Spectral dynamics of two frequency agile laser materials: Alexandrite and barium-ytterbium-2 - fluoride-8:holmium(3+)

Scope of study. Four-wave mixing techniques were used to establish and probe population gratings of Cr{dollar}sp{lcub}3+{rcub}{dollar} ions in alexandrite crystals at temperatures between 10 and 300 K. The results were interpreted in terms of the interaction of the laser radiation with a two-level atomic system. They provide information about the characteristics of the four-wave mixing signals for this physical situation as well as being useful in characterizing the properties of energy transfer and dephasing within the ensemble of Cr{dollar}sp{lcub}3+{rcub}{dollar} ions. The optical spectroscopic properties, energy transfer and upconversion transitions, and lasing dynamics of BaYb{dollar}sb2{dollar}F{dollar}sb8{dollar}:Ho{dollar}sp{lcub}3+{rcub}{dollar} crystals were measured. The positions of the various Stark components of the different J-manifolds of Ho{dollar}sp{lcub}3+{rcub}{dollar} are identified, and the radiative decay rates and branching ratios were calculated for the Ho{dollar}sp{lcub}3+{rcub}{dollar} levels. The ion-ion interactions were analyzed with several energy transfer theories and with two rate equation models. The 2.9 {dollar}mu{dollar}m laser action resulting from 1.0 {dollar}mu{dollar}m excitation was measured.; Findings and conclusions. The exciton transport properties of the mirror site Cr{dollar}sp{lcub}3+{rcub}{dollar} ions are consistent with a quasi-coherent exciton motion. The temperature dependence of the ion-ion interaction rate is found to be consistent with the excited state lifetime and linewidth data. The temperature dependence of the exciton scattering rate is found to be T{dollar}sp{lcub}3/4{rcub}{dollar}, and the temperature dependence of the resulting diffusion coefficient is T{dollar}sp{lcub}-1/2{rcub}{dollar}, consistent with the mechanism of scattering of the excitons by acoustic phonons. The laser-induced change in the absorption coefficient was found to significantly affect the LIG signal characteristics. The dephasing time of the atomic system for direct excitation of the {dollar}sp2{dollar}E level was found to be 55 ps, consistent with a pure dephasing process. The dephasing time for the case of {dollar}sp4{dollar}T{dollar}sb2{dollar} excitation is consistent with a population relaxation process, the nonradiative relaxation from the {dollar}sp4{dollar}T{dollar}sb2{dollar} to {dollar}sp2{dollar}E level. A theoretical model is presented to explain the nonradiative decay dynamics. The ion-ion interaction mechanism between Yb{dollar}sp{lcub}3+{rcub}{dollar} and Ho{dollar}sp{lcub}3+{rcub}{dollar} ions in BaYb{dollar}sb2{dollar}F{dollar}sb8{dollar}:Ho{dollar}sp{lcub}3+{rcub}{dollar} is dominated by the dipole-dipole mechanism. The diffusion of the excitation energy among Yb{dollar}sp{lcub}3+{rcub}{dollar} ions is a thermally assisted incoherent hopping process. The efficiencies of the different laser transitions, at 2.0 {dollar}mu{dollar}m, 2.9 {dollar}mu{dollar}m, and 0.55 {dollar}mu{dollar}m, were found to be dependent upon the pump power.