Optical absorption and color centers in large Ti: Sapphire crystals grown by horizontally directed crystallization under reducing conditions

The optical absorption in large Ti: sapphire crystals (up to 175 $\times$ 175 $\times$ 40 mm in size) grown by horizontally directed crystallization with the use of zone leveling of the activator distribution in different reducing atmospheres has been investigated. It has been shown that there is a high uniformity of the optical characteristics and the distribution of color centers based on Ti$^{3+}$, Ti$^{4+}$, and activator-vacancy complexes in grown crystals, which is largely determined by the stability of the reduction potential of the growth medium. It has been established that the activator concentration in the Ti$^{4+}$ charged state in crystals grown in the CO + H$_2$ low-pressure atmosphere does not exceed 1.5% and accounts for 0.2–0.5% of the total activator amount in an Ar atmosphere. The Ti$^{4+}$ concentration decreases to $\sim$ 0.01% after the additional reducing annealing.