Evolution of the optical properties of chromium doped calcium tetraborate glass under high pressure

In this contribution, we present luminescence properties of calcium tetraborate glass (CaB4O7) activated with Cr3+ ions. Excitation spectra, steady state and time resolved luminescence spectra at temperatures between 10 K and 300 K and at high hydrostatic pressure up to 120 kbar were measured. The excitation spectrum consists of two broad bands peaking at 420 nm and 580 nm related to transitions from the 4A2g ground state to 4T1g and 4T2g excited states, respectively. Ambient pressure luminescence spectrum consists of two bands peaking at 690 nm and 850 nm. First band is related to the spin forbidden 2Eg→4A2g transition, whereas the second broad band is related to the spin allowed 4T2g→4A2g transition. Widths of both bands are significantly greater than natural due to inhomogeneous broadening. The ratio between intensities of these bands is strongly temperature and pressure dependent. At pressure below 50 kbar relative contribution of the 2Eg→4A2g luminescence decreases with increasing temperature and increases when pressure is applied. For pressure higher than 50 kbar only the emission related to the 2Eg→4A2g transition is observed. Analysis of luminescence lineshape and kinetics allowed to estimate the width of the crystal field distribution and show that even at ambient pressure most of the Cr3+ ions occupy high field sites with energy of the 4T2g higher than the energy of the 2Eg state.