Laser used for solid-state refrigeration of a semiconductor

June 26, 2020 //By Julien Happich
Laser used for solid-state refrigeration of a semiconductor
Researchers from the University of Washington used an infrared laser to cool a solid semiconductor cantilever by at least 20 degrees C below room temperature.

Lasers that can cool materials could revolutionize fields ranging from bio-imaging to quantum communication. The discovery was first made in 2015, whereby a laser was used to cool water and other liquids below room temperature. Now that same team has used a similar approach to refrigerate a solid semiconductor.

The findings were published in Nature Communications under the title “Solid-state laser refrigeration of a composite semiconductor Yb:YLiF4 optomechanical resonator". The device is a cantilever which can vibrate at a specific frequency in response to thermal energy, at room temperature. Devices like these could make ideal optomechanical sensors, where their vibrations can be detected by a laser. But that very laser also heats up the cantilever, which dampens its performance.

"Historically, the laser heating of nanoscale devices was a major problem that was swept under the rug," explains senior author Peter Pauzauskie, a UW professor of materials science and engineering 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 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."

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

An image of the team's experimental setup, taken using a
bright-field microscope. The silicon platform, labelled "Si," is
shown in white at the bottom of the image. The nanoribbon
of cadmium sulfide is labelled "CdSNR." At its tip is the
ceramiccrystal, labeled "Yb:YLF." Scale bar is 20 micrometers.
Credit: Pant et al. 2020, Nature Communications.

The results have wide potential applications due to both the improved performance of the resonator and the method used to cool it. Reduced interference could improve performance of cantilever-based sensors. In addition, using a laser to cool the resonator is a much more targeted approach to improve sensor performance compared to trying to cool an entire sensor. In their experimental setup, a nanoribbon of cadmium sulfide extended from a block of silicon and would naturally undergo thermal oscillation at room temperature.

At the end of this diving board, the team placed a tiny ceramic crystal containing a specific type of impurity, ytterbium ions. When the team focused an infrared laser beam at the crystal, the impurities absorbed a small amount of energy from the crystal, causing it to glow in light that is shorter in wavelength than the laser color that excited it. This "blueshift glow" effect cooled the ceramic crystal and the semiconductor nanoribbon it was attached to.

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