Phonon-induced Raman processes connecting the Ē(2E) and 2Ā(2E) states in ruby

The dynamics of near-zone-boundary phonons is investigated in ruby at low temperature by use of a technique of modulated pumping. The phonons are generated in the nonradiative decay to ${}_{}{}^2{}_{}{}^{}\mathrm{E}$ following optical excitation of ${}_{}{}^4{}_{}{}^{}\mathrm{T}_1^{}{}_{}{}^{}$ or ${}_{}{}^4{}_{}{}^{}\mathrm{T}_2^{}{}_{}{}^{}$. The detection is via phonon-induced Raman transitions from Ē${(\mathrm{}}^2$E) to 2Ā${(\mathrm{}}^2$E), and the associated enhancement of the $R_2$ luminescence. The phonon occupation is linear in the optical-pumping intensity, but becomes quadratic in case of significant secondary optical pumping starting from the ${}_{}{}^2{}_{}{}^{}\mathrm{E}$ states, and reaches values of up to ${10}^{\mathrm{-}7}$. The prevailing decay mechanism appears to be ballistic flight out of the pumped zone over distances of up to 0.3 mm, corresponding to lifetimes of over 200 ns. An additional result is the optical feeding constant into 2Ā${(\mathrm{}}^2$E). .AE