OPTICAL-DETECTION AND PHONON BOTTLENECK OF THE DIRECT DECAY WITHIN THE EBAR(2E) STATE OF RUBY

Direct relaxation within the optically excited Ē(E2) level of 700-ppm ruby is investigated in magnetic fields up to 65 kG making an angle to the c axis. At low pump intensities, the experimental relaxation rates are found to be in excellent agreement with the theory by Blume et al., predicting a fifth-power dependence on field and a cos3θsin2θ dependence on orientation. The observed proportionality constant of 4.7 × 10-19 G-5s-1 is close to the theoretical estimate based on static experiments in ruby. When increasing optical pumping, the phonon-bath conduction is demonstrated to become the limiting process due to resonant trapping of the phonons. Following pulsed excitation, a prolonging of the decay time by a factor up to 13 is attained at excited-state concentrations N*∼2×1017/cm3. In stationary experiments, a nonlinear increase with N* is observed of the equilibrium population of the upper Zeeman level of Ē to over a decade above the nonbottlenecked value, corresponding to bottlenecking factors of over ∼200. Under the assumption of a constant phonon lifetime of the order of the flight time through the illuminated region, rate equations appear to consistently account for the experiments. A Gaussian-type resonance line, fully homogenized supposedly by magnetic interactions with the surrounding Al nuclei and Cr3+ ions, explains the lattice-bath equilibration. Finally, the prospect is discussed of using the Ē direct transition in ruby as a tunable phonon generator and detector with bandwidth 0.002 cm-1 in the practical frequency range of 2-15 cm-1. © 1979 The American Physical Society.