  We investigate a system composed of an ensemble of room temperature
rare-earth ions embedded in a bulk crystal, intrinsically coupled to internal
strain via their sensitivity to the surrounding crystal field. We evidence the
generation of a mechanical response under resonant atomic excitation. We find
this motion to be the sum of two fundamental, resonant optomechanical
backaction processes: a conservative, piezo-orbital mechanism, resulting from
the modification of the crystal field associated with the promotion of the ions
to their excited state, and a dissipative, non-radiative photothermal process
related to the phonons generated throughout the atomic population relaxation.
Our work opens new research avenues in hybrid optomechanics, and highlights new
interactions that may be key for understanding the dephasing dynamics of
ultra-coherent rare-earth ions.
