Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Generation of terahertz radiation from ionizing two-color laser pulses
  in Ar filled metallic hollow waveguides   The generation of THz radiation from ionizing two-color femtosecond pulses
propagating in metallic hollow waveguides filled with Ar is numerically
studied. We observe a strong reshaping of the low-frequency part of the
spectrum. Namely, after several millimeters of propagation the spectrum is
extended from hundreds of GHz up to ~150 THz. For longer propagation distances,
nearly single-cycle near-infrared pulses with wavelengths around 4.5 um are
obtained by appropriate spectral filtering, with an efficiency of up to 0.25%.

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The categories of this paper are: stat
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The categories of this paper are: gr-qc
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The categories of this paper are: eess
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The categories of this paper are: hep-lat
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The categories of this paper are: atom-ph
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The categories of this paper are: nucl-ex
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The categories of this paper are: math
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The categories of this paper are: bayes-an
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The categories of this paper are: mtrl-th
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The categories of this paper are: econ
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The categories of this paper are: q-alg
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The categories of this paper are: comp-gas
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The categories of this paper are: alg-geom
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The categories of this paper are: acc-phys
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The categories of this paper are: hep-ph
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The categories of this paper are: quant-ph