Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Coherent Radiation by a Spherical Medium of Resonant Atoms   Radiation by the atoms of a resonant medium is a cooperative process in which
the medium participates as a whole. In two previous papers \cite{PG00,GP00}, we
treated this problem for the case of a medium having slab geometry, which,
under plane wave excitation, supports coherent waves that propagate in one
dimension. We extend the treatment here to the three-dimensional problem,
focusing principally on the case of spherical geometry. By regarding the
radiation field as a superposition of electric and magnetic multipole fields of
different orders, we express it in terms of suitably defined scalar fields. The
latter fields possess a sequence of exponentially decaying eigenmodes
corresponding to each multipole order. We consider several examples of
spherically symmetric initial excitations of a sphere. Small uniformly excited
spheres, we find, tend to radiate superradiantly, while the radiation from a
large sphere with an initially excited inner core exhibits temporal
oscillations that result from the participation of a large number of coherently
excited amplitudes in different modes. The frequency spectrum of the emitted
radiation possesses a rich structure, including a frequency gap for large
spheres and sharply defined and closely spaced peaks caused by the small
frequency shifts and even smaller decay rates characteristic of the majority of
eigenmodes.

22
The categories of this paper are: chao-dyn
0
The categories of this paper are: hep-ph
20
The categories of this paper are: acc-phys
27
The categories of this paper are: comp-gas
1
The categories of this paper are: cs
23
The categories of this paper are: solv-int
24
The categories of this paper are: dg-ga
31
The categories of this paper are: ao-sci
12
The categories of this paper are: nucl-th
6
The categories of this paper are: astro-ph
4
The categories of this paper are: cond-mat
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The categories of this paper are: nucl-ex
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The categories of this paper are: bayes-an
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The categories of this paper are: adap-org
5
The categories of this paper are: gr-qc
13
The categories of this paper are: quant-ph