Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Generation of harmonics by a focused laser beam in vacuum   Generation of odd harmonics by a super strong focused laser beam in vacuum is
considered. The process occurs due to the plural light-by-light scattering
effect. In the leading order of perturbation theory, generation of $(2k+1)$th
harmonic is described by a loop diagram with $(2k+2)$ external incoming, and
two outgoing legs. A frequency of the beam is assumed to be much smaller than
the Compton frequency, so that the approximation of a constant uniform
electromagnetic field is valid locally. Analytical expressions for angular
distribution of generated photons, as well as for their total emission rate are
obtained in the leading order of perturbation theory. Influence of higher-order
diagrams is studied numerically using the formalism of Intense Field QED. It is
shown that the process may become observable for the beam intensity of the
order of $10^{27}\rm{W/cm}^2$.

5
The categories of this paper are: gr-qc
10
The categories of this paper are: q-bio
21
The categories of this paper are: adap-org
26
The categories of this paper are: bayes-an
34
The categories of this paper are: chem-ph
9
The categories of this paper are: nlin
3
The categories of this paper are: physics
31
The categories of this paper are: ao-sci
33
The categories of this paper are: plasm-ph
37
The categories of this paper are: supr-con
22
The categories of this paper are: chao-dyn
35
The categories of this paper are: mtrl-th
25
The categories of this paper are: patt-sol
30
The categories of this paper are: q-alg
23
The categories of this paper are: solv-int
36
The categories of this paper are: cmp-lg