Upconversion enabled optical limiting behaviour in Y2O3: Yb, Er nanophosphors under 532 nm and 1064 nm laser excitation

Y₂O₃: Yb, Er nanophosphors were synthesized by sol-gel approach and preliminary characterization confirms the existence of lanthanide dopants and the host material in the appropriate ratio with nanosphere-like morphology. Linear absorption displays visible and NIR absorption regions due to the sub-bandgap states involved in f-f transitions of Er- Yb ions. PL study shows more intense red emissions than blue and green emissions due to the combination of energy transfer and cross-relaxation process in Er ions. Wavelength-dependent nonlinear optical response of Y₂O₃: Yb, Er was examined by adapting the intensity-dependent Z-scan technique (open aperture) using nano pulsed Nd: YAG laser. Remarkably Y₂O₃: Yb, Er nanophosphors show reverse saturable absorption ascribed due to the two-photon absorption and two/three-photon absorption at 532 nm and 1064 nm respectively. The nonlinear absorption coefficient reliant on the intensity of the laser unambiguously demonstrates the presence of a sequential multi-photon absorption process. The results from the Z-scan experiment demonstrate the influence of the sub-bandgap energy states of the Y₂O₃ matrix due to the Yb and Er dopants in the excited state absorption behaviour. Upconversion integrated optical limiting of Y₂O₃: Yb, Er nanophosphors provide a potential origin for designing high-performance broadband solid-state optical limiters for laser protection devices.