Laser applications may benefit from crystal research by scientists at the National Institute of Standards and Technology (NIST) and China’s Shandong University. They have discovered a potential way to sidestep longstanding difficulties with making the crystals that are a crucial part of laser technology. But the science behind their discovery has experts scratching their heads.
The findings, published today in Science Advances, suggest that the relatively large crystals used to change several properties of light in lasers – changes that are crucial for making lasers into practical tools – might be created by stacking up far smaller, rod-shaped microcrystals that can be grown easily and cheaply.
So far, the team’s microcrystals outperform conventional crystals in some ways, suggesting that harnessing them could signal the end of a long search for a fast, economical way to develop large crystals that would otherwise be prohibitively expensive and time-consuming to create. But the microcrystals also challenge conventional scientific theory as to why they perform as they do.
The color you see in a laser’s light is often different than the one it initially generates. Many lasers create infrared light, which then passes through a crystal converting its energy – and therefore its wavelength – to light of a visible color like green or blue.
Frequently, that crystal is made of potassium diphosphate (KDP), a common material that has properties that make it invaluable: Not only can a KDP crystal alter the light’s color, but it also can act as a switch that changes the light’s polarization (the direction in which its electric field vibrates) or prevent it from passing through the crystal until just the right moment. The data carried by laser light through fiber-optic cables depends on the light’s polarization, and many applications depend on a laser pulse’s timing.
Small KDP crystals are easy to make, and these find use in pocket laser pointers and telecommunications systems alike. But for higher-energy applications, scientists have searched for decades for a way to make large, high-quality crystals that can survive repeated exposure to intense laser pulses, but a solution has remained elusive.
The team has found useful results by growing KDP crystals in solution. These crystals take the form of hexagonal-shaped hollow tubes and long rods just a few micrometers wide. Individually, these KDP microcrystals have an energy-conversion efficiency surpassing even the best KDP crystals under the same conditions, raising the possibility of directly growing crystals for use in telecommunications.
The team also suggests the rods could be stacked up like firewood, building a larger piece out of billions of the tiny filaments. Before they are stacked together they could be coated by a thin layer of conductive material that carries heat away, rendering them capable of handling repeated pulses of high-intensity laser light – potentially broadening their application range if a way can be found to stack them.
The mystery is why the microcrystals perform as they do. Basic physics says they shouldn’t. Conventional physics models indicate that an optical medium like a crystal must not be symmetric about its center if it is to convert energy efficiently, yet these microcrystals appear to break this rule.
“We’ve spoken to a number of experts in different fields worldwide, and none of them can explain it,” says NIST physicist Lu Deng. “Currently no theory can explain the initial growth mechanism of this exotic crystal. It’s challenging our current understanding in fields from crystallography to condensed matter physics.”
While theory catches up with data, Deng said the team is concentrating on the engineering challenges of growing stackable microcrystal rods.
“We can grow more than 1,000 microstructures every 10 minutes or so on a single glass slide, so growing a large amount is not a problem,” he said. “What we need to figure out is how to grow a large fraction of them with nearly uniform cross-sections since that will be important in the final assembly stage.”
Shandong University (simplified Chinese: 山东大学; traditional Chinese: 山東大學; pinyin: Shāndōng Dàxué, abbreviated as Shanda, Chinese: 山大; pinyin: Shāndà, English acronym SDU) is a public comprehensive university in Shandong, China.
It is one of the largest universities in China by student population (57,500 full-time students in 2009) and is supported directly by the national government.
Present-day Shandong University is the result of multiple mergers as well as splits and restructurings that have involved more than a dozen academic institutions over time. The oldest of Shandong University’s precursor institutions, Cheeloo University, was founded by American and English mission agencies in the late 19th century (as Tengchow College of Liberal Arts in Penglai).
Tengchow College was the first modern institution of higher learning in China. Shandong University derives its official founding date from the Imperial Shandong University (Chinese: 山东大学堂; pinyin: Shāndōng Dàxué Táng) established in Jinan in November 1901 as the second modern national university in the country.
Shandong University has seven campuses, all but one of which are located in the provincial capital city of Jinan. A campus to the northeast of the port city of Qingdao is under construction. The university has been classified as a National Key University by the Chinese Ministry of Education since 1960. It has been included in major national initiatives seeking to enhance the international competitiveness of the top-tier universities in China such as Project 985 and Project 211.
Shandong University offers master and doctoral degree programs in all major academic disciplines covering the humanities, science and engineering, as well as medicine.
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