Lasers can Redefine Bio-Imaging and Quantum Communication

According to common knowledge, lasers heat objects and this understanding would be right in a general sense. However, lasers also have the potential to do the exact opposite—that is, to cool materials. Lasers that are capable of cooling materials could redefine many fields, spanning from quantum communication to bio-imaging.

Back in 2015, scientists from the University of Washington announced that a laser can be used to cool water as well as other kinds of liquids at less than room temperature. Now, the same research team has applied a similar technique to cool something that is relatively different—a solid semiconductor.

As demonstrated by the researchers in a study published in Nature Communications on June 23^rd, 2020, an infrared laser could be used to refrigerate the solid semiconductor by a minimum of 36 °F, or 20 °C, below room temperature.

This cantilever device is analogous to a diving board. Just like how a diving board vibrates after a diver jumps off into the water, the cantilever can vibrate at a particular frequency. But in this case, a diver is not required by the cantilever to vibrate.

The cantilever can oscillate at room temperature in reaction to heat energy, or thermal energy. Such devices could serve as perfect optomechanical sensors, in which their vibrations can be identified by a laser. However, that laser can also heat the cantilever, which reduces its performance.

Historically, the laser heating of nanoscale devices was a major problem that was swept under the rug. 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.

Peter Pauzauskie, Study Senior Author and Professor of Materials Science and Engineering, University of Washington

Pauzauskie is also a senior scientist at the Pacific Northwest National Laboratory.

Pauzauskie further added that the researchers are the first to show the “solid-state laser refrigeration of nanoscale sensors.” He is also a faculty member at the Molecular Engineering & Sciences Institute at the University of Washington and the Institute for Nano-engineered Systems at the same university.

The study results have immense potential applications because of the enhanced performance of the resonator as well as the technique used for cooling it.

Since the semiconductor resonators are able to vibrate, they are handy as mechanical sensors to identify temperature, mass, acceleration, and other properties in a wide range of electronics—like accelerometers to identify the direction being faced by a smartphone. Decreased interference may possibly enhance the performance of these sensors.

Using a laser to refrigerate the resonator is also a relatively more targeted method to enhance the performance of sensors rather than attempting to cool a whole sensor.

In the researchers’ experimental setup, a nanoribbon, or tiny ribbon, of cadmium sulfide extended from a silicon block; at room temperature, this ribbon would naturally experience a thermal oscillation.

The researchers placed a minute ceramic crystal at the end of this diving board. This crystal contains a certain type of impurity, that is, ytterbium ions. When the researchers directed a beam of infrared laser at the crystal, an insignificant amount of energy from the crystal was absorbed by the impurities. This caused the crystal to glow in light that is shorter in wavelength when compared to the laser color that activated it.

As a result of this “blueshift glow” effect, the ceramic crystal and the semiconductor nanoribbon to which it was fixed were eventually cooled down.

“These crystals were carefully synthesized with a specific concentration of ytterbium to maximize the cooling efficiency,” stated Xiaojing Xia, the study’s co-author and doctoral student in molecular engineering at the University of Washington.

The scientists employed a couple of techniques to quantify the extent to which the semiconductor is cooled by the laser. They initially noted variations in 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.

Peter Pauzauskie, Study Senior Author and Professor of Materials Science and Engineering, University of Washington

The researchers further noted that the light produced by the crystal moved on average to longer wavelengths as they raised the laser power, which also denoted the cooling effect.

Through these two techniques, the team estimated that the temperature of the resonator had reduced considerably by 20 °C below room temperature. It took less than 1 ms to trigger the refrigeration effect and this effect persisted, provided that the excitation laser was on.

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.

Anupum Pant, Study Lead Author and Doctoral Student, Materials science and Engineering, University of Washington

According to the scientists, the technique also holds other promising applications. It could form the core of extremely accurate scientific instrumentation, using oscillation changes of the resonator to accurately quantify the mass of an object, like a single virus particle.

Lasers that are capable of cooling solid components may also be used to create cooling systems that prevent overheating of major components integrated into electronic systems

E. James Davis, professor emeritus of chemical engineering at the University of Washington, is an additional co-author of the study.

The study was financially supported by the National Science Foundation, the Air Force Office of Scientific Research, the University of Washington, and the National Institutes of Health.

Journal Reference:

Pant, A., et al. (2020) Solid-state laser refrigeration of a composite semiconductor Yb:YLiF₄ optomechanical resonator. Nature Communications. doi.org/10.1038/s41467-020-16472-6.