ESA spectra and polarizability changes in Cr3+ doped laser materials

Refractive index changes caused by polarizability variations induced in highly pumped rare-earth and transition-metal ion doped laser materials are now actively investigated by several groups for several applications including laser-beam combining, laser beam clean-up and self-Q switching. This paper presents two kinds of measurement which are ground-state and excited-state absorption measurements in the near-UV as well as in the visible and the near-IR spectral domains, which has never been done so far, as systematically , and Z-scan measurements. The investigated materials areare Cr³⁺ doped Al₂O₃ (ruby), BeAl₂O₄ (alexandrite), LaMgAl₁₁O₁₉ (LMA), Y₃Al₅O₁₂ (YAG), Gd₃Ga₅O₁₂ (GGG) and Gd₃Sc₂Ga₃O₁₂ (GSGG). Excited state absorption UV spectrum obtained in the case of Cr:GSGG show that the ESA band is shrinking and decreasing by about 25%. This means that, at room temperature, when the ⁴T₂ and ²E energy levels are equally populated, about 75% of the intensity is coming from an ESA process involving the ⁴T₂ energy level. Considering that the ground-state for the Cr³⁺ ion is the spin- quartet ⁴A₂, the oscillator strength for the charge transfer transition starting from this ground state is assumed to be about the same as derived from the ⁴T₂, and doing so it is possible to deduce the polarizability change versus the wavelength. At wavelength lambda=647 nm the polarizability change using a Z-scan experiment is measured to be Deltaalpha_p =3.5 10^-25 cm³.