Instead of using lasers to heat targets, now researchers are shooting light beams that cool what they shine on.
Scientists want to see if novel refrigerators based on this research could reach temperatures just a few degrees above the coldest possible—absolute zero.
Normal refrigerators use motors to pump compressed gas through tubes lining them. As these gases expand, they remove heat from inside the appliance.
The technique whereby lasers can be used to cool targets is called optical refrigeration. It works like this: lasers focused on an object can make it fluoresce, which causes it to lose energy as light and decrease in temperature. Optical refrigerators potentially have a number of advantages over conventional units. Since they require neither gas nor moving parts, they can be more compact, free from vibration and not prone to mechanical failure.
Although optical refrigeration of solids was first predicted in 1929, it was not seen experimentally until 1995 with glassy and crystalline materials doped with rare earth metals. In less than 20 years, research using these materials has advanced optical refrigeration enough to achieve cooling from room temperature to about -260 degrees Fahrenheit. Still, that is significantly warmer than the -321 degrees F at which liquid nitrogen boils.
Scientists wanted to accomplish optical refrigeration with semiconductors instead of rare-earth-metal-doped glasses and crystals. Semiconductors are the basis of modern electronics, and so researchers strove to directly integrate optical refrigerators into existing devices. Calculations also suggested that optical refrigerators that used semiconductors could achieve much lower temperatures. However, attempts to develop the units based on the semiconductor gallium arsenide failed. Although this semiconductor does fluoresce when hit by lasers, the emitted light does not escape from the material efficiently, so it heats up instead of cooling down.
Now, using the semiconductor cadmium sulfide, researchers have shown that lasers could trigger cooling, from about 62 degrees F to about -9 degrees F.
"This came to us as a surprise," says Qihua Xiong, a physicist and optical spectroscopist at Nanyang Technological University in Singapore. He and his colleagues were originally investigating how the semiconductor responds to lasers for completely different purposes.
The laser excites electrons in the semiconductor, dislodging them and leaving behind a “hole.” This electron-hole pair is known as an “exciton,” which is surrounded by vibrating atoms. Extremely strong interactions between the excitons and bundles of these vibrations, known as phonons, lead to fluorescence that cools the semiconductor very efficiently.
"It was a very welcome surprise to see that these types of bulk semiconductors can be laser-cooled," says University of New Mexico physicist Mansoor Sheik-Bahae, who did not participate in this study. "Initially we were skeptics, but when we looked at their methodology — the way they measured temperatures, the way they did experiments — it looked very solid to us."
The researchers achieved cooling using specific wavelengths of green light. They precisely engineered the semiconductor into thin ribbons less than 10 microns wide, or less than one-tenth the width of a human hair, and only 100 nanometers thick, less than a wavelength of visible light, to confine the interactions within the semiconductor and accomplish cooling.
"People may think the nanobelt is too small, that it is not going to be useful," Xiong says. But, by showing that laser cooling of semiconductors is possible, more practical devices may emerge "once we find better materials, pushing the cooling to bulk materials."
Important future uses
"The big potential users of this cooling technology are night-vision goggles, and infrared cameras on satellites, where weight is very important and you would not want the motors and pumps and vibrations that come with regular coolers if you can," says Richard Epstein, a University of New Mexico physicist and CEO at technology startup ThermoDynamic Films, who did not take part in this research.
"There are still lots of questions of why laser cooling of nanobelts works so well here — research should try different thicknesses and different-shaped structures to understand why laser cooling was so successful here," Epstein says. "On a practical level, one would then like to start incorporating these cooling cadmium sulfide nanobelts with some actual electronics."
The researchers want to see if semiconductor optical refrigerators can reach far lower temperatures, even reaching down to that of liquid helium, which is at least -452 degrees F, just a few degrees above absolute zero.
"Rare-earth-doped glasses or crystals always reached a fundamental limit of about 110 Kelvin (-261 degrees F), while semiconductors are our hope to reach liquid helium temperature by laser cooling," Xiong says.
The scientists detailed their findings in the Jan. 24 issue of the journal Nature.