Lifetime and Linewidth of Resonant 40-cm−1 Phonons in Alexandrite

Abstract

The lifetime and linewidth of 40-cm−1 phonons resonant between the optically excited Ē(2E) and \( \bar 2\bar A \)(2E) states of Cr3+ in alexandrite (BeAl2O4:Cr3+; about 600ppm) have been determined by observing the bottlenecking of the phonons via the R1 and R2 fluorescent lines under the conditions of both intense cw and pulsed optical excitation into the broad bands and at temperatures where thermalization is absent (1.5 K). BeAl2O4 is of the orthorombic structure [1], with the Al ions occupying sites with either mirror or inversion symmetry. The active Cr3+ ions in the present study substitutionally replace Al at mirror-symmetry sites. The relevant part of the energylevel diagram closely resembles that of ruby. The broad bands occur around 590 nm above the ground state for the 4T2, and 420 nm for the 4T1; at 1.5 K the R1 fluorescence is at 679 nm and the \( \bar 2\bar A \)−Ē splitting is 40 cm−1.