Recently, air plasma, produced by focusing an
intense laser beam to ionize atoms or molecules, has been
demonstrated to be a promising source of broadband
terahertz waves. However, simultaneous broadband and
coherent detection of such broadband terahertz waves is
still challenging. Electro-optical sampling and photocon-
ductive antennas are the typical approaches for terahertz
wave detection. The bandwidth of these detection methods
is limited by the phonon resonance or carrier’s lifetime.
Unlike solid-state detectors, gaseous sensors have several
unique features, such as no phonon resonance, less
dispersion, no Fabry-Perot effect, and a continuous
renewable nature. The aim of this article is to review the
development of a broadband terahertz time-domain
spectrometer, which has both a gaseous emitter and sensor
mainly based on author’s recent investigation. This
spectrometer features high efficiency, perceptive sensitiv-
ity, broad bandwidth, adequate signal-to-noise ratio,
sufficient dynamic range, and controllable polarization.
The detection of terahertz waves with ambient air has
been realized through a third order nonlinear optical
process: detecting the second harmonic photon that is
produced by mixing one terahertz photon with two
fundamental photons. In this review, a systematic inves-
tigation of the mechanism of broadband terahertz wave
detection was presented first. The dependence of the
detection efficiency on probe pulse energy, bias field
strength, gas pressure and third order nonlinear suscept-
ibility of gases were experimentally demonstrated with
selected gases. Detailed discussions of phase matching and
Gouy phase shift were presented by considering the
focused condition of Gaussian beams. Furthermore, the
bandwidth dependence on probe pulse duration was also
demonstrated. Over 240 times enhancement of dynamic

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• Investigation of ultra-broadband terahertz time-domain spectroscopy with terahertz wave gas photonics