Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Plasma-Induced Frequency Chirp of Intense Femtosecond Lasers and Its
  Role in Shaping High-Order Harmonic Spectral Lines   We investigate the self-phase modulation of intense femtosecond laser pulses
propagating in an ionizing gas and its effects on collective properties of
high-order harmonics generated in the medium. Plasmas produced in the medium
are shown to induce a positive frequency chirp on the leading edge of the
propagating laser pulse, which subsequently drives high harmonics to become
positively chirped. In certain parameter regimes, the plasma-induced positive
chirp can help to generate sharply peaked high harmonics, by compensating for
the dynamically-induced negative chirp that is caused by the steep intensity
profile of intense short laser pulses.

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The categories of this paper are: hep-th
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The categories of this paper are: nucl-ex
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The categories of this paper are: nlin
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The categories of this paper are: patt-sol
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The categories of this paper are: chao-dyn
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The categories of this paper are: dg-ga
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The categories of this paper are: solv-int
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The categories of this paper are: econ
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The categories of this paper are: math
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The categories of this paper are: supr-con
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The categories of this paper are: nucl-th
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The categories of this paper are: hep-lat
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The categories of this paper are: eess
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The categories of this paper are: hep-ex
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The categories of this paper are: quant-ph
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The categories of this paper are: math-ph