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 Cr
³⁺
doped Al
₂
O
₃
(ruby), BeAl
₂
O
₄
(alexandrite), LaMgAl
₁₁
O
₁₉
(LMA), Y
₃
Al
₅
O
₁₂
(YAG), Gd
₃
Ga
₅
O
₁₂
(GGG) and Gd
₃
Sc
₂
Ga
₃
O
₁₂
(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
⁴
T
₂
and
²
E energy levels are equally populated, about 75% of the intensity is coming from an ESA process involving the
⁴
T
₂
energy level. Considering that the ground-state for the Cr
³⁺
ion is the spin- quartet
⁴
A
₂
, the oscillator strength for the charge transfer transition starting from this ground state is assumed to be about the same as derived from the
⁴
T
₂
, 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
cm
³
.