ABSTRACT
The most popular centres in inorganic solids are probably Cr3+ optical ones. As with many transition metal ions, Cr3+ is used as a colour agent in the glass industry and in jewellery where beautiful mineral stones are used such as ruby (Al2O3 sapphire), alexandrite (BeAl2O4 chrysoberyl), emerald [Be3Al2(Si03)6], as well as several garnets containing aluminium or gallium. Colour characteristics can easily be connected to our light environment, with visible and strong broadband absorption. In solid-state laser sources, Cr3+ ions, which are dispersed in a host crystal as an impurity in the place of aluminium octahedral symmetry sites, can be pumped inside the usual absorption bands with an appropriate xenon flashlamp or with argon and krypton laser energy sources. The optical excitation energy is absorbed by raising electrons of 3d3 Cr3+ configuration from the ground level to one of the pump absorption bands. It is then transformed by fast radiation-less transitions into the lowest excited level. During this process, the energy lost by the electron of the excited Cr3+ ion is carried into the lattice by phonons. In the last step of the process, the electrons return to the ground state through spontaneous emission or fluorescence if pumping intensity is below the laser threshold. Laser emission, that is to say, stimulated radiation which produces the expected output beam, only occurs when the pump intensity is above the laser threshold, and then the necessary inversion of population begins to be reached between the ground level and the involved excited level. One of the advantages of Cr3+-doped crystals is to show visible emission in the deep red spectral range where eyes are still sensitive. It is truly fascinating to detect the passage between spontaneous emission in all space and the directional stimulated emission in ruby at 694.3 nm (0.6943 µm). Theodore H Maiman [1] saw it for the first time in 1960 at the Hughes Research Laboratories in Malibu, California, by using a rod of synthetic ruby coated on the two opposite sides and pumped by a pulsed helical xenon flashlamp. Maiman’s demonstration of the ruby laser, before the gas laser and the dye laser, opened the laser era.
