Two distinct simultaneous NIR looping behaviours of Er3+ singly doped BiOBr: The underlying nature of the Er3+ ion photon avalanche emission induced by a layered structure

Traditional photon avalanche (PA) emission of rare earth (RE) ions is a consequence of the efficient cross-relaxation (CR) process between excited energy levels. Here, we report the simultaneous visible and near-infrared (NIR) PA emission of Er³⁺-doped BiOBr layered crystals showing a different construction mechanism. Over the threshold power point of a 980 nm laser, seven-photon PA upconversion (UC) behaviour and four-photon NIR emission of Er³⁺ ions are observed simultaneously, but only the latter significantly depends on the concentration. Compared to the reported BiOCl system, the great divergences in the Er³⁺-activated PA emission in BiOBr enable us to reconsider the basic mechanism underlying this emission and rediscover the special photo-physical nature of these layered crystals. According to the same power dependence of the PA emission and the host photocurrents, we show that the visible PA emission indeed is a deexcitation process of Er³⁺ ions from the NIR-excited band transition of BiOBr; but the NIR one retains traditional efficient CR between Er³⁺ ions, which demonstrates a new mechanism for PA emission different from the one previously reported. The results of this work not only offer new insight into the nature of PA emission but also promote potential applications of PA UC phosphors in solid-state lasers and optical switches.