Ti:sapphire, also denoted as Ti:Al2O3, is a commonly used tunable solid state laser. This laser is unique in its wide tunability range (600-1100 nm), peaking at ~800nm, as well as its relatively high emission peak cross-section.2 Ti3+ belongs to the family of transition-metal ions, with d1 configuration. This laser combines the advantages of an Al2O3 host with those of a Ti3+ dopant ion. Table 5.1(a) presents the physical and optical properties of Al2O3, and Table 5.1(b) presents the mechanical and thermal properties. The host Al2O3 or sapphire is a unique laser and optical material in terms of both good optical and mechanical properties. It should be emphasized that sapphire is a very hard material; its hardness (in Mohs units) is M = 9, whereas for diamond M = 10.
The crystal lattice is composed of O2- and Al3+ ions arranged in layers, where the structure is displaced and repeated in the first and third layers. Figure 5.1 presents the layered structure of an Al2O3 crystal. The crystalline structure is such that each of the Al3+ ions are located in the octahedral voids between the close-packed O2- ions in hexagonal arrangement. This octahedron is distorted vertically because the distance between the O2--O2- ions located in two layers is much larger than the O2--O2- distance within a layer. This distortion is called trigonal distortion: It is a hexagonal, closepacking system, uniaxial, and birefringent with a wide transmission range (~0.15 to 6.00 m), which provides flexibility in using various pumping sources for efficient laser operation.
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