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Lasers Cool Gas Clouds
Mar 2012
COPENHAGEN, Denmark, March 5, 2012 — A novel method for laser cooling semiconductor membranes was developed, paving the way to efficient cooling of components for future ultrasensitive sensors and quantum computers.

Researchers in the quantum optical laboratories of the Quantop research group at Neils Bohr Institute have conducted experiments on laser cooling of atoms for the past several years to understand optomechanics — the interaction between a mechanical motion and optical radiation or light — according to Eugene Polzik, head of the Center of Excellence Quantop.

Koji Usami shows a holder with the semiconductor nanomembrane. The holder measures about 1 x 1 cm, while the nanomembrane itself has a surface area of 1 x 1 mm and a thickness of 160 nm. (Images: Ola J. Joensen)

The scientists developed the new cooling method, which works paradoxically by heating the material, by combining quantum mechanics with nano physics. They produced semiconductor nanomembranes with a thickness of 160 nm and a surface area of 1 x 1 mm.

“We let the membrane interact with the laser light in such a way that its mechanical movements affected the light that hit it,” said Koji Usami, an associate professor at Quantop at the Niels Bohr Institute.

Koji Usami is working in the Quantop laboratories at the Niels Bohr Institute.

The team examined the physics and discovered that the reflected light from the membrane was reflected back once more using a mirror, which makes the light move back and forth within the space, creating an optical resonator, Usami said. They were able to cool the membrane to -269 °C.

Some of the light absorbed by the membrane causes the release of free electrons. When the freed electrons decay, they heat the membrane, resulting in thermal expansion. In this manner, the distance between the membrane and the mirror continuously fluctuate, he said.

The researchers observed that the membrane’s certain oscillation mode is cooled down to -269 °C from room temperature due to the complicated interaction between the optical resonances, the membrane movements and the properties of the semiconductor material, he said.

The laser light that hits the semiconducting nanomembrane is controlled with a forest of mirrors.

“This is a new optomechanical mechanism, which is central to the new discovery,” Usami said. “The paradox is that even though the membrane as a whole is getting a little bit warmer, the membrane is cooled at a certain oscillation, and the cooling can be controlled with laser light. So it is cooling by warming!” he said.

The new discovery could lead to the development of new sensors for electric current and mechanical forces. In some cases, they could even replace expensive cryogenic cooling used today, which could result in extremely sensitive sensors that are limited only by quantum fluctuations, Polzik said.

The results were published in Nature Physics.

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