As an excellent oscillation and amplification medium for ultrafast, ultra-strong femtosecond lasers and high-power tunable laser systems, titanium sapphire (Ti∶Al₂O₃) crystals are among the laser crystals with the widest tuning range (700?1000 nm). Through the chirped pulse amplification (CPA) technology, laser pulse output of less than 10 fs can be achieved. They have extensive applications in high-energy physics, environmental pollutant detection, military defense, laser spectroscopy, and other fields, and become the preferred working medium for many ongoing ultra-intense and ultra-short laser engineering systems. The optical properties of Ti∶Al₂O₃ crystals are highly dependent on the concentration and uniform distribution of doped titanium ions (Ti3?). However, first, the coefficient of segregation of titanium ions in Al₂O₃ melt is only 0.16, which easily leads to the formation of axial and radial concentration gradients during crystal growth, resulting in optical inhomogeneity. Second, after titanium ions are doped, the viscosity of the Al₂O₃ melt significantly increases. The relatively slow convection of the melt leads to many scattering defects such as bubbles inside the crystal, making it difficult to prepare Ti∶Al₂O₃ crystals with uniform composition and high quality. Studies have shown that when the doping mass fraction exceeds 0.25%, it is easy to form Ti3?-Ti3? ion pairs and Ti?? impurity centers, significantly increasing the parasitic absorption in the 575 nm (visible light) and 800 nm (near-infrared) bands, and reducing the figure of merit (FOM) of Ti∶Al₂O₃ crystals. Therefore, it is extremely necessary to grow large-sized, high-quality and high-factor Ti∶Al₂O₃ crystals.

Heat exchange method and Kyropoulos method are currently the most mainstream growth methods for large-sized and high-quality Ti∶Al₂O₃ crystals. The heat exchange method involves using liquid helium in the heat exchanger to remove heat, creating a longitudinal temperature gradient in the crystal growth zone from the bottom to the top. At the same time, the temperature gradient is controlled by adjusting the size of the gas (He cooling source) flow rate in the heat exchanger, adjusting the structure of the heat exchanger, and changing the heating power, which enables the melt in the crucible to gradually solidify from bottom to top into a crystal. In contrast, the Kyropoulos method is used to form temperature gradients in the axial and radial directions by adjusting the power of the heating zones around the crucible—for example, by cooling the top heater while heating the side heater. First, the raw material is heated to its melting point. Next, a seed crystal is brought into contact with the melt surface and slowly withdrawn. By controlling the cooling rate, the single crystals gradually solidify from top to bottom and finally grow into a whole single crystal. Through the self-designed heat exchange furnaces and the Kyropoulos furnaces, large-sized and high-quality Ti∶Al₂O₃ crystals with diameters of 380 mm×260 mm and 380 mm×360 mm are successfully grown.

Large Ti∶Al₂O₃ crystals with 100 kg and the diameter up to 300 mm are grown successfully with the heat exchange method and the Kyropoulos method, respectively. The Ti∶Al₂O₃ crystals by the heat exchange method have a transparent red center, and under laser irradiation, scattered particles can be locally distributed. The color of the crystal gradually deepens along the direction of crystal growth, mainly due to the condensation of titanium ions, which leads to the gradual increase of doping concentration. In the central region of the crystal, the absorption coefficient at 532 nm from the seed crystal to the top of crystal gradually increases from 2.01 cm^-1 to 3.98 cm^-1. The Ti∶Al₂O₃ crystals by the Kyropoulos method also gradually deepen in color during the crystal growth process. The testing results of absorption coefficients at different heights from the seed crystal to the crystal bottom show that the absorption coefficient at 532 nm gradually increases from 0.6 cm^-1 to 5.5 cm^-1. The average stress of the wafer with 4 inch (1 inch=2.54 cm) diameter and 1 mm thickness is reduced from 183 nm/cm to below 10 nm/cm after hydrogen annealing. The full width at half maxima (FWHM) of the double swing curve is 14.04″, which shows that the sample lattice is intact and the single crystal performance is good. After polishing and coating, the laser energy amplification experiment is operated with 50 mm×35 mm sized samples, which achieves the laser energy amplification of 3 J , the pulse duration of 500 ps, and the peak power of up to 6 GW.

High-quality and large-size Ti∶Al₂O₃ crystals grown by the heat exchange method and the Kyropoulos method are now on the market.