Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Generation of Kerr combs centered at 4.5{\mu}m in crystalline
  microresonators pumped by quantum cascade lasers   We report on the generation of mid-infrared Kerr frequency combs in
high-finesse CaF$_2$ and MgF$_2$ whispering-gallery mode resonators pumped with
continuous wave room temperature quantum cascade lasers. The combs were
centered at 4.5$\mu$m, the longest wavelength to date. A frequency comb wider
than a half of an octave was demonstrated when approximately 20mW of pump power
was coupled to an MgF2 resonator characterized with quality factor exceeding
10$^8$.

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The categories of this paper are: q-fin
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The categories of this paper are: nucl-th
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The categories of this paper are: dg-ga
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The categories of this paper are: ao-sci
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The categories of this paper are: q-bio
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The categories of this paper are: hep-lat
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The categories of this paper are: adap-org
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The categories of this paper are: cs
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The categories of this paper are: astro-ph
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The categories of this paper are: plasm-ph
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The categories of this paper are: chem-ph
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The categories of this paper are: solv-int
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The categories of this paper are: math-ph
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The categories of this paper are: comp-gas
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The categories of this paper are: patt-sol
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The categories of this paper are: chao-dyn