Disorder and the optical spectroscopy of Cr³⁺-doped glasses: I. Silicate glasses

This paper reports optical absorption and luminescence studies of Cr³⁺-doped silicate glasses with compositions chosen to vary the octahedral crystal-field splitting at the Cr³⁺ site. The extensive site-to-site disorder in the glasses is signalled by the inhomogeneous broadening of radiative transitions and by non-exponential fluorescence decay patterns. Laser-excited luminescence shows that the octahedral crystal-field splitting Dq varies between the different sites occupied by Cr³⁺ ions. Both R-line and ⁴T₂ to ⁴A₂ emissions are inhomogeneously broadened by site-to-site disorder. Fluorescence line narrowing (FLN) of the R line gives a particularly dramatic demonstration of the site multiplicity. Measurement of the width of the FLN component of the R line as a function of temperature has been used to probe the density of two-level vibrational systems coupled to the electronic levels of the Cr³⁺ ion.