Fig. 1. Schematic of Al-Ti-O ternary system^[23]

Fig. 2. Formation energy of each defect in Ti∶sapphire versus chemical potential^[23]. (a) Substitutional defect; (b) interstitial Ti defect

Fig. 3. Bonding energy in Ti∶Al₂O₃ crystal versus distance between defects^[33]

Fig. 4. Schematic of formation of defect cluster or ion pair in α-Al₂O₃ crystal structure^[38]

Fig. 5. Optical absorption spectra of UC3 in Ti∶sapphire sample^[20]. (a) Decomposed absorption spectra in near IR and visible region; (b) spectral decomposition at infrared region in Fig. 5(a)

Fig. 6. Optical absorption spectra^[20]. (a) π-polarized optical absorption spectra of Ti∶sapphire crystals with different Ti concentrations; (b) normalized optical absorption spectra ( absorption coefficient at 490 nm is 1)

Fig. 7. Optical absorption spectra^[20] . (a) π-polarized optical absorption spectra of Ti∶sapphire crystals with different Ti concentrations ( “E” band and Ti³⁺ band have been subtracted); (b) optical absorption spectra in log coordinate

Fig. 8. Optical absorption spectra and decomposed results of highly Ti-doped sapphire crystal after reduced-anneal. Red line: optical absorption spectra of TiAl3+; blue line: optical absorption spectra of

Fig. 9. Emission spectra of Ti∶Al₂O3[44]

Fig. 10. Schematic of energy levels of Tii3+ and