Optical hole burning and spectral diffusion in ruby

Frequency-domain optical hole burning of the ${\mathit{R}}_1$ line ${[}^4$ ${\mathit{A}}_2$(-1/2)⇆E¯(-1/2) transition] in dilute ruby is studied at 2 K and with a field of 3.6 kG applied along the c axis. A pump-probe (PP) technique is used with precise probe-frequency shifting obtained by an acousto-optic modulator. For sufficiently short PP pulses (50 μs), two holes (narrow and wide) are observed. The narrow (∼20 kHz half width at half maximum at low power) hole-width dependence on Rabi frequency approximately agrees with that inferred from free-induction-decay measurements. For the present concentration (0.0034 wt. % ${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$) and field, the narrow hole width is determined by fluctuating fields produced by Cr-Cr spin flips [A. Szabo, T. Muramoto, and R. Kaarli, Phys. Rev. B 42, 7769 (1990)]. When the PP delay time is varied, the amplitude of the narrow hole decays with a 1/e time constant of ∼300 μs compared to the E¯ fluorescence lifetime of 4 ms. The wide hole (∼500 kHz half width at half maximum) is due to Al-Cr superhyperfine interactions and is created by a spectral diffusion mechanism driven by Al-spin flipping in the host lattice.