Details of the optical-spectroscopic properties of alexandrite (${\mathrm{BeAl}}_2$${\mathrm{O}}_4$:${\mathrm{Cr}}^{3+}$) crystals were studied by different laser-spectroscopy techniques. The temperature dependences of the fluorescence lifetimes and widths of the zero-phonon lines were found to be quite different for ${\mathrm{Cr}}^{3+}$ ions in the mirror and inversion crystal-field sites. The results indicate that direct phonon-absorption processes dominate both thermal line broadening and lifetime quenching for ions in the mirror sites while phonon-scattering processes dominate the line broadening of inversion-site ions and leave their lifetime independent of temperature. Tunable-dye-laser site-selection methods were used to obtain the excitation spectra of the ${\mathrm{Cr}}^{3+}$ ions in inversion sites at low temperature and to identify six types of exchange-coupled pairs of ${\mathrm{Cr}}^{3+}$ ions in the lattice. Time-resolved site-selection spectroscopy was used to monitor the energy transfer between ${\mathrm{Cr}}^{3+}$ ions in mirror and inversion sites at both low and high temperature. Finally, high-power, picosecond pulse excitation was used to produce two-photon absorption, and the resulting emission spectrum was found to exhibit a new fluorescence band in the 400-nm spectral region.

• Received 11 March 1985

DOI:https://doi.org/10.1103/PhysRevB.32.2788