By doping alumina crystals with neodymium ions, engineers at the University of California San Diego (UCSD; San Diego, CA) have developed a new laser material that is capable of emitting ultrashort, 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, has 24X higher thermal shock resistance than one of the leading solid-state laser gain materials.
The neodymium-alumina laser gain material is shown. (Courtesy: Elias Penilla)
Neodymium, used to make high-power lasers, and alumina, a host material for light-emitting ions, are two of the most widely used components in solid-state laser materials. 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, ultrashort pulses, and superior thermal shock resistance," says Javier Garay, a mechanical engineering professor at the UCSD 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," explains first author 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 300ºC per minute until it reaches 1260ºC. 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 300ºC 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 (IR) light (806 nm). The material emitted amplified light (gain) at a lower-frequency IR light at 1064 nm.
Neodymium-alumina (left) shows no signs of cracking at 40 W applied optical pumping at 808 nm, while neodymium-YAG (right) cracks at 25 W. (Courtesy: Elias Penilla)
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," Garay says.
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," Garay says.
The research, which was supported by the High Energy Laser – Joint Technology Office administered by the Army Research Office, was published in the journal Light: Science & Applications. The team will also present their work at the 2018 SPIE Optics + Photonics Conference, to take place August 19-23 in San Diego.