Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Spectroscopy, upconversion dynamics, and applications of Er3+-doped
  low-phonon materials   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.

24
The categories of this paper are: dg-ga
0
The categories of this paper are: hep-ph
1
The categories of this paper are: cs
15
The categories of this paper are: nucl-ex
21
The categories of this paper are: adap-org
11
The categories of this paper are: hep-lat
20
The categories of this paper are: acc-phys
37
The categories of this paper are: supr-con
5
The categories of this paper are: gr-qc
33
The categories of this paper are: plasm-ph
30
The categories of this paper are: q-alg
29
The categories of this paper are: funct-an
14
The categories of this paper are: math-ph
23
The categories of this paper are: solv-int
8
The categories of this paper are: hep-ex
17
The categories of this paper are: q-fin