Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Stimulated vacuum emission and photon absorption in strong
  electromagnetic fields   According to quantum electrodynamics (QED), a strong external field can make
the vacuum state decay producing electron-positron pairs. Here we investigate
emission of soft photons which accompanies a nonperturbative process of pair
production. Our analysis is carried out within the Furry picture to first order
in the fine-structure constant. It is shown that the presence of photons in the
initial state gives rise to an additional (stimulated) channel of photon
emission besides the pure vacuum one. On the other hand, the number of final
(signal) photons includes also a negative contribution due to photon absorption
within the pair production process. These contributions are evaluated and
compared. To obtain quantitative predictions in the domain of realistic field
parameters, we employ the WKB approach. We propose to use an optical probe
photon beam, whose intensity changes as it traverses a spatial region where a
strong electric component of a background laser field is present. It is
demonstrated that relative intensity changes on the level of $1 \%$ can be
experimentally observed once the intensity of the strong background field
exceeds $10^{27}~\text{W/cm}^2$ within a large laser wavelength interval. This
finding is expected to significantly support possible experimental
investigations of nonlinear QED phenomena in the nonperturbative regime.

19
The categories of this paper are: eess
37
The categories of this paper are: supr-con
4
The categories of this paper are: cond-mat
20
The categories of this paper are: acc-phys
2
The categories of this paper are: math
15
The categories of this paper are: nucl-ex
5
The categories of this paper are: gr-qc
6
The categories of this paper are: astro-ph
30
The categories of this paper are: q-alg
11
The categories of this paper are: hep-lat
3
The categories of this paper are: physics
27
The categories of this paper are: comp-gas
33
The categories of this paper are: plasm-ph
17
The categories of this paper are: q-fin
35
The categories of this paper are: mtrl-th
22
The categories of this paper are: chao-dyn