Materials capable of producing laser light need to have a variety of properties, related both to how their electrons behave and how they cope with the stresses of large amounts of energy passing through them. Researchers at the University of California San Diego have devised a method of combining alumina crystals with neodymium ions to produce a material that can deliver very short, high power pulses and is also tuneable across a range of light wavelengths and can resist thermal shock.
Both neodymium and alumina are common materials in solid-state lasers. The former is used to make high-power lasers; the latter, used as a matrix for metal ions that can emit light, makes lasers that emit short pulses and can withstand rapid changes of temperature and high heat loads. However, they are incompatible in size: alumina can only host small ions, and neodymium is large.
The San Diego team, led by mechanical engineer Javier Garay at the Jacobs School of Engineering, realised that the key to combining the two materials was in finessing the conditions in the material processing. Traditionally, alumina is doped – treated to disperse foreign ions within its structure – by melting two materials together and cooling them slowly so that the mixture crystallises. But if a molten mixture of alumina and neodymium is cooled too slowly, the neodymium crashes out of the solution. The first author of the team’s paper in the journal Light: Science & Applications, Elias Penilla, devised a new method based on speeding up both the heating and cooling steps fast enough to prevent neodymium ions escaping from the melt.
The new process involves placing a mixture of alumina and neodymium powders under high pressure and heating them at a rate of 300°C per minute until the mixture reaches 1260°C. This forces a high concentration of neodymium ions into the alumina matrix, creating a solid solution. The solution is held at 1260 °C for five minutes and then cooled, also at 300°C per minute.
After characterising the structure of the combined crystal with x-ray diffraction and electron microscopy, Garay’s team optically pumped the crystal with infrared light at a wavelength of 806nm. The crystal produced laser light at 1064nm. Moreover, they showed that the materials thermal shock resistance is 24 times higher than a standard neodymium-containing laser medium, where the ion is dispersed in the mineral yttrium aluminium garnet.
“This means we can pump this with more energy before it cracks, which we can use it to make a more powerful laser,” Garay explained. “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.”
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