Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Possible Realization of Directional Optical Waveguides in Photonic
  Crystals with Broken Time-Reversal Symmetry   We show how in principle to construct analogs of quantum Hall edge states in
"photonic crystals" made with non-reciprocal (Faraday-effect) media. These form
"one-way waveguides" that allow electromagnetic energy to flow in one direction
only.

28
The categories of this paper are: alg-geom
5
The categories of this paper are: gr-qc
9
The categories of this paper are: nlin
15
The categories of this paper are: nucl-ex
25
The categories of this paper are: patt-sol
26
The categories of this paper are: bayes-an
30
The categories of this paper are: q-alg
32
The categories of this paper are: atom-ph
3
The categories of this paper are: physics
8
The categories of this paper are: hep-ex
24
The categories of this paper are: dg-ga
19
The categories of this paper are: eess
21
The categories of this paper are: adap-org
12
The categories of this paper are: nucl-th
36
The categories of this paper are: cmp-lg
14
The categories of this paper are: math-ph