Nanohighway for Light

Illustration of a nano-opto-electro-mechanical switch as it could be used for future filtering of colors for sensing or communications. (Source: S. Kelley, NIST)

Researchers at the ETH Zurich in Switzerland and National Insti­tute of Standards and Tech­nology NIST have developed an optical switch that routes light from one computer chip to another in just 20 billionths of a second – faster than any other similar device. The compact switch is the first to operate at voltages low enough to be inte­grated onto low-cost silicon chips and redirects light with very low signal loss.

The switch’s record-breaking performance is a major new step toward building a computer that uses light instead of electricity to process infor­mation. Relying on photons to transport data within a computer offers several advantages over electronic communi­cations. Photons travel faster than electrons and don’t waste energy by heating up the computer components. Managing that waste heat is a major barrier to improving computer per­formance. Light signals have been used for decades to transmit information over great distances using optical fibers, but the fibers take up too much room to be used to carry data across a computer chip.

The new switch combines nano­meter-scale gold and silicon optical, electrical and mechanical components, all densely packed, to channel light into and out of a miniature racetrack, alter its speed, and change its direction of travel. The device has myriad applications, notes Christian Haffner of NIST, ETH Zurich and the Univer­sity of Maryland. In driverless cars, the switch could rapidly redirect a single light beam that must conti­nually scan all parts of the roadway to measure the distance to other automobiles and pedes­trians. The device could also make it easier to use more powerful light-based circuits instead of elec­tricity-based ones in neural networks. These are artificial intelli­gence systems that simulate how neurons in the human brain make decisions about such complex tasks as pattern recognition and risk manage­ment.

The new tech­nology also uses very little energy to redirect light signals. This feature may help realize the dream of quantum computing. A quantum computer processes data stored in the subtle inter­relations between specially prepared pairs of subatomic particles. However, these relation­ships are extremely fragile, requiring that a computer operate at ultralow tempera­tures and low power so that the particle pairs are disturbed as little as possible. Because the new optical switch requires little energy unlike previous optical switches it could become an integral part of a quantum computer.

Haffner and his colleagues say their findings may come as a surprise to many in the scientific community because the results contradict long-held beliefs. Some researchers have thought that opto-electro-mechanical switches would not be practical because they would be bulky, operate too slowly and require voltages too high for the components of a computer chip to tolerate. The switch exploits the wave nature of light. When two identical light waves meet, they can superpose such that the crest of one wave aligns or reinforces the crest of the other, creating a con­structive inter­ference. The two waves may also be exactly out of step, so that the valley of one wave cancels the crest of the other, resulting in a destruc­tive inter­ference.

In the team’s setup, a light beam is confined to travel inside a miniature waveguide. This linear highway is designed so that it has an off-ramp – some of the light can exit into a racetrack-shaped cavity, just a few nano­meters away, etched into a silicon disk. If the light has just the right wave­length, it can whip around the racetrack many times before leaving the silicon cavity.

The switch has one other crucial component: a thin gold membrane suspended just a few tens of nanometers above the silicon disk. Some of the light traveling in the silicon racetrack leaks out and strikes the membrane, inducing groups of electrons on the membrane’s surface to oscillate. These oscil­lations, the plasmons, are a kind of hybrid between a light wave and an electron wave: The oscillating electrons resemble the incoming light wave in that they vibrate at the same frequency, but they have a much shorter wavelength. The shorter wavelength lets researchers manipulate the plasmons over nanoscale distances, much shorter than the length of the original light wave, before converting the oscil­lations back into light. This, in turn, allows the optical switch to remain extremely compact.

By changing the width of the gap between the silicon disk and the gold membrane by only a few nano­meters, the researchers could delay or advance the phase of the hybrid light wave – the point in time when the wave reaches a crest or valley. Even minuscule variations in the width of the gap, which the team accomplished by electro­statically bending the gold membrane, dramatically altered the phase. Depending on how much the team had advanced or delayed the phase of the wave, when it recombined with light still traveling in the tube-shaped highway, the two beams interfered either constructively or destruc­tively. If the light beams match up to interfere con­structively, the light will continue in its original direction, traveling down the tube. But if the light beams interfere destruc­tively, canceling each other out, that pathway is blocked. Instead, the light must move in another direction, determined by the orien­tation of other wave­guides, or routes, placed close to the blocked pathway. In this way, the light can be switched at will to any of hundreds of other computer chips.

Scientists had once thought that a plasmonic system would greatly attenuate light signals because photons would penetrate the interior of the gold membrane, where electrons would absorb much of the light energy. But the researchers have now proved that assump­tion wrong. The compact­ness of the device and a design that ensured that few photons would penetrate the membrane resulted in a loss of just 2.5% of the light signal, compared with 60% with previous switches. That puts the switch, although still a proto­type, within reach of commercial appli­cations. The team is now working to make the device even smaller by shortening the distance between the silicon disk and the gold membrane. This would further reduce signal loss, making the tech­nology even more appealing to industry. (Source: NIST)

Reference: C. Haffner et al.: Nano–opto-electro-mechanical switches operated at CMOS-level voltages, Science 366, 860 (2019); DOI: 10.1126/science.aay8645

Link: Institute of Electromagnetic Fields IEF, ETH Zurich, Zurich, Switzerland Physical Measurement Laboratory, National Institute of Standards and Technology NIST, Gaithersburg, USA

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