Broadband luminescence operating at ∼2.0 μm wavelength and its correlation with the competitive incorporation of Tm3+/Ho3+ into sol-gel nano-glass-ceramics.

In this study, we report the results of systematic research on the luminescent properties of SiO2-LaF3 nano-glass-ceramics co-activated by Tm3+/Ho3+ ions (GC1Ho:xTm), fabricated using the sol-gel technique. Based on the XRD and TEM measurements, the crystallization of the LaF3 nanophase was verified for all samples, regardless of the established Ho3+ : Tm3+ molar ratios. The prepared GC1Ho:xTm materials revealed two luminescence bands in the NIR spectral region: a band at ∼1.45 μm (3H4 → 3F4 (Tm3+)) and a broader band spanning from ∼1.6 μm to ∼2.2 μm (arising from the mutual superposition of the 3F4 → 3H6 (Tm3+) and the 5I7 → 5I8 (Ho3+) emissions). Since such a broadband emission is generally considered for application, e.g., in LIDAR devices or atmospheric sensors, we analyzed the FWHM parameter. For samples with the molar ratio of Ho3+ : Tm3+ changing from 0.012 : 0.006 (GC1Ho:0.5Tm) to 0.012 : 0.06 (GC1Ho:5Tm), the FWHM values were estimated to be ≤168 nm; however, for GC1Ho:10Tm with the highest Tm3+ concentration (Ho3+ : Tm3+ = 0.012 : 0.12), the FWHM rapidly reached nearly a 2-fold higher value (343 nm). Additionally, the occurrence of energy transfers in two opposite directions was observed, i.e., Tm3+ → Ho3+ (ETTm→Ho) and Ho3+ → Tm3+ (ETHo→Tm). The first energy transfer process is responsible for efficiently enhancing the 'eye-safe' luminescence of Ho3+ at ∼2.0 μm, and the τavg(5I7) decay time via ETTm→Ho was even 1.5 times longer than that obtained via the direct excitation pathway. Importantly, some spectroscopic effects (like the unusual prolongation of the τavg(3F4) lifetime for the GC1Ho:10Tm sample (2.75 ms) compared to the decay time for GC10Tm (1.48 ms)) suggest a mutual competition between Tm3+ and Ho3+ for incorporation into the LaF3 parent crystalline phase.