Ground state absorption in the lasing wavelength region of alexandrite: Theory and experiment

The ground state absorption cross section σ 1 of alexandrite is determined over the tunable lasing region from 12 000 cm -1 (830 nm) to 14 500 cm -1 (690 nm) both from theory and from experiment. σ 1 varies from 4 \times 10^{-27} to 2 \times 10^{-21} cm 2 over this range. The theory, which relates the emission and the ground state absorption cross sections, is verified by experiment over the entire range. This result supports the assumption of quasi-thermodynamic equilibrium among the excited multiplets of the Cr ion.