Transparent glass-ceramics based on Co2+-doped γ-GaxAl2−xO3 spinel nanocrystals for passive Q-switching of Er lasers

ABSTRACT Transparent glass-ceramics (GCs) based on Co2+-doped γ-GaxAl2−xO3 spinel nanocrystals were developed in the lithium aluminogallosilicate glass system with TiO2 as a nucleating agent. The initial glass contains spinel nanocrystals with sizes less than 3 nm. Secondary heat-treatments in the temperature range of 680-800 °C result in fabrication of transparent GCs based on γ-GaxAl2−xO3 crystals enriched in aluminum with cubic spinel structure (sp. gr. Fd3m) and a mean size up to 5.5 nm. At higher temperatures, GCs become translucent and then opaque due to additional crystallization of lithium aluminosilicate solid solutions (ss) with β-quartz structure and lithium aluminogallium silicate ss with β-spodumene (keatite) structure. The introduction of gallium oxide improves the temperature stability of spinel crystals that are not decomposed at high heat-treatment temperatures when aluminosilicate crystals evolve. The Co2+ ions in GCs enter the spinel crystals predominantly in tetrahedral sites. Transparent GCs feature broadband absorption (1.15-1.7 μm) owing to the 4A2(4F) → 4T1(4F) transition of IVCo2+ species in spinel nanocrystals. They exhibit saturable absorption and high ground-state absorption (GSA) cross-section σGSA of 3.1±0.4×10-19 cm2 at 1.54 μm. Glass-ceramic based on γ-GaxAl2−xO3 crystals is employed as a saturable absorber of an eye-safe diode-side-pumped Er3+,Yb3+:glass laser delivering 1.1 mJ/42 ns pulses at 1535 nm.