(Invited) Non-Thermal Detrapping in Persistent Phosphors: Drawbacks and Opportunities

Abstract

Persistent phosphors, also called glow-in-the-dark materials, are a specific type of luminescent materials. They can emit light long after the excitation ended, which is realized by temporarily storing energy in the crystal lattice. Ambient heat can release the trapped charge carriers, after which recombination and light emission can occur. For many applications, such as in emergency signage, the storage capacity of persistent phosphors should further be increased. This would open new application areas, such as glowing road marks [1]. We show that the excitation of the europium center in the blue emitting Sr2MgSi2O7:Eu,Dy by near-UV light not only leads to charge trapping – essential to the persistent luminescence - but also to optically stimulated release of previously trapped charges and subsequent luminescence (OSL) [2]. This is substantiated by specific charging-OSL experiments making use of the presence of a thermal barrier for trapping at low temperature. Furthermore, the optical detrapping is observed to be significantly more important when a larger fraction of the traps is already filled, suggesting OSL is the limiting factor in the storage capacity of persistent phosphors. Not only photons can lead to detrapping, also pressure can in certain (mechanoluminescent (ML)) phosphors lead to light emission. Consequently, these materials can be used as pressure gauges or stress indicators [3]. Here we focus on the use of ML phosphors as a visualisation tool for ultrasound pressure fields [4]. In the case of BaSi2O2N2:Eu, a bluish green ML signal is observed proportional to the ultrasound intensity. An approach is presented to construct a 3D representation of the ultrasound pressure field. The obtained results are compared to numerical simulations, showing an excellent match [4]. [1] J. Botterman and P. F. Smet, "Persistent phosphor SrAl2O4:Eu,Dy in outdoor conditions: saved by the trap distribution," Opt. Express 23, A868-A881 (2015) [2] C. Tydtgat et al., "Optically stimulated detrapping during charging of persistent phosphors," Opt. Mater. Express 6, 844-858 (2016). [3] C.N. Xu et al., "Direct view of stress distribution in solid by mechanoluminescence, " Appl. Phys. Lett. 74, 2414-2416 (1999) [4] M. Kersemans et al., "Fast reconstruction of a bounded ultrasonic beam using acoustically induced piezo-luminescence, " Appl. Phys. Lett. 107, 234102 (2015)