Study of the excited-state Raman process in optically pumped ruby

Raman-scattering transitions between the excited states Ē (2E) and 2Ā (2E) of Cr3+ in αAl2O3 are studied, with the laser radiation (λ=514.5 nm) simultaneously serving as an optical pump and a Raman probe. This gives rise to a quadratic dependence of the Raman signal on the intensity of the incident radiation at low laser powers, which tends to become linear at higher power levels due to the onset of saturation effects. An analysis is made of the tensorial symmetry properties of the scattering, based on group-theoretical selection rules as well as on a perturbative expansion in the spin–orbit coupling parameter. In comparing the observed polarized spectra with the results of the theoretical analysis, proper account is taken of the dichroic features of the broad absorption bands in ruby, which affect the pumping process and the accompanying population in the excited state. An alternative method for measuring any asymmetry in the scattering tensor is described and employed, making use of the relation between Stokes and anti-Stokes Raman intensities for a fixed scattering geometry.