More Light From Metamaterials

Hyperbolic metasurface gives new control over lightwaves

2 min read
More Light From Metamaterials
Image: Harvard University

A material that manipulates light in unusual ways could lead to a whole variety of exotic devices, including microscopes capable of seeing inside cells, optical circuits for quantum computers, and invisibility cloaks.

The material in question is a hyperbolic metasurface, a two-dimensional type of metamaterial with a negative index of refraction that bends light in directions it would not normally travel, sending it along a hyperbola rather than an ellipse. One difficulty with metamaterials is that they often contain metals that absorb photons, limiting the distance light can travel to a few hundred nanometers. Diffraction, in which light bends around objects in its path, can also distort a lightwave in a metamaterial, limiting the reach of a light-based signal.

Hyperbolic metasurfaces could steer light according to its wavelength or by a magnetic field

This new metasurface overcomes the distance problem by sending light along the surface of a metal grating rather than through it, avoiding absorption. Harvard chemistry and physics professor Hongkun Park and his team describe the metasurface in the current issue of Nature.

They started by growing a single crystal of silver on top of a piece of silicon; then they used plasma to etch a grating into the silver. When they shone a laser onto the grating, surface plasmon polaritons—oscillations of electron density—formed at the interface of the silver and surrounding air, carrying the light along. The single crystal meant the surface was so smooth that there were no defects to absorb the light. The team’s new technique increased the propagation distance of the light by two orders or magnitude, Park says. The light was also free of diffraction, which means it could be used to image objects much smaller than its wavelength.

Alexander High, a postdoctoral research in Park’s group, says the metasurface is relatively easy to build. “There’s no single aspect of it that is incredibly challenging or difficult to implement,” he says.

The hyperbolic metasurface gives new levels of control over the propagation of light. The size of the grating determines which wavelengths are negatively refracted, so the metasurface can be used to route light depending on its wavelength. It also makes possible control of light through a property of quantum mechanics known as spin. Such spin control would allow the direction of light to be switched by applying a magnetic field. Finally, it provides a way of delivering light to other tiny optical elements, such as quantum dots.

It even gives fine-grained enough control that it would be possible to build single-photon transitors. Such devices could be elements of a quantum computer. What’s more, the new metasurface is a step further on the road to invisibility cloaks; most of the demonstrations of optical cloaking have been at infrared or microwave wavelengths, whereas this would bring it into the visible spectrum. “There are lots of different possibilities,” Park says.

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Two Startups Are Bringing Fiber to the Processor

Avicena’s blue microLEDs are the dark horse in a race with Ayar Labs’ laser-based system

5 min read
Diffuse blue light shines from a patterned surface through a ring. A blue cable leads away from it.

Avicena’s microLED chiplets could one day link all the CPUs in a computer cluster together.

Avicena

If a CPU in Seoul sends a byte of data to a processor in Prague, the information covers most of the distance as light, zipping along with no resistance. But put both those processors on the same motherboard, and they’ll need to communicate over energy-sapping copper, which slow the communication speeds possible within computers. Two Silicon Valley startups, Avicena and Ayar Labs, are doing something about that longstanding limit. If they succeed in their attempts to finally bring optical fiber all the way to the processor, it might not just accelerate computing—it might also remake it.

Both companies are developing fiber-connected chiplets, small chips meant to share a high-bandwidth connection with CPUs and other data-hungry silicon in a shared package. They are each ramping up production in 2023, though it may be a couple of years before we see a computer on the market with either product.

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