Getting Cooler With Lasers

NTU research team makes groundbreaking discovery on use of lasers.

By Ronald Loh

For the first time, scientists have successfully used lasers to cool a solid.

While the cooling of gases and glasses by lasers is already common practice, the research team from the School of Physical and Mathematical Sciences (SPMS) were the first in the world to cool a solid semiconductor by 40 degrees Celsius.

Their groundbreaking research, which took three years to complete, was published in the international research journal Nature last month.

“This is the first time we have managed to reduce the temperature of a solid, and more notably a semiconductor — commonly used in electrical devices,” said Assistant Professor Xiong Qihua, the leading scientist of the study.

His team also includes research fellow Dr Zhang Jun, PhD student Li Dehui, and researcher Chen Renjie.

Today, cooling components — such as fans or electrical conductors — contribute to most of the weight and bulkiness in our everyday devices.

Chunky refrigerators and noisy air-conditioning compressors hence could soon be obsolete, if this discovery is successfully replicated on a larger scale. Handphones and laptops could also come in lighter, more compact models.

Beyond the compactness, electronic devices — which slow considerably when heated — could also be made more powerful and to last longer after the research team was able to cool their Cadmium Sulfide (CdS) sample to -20 deg C.

“Removing the need for compressors and coolants in airconditioning and refrigerators could also reduce the emission of greenhouse gases harmful to our ozone layer,” said Asst Prof Xiong.

Reducing heat

The process of cooling normally comes about when the laser interacts with gas particles to reduce their motion, and subsequently the gas’s energy, thus reducing its heat.

But the process of working with solids is entirely different.

In order to cool a solid, phonons — vibrations of the tightly-packed atoms that propagate through the material — have to be terminated.

The physicists found that a laser — calibrated at the right wavelength — could nullify the semiconductor’s phonons completely, hence lowering the temperature of the test strips.

But Asst Prof Xiong said his team’s work is far from done.

“Our biggest challenge right now is finding other materials that will produce the same effect as the CdS sample,” he said.

While the tests were successful on a semiconductor 3 micrometres in size, or about one-sixth the width of a human hair, Asst Prof Xiong said their task was also to reproduce the same results on a much larger scale.

“After that, there is a need to design a system that can incorporate the laser cooler and manage the heat flow.”

As of now, Asst Prof Xiong said it will be still “years and years” before the laser coolers make it to the shelves.

Should they succeed, its capabilities are endless, he said.

“Besides these gadgets, we hope to achieve a greater cooling effect of 100 deg C. This could replace the use of liquid helium in MRI scanners,” said Asst Prof Xiong. “We are running out of liquid helium (in the next 20-25 years), so this could be a replacement.”

“This discovery also translates into the ability to build miniaturised coolers for infrared sensors in imaging satellites,” he added.

The significance of this groundbreaking project was also highlighted by Dr Ho Shen Yong, a lecturer at SPMS.

“It may not be common knowledge, but low-temperature physics is one of the hottest areas in research now,” he said of the team’s breakthrough.

He added that if engineers are able to create a mechanism that can sustain lowered temperatures for long periods, it could pave the way for more powerful computers that could even surpass supercomputers.

“A more exciting speculation is whether this optical refrigeration technology can develop into the central component of possible solid-state quantum computers.”