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Details of the optical-spectroscopic properties of alexandrite (BeA12O~.Cr +) 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 Cr + 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 Cr'+ ions in inversion sites at low temperature and to identify six types of exchange-coupled pairs of Cr + ions in the lattice. Time-resolved site-selection spectroscopy was used to monitor the energy transfer between Cr + 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.

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Excited-state absorption in alexandrite has been measured from 420 to 680 nm by using pulsed flash lamp pumping and a dye laser probe. The excited-state absorption cross section is generally of the same order of magnitude as the ground-state absorption cross section (10-19 cm 2 ) and therefore may represent a large loss of energy from the pump source in a laser system. The results were confirmed at two wavelengths with a cw experiment in which 568-or 590-nm radiation was used for pumping and probing.

Broadly tunable laser operation has been obtained at room temperature from the vibronic bands of single-crystal BeAl(2)O(4):Cr(3+) under Xe-flashlamp excitation. Continuous tuning is obtained from 701 to 794 nm, with maximum power at 750 nm. Long-pulse operation employing 6.3-mm x 76-mm rods easily provides 500 mJ in 200-microsec bursts, whereas 70-mJ, 120-nsec pulses are obtained under Q-switch operation.

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