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

Transparent glass-ceramics (GCs) based on Co2+-doped gamma-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 degrees C result in fabrication of transparent GCs based on gamma-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 beta-quartz structure and lithium aluminogallium silicate ss with beta-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 mu m) owing to the (4)A(2)(F-4) -> T-4(1)(F-4) transition of Co-IV(2+) species in spinel nanocrystals. They exhibit saturable absorption and high ground-state absorption (GSA) cross-section sigma(GSA) of 3.1 +/- 0.4 x 10(-19) cm(2) at 1.54 mu m. Glass-ceramic based on gamma-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.