New Class of Laser Materials

By doping alumina crystals with neodymium ions, engineers at the Univer­sity of Cali­fornia San Diego have developed a new laser material that is capable of emitting ultra-short, high-power pulses: a combi­nation that could poten­tially yield smaller, more powerful lasers with superior thermal shock resis­tance, broad tuna­bility and high-duty cycles. To achieve this advance, engineers devised new materials processing strategies to dissolve high concen­trations of neodymium ions into alumina crystals. The result, a neo­dymium-alumina laser gain medium, is the first in the field of laser materials research. It has 24 times higher thermal shock resistance than one of the leading solid-state laser gain materials.

By doping alumina crystals with neodymium ions, engineers at the University of California San Diego have developed a new laser material that is capable of emitting ultra-short, high-power pulses. (Source: E. Penilla)

Neo­dymium and alumina are two of the most widely used compo­nents in today’s state-of-the-art solid-state laser materials. Neo­dymium ions, a type of light-emitting atoms, are used to make high-power lasers. Alumina crystals, a type of host material for light-emitting ions, can yield lasers with ultra-short pulses. Alumina crystals also have the advan­tage of high thermal shock resis­tance, meaning they can withstand rapid changes in tempera­ture and high loads of heat.

However, combining neodymium and alumina to make a lasing medium is chal­lenging. The problem is that they are incom­patible in size. Alumina crystals typically host small ions like titanium or chromium. Neo­dymium ions are too big. They are normally hosted inside a crystal called yttrium aluminum garnet (YAG). “Until now, it has been impossible to dope suffi­cient amounts of neo­dymium into an alumina matrix. We figured out a way to create a neo­dymium-alumina laser material that combines the best of both worlds: high power density, ultra-short pulses and superior thermal shock resistance,” said Javier Garay, a mechanical engi­neering professor at the UC San Diego Jacobs School of Engi­neering.

The key to making the neodymium-alumina hybrid was by rapidly heating and cooling the two solids together. Tradi­tionally, researchers dope alumina by melting it with another material and then cooling the mixture slowly so that it crystal­lizes. “However, this process is too slow to work with neo­dymium ions as the dopant. They would essen­tially get kicked out of the alumina host as it crystal­lizes,” explained Elias Penilla, a post­doctoral researcher in Garay’s research group. So his solution was to speed up the heating and cooling steps fast enough to prevent neodymium ions from escaping.

The new process involves rapidly heating a pres­surized mixture of alumina and neodymium powders at a rate of 300 C per minute until it reaches 1,260 C. This is hot enough to dissolve a high concen­tration of neodymium into the alumina lattice. The solid solution is held at that tempera­ture for five minutes and then rapidly cooled, also at a rate of 300 C per minute. Researchers charac­terized the atomic structure of the neodymium-alumina crystals using X-ray diffrac­tion and electron micro­scopy. To demonstrate lasing capa­bility, researchers opti­cally pumped the crystals with infrared light (806 nm). The material emitted amplified light at a lower frequency infrared light at 1064 nm.

In tests, researchers also showed that neodymium-alumina has 24 times higher thermal shock resis­tance than one of the leading solid-state laser gain materials, neodymium-YAG. “This means we can pump this material with more energy before it cracks, which is why we can use it to make a more powerful laser,” said Garay. The team is working on building a laser with their new material. “That will take more engi­neering work. Our experiments show that the material will work as a laser and the funda­mental physics is all there,” said Garay. (Source: UCSD)

Reference: E. Penilla et al.: Gain in Polycrystalline Nd-doped Alumina: Leveraging Length Scales to Create a New Class of High-Energy, Short Pulse, Tunable Laser Materials, Light: Sci. & App. 7, 33 (2018); DOI: 10.1038/s41377-018-0023-z

Link: Jacobs School of Engineering, University of California San Diego, San Diego, USA

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