"These crystals were carefully synthesized with a specific concentration of ytterbium to maximize the cooling efficiency," said co-author Xiaojing Xia, a UW doctoral student in molecular engineering.
The researchers used two methods to measure how much the laser cooled the semiconductor. First, they observed changes to the oscillation frequency of the nanoribbon.
"The nanoribbon becomes more stiff and brittle after cooling—more resistant to bending and compression. As a result, it oscillates at a higher frequency, which verified that the laser had cooled the resonator," said Pauzauskie.
The team also observed that the light emitted by the crystal shifted on average to longer wavelengths as they increased laser power, which also indicated cooling. Using these two methods, the researchers calculated that the resonator's temperature had dropped by as much as 20 degrees C below room temperature. The refrigeration effect took less than 1 millisecond and lasted as long as the excitation laser was on.
The researchers anticipate their findings will impact several fields including scanning probe microscopy, the sensing of weak forces, the measurement of atomic masses, and the development of radiation-balanced solid-state lasers.
Optically refrigerated resonators may also be used in the future as a promising starting point to perform motional cooling for exploration of quantum effects at mesoscopic length scales, temperature control within integrated photonic devices, and solid-state laser refrigeration of quantum materials.
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