Engineers Create New Tunable Laser Materials

Engineers doped alumina (Al₂O₃) crystals with neodymium (Nd) ions to develop a new laser material capable of emitting ultrashort, high-power pulses. Their approach to materials processing resulted in a Nd-Al₂O₃ laser gain medium that has 24× higher thermal shock resistance than one of the leading solid-state laser gain materials.

Nd and Al₂O₃ are two of the most widely used components in today’s solid-state laser materials. However, alumina crystals typically host small ions like titanium or chromium. Neodymium ions are too big — they are normally hosted inside a yttrium aluminum garnet (YAG) crystal.

To address this issue, the team from the University of California, San Diego tailored the crystallite size to other important length scales, i.e., the wavelength of light and interatomic dopant distances, which minimized optical losses and allowed successful Nd doping.

By doping alumina crystals with neodymium ions, engineers at the University of California, San Diego have developed a laser material 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. Courtesy of Elias Penilla.

The new process involves rapidly heating a pressurized mixture of Al₂O₃ and Nd 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 Nd into the Al₂O₃ 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 team characterized the atomic structure of the Nd-Al₂O₃ crystals using x-ray diffraction and electron microscopy. To demonstrate lasing capability, researchers optically pumped the crystals with IR light (806 nm). The material emitted amplified light (gain) at a lower frequency IR light at 1064 nm.

In tests, researchers showed that Nd-Al₂O₃ has 24× higher thermal shock resistance than Nd-YAG, one of the leading solid-state laser gain materials.

“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 professor Javier Garay.

Traditionally, alumina is doped 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,” said researcher Elias Penilla.

The team speeded up the heating and cooling steps enough to prevent neodymium ions from escaping. The Nd-Al₂O₃ hybrid was made by rapidly heating and cooling the two solids together.

Neodymium-alumina (left) shows no signs of cracking at 40 W with applied optical pumping at 808 nm, while neodymium-YAG (right) cracks at 25 W. Courtesy of Elias Penilla.

“Until now, it has been impossible to dope sufficient amounts of neodymium into an alumina matrix," Garay said. "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.”

The team is working on building a laser with their new material.

“That will take more engineering work," Garay said. "Our experiments show that the material will work as a laser and the fundamental physics is all there.”

The successful demonstration of gain and high bandwidth in a medium with superior R_s could lead to the development of lasers with previously unobtainable high-peak powers, short pulses, tunability, and high-duty cycles.

The research was published in Light Science & Applications (doi:10.1038/s41377-018-0023-z).

Penilla will be presenting this work on Aug. 19 at the 2 018 SPIE Optical Engineering + Application Meeting in San Diego.