Modulation of upconversion luminescence in Er3+, Yb3+-codoped lanthanide oxyfluoride (YOF, GdOF, LaOF) inverse opals

Abstract

In this work, novel upconversion lanthanide oxyfluoride (LnOF:Yb³⁺, Er³⁺, Ln = La, Y, Gd) inverse opal photonic crystals (IOPCs) were successfully fabricated by the sol–gel method combined with polymethylmethacrylate (PMMA) template technique and the modulation of the photonic stop band (PSB) on the green emissions ²H_11/2/⁴S_3/2 → ⁴I_15/2 for Er³⁺ ions were systemically studied under 980 nm excitation. The results showed that the LaOF IOPCs (annealed at 500 °C) were of cubic phase while GdOF and YOF matrices were of rhombohedral phase, and the LaOF IOPCs demonstrated more efficient upconversion luminescence (UCL) than GdOF and YOF due to the phase transition. In contrast to the ground reference (REF) samples, strong suppression of UCL was observed in the IOPCs while the PSB overlapped with the ²H_11/2/⁴S_3/2 → ⁴I_15/2 lines. Furthermore, the spontaneous decay rates (SDRs) of ²H_11/2/⁴S_3/2 → ⁴I_15/2 were suppressed in the IOPCs, independent of the location of the PSB. In LaOF IOPCs, the decay time constants of ⁴S_3/2 → ⁴I_15/2 were increased by as much as 9 times in contrast to the corresponding REFs. It was also significant to observe that in the IOPCs the local thermal effect was greatly suppressed. In addition, broadband UCL extending into the visible range was observed in LnOF:Yb³⁺, Er³⁺ REF samples under high excitation power, and the origin was identified by electron paramagnetic resonance (EPR) spectroscopy.