Researchers at the University of California San Diego (UCSD) have developed a new laser material, neodymium (Nd)-doped alumina crystals, that is capable of emitting ultrashort (theoretically down to 7.7 fs), high-power pulses, a combination that could potentially yield smaller, more powerful lasers with superior thermal shock resistance, broad tunability, and high-duty cycles.1
To achieve this advance, engineers devised new materials-processing strategies to dissolve high concentrations of Nd ions into alumina crystals. The result, a Nd:alumina laser gain medium, is the first in the field of laser materials research.
Nd (dopant) and alumina (host material) are, separately, two of the most widely used components in today's state-of-the-art solid-state laser materials. However, combining Nd 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; Nd ions are too big.
The key to making the Nd: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 Nd ions as the dopant—they would essentially get kicked out of the alumina host as it crystallizes," explains Elias Penilla, a postdoctoral researcher. So his solution was to speed up the heating and cooling steps fast enough to prevent Nd ions from escaping.
The new process involves rapidly heating a pressurized mixture of alumina and Nd powders at a rate of 300 °C per minute until it reaches 1260 °C. This is hot enough to dissolve a high concentration of Nd 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 Nd:alumina crystals using x-ray diffraction and electron microscopy. To demonstrate lasing capability, researchers optically pumped the crystals with infrared light at 806 nm. The material emitted amplified light at 1064 nm.
24 times higher thermal shock resistance than Nd:YAG
In tests, researchers also showed that Nd:alumina has 24 times higher thermal shock resistance than the leading solid-state laser gain material, Nd: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," says Javier Garay, a mechanical engineering professor.
The team is working on building a laser with the new material.
Penilla will be presenting this work as an Invited Oral Lecture on Aug. 19 at the 2018 SPIE Optical Engineering + Application Meeting within the Photonic Fiber and Crystal Devices: Advances in Materials and Innovations in Device Applications XII Section.
1. Elias H. Penilla et al., Nature, Light: Science & Applications (2018); https://doi.org/10.1038/s41377-018-0023-z