  In this work I summarize some of the recent work carried out by our group on
the upconversion dynamics of Er3+-doped potassium lead halide crystals, which
possess very small phonons and present very efficient blue and green
upconversion. Furthermore, a non-conventional application of these RE-doped
low-phonon materials in optical refrigeration of luminescent solids is also
discussed, paying especial attention to new pathways for optical cooling that
include infrared-to-visible upconversion. Finally, I conclude with some hints
of what I think it is the next step into improving the luminescence efficiency
of solids: the use of RE-doped nanoscale photonic heterostructures for
controlling the density of photonic states.
