Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Optically unstable phase from ion-ion interactions in an erbium doped
  crystal   We demonstrate an optical unstable phase for a laser driven erbium ion
ensemble. The transmitted light through an erbium-doped yttrium orthosillicate
crystal becomes dynamically unstable when illuminated by a strong
continuous-wave laser. Transient net gain was recorded if the light passes the
sample twice. The experimental results are understood in the framework of a
many-body system interacting with a classical field, where the ion-ion
interaction becomes significant as a result of the high erbium concentration. A
Bloch-equation model that includes the excitation-induced frequency-shift is
introduced to discuss the conditions of the instabilities.

28
The categories of this paper are: alg-geom
21
The categories of this paper are: adap-org
26
The categories of this paper are: bayes-an
27
The categories of this paper are: comp-gas
23
The categories of this paper are: solv-int
34
The categories of this paper are: chem-ph
30
The categories of this paper are: q-alg
12
The categories of this paper are: nucl-th
37
The categories of this paper are: supr-con
29
The categories of this paper are: funct-an
25
The categories of this paper are: patt-sol
6
The categories of this paper are: astro-ph
4
The categories of this paper are: cond-mat
17
The categories of this paper are: q-fin
1
The categories of this paper are: cs
9
The categories of this paper are: nlin