Spectroscopic investigation of CaAl12O19:: M3+ upon UV/vacuum-UV excitation:: a comparison with SrAl12O19:: M3+ (M = Pr, Cr)

In order to evaluate the possibility of using Cr3+ co-doping to modify the emission properties of Pr3+-based quantum cutting phosphors, which emit one ultraviolet (UV) and one visible photon from each vacuum-UV photon absorbed, into phosphors that emit two visible photons, the optical spectroscopies between 150 and 750 nm of CaAl12O19:M3+ (M = Pr, Cr) were investigated and compared with those of SrAl12O19:M3+ using synchrotron radiation as one of the excitation sources. When Pr3+ is excited to its 4f5d states, cascade emission of UV photons from the S-1(0) state followed by visible photons from the P-3(0) state are both observed in (Ca, Sr) Al12O19:Pr3+. Similar to SrAl12O19:M3+, there exist desirable spectral overlaps between Pr3+ S-1(0) emissions and the Cr3+ higher multiplet absorptions in CaAl12O19, which should be favourable to the energy transfer from Pr3+ S-1(0) to Cr3+, converting the UV photon from Pr3+ S-1(0) into the red Cr3+ emission. However, the host absorption band and the Cr3+-related charge transfer band have an unfavourable overlap with the Pr3+ 4f5d states in the vacuum-UV region, preventing efficient selective Pr3+ excitation under radiation of Xe discharge, a prerequisite for the quantum cutting process.