Transparent ceramics of cobalt-doped zinc aluminium spinel (gahnite), Co2+:ZnA2O4, are synthesized by hot pressing at 1520 °C for 4 h in the presence of zinc fluoride, ZnF2, as a sintering additive. The effect of the ZnF2 content (3–10 wt%) on the microstructure, Raman spectra, optical absorption and luminescence of ceramics is studied. The ceramics feature clean grain boundaries, the absence of pores and a narrow grain size distribution (mean grain size: 70-100 μm) resulting in high in-line transparency close to the theoretical limit. The obtained ceramics are suitable for fabrication of saturable absorbers of erbium lasers.
There is strong demand for effective gain materials for the 3.0-3.9 μm spectral range not nicely covered by Cr:ZnSe and Fe:ZnSe amplification bands. We characterized, Fe:ZnAl2O4 ceramic sample, Fe:MgAl2O4 and Fe:InP single crystals as promising laser materials for this mid-IR spectral range. In all crystals, the absorption bands corresponding to 5E↔5T2 transition of Fe2+ ions in the tetrahedral sites were measured. In addition, absorption band of Fe2+ ions in the octahedral sites were observed in Fe:ZnAl2O4 sample with maximum absorption cross-section at ~1.0 μm. From the absorption measurements, the radiative lifetime of Fe:MgAl2O4 was calculated to be 60 μs. Saturation absorption of Fe2+ ions in Fe:ZnAl2O4 was studied using Ho:YAG@2.09 μm and Er:YAG@2.94 μm lasers. Saturation measurements were taken up to energy density of 2 J/cm2 and showed no saturation of absorption. This can be explained by a fast non-radiative (<100 ps) relaxation time from the 5T2 level of Fe2+ ions in the Fe:ZnAl2O4 sample at RT. A strong mid-IR photoluminescence (PL) signal in Fe:InP crystal was detected under the direct excitation of the 5E↔5T2 transition of Fe2+ ions using Er:YAG@2.94 μm laser as well as excitation using photo-ionization process under radiation from Nd:YAG@1.064 μm laser. This indicates that Fe:InP crystals could become promising mid-IR laser media with optimization of fabrication technology.