Sub-bandgap energy states induced multiphoton absorption in CeO2: Yb, Er nanophosphors for designing broadband optical limiters

The upconversion nanophosphors ceria (CeO₂) integrated with Yb, Er ions were synthesized by the Pechini sol-gel technique and their broadband optical limiting behaviour in the visible and near-infrared regions was examined. Formation of the fluorite cubic phase of CeO₂: Yb, Er with spherical morphology, having uniform elemental distribution of lanthanide ions and the presence of both oxidation states of Ceria (Ce³⁺ and Ce⁴⁺) with rich oxygen defect states was validated by structural characterizations. Absorption study reveals distinct peaks at visible (blue, green and red) and NIR regions owing to the 4f transitions of Yb and Er ions in the CeO₂ host. Emission spectra analysis demonstrates intense blue emission due to the grouping of the transitions explicitly from the 4f state (Ce³⁺) to the valence band of CeO₂ and due to the defect state transitions present in the CeO₂: Yb, Er nanophosphors. Existence of sub-bandgap energy states with a very long lifetime of the excited states (µs) favours the occurrence of excited state absorption. The intensity-dependent open aperture (OA) Z-scan experiment was applied for assessing the broadband optical nonlinearity of CeO₂: Yb, Er, by employing a pulsed Nd: YAG laser. Prominently reverse saturable absorption was witnessed in CeO₂: Yb, Er nanophosphors at both pulsed excitation wavelengths, i.e., 532 nm and 1064 nm, where two-photon absorption (2PA) and two/three-photon absorption (2PA/3PA) were accountable, respectively. Also, the obtained nonlinear absorption coefficients (β and γ) contingent on the input pulsed laser intensities indicates that the process of sequential multi-photon absorption occurs in the nanophosphors. The outcomes from the Z-scan experimentation validate the excited state absorption action, which is contributed by the Yb and Er ions owing to the sub-bandgap energy levels induced in the CeO₂ medium. Thus, sub-bandgap states induced broadband optical limiting performance of CeO₂: Yb, Er swayed real-world deals for designing, developing and fabricating rare-earth elements integrated laser safety gadgets.