NIR Luminescence from Sol-Gel Er³⁺Doped SiO₂:GeO₂Transparent Gels, Nanostructured Powders and Thin Films for Photonic Applications

Optical polarizability of erbium-oxygen complexes in sol-gel-based silica films

“…For the germania and germania-silica sol-gel films, including co-doping with Al and P, studied in [54], experimental resolution is insufficient to discern changes from erbium doping. Refractive index data reported for erbium-doped sol-gel silica alloyed with hafnia [55], titania [15] and germania [42], have similarly insufficient experimental resolution. Erbium doping of the Te-B-Zn-oxide glasses studied in [56] is interpreted as forming non-bridging oxygens, which obscures changes associated with cation polarizabilities in this silica-free telluritebased glass.…”

“…Thermal annealing at higher temperatures densifies the sol-gel material, increases the refractive index, and activates optical emission from Er 3+ [13]. Since N and Cl are among the dopant species known to increase the refractive index of silica [27], the sol-gel formulation based on erbium oxide and acetic acid, introduced in [13], is perhaps desirable for analysis compared to alternatives containing nitrate and chloride ions, although the colloids formed by hydrolysis of SiC 8 H 20 O 4 (TEOS) could contain residual C. A 700 • C anneal optimally activates Er 3+ in Er-Ge codoped silica [21] and is used for optically active Er +3 in solgel germania-silica films [42], while 670 • C activates solgel silica-titania films [15]. Annealing at temperatures much above 700 • C, however, may cause short-range redistribution and clustering of the Er impurities, although higher temperatures are used for depleting the trace OH impurities capable of degrading performance at 1.54 µm [12,13] and for bringing residual porosity to very low levels [15].…”