Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Unveiling spectral purity and tunability of terahertz quantum cascade
  laser sources based on intra-cavity difference frequency generation   Terahertz sources based on intra-cavity difference-frequency generation in
mid-infrared quantum cascade lasers (THz DFG-QCLs) have recently emerged as the
first monolithic electrically-pumped semiconductor sources capable of operating
at room-temperature (RT) across the 1-6 THz range. Despite tremendous progress
in power output, that now exceeds 1mW in pulsed and 10 {\mu}W in
continuous-wave regime at room-temperature, knowledge of the major figure of
merits of these devices for high precision spectroscopy, such as spectral
purity and absolute frequency tunability, is still lacking. Here, by exploiting
a metrological grade system comprising a terahertz frequency comb synthesizer,
we measure, for the first time, the free-running emission linewidth (LW), the
tuning characteristics, and the absolute frequency of individual emission lines
of these sources with an uncertainty of 4 x 10-10. The unveiled emission LW
(400 kHz at 1ms integration time) indicates that DFG-QCLs are well suited to
operate as local oscillators and to be used for a variety of metrological,
spectroscopic, communication, and imaging applications requiring
narrow-linewidth THz sources.

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The categories of this paper are: funct-an
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The categories of this paper are: ao-sci
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The categories of this paper are: comp-gas
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The categories of this paper are: acc-phys
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The categories of this paper are: adap-org
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The categories of this paper are: nucl-th
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The categories of this paper are: econ
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The categories of this paper are: chem-ph
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The categories of this paper are: q-bio
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The categories of this paper are: math
0
The categories of this paper are: hep-ph
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The categories of this paper are: eess
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The categories of this paper are: patt-sol
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The categories of this paper are: mtrl-th
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The categories of this paper are: stat
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The categories of this paper are: quant-ph