Time-resolved photo-phonon spectroscopy of exchange coupled Cr$\mathbf{^{3+}}$ — pairs in ruby

A rather simple method is used to detect at once both optical absorption spectra and excited-states nonradiative transitions in 0.03 at.% and 0.16 at.% ruby at temperature 2 K. The technique utilizes a time-resolved bolometer detection of phonons, generated by the excited-state nonradiative relaxation following optical excitation with a pulsed tunable dye laser. The observed excitation spectra of phonons well coincide with already known absorption spectra of both single chromium ions and pairs of the nearest neighbors. For the first time the fast ($\leqslant 0,5\,{\mu}$s)resonant energy transfer from single chromium ions to the fourth-nearest neighbors is observed directly. The new strongly perturbed Cr-single-ion sites are observed.