Growth, Thermal, and Spectroscopic Properties of Er³⁺-Doped Ca₉Yb(VO₄)₃(PO₄)₄ Crystal for 1.55 μm Laser

A Ca9Yb(VO4)3(PO4)4 crystal doped with (4.5 at. %) Er3+ was grown by the Czochralski method. A comprehensive investigation of the thermal and spectroscopic properties was conducted. The average thermal expansion coefficients of the crystal are 11.2 and 16.9 × 10–6 K–1 along the crystallographic a and c axes, respectively. Additionally, the thermal conductivities of the crystal along a and c axes are both about 1.2 W m–1 K–1 at room temperature and increase to 1.43 and 1.37 W m–1 K–1 at 400 °C, respectively. For σ polarization, the crystal exhibits a maximum absorption cross section of 1.76 × 10–20 cm2 at a wavelength of 980 nm and a maximum emission cross section of 0.78 × 10–20 cm2 at 1533 nm. The fluorescence lifetime of the upper laser level 4I13/2 is 4.68 ms, while the Yb3+→Er3+ energy transfer efficiency is approximately 63.9%. The full widths at half-maximum of the emission bands around 1.55 μm are measured to be 34 nm for σ polarization and 52 nm for π polarization. Combining a large σ-polarized absorption coefficient of 27.4 cm–1 at a wavelength of 980 nm as well as a wide emission band, the Er:Ca9Yb(VO4)3(PO4)4 crystal has the potential to serve as a microchip gain medium for broadband-tunable and ultrashort-pulse lasers operating at 1.55 μm.