Oliver S. Wenger, G. Mackay Salley, Rafael Valiente and Hans U. Güdel
Luminescence upconversion under hydrostatic pressure in the 3d-metal systems Ti²⁺:NaCl and Ni²⁺:CsCdCl₃
Phys. Rev. B 65, 212108/1-4 (2002)                 

Abstract: We present a study of upconversion materials and processes under external hydrostatic pressure. The near-infrared to visible photon upconversion properties of Ti²⁺-doped NaCl and Ni²⁺-doped CsCdCl₃ at 15 K are studied as a function of external hydrostatic pressure. It is found that in Ti²⁺:NaCl pressure can be used to switch on an efficient upconversion mechanism, which is inactive at ambient pressure, leading to an order-of-magnitude enhancement of the overall upconversion efficiency of this material. For Ni²⁺:CsCdCl₃ it is demonstrated that upconversion luminescence excitation spectroscopy can be used to study the pressure dependence of excited state absorption transitions. The results demonstrate the ability to tune upconversion properties by altering the local crystal field of active ions, in addition to probing the pressure dependence of excited state absorption transitions via upconversion spectroscopy.