Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Enhanced harmonic generation in relativistic laser plasma interaction   We report the enhancement of individual harmonics generated at a relativistic
ultra-steep plasma vacuum interface. Simulations show the harmonic emission to
be due to the coupled action of two high velocity oscillations -- at the
fundamental $\omega_L$ and at the plasma frequency $\omega_P$ of the bulk
plasma. The synthesis of the enhanced harmonics can be described by the
reflection of the incident laser pulse at a relativistic mirror oscillating at
$\omega_L$ and $\omega_P$.

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The categories of this paper are: hep-ph
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The categories of this paper are: gr-qc
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The categories of this paper are: cond-mat
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The categories of this paper are: comp-gas
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The categories of this paper are: chao-dyn
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The categories of this paper are: math-ph
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The categories of this paper are: hep-ex
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The categories of this paper are: nlin
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The categories of this paper are: q-bio
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The categories of this paper are: stat
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The categories of this paper are: ao-sci
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The categories of this paper are: nucl-th
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The categories of this paper are: eess
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The categories of this paper are: nucl-ex
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The categories of this paper are: quant-ph
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The categories of this paper are: astro-ph