Spectral Wipeout of Imprisoned 29-cm-1 Phonons in Ruby

Abstract

The relaxation of 29-cm-1 phonons resonant with the E(2E)-2Ā(2E) transition of optically excited Cr3+ in Al2O3 (ruby) has yielded a remarkable enigma: While at low concentrations N* of excited Cr3+ in a narrow-pumped zone the phonon relaxation timeτ increases with N*, such as expected for boundary-limited decay of trapped phonons, τ declines with N* at high N*. This is straightforwardly derived [1] from the effective decay time Teff of the 2Ā(2E) level. Under the conditions met in the experiments, Teff scales with N*τ and tends to saturate at high N* [2]. Apparently, at high N* additional scattering processes that are able to bring the phonon excitation out of range become operative.