Could these crystals be the next leap forward in China’s laser technology?

Stephen Chen

Scientists in southeast China say they have synthesised a crystal with the potential to significantly improve the performance of lasers used in consumer and military equipment.

Crystals of caesium bismuth germanate (CBGO) can turn low-energy beams into high-energy emissions with unparalleled efficiency, according to Professor Mao Jianggao, team leader at the Fujian Institute of Research on the Structure of Matter, part of the Chinese Academy of Sciences, in Fuzhou.

The team looked at several candidate crystals in their experiments. Compared to widely used potassium dihydrogen phosphate crystals, the CBGO crystal was 13 times more efficient at turning infrared lasers into more energised green beams.

“This is a record performance,” Mao said on Tuesday. “This is why we think the crystal may have potential.”

China’s prototype Guanlan anti-submarine warfare satellite uses a high-energy laser to sweep beneath the sea to a depth of 500 metres. Illustration: SCMP

Their findings were published in the German weekly scientific peer-reviewed journal Angewandte Chemie, or Applied Chemistry, last month.

The researchers said CBGO crystals could be a way around a problem that has limited the performance of lasers – the huge amount of electricity needed to power them.

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The demand on power is great because existing technology is not efficient at converting low-energy beams to high-energy ones – one reason that laser weapons are not yet as common as scientists predicted in the 1960s.

CBGO belongs to a family known as non-linear crystals, which cause abrupt changes to energy that passes through them. The scientists found that CBGO crystals could double the frequency of a laser beam.

As high-energy lasers can be created by merging two low-energy photons, or particles of light, a process known as frequency doubling, CBGO crystals are an ideal medium, and the higher frequency of the laser, the more energy it carries.

Many military and civilian applications required high-energy beams, they said. These included directed energy weapons designed to destroy drones or missiles, or China’s prototype Guanlan anti-submarine warfare satellite, which will use a green laser to penetrate water to a depth of 500 metres (1,640 feet) to detect a target.

Mao said his team’s research was at an early stage and that years of testing would be needed before the CBGO crystals found their way to market.

The CBGO crystal grown in the Fuzhou laboratory was the size of a grain of sand, he said. For industrial use, crystals would need to be at least the size of a dice.

Growing them was a very slow and challenging job, and there was no certainty that CBGO crystals could be grown on an industrial scale, Mao said.

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China is a world leader in crystal research, and some of those most commonly used in laser devices were developed by Chinese scientists thanks to heavy investment from central government.

Professor Li Qiang, deputy director with the Institute of Laser Engineering at Beijing University of Technology, said the discovery of CBGO was encouraging, but its success should be evaluated not only on its efficiency, but also on attributes such as mechanical strength, tolerance to laser damage, and stability in extreme environments such as high humidity.

“Lots of crystals have been proposed over the decades, but only a handful are useful. It’s a high-risk business,” Li said. “China has achieved a leading position in this field not because of luck, but by continuous effort by several generations of researchers through countless failures.”

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