Critical slowing down and off-resonant enhancement of avalanche nonlinearities

We report two-beam coupling measurements of temporal response and intensity-dependent index changes in the avalanche crystal LiYF/sub 4/:Tm(3+). Surprisingly, the induced nonlinear response on the excited state resonance is dominated by a real portion arising from a non-resonant ground state transition. Critical slowing down of excited state dynamics is also predicted and observed.