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Scientists create liquid light, get one step closer to spintronics

Strange things go on when you push physics to extremes. Extending Moore’s Law to its physical conclusion, we run into problems like the traces in circuits being so small that electrons can quantum tunnel between them. But electrons aren’t the only thing we can use to carry data through circuits. Researchers from Cambridge University have created a semiconductor assembly that blurs the line between electricity and light, and they think we can commercialize it to make optical spintronics — using electron spin in electronics — a reality.

“We have made a field-effect light switch that can bridge the gap between optics and electronics,” says Dr. Hamid Ohadi, coauthor, from the Cavendish Laboratory at Cambridge. “We’re reaching the limits of how small we can make transistors, and electronics based on liquid light could be a way of increasing the power and efficiency of the electronics we rely on.”

It started when researchers caught a laser with a thin slice of semiconductor material in a tiny, mirrored microcavity. This arrangement forced the photons to interact with the semiconductor excitons (excited electrons, bound to the “hole” created when they become excited) and produce a superfluid made of half-light, half-matter chimera quasi-particles called polaritons.

Polaritons result from imposing a dipole on an electromagnetic wave. It’s the same thing that happens when you circularly polarize light. The clockwise or counterclockwise rotation confers a dipole unto the polaritons, giving them orientation and angular momentum in 3-space.

At the cryogenic temperatures these researchers were using, when lots of polaritons are generated in a confined space, they start doing wibbly-wobbly waveform interference stuff, and condense together like water vapor does onto the bathroom mirror. What results is called a polariton Bose-Einstein condensate, which is a superfluid just like a regular Bose-Einstein condensate. The polariton fluid emits light with clockwise or counterclockwise spin. The researchers were able to switch between spin directions by controlling an electric field that they induced within the condensate.

All this matters because spin encoded light can carry data as optical signals, which have advantages over electrical signals at the nanoscale, as well as in security, bandwidth and power consumption. This liquid-light switch could act sort of like a nanophotonic torque converter, translating information from the electrical regime into optical signals. The electric field switching that the researchers used to control their polariton condensate consumed less than 0.5 fJ, which is an amount of power so small that it both defies casual comprehension and makes researchers drool.

Cryogenic temperatures, superfluids, and femto-Joule power consumption are fine for in the lab to prove a concept. They’re less helpful when it comes to real-world consumer devices accessible to mere mortals. Theoretically, this is a great development that could much accelerate fiber-to-the-home, but in practice it’s still a handful of dudes with a laser they can’t take out of the lab. But the team is already working on ways to make this system operable at room temperatures. They’re optimistic: coauthor Pavlos Savvidis of the FORTH institute in Crete says, “Since this prototype is based on well-established fabrication technology, it has the potential to be scaled up in the near future.”

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