  We demonstrate super-resolution imaging of single rare-earth emitting
centers, namely, trivalent cerium, in yttrium aluminum garnet (YAG) crystals by
means of stimulated emission depletion (STED) microscopy. The achieved
all-optical resolution is $\approx$ 80nm. Similar results were obtained on H3
color centers in diamond with resolution of $\approx$ 60nm. In both cases, STED
resolution is improving slower than the inverse square-root of the depletion
beam intensity. This is caused by excited state absorption (ESA) and
interaction of the emitter with non-fluorescing crystal defects in its near
surrounding.
