Red laser in the dark

Solid semiconductor sensor cooled with laser

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Scientists at the University of Washington have used an infrared laser to cool a solid semiconductor material by at least 20°C below room temperature.

Laser cooling is a family of techniques for using collimated light beams to reduce the temperature of substances, such as by reducing the momentum of atoms through collisions with photons from a laser source. Laser cooling has made possible the study of cold atoms and holds the potential to transform fields such as quantum communication and bioimaging.

Now, a team of scientists at the University of Washington - which had previously used laser cooling to reduce cool water to below room temperature - have demonstrated laser cooling of a solid semiconductor. Using an infrared laser, they were able to reduce the temperature of the semiconductor far below room temperature.

The scientists described their technique in a Nature Communications paper.

The team used a cantilever (a horizontal structure supported at one end, like a diving board) made from a nanoribbon of cadmium sulphide extending from a silicon block. This oscillates in response to thermal energy at room temperature and is sometimes used as a sensor. At the end of the strip, they placed a ceramic crystal containing an impurity (ytterbium ions).

When the scientists focused an infrared laser beam at the crystal, the impurities absorbed some thermal energy from the crystal, causing it to emit photons of a higher frequency (therefore of a higher energy) than the laser light. This 'blueshift glow' cooled the crystal and the nanoribbon to which it was attached.

They used two methods to measure the extent of the laser cooling, first observing changes to the nanoribbon’s oscillation frequency and then changes to the frequency of the light emitted by the crystal. This showed that the temperature of the semiconductor material had dropped by as much as 20°C below room temperature. This took less than 1ms and lasted for as long as the laser was being used.

This was the first-ever demonstration of solid-state laser cooling of nanoscale sensors.

“Historically, the laser heating of nanoscale devices was a major problem that was swept under the rug,” said Professor Peter Pausauskie, a professor of materials science and engineering and senior author of the study.

“We are using infrared light to cool the resonator, which reduces interference or 'noise' in the system. This method of solid-state refrigeration could significantly improve the sensitivity of optomechanical resonators, broaden their applications in consumer electronics, lasers and scientific instruments and pave the way for new applications, such as photonic circuits.”

This work has wide potential applications, not just because of the cooling method – which allows for very precise targeting – but also because of the improved performance of the sensor. The method could have applications in precision measurement, using changes in oscillation frequency to measure the mass of tiny objects such as a single virus particle, as well in cooling systems.

“In the coming years, I will eagerly look to see our laser cooling technology adapted by scientists from various fields to enhance the performance of quantum sensors,” said Anupum Pant, University of Washington doctoral student and lead author of the Nature Communications paper.

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