Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Nondifractive Propagation of Light in Photonic Crystals   We show that diffraction of electromagnetic radiation (in particular of a
visible light) can disappear in propagation through materials with periodically
in space modulated refraction index, i.e. photonic crystals. In this way the
light beams of arbitrary width can propagate without diffractive broadening
and, equivalently, arbitrary light patterns can propagate without diffractive
smearing.

8
The categories of this paper are: hep-ex
9
The categories of this paper are: nlin
21
The categories of this paper are: adap-org
28
The categories of this paper are: alg-geom
14
The categories of this paper are: math-ph
4
The categories of this paper are: cond-mat
6
The categories of this paper are: astro-ph
29
The categories of this paper are: funct-an
20
The categories of this paper are: acc-phys
22
The categories of this paper are: chao-dyn
13
The categories of this paper are: quant-ph
18
The categories of this paper are: econ
10
The categories of this paper are: q-bio
1
The categories of this paper are: cs
32
The categories of this paper are: atom-ph
15
The categories of this paper are: nucl-ex