Shock of the new neodymium-doped alumina crystals

By doping alumina crystals with neodymium ions, engineers at the University of California (UC) San Diego have developed a new laser material that is capable of emitting ultra-short, high-power pulses. This combination could potentially yield smaller, more powerful lasers with superior thermal shock resistance, broad tunability and high-duty cycles.

To achieve this advance, the engineers devised new materials processing strategies for dissolving 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 and is described in a paper in Light: Science & Applications. It has 24 times higher thermal shock resistance than one of the leading solid-state laser gain materials.

Neodymium and alumina are two of the most widely used components in today's state-of-the-art solid-state laser materials. Light-emitting neodymium ions 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, and so 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 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 first author Elias Penilla, a postdoctoral researcher in Garay's research group. 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 the mixture reaches 1260°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.

The researchers characterized the atomic structure of the neodymium-alumina crystals using X-ray diffraction and electron microscopy. To demonstrate lasing capability, they optically pumped the crystals with infrared light (806nm), finding that the material emitted amplified light (gain) at a lower frequency of 1064nm.

In tests, the 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 now 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.

This story is adapted from material from the University of California San Diego, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.