Bubbles behavior in titanium doped Sapphire crystals: from micro to macro scale

Abstract

This work presents a comprehensive experimental and numerical investigation of bubble formation, motion, and entrapment in titanium-doped sapphire (Ti:Al₂O₃) crystals from micro to large-scale. Crystals of 5 mm, 30 mm, and 260 mm diameters were grown using distinct furnace configurations, enabling systematic comparison of bubble morphology and density across crystal scale. A global finite element model was developed to simulate coupled heat and mass transfer in the growth systems, including heat conduction, buoyant and thermocapillary (Marangoni) convection, internal and surface radiation, and forced convection induced by pulling or rotation. In µ-PD sapphire rods (D ≈ 5 mm), the strong thermal gradients (270-680 K/cm), forced convection induced by rod pulling and Marangoni convection close to the meniscus promote bubble accumulation and entrapment. Optical microscopy revealed bubbles ranging from 10 to 70 µm, with different morphology and size evolving with pulling rate and Ti concentration. In contrast, Cz-grown crystals (D ≈ 30 mm) exhibited lower temperature gradients (150-300 K/cm), and efficient bubble removal due to enhanced upward flow near the crucible sidewall and large free surface area, resulting in a nearly bubble-free central zone. For large-diameter crystals (D ≈ 260 mm), highly convex interface and a small upward buoyant flow with small free surface area (compared to the crucible cross section) limits gas evacuation, leading to localized peripheral bubble zones. The strong correlation between simulations and experiments demonstrates that bubble dynamics in Ti:sapphire growth are governed by the interaction of Marangoni, buoyant, and forced convection and titanium concentration. These insights provide key guidelines for optimizing furnace design and growth parameters to minimize gas inclusions and improve optical quality in high-performance laser crystals.