Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: Terahertz radiation by ultrafast spontaneous polarization modulation in
  multiferroic BiFeO$_3$ thin films
6
The categories of this paper are: astro-ph
9
The categories of this paper are: nlin
7
The categories of this paper are: hep-th
31
The categories of this paper are: ao-sci
10
The categories of this paper are: q-bio
25
The categories of this paper are: patt-sol
27
The categories of this paper are: comp-gas
34
The categories of this paper are: chem-ph
21
The categories of this paper are: adap-org
17
The categories of this paper are: q-fin
18
The categories of this paper are: econ
2
The categories of this paper are: math
8
The categories of this paper are: hep-ex
24
The categories of this paper are: dg-ga
22
The categories of this paper are: chao-dyn
33
The categories of this paper are: plasm-ph