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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3D-Stacked CMOS Takes Moore’s Law to New Heights

When transistors can’t get any smaller, the only direction is up

10 min read
An image of stacked squares with yellow flat bars through them.
Emily Cooper
Green

Perhaps the most far-reaching technological achievement over the last 50 years has been the steady march toward ever smaller transistors, fitting them more tightly together, and reducing their power consumption. And yet, ever since the two of us started our careers at Intel more than 20 years ago, we’ve been hearing the alarms that the descent into the infinitesimal was about to end. Yet year after year, brilliant new innovations continue to propel the semiconductor industry further.

Along this journey, we engineers had to change the transistor’s architecture as we continued to scale down area and power consumption while boosting performance. The “planar” transistor designs that took us through the last half of the 20th century gave way to 3D fin-shaped devices by the first half of the 2010s. Now, these too have an end date in sight, with a new gate-all-around (GAA) structure rolling into production soon. But we have to look even further ahead because our ability to scale down even this new transistor architecture, which we call RibbonFET, has its limits.

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