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.