Cross-relaxation kinetics in the presence of energy migration

A. Suchocki, Guy D. Gilliland, Richard C. Powell

Four-wave-mixing techniques were used to establish and probe population gratings of ${\mathrm{Cr}}^{3+}$ ions in both mirror and inversion sites in ${\mathrm{BeAl}}_{2}$${\mathrm{O}}_{4}$:${\mathrm{Cr}}^{3+}$ crystals as a function of temperature between about 6 and 300 K. The four-wave-mixing signal intensity and decay rate were monitored as a function of the crossing angle of the laser ``write'' beams. The variation of the signal intensity with crossing angle is explained theoretically with use of a model based on the interaction between the laser and a two-level atomic system. Theoretical fits to the results provide information concerning the relative importance of the absorption and dispersion contributions to the signal, and the dephasing time of the atomic system. The latter is found to be 2.2 psec for the mirror site ions and 80 psec for the inversion site ions. The signal decay rate of the inversion site ions was found to be independent of crossing angle at all temperatures, while the signal decay rate for ions in the mirror site increased with increasing crossing angle at temperatures below about 150 K. This indicates the presence of long-range energy migration, and the diffusion coefficient describing this process was found to increase to about 4.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}7}$ ${\mathrm{cm}}^{2}$?${\mathrm{c}}^{\mathrm{\ensuremath{-}}1}$ at 6 K.

R. S. Lenk

Hui Ni

This thesis reports original observations and theory of strong, induced excited state absorption and emission, known as avalanche upconversion, in thulium doped crystals. The nonlinear dynamics responsible for this phenomenon and its characteristic threshold are studied in great detail, and shown to consist of two distinct cooperative processes, namely near-neighbor cross relaxation(s) and excitation migration among more distant ion pairs. The temperature dependence of avalanche upconversion and cross relaxation in these materials is explained for the first time. Also, phase conjugation with threshold behavior and potential motion sensitivity for novelty filtering is demonstrated utilizing an avalanche nonlinearity. Complex, transverse ring structures observed in Gaussian beam propagation through avalanche media are explained on the basis of numerical simulations of avalanche-induced absorption and dispersion. Evidence is presented of coherent delocalization in Tm-doped solids, in the form of unexpected oscillations in the transient response to incident light pulses. Two theoretical approaches for describing avalanche behavior were compared. A rate equation analysis suitable for weak-coupled impurity ions was used to predict basic avalanche features, including increasing absorption and upconversion emission above a threshold, temporal delay of absorption, and temperature dependence of avalanche threshold. A density matrix approach suitable for arbitrary coupling reproduced these features but also predicted oscillations in excited state populations in the presence of significant interion coherence. This theory incorporated both near-neighbor cross relaxation which gives rise to induced absorption, and excitation migration among distant pair ions necessary for immensely magnifying the effect, in a single formalism. Studies of the concentration and temperature dependence of fluorescence decays of Tm ions from $\sp1$G$\sb4$ and $\sp3$H$\sb4$ states provided firm evidence of pair-wise cross relaxations of ions in both these states. Transient four-wave mixing experiments directly revealed energy migration among $\sp3$F$\sb4$ state Tm ions at a rate rapid enough to support the content ion that migration plays an important role in avalanche dynamics. Additional experiments yielded a temperature dependence of avalanche threshold in good agreement with theory, as well as avalanche phase conjugation and transverse propagation effects due to avalanche dynamics in accord with predictions.

J. Hevesi, E. Lehotski, E. Balint

David A. Hostutler, Gordon D. Hager, Michael C. Heaven

Optically pumped alkali vapor lasers have been constructed using excitation of the D2 line (2P3/2-2S1/2) followed by lasing on the D1 line (2P1/2-2S1/2). Collisional relaxation is used to transfer population from 2P3/2 to the 2P1/2 level. The collision partner used for this step must have a large cross section for inducing transfer between the 2PJ levels, combined with a very small cross section for electronic quenching of the form M(2PJ)+Q→M(2S1/2)+Q (where M is an alkali metal and Q is the energy transfer agent). Ethane has proved to be an effective energy transfer agent for optically pumped Rb and Cs lasers. However, modeling of data for the Rb/C2H6 laser with the literature value for the quenching rate constant was unsuccessful. We have reexamined the quenching of Rb(2PJ) by C2H6 using time-resolved fluorescence techniques. Radiation trapping was significant under the conditions of our measurements, and an analysis of the interplay between the kinetics of trapping and quenching was carried out. It was found that quenching of Rb(2PJ) by C2H6 was very inefficient. The upper bound established for the quenching cross section was two orders of magnitude lower than that indicated by the previous determination.

Nobuyasu Ito, S. Fukushima, H. Watanabe +1 more