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Upconversion processes of Er3+ in ZrO2-CaO eutectic crystals

In this work we report the infrared to visible upconversion luminescence in Er3+-doped ZrO2-CaO eutectic crystals with an ordered microstructure. The microstructure consists of alternating 2 μm thick lamellae of calcia-stabilized zirconia (CaSZ) and CaZrO3 crystals. Er3+ ions enter both phases but at a higher concentration in CaSZ. Site selective excitation at different wavelengths along the 4I15/2→4I9/2 absorption band allows to select the upconverted emission from Er3+ ions corresponding to the two components of the eutectic samples. Depending on the excitation wavelength the upconversion emission spectrum is dominated either by the CaZrO3 contribution, with an intense green emission characterized by sharp peaks from (2H11/2,4S3/2) levels together with a more weak red emission from 4F9/2 level, or by the CaSZ phase with broad green and red emissions with an enhancement of the red emission from level 4F9/2. The possible upconversion mechanisms are discussed in terms of excitation power dependence and lifetimes of the upconversion emissions.