Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Group delay in THz spectroscopy with ultra-wideband log-spiral antennae   We report on the group delay observed in continuous-wave terahertz
spectroscopy based on photomixing with phase-sensitive homodyne detection. We
discuss the different contributions of the experimental setup to the phase
difference \Delta\phi(\nu) between transmitter arm and receiver arm. A simple
model based on three contributions yields a quantitative description of the
overall behavior of \Delta\phi(\nu). Firstly, the optical path-length
difference gives rise to a term linear in frequency. Secondly, the
ultra-wideband log-spiral antennae effectively radiate and receive in a
frequency-dependent active region, which in the most simple model is an annular
area with a circumference equal to the wavelength. The corresponding term
changes by roughly 6 pi between 100 GHz and 1 THz. The third contribution stems
from the photomixer impedance. In contrast, the derivative (d\Delta\phi / d\nu)
is dominated by the contribution of periodic modulations of \Delta\phi(\nu)
caused by standing waves, e.g., in the photomixers' Si lenses. Furthermore, we
discuss the Fourier-transformed spectra, which are equivalent to the waveform
in a time-domain experiment. In the time domain, the group delay introduced by
the log-spiral antennae gives rise to strongly chirped signals, in which low
frequencies are delayed. Correcting for the contributions of antennae and
photomixers yields sharp peaks or "pulses" and thus facilitates a
time-domain-like analysis of our continuous-wave data.

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The categories of this paper are: gr-qc
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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: acc-phys
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The categories of this paper are: stat
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The categories of this paper are: supr-con
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The categories of this paper are: ao-sci
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The categories of this paper are: quant-ph
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The categories of this paper are: math-ph
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The categories of this paper are: funct-an
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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: plasm-ph
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The categories of this paper are: hep-ex
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The categories of this paper are: eess
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The categories of this paper are: cs