  Material strain has recently received growing attention as a complementary
resource to control the energy levels of quantum emitters embedded inside a
solid-state environment. Some rare-earth ion dopants provide an optical
transition which simultaneously has a narrow linewidth and is highly sensitive
to strain. In such systems, the technique of spectral hole burning, in which a
transparent window is burnt within the large inhomogeneous profile, allows to
benefit from the narrow features, which are also sensitive to strain, while
working with large ensembles of ions. However, working with ensembles may give
rise to inhomogeneous responses among different ions. We investigate
experimentally how the shape of a narrow spectral hole is modified due to
external mechanical strain, in particular, the hole broadening as a function of
the geometry of the crystal sites and the crystalline axis along which the
stress is applied. Studying these effects are essential in order to optimize
the existing applications of rare-earth doped crystals in fields which already
profit from the more well-established coherence properties of these dopants
such as frequency metrology and quantum information processing, or even suggest
novel applications of these materials, for example as robust devices for
force-sensing or highly sensitive accelerometers.
