Halide double perovskites are gaining increasing attention for various optoelectronic applications, such as photovoltaics, light-emitting diodes, and sensors, due to their unique properties. They offer advantages over traditional lead-based perovskites, including enhanced stability and reduced toxicity. Lanthanides, with their distinct electronic configurations, enhance functionality by introducing luminescence, magnetism, and stability. This study focuses on synthesizing and characterizing ytterbium- and erbium-based halide double perovskites for near-infrared (NIR) optical amplifiers and lasers, contributing to advancements in solid-state photonics. 

Polycrystalline powders of double perovskites Cs2NaxAg1-xLnyBizIn1-y-zCl6 (0≤𝑥≤1, 0≤𝑦≤1, 0≤𝑧≤1) were synthesized by solvent evaporation of acidic solutions containing precursor salts. Comprehensive characterization of the materials was conducted using techniques such as powder X-ray Diffraction (pXRD), Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), optical absorption spectroscopy, Raman spectroscopy, photoluminescence excitation (PLE) spectroscopy, photoluminescence quantum yield (PLQY) spectroscopy, and time-resolved photoluminescence (PL) spectroscopy with sub-nanosecond resolution. Additionally, single crystals of Cs2NaYbCl6 were grown using a vertical Bridgman furnace and characterized both structurally and spectroscopically. 

pXRD analysis confirmed a single cubic phase in all samples, while precise control over materials composition was demonstrated via ICP-OES analysis. Materials rich in indium exhibited PLQYs reaching 90% for warm-white emission from self-trapped excitons and 10% for lanthanide emission. Using NIR-PL lifetime measurements and resonant absorption spectroscopy, we determined the lanthanide photophysical parameters, including radiative lifetimes and absorption/emission cross-section spectra, within the halide octahedral crystal field. Modeling optical gain enabled estimation of the figure-of-merit (FOM) of lanthanide-based double perovskites as NIR optical gain media. Structural and spectroscopic data on Cs2NaYbCl6 powder were confirmed in Cs2NaYbCl6 single crystals. 

In summary, halide double perovskites demonstrated minimal lanthanide luminescence concentration quenching for Er and Yb concentrations up to 100 at.% (𝑦≈1), attributed to the significant interionic distances within the double perovskite matrix. Evidence suggested a reduction in phonon-assisted relaxation within low-lying Er(III) electronic multiplets in Er-based double perovskites. Additionally, Er-based lasing at 1570 nm under 1530 nm optical pumping demonstrated an excellent FOM, distinguishing the halide double perovskite (HDP) crystal matrix from conventional crystal matrices such as yttrium aluminum garnet (YAG). Successful growth of high-purity single crystals is essential for advancing lanthanide-based halide double perovskites in the development of a new class of NIR solid-state lasers.