists can simply choose two materials with opposite permittivities (one positive, one negative) and use them as metamaterial bits. Arranging those bits in various combinations, called bytes, could then achieve the desired effect on incoming light waves.

The researchers ran simulations to show that arrangements of two bits, silver and glass, can replicate the performance of metamaterial devices that required more materials or more complex engineering. “The biggest impact is the simplicity it offers in future nanofabrication,” Engheta says.

Steven Cummer, an electrical engineer at Duke University, worries that the proposal doesn’t offer any obvious shortcut for metamaterial researchers. “I find the high-level idea compelling and interesting,” he says. “But I’m really struggling to come up with what’s the big new thing here.” He notes that some of Engheta’s designs still require complex fabrication techniques, despite using only two materials.

Engheta says his work has drawn interest from researchers building metamaterials, and he says that it can serve as a recipe for cooking up devices in a more efficient way.

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