The spectroscopic properties of Cr 4 + in MgCaBa aluminate glass at 15 K have been studied using time-resolved fluorescence measurements. It is found that the luminescence of Cr 4 ÷ in the glass is from Cr 4 ÷ ions in both high and low crystalline field sites. With increasing delays the broadband emission observed at early times gradually evolves into a narrow band with a peak wavelength at 1.18 ixm. The time-resolved spectra are compared with the theoretical prediction of a model taking into account both the site distribution and the admixture of the 3T 2 state into ~E state through spin-orbit interaction.
The discovery of near-infrared tunable laser action in Cr4+-doped Mg2SiO4 [ 1 ] has stimulated the investigation on tetravalent chromium ions in solids. In recent years spectroscopic studies on Cr 4+ in some silicates and garnets have been carried out . The fluorescence from Cr 4+ in these host materials are broad bands due to the transition 372 --) 3A 2 of d 2 ions. The energy levels of Cr 4+ ions are generally assigned close to the lE-3W states crossing point in Tanabe-Sugano diagram . In low-field sites the 3T 2 level lies below the 1E level, whereas in high-field sites the order is reversed. An example of the high-field case is the narrow line 1E ~ 3A z emission observed in the spec-traofCd + in BazTiO4 .
Efforts have been made to expand the investigation into glass hosts: the spectroscopic properties of Cr 4+doped aluminate and alumino-silicate glasses have been measured [ 10:-12]. In these glasses, Cr 4+ ions are at low-field sites and a broadband 3T 2 --~ 3A 2 emission spanning the 1100-1600 nm region was observed.