Nanostructured transparent Fe$^{2+}$-doped lithium aluminosilicate glass-ceramics with tunable optical properties: effect of heat-treatment regimes on near-infrared absorption

This study explores optical tuning of nanostructured transparent lithium aluminosilicate glassceramics nucleated by titania and doped with Fe$^{2+}$ ions. The glass was melted at 1620$^\circ$C and heat-treated between 660$^\circ$C and 800$^\circ$C, yielding nanocrystals of $\gamma$-Al$_2$O$_3$ (2 – 23 nm) and -quartz solid solutions (8 – 40 nm). Fe$^{2+}$ ions in octahedral coordination in the initial glass are responsible for absorption in the 1000 – 1400 nm range. Tetrahedrally coordinated Fe$^{2+}$ ions in $\gamma$-Al$_2$O$_3$ are responsible for absorption at 1550 – 2300 nm. Crystallization of $\beta$-quartz solid solutions leads to decreasing the $\gamma$-Al$_2$O$_3$ fraction and corresponding decrease of absorption at 1550 – 2300 nm. Differential scanning calorimetry, scanning electron microscopy, X-ray diffraction, Raman and optical spectroscopy reveal the relationship between heat-treatment regimes, crystalline phase development, and optical performance, highlighting the potential of Fe$^{2+}$-doped LAS glass-ceramics for advanced photonic applications. The glass-ceramics exhibit customizable optical properties, promising for saturable absorbers in passive Q-switching lasers.