The growth of titanium doped sapphire for laser application

Abstract

The main application of Ti doped sapphire (Ti:sapphire) lies in the field of lasers, thanks to its outstanding production of ultra-short pulses due to the presence of doping Ti³⁺ ions. The absorption and emission mechanisms of this crystal are intricate, necessitating consideration of point defects existing in the grown crystal. A plethora of liquid-phase growth methods yield crystals of diverse sizes and quality. This paper gives a comprehensive review of the literature on managing dopants during the growth of Ti doped bulk sapphire crystals.

Substantial research has indicated that the presence of detrimental Ti⁴⁺ ions diminishes the crystal laser efficiency due to their residual absorption. Although annealing under reducing atmosphere is an efficient way to increase the Ti³⁺/Ti⁴⁺ ratio, this process becomes increasingly time-consuming as the demand for larger optical components increases. Consequently, it would be more practical and convenient to control this ratio directly during the growth processes. However, the conversion mechanisms between the two Ti ions valences during crystal growth and annealing remain largely unexplored.

A study of the thermodynamics of the Al₂O₃/TiO₂ and Al₂O₃/Ti₂O₃ solid and liquid solutions as a function of the partial pressure ($p_{O_2}$) and oxygen activity is crucial for understanding these mechanisms. This paper presents corrected, reliable phase diagrams that enable quantitative prediction of the effect of $p_{O_2}$ on the melt concentrations of the two ions. Consequently, a novel value of the absorption coefficient constant, pertinent to Ti⁴⁺ concentration measurement, is proposed. Equilibrium with the solid solution yields segregation coefficients that appear distinct for the two ions. Given their influence on oxygen activity during growth, the effect of surrounding furnace parts, such as graphite casing or Mo crucible, is also important.

Understanding the behavior of Ti³⁺and Ti⁴⁺ ions in the grown crystal as a function of pulling time and considering the $p_{O_2}$ levels in the furnace atmosphere, requires the knowledge of solid-state electrochemistry, including the charge carriers and the Al and O vacancies. This foundation allows the development of a physico-chemical model illustrating the evolution of ion valence during growth. Analysis of experimental results from existing literature gives the necessary diffusion coefficients and reaction rate constants. Investigating crystal-atmosphere interaction provides the required boundary condition for solving the problem. The findings exhibit qualitative agreement with experimental measurements of Ti³⁺ and Ti⁴⁺ concentrations in grown Ti:sapphire crystals.