New solid state laser crystals created by epitaxial growth

• Main Author: Kumaran, Raveen
• Language: English
• Published: University of British Columbia 2012
• Online Access: http://hdl.handle.net/2429/43270

Rare-earth-doped oxide crystals such as Nd:YAG are often used as the gain medium in solid state lasers because they produce a collection of sharp emission peaks, some of which have strong gain. Rather than the typical bulk form of these crystals, thin films with planar waveguide geometry are promising alternatives for compact devices with lower lasing thresholds and better heat extraction. Such benefits motivated the growth of these oxide films by molecular beam epitaxy, a technique capable of films with precise composition, thickness and structure due to its independently controlled elemental sources. Two sets of material systems were attempted, Al-Ga-O and Y-Al-O, with Nd as the sole dopant. The films were grown on sapphire substrates of various orientations, then analyzed by x-ray diffraction and photoluminescence where peaks signaled long-range and short-range order respectively. Work on the Al-Ga-O system yielded Nd:α-Al₂O₃ (Nd:sapphire) and Nd:α-Ga₂O₃, two new laser crystals with unique collections of sharp emission peaks only observable from single-crystal films grown at temperatures far below the melting point. These conditions were required to confine the much-larger Nd dopants into Al and Ga sites with short-range order, and are unlikely reproducible by bulk methods. Due to their shared corundum structure, the emission spectra from Nd:sapphire and Nd:α-Ga₂O₃ appear similar but wavelength-shifted. The dominant Nd:sapphire peak has strong gain comparable to its counterpart from Nd:YVO₄, one of the highest available, while the blue-shifted Nd:α-Ga₂O₃ peak is likewise comparable to Nd:YAG. Alloying both produced corundum-structure crystals with peaks that shifted linearly with unit cell volume, which in turn depended on Ga/Al ratio and film stress. These Al-Ga-O crystals are promising for applications involving compositional-tuning and graded-index layers. Ternary Y-Al-O is not tunable because it has three stable phases YAM, YAP and YAG with different compositions, structures and thus emission spectra. Single-phase films of each phase were grown without single-crystal structure but still yielded sharp peaks consistent with their bulk counterparts. Short-range order was achievable because the Nd dopants were size-compatible with the Y sites. While the peaks did not shift with Y/Al ratio, peak sharpness improved when the ratio approached bulk stoichiometric values.