  We report the first demonstration of solid-state optical refrigeration of a
Ho-doped material. A 1 mol% Ho-doped Yttrium Lithium Fluoride (YLF) crystal is
cooled by mid-IR laser radiation, and its external quantum efficiency and
parasitic background absorption are evaluated. Using detailed
temperature-dependent spectroscopic analysis the minimum achievable temperature
of a 1% Ho:YLF sample is estimated. Owing to its narrower ground- and
excited-state manifolds, larger absorption cross-section, and coincidence of
optimum cooling wavelength of 2070 nm with commercially available high-power
and highly efficient Tm-fiber lasers, Ho^3+-doped crystals are superior to
Tm^3+-doped systems for mid-IR optical refrigeration. With further improvement
in material purity and increased doping concentration, they offer great
potential towards enhancing the cooling efficiency nearly two-fold over the
best current Yb:YLF systems, achieving lower temperatures as well as for
realization of eye-safe mid-IR high-power radiation balanced lasers.
