Crystal growth design for enhancing Ti3+ concentration and photoluminescence in Ti:sapphire single crystals

Achieving a high concentration of optically active Ti 3+ ions remains a central challenge in Ti:sapphire laser crystal growth. Here, we present a comparative study investigating the influence of growth conditions on Ti 3+ concentration and luminescence in Ti:Al 2 O 3 single crystals. Three Ti:Al 2 O 3 single crystals with identical nominal Ti content (0.30 mol%) were grown under varying starting materials and atmospheres: (1) using 0.30 mol% Ti metal powder, grown in a reducing atmosphere (93% Ar + 7% H 2 ), denoted as the Ti: Al 2 O 3 (Ar+H 2 ) crystal; (2) using 0.30 mol% TiO 2 powder, grown in a pure Ar atmosphere, denoted as Ti: Al 2 O 3 (Ar); and (3) using 0.30 mol% TiO 2 powder, grown in a pure O 2 atmosphere, denoted as Ti: Al 2 O 3 (O 2 ). The balance material in three cases was Al 2 O 3 powder. XPS results indicate that the ratio of the Ti³ ⁺ and Ti 4+ concentrations (Ti 3+ /Ti 4+ ) follows the order: Ti: Al 2 O 3 (Ar+H 2 ) > Ti: Al 2 O 3 (Ar) > Ti: Al 2 O 3 (O 2 ). The Ti: Al 2 O 3 (Ar+H 2 ) crystal, which exhibits the highest Ti³ ⁺ concentration, shows the strongest characteristic Ti³ ⁺ photoluminescence emission broad band (Ti 3+ : 2 E → 2 T 2 ) centered at 700 nm under both 468 nm and X‑ray excitation, as well as the highest transmittance (83%). These findings demonstrate that the reducing atmosphere combined with metallic Ti precursor most effectively promotes Ti³ ⁺ incorporation and optimizes photoluminescence properties, positioning Ti: Al 2 O 3 (Ar+H 2 ) as a promising gain medium for ultrafast and ultra‑intense laser applications. In contrast, the Ti: Al 2 O 3 (O 2 ) crystal exhibits the highest thermoluminescence (TL) intensity, which is attributed to its highest Ti 4+ concentration. In the oxygen atmosphere, oxygen incorporation at high temperature increases Ti 4+ concentration and simultaneously generates aluminum vacancies (V Al ) as charge compensators, thereby enhancing the density of TL‑active centers.