Czochralski growth of Ti:Sapphire laser crystals

The laser efficiency of Ti3+:Al2O3 (Ti:Sapphire) is affected by residual absorption in the infrared region (800 nm) of the laser emission caused by Ti3+-Ti4+ pairs. Consequently, the ratio of the absorption coefficients at pump wavelength and at maximum of residual absorption forms the most important figure of merit (FoM) of Ti:Sapphire laser crystals. In general, to achieve sufficient FoM commercial Ti;Sapphire crystals are subject to a post-growth annealing under strongly reducing atmosphere to shift the balance between Ti3+ and Ti 4+ ions in favour of the former ion. However, due to the low diffusion velocities under these conditions, this process is very time-consuming, especially for larger crystals. To save this step the crystal growth process of Ti:Sapphire was performed in a special gas mixture to stabilize the Ti3+ ions during the growth. The stability range of Ti3+ at growth temperature was estimated on the basis of thermodynamic equilibrium calculations. Ti3+ ions exist at about 2050 °C only in very small "window" regarding the oxygen partial pressure. This "window" remains very small during cooling too. To meet these conditions, i.e. to create the optimum growth atmosphere the oxygen partial pressure of different gas mixtures was calculated. That way a gas mixture was found which allowed the growth of Ti:Sapphire crystals with diameter up to 55 mm and a FoM > 100 without subsequent annealing. Under these conditions the formation of aluminium suboxides and therewith of oxygen which can be trapped in the crystal as gaseous inclusions was suppressed efficiently.