THz time domain spectroscopy has been largely applied on the measurement of semiconductor, electro-optic crystals,
and selected chemical, biological and explosive materials. The objective of this paper is to report THz gas photonics and
its applications, with an emphasis on remote sensing capabilities. The most recent results of using air (and selected
gases) as the emitter and sensor material for both generation and detection of broadband THz waves will be reported.
Air, especially ionized air (plasma), has been used to generate intense peak THz waves (THz field > 1.5 MV/cm) with a
broadband spectrum (10% bandwidth from 0.1 THz to 46 THz). THz-enhanced-fluorescence (TEF) and THz-enhanced
acoustic (TEA) techniques have been developed for remote sensing purpose. By "seeing" the fluorescence, or "hearing"
the sound, coherent detection of THz waves at standoff distance is feasible.
The significant scientific and technological potential of terahertz (THz) wave sensing and imaging has been attracted
considerable attention within many fields of research. However, the development of remote, broadband THz wave
sensing technology is lagging behind the compelling needs that exist in the areas of astronomy, global environmental
monitoring, and homeland security. This is due to the challenge posed by high absorption of ambient moisture in the
THz range. Although various time-domain THz detection techniques have recently been demonstrated, the requirement
for an on-site bias or forward collection of the optical signal inevitably prohibits their applications for remote sensing.
The objective of this paper is to report updated THz air-plasma technology to meet this great challenge of remote
sensing. A focused optical pulse (mJ pulse energy and femtosecond pulse duration) in gas creates a plasma, which can
serve to generate intense, broadband, and directional THz waves in the far field.
In general, a reflective spectrometer is more suitable for the spectroscopic measurement for highly absorptive samples.
We report the design and evaluation of a reflective terahertz time-domain spectroscopy (R-THz-TDS), using air as THz
wave emitter and sensor, together with air-biased-coherent-detection (ABCD) method. With an 85 fs pulse amplified
laser, we demonstrate a usable bandwidth from 0.5 THz to 12 THz, together with a peak dynamic range (DR) better than
2000:1 and a peak THz electrical field greater than 30 kV/cm. With a 32 fs pulse amplified laser, the usable bandwidth is
further expanded to a continuous 35 THz. Several far-infrared optical properties such as phonon resonance and plasma
resonance in various samples are reported. We also compared both transmission and reflection measurements.
Furthermore, the time-resolved optical pump-THz probe experiment is performed. The evolution of carrier dynamics of
GaAs and InSb samples are demonstrated in this study. Finally, the uniqueness and advantage of this R-THz-TDS
spectrometer are comprehensively compared with traditional THz-TDS and Fourier transform infrared (FTIR)
spectroscopy, including radiation source, detector, DR, bandwidth, resolution, peak power, and other features. In terms
of signal-to-noise ratio (SNR), this study provides the SNR variation with frequency in both broadband R-THz-TDS and