ALBERT

Notes: Nd:Sr5(VO4)3Cl has been successfully lased using a Ti:sapphire pump laser operating at 810 nm. A slope efficiency of 52% for the 1.065 μm laser line and a lasing threshold of only a few tens of milliwatts have been measured. These favorable results are comparable to another Nd-doped vanadate apatite, Sr5(VO4)3F. A brief survey of the spectroscopy of Nd3+ in Sr5(VO4)3Cl, Sr5(VO4)3F, and Ca5(VO4)3F reveals properties which suggest the vanadate-based apatite crystals as a whole may represent a useful new class of Nd-laser hosts.

Notes: A new class of Yb-lasers is summarized in this article. The apatite family of crystals, based on the hexagonal structure of the mineral fluorapatite, has been found to impose favorable spectroscopic and laser properties on the Yb3+ activator ion. Crystals of Yb-doped Ca5(PO4)3F, Sr5(PO4)3F, CaxSr5−x(PO4)3F, and Sr5(VO4)3F have been grown and investigated. Several useful laser crystals have been identified which offer a variety of fundamental laser parameters for designing diode-pumped systems. In general, this class of materials is characterized by high emission cross sections (3.6–13.1×10−20 cm2), useful emission lifetimes (0.59–1.26 ms), a strong pump band (σabs=2.0–10.0×10−20 cm2), and pump and extraction wavelengths near 900 and 1045 nm, respectively. Efficient lasing has been demonstrated for several of the members of this class of materials, and high optical quality crystals have been grown by the Czochralski method. A summary of the laser parameters and a discussion of the Yb:apatite class of lasers is presented.

Notes: Titania is a material with structural flexibility, and as a result, readily forms both crystalline polymorphs and an amorphous structure in thin films grown near room temperature. The goal of this study is to correlate fundamental optical absorption edge characteristics with the phase constituency of titania films. To that end, films with coexistent rutile, anatase, and amorphous constituents were sputter deposited onto fused silica and 〈111〉-Si substrates. The films were then subjected to cyclic annealing in air at moderate temperature (700 and 1000 °C) to affect phase changes. Bragg–Brentano x-ray diffraction was used for phase identification and near ultraviolet-visible transmission and reflection spectrophotometry was used to determine the optical absorption coefficient at the onset of interband transitions. The optical absorption coefficient was modeled within the framework of the coherent potential approximation (CPA), with Gaussian site disorder introduced into the valence and conduction bands of a perfect virtual crystal. Two parameters of the disordered crystal were defined: the optical band gap, Ex, and the slope of absorption edge, Eo. The results are discussed in terms of two extreme cases: (1) film states containing a large rutile volume fraction (0.70–1) share a rutile virtual crystal, with Eg=3.22 eV. Data for these states was combined with single crystal data to develop an expression interrelating Eg, Ex, and Eo. This expression is applicable to any structure with a rutile virtual crystal. The relationship between structural disorder (i.e., the volume fraction of amorphous material) and electronic disorder (i.e., Eo), is quantitatively consistent with the CPA model. (2) Film states containing a small rutile volume fraction (0.02–0.17), and hence a large anatase+amorphous component, share a nonrutile virtual crystal, with Eg=3.41 eV. The effect of increasing the structural disorder (i.e., the rutile volume fraction), in these states is to shift Ex to lower values, which is qualitatively consistent with the CPA model. Furthermore, anatase and amorphous components can be modeled using the same nonrutile virtual crystal, indicating these structures have a common short-range order in the sputter deposited films of this study. © 1999 American Institute of Physics.

Notes: A mixed cation interfacial structure in ZrO2–TiO2 nanolaminate films with ultrathin bilayer periodicity grown by sputter deposition at 297 K was identified by x-ray diffraction and nonresonant Raman spectroscopy. This structure consists of an amorphous phase at a ZrO2-on-TiO2 bilayer interface, followed by an extensive crystalline monoclinic (Zr,Ti)O2 solid solution predicted by Vegard's law. Monoclinic (Zr,Ti)O2 has previously been reported only once, in bulk powder of a single composition (ZrTiO4) at high pressure. Its stabilization in the nanolaminates is explained by the Gibbs–Thomson effect. This complex interfacial structure is shown to be a means of accommodating chemical mixing in the absence of a driving force for heteroepitaxy. © 2002 American Institute of Physics.