The laser enables solid-state cooling of a semiconductor material

For the general public, lasers heat objects. And in general, that would be correct.

But lasers also promise to do just the opposite: cool materials. Lasers that can cool materials could revolutionize fields ranging from bioimaging to quantum communication.

In 2015, researchers at the University of Washington announced that they can use a laser to cool water and other liquids below room temperature. Now that same team has used a similar approach to refrigerate something quite different: a solid semiconductor. As the team shows in an article published on June 23 in Nature’s CommunicationsThey could use an infrared laser to cool the solid semiconductor to at least 20 degrees C, or 36 F, below room temperature.

The device is a cantilever, similar to a trampoline. Like a springboard after a swimmer jumps into the water, the cantilever can vibrate at a specific frequency. But this cantilever does not need a diver to vibrate. It can oscillate in response to thermal energy, or thermal energy, at room temperature. Devices like these could be ideal optomechanical sensors, where their vibrations can be detected by a laser. But that laser also heats the overhang, which reduces its performance.

“Historically, laser heating of nanoscale devices was a major problem that went unnoticed,” said lead author Peter Pauzauskie, a professor of materials science and engineering at UW and a senior scientist at the Pacific Northwest National Laboratory. “We are using infrared light to cool the resonator, which reduces interference or ‘noise’ in the system. This solid-state cooling method could significantly improve the sensitivity of optomechanical resonators, expanding its applications in consumer electronics, laser and scientific instruments, and pave the way for new applications, such as photonic circuits. “

The team is the first to demonstrate “solid-state laser cooling of nanoscale sensors,” added Pauzauskie, who is also a faculty member at the UW Institute of Engineering and Molecular Sciences and the Institute of Nano Engineering Systems at the UW.

The results have wide potential applications due to both the improved performance of the resonator and the method used to cool it. The vibrations of semiconductor resonators have made them useful as mechanical sensors to detect acceleration, mass, temperature, and other properties in a variety of electronic components, such as accelerometers to detect the direction a smartphone is headed. Reducing interference could improve the performance of these sensors. Also, using a laser to cool the resonator is a much more specific approach to improving sensor performance compared to trying to cool an entire sensor.

In its experimental setup, a small ribbon, or nanoribon, of cadmium sulfide stretched from a block of silicon, and would naturally undergo thermal oscillation at room temperature.

At the end of this trampoline, the team placed a small ceramic crystal that contains a specific type of impurity, ytterbium ions. When the team focused an infrared laser beam on the crystal, the impurities absorbed a small amount of energy from the crystal, causing it to shine in light that is shorter in wavelength than the color of the laser that excited it. This “blue glow” effect cooled the ceramic crystal and the semiconductor nanoribon to which it was attached.

“These crystals were carefully synthesized with a specific concentration of ytterbium to maximize cooling efficiency,” said co-author Xiaojing Xia, a PhD student in molecular engineering at UW.

The researchers used two methods to measure how much the semiconductor cooled the laser. First, they observed changes in the oscillation frequency of the nanoribon.

“The nanoribon becomes stiffer and more 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 the laser power increased, which also indicated cooling.

Using these two methods, the researchers calculated that the resonator temperature had dropped to 20 degrees C below ambient temperature. The cooling effect took less than 1 millisecond and lasted while the excitation laser was on.

“In the coming years, I will look forward to seeing our laser cooling technology adapted by scientists from various fields to improve the performance of quantum sensors,” said lead author Anupum Pant, a Ph.D. student at the University of Washington in science and materials engineering.

The researchers say the method has other potential applications. It could form the heart of highly precise scientific instruments, using changes in resonator oscillations to accurately measure the mass of an object, such as a single virus particle. Lasers that cool solid components could also be used to develop cooling systems that prevent overheating of key components in electronic systems.