Quantum Cascade Laser Spectroscopy
381
8 Application of QCL Spectroscopy in Terahertz Spectral
Range
The invention of quantum cascade lasers (QCL) in terahertz (THz) spectral region (in
2002) [107] has opened the opportunity to explore the high-resolution spectroscopy
of molecules, atoms and ions by investigating their characteristic rotational and
fine-structure transitions. Moreover, the advent of THz-QCL coupled spectrometer
has become a popular analytical tool for analysis of structure and energy levels
of the molecules and atoms. The extraordinary high-resolution THz spectra also
provide important information about Doppler effect and pressure-broadening effect
of the molecules without affected by the instrumental resolution. The key features
of the THz-QCLs, such as compact sizes, broad spectral-coverage from 1 to 5 THz
[108], high-output powers (several mW), coherent-emission with quantum noiselimited linewidths make it most admired optical source among the researcher. Earlier
studies mainly focused on the development of THz-QCLs coupled high-resolution
spectroscopic methods such as frequency and wavelength modulation spectroscopy
[109], differential spectroscopy [110], saturation spectroscopy [111], photo-acoustic
spectroscopy [112] and frequency-comb assisted spectroscopy [113] for molecular
spectroscopic studies.
Recently, THz-QCLs has been served in Heterodyne spectrometer as a local oscillator (LO) for atmospheric sciences applications and remote sensing in astronomy
[114]. In 2015, THz QCL was exploited as a LO in the German Receiver for
Astronomy at Terahertz Frequencies (GREAT), which was placed on board Stratospheric Observatory for Infrared Astronomy (SOFIA) to detect the interstellar atomic
oxygen by probing fine-structure transition at 4.7 THz [115, 116]. However, others
spaceborne missions such as the Galactic/Extra galactic Ultra long Duration Balloon
Spectroscopic-Stratospheric Terahertz Observatory (GUSTO); Low-Cost UpperAtmosphere Sounder (LOCUS); the Far Infrared Spectroscopic Explorer for probing
the lifecycle of the interstellar medium (FIRSPEX) and the Heterodyne Receiver for
the Origins Space Telescope (HERO) exploited the QCL as a LO [117–121]. Moreover, the high output power emission with frequencies, which have low atmospheric
attenuation, has enabled THz QCL suitable for imaging spectroscopy. Different THz
imagining strategies have exploited QCL for chemical applications [122] (Fig. 10).
For example, in Paul dean et al., they utilised a continuum THz QCL source
operating at 2.8 THz in absorption-sensitive diffuse reflection imaging of powdered
admixtures of polystyrene and polymethyl methacrylate (PMMA) [123]. In another
study, they used the dual-frequency imaging using a tunable terahertz quantum
cascade laser for imaging organic explosive material pentaerythritol tetranitrate
(PETN) around 3THz spectral region [124].
Furthermore, high-spectral-resolution terahertz imaging system coupled with a
multi-mode quantum-cascade laser (QCL) was utilised to investigate the dynamics
of the CH 3 OH gas leaking from a terahertz-transparent tube into the evacuated cell
at 3.38 THz [125]. However, few disadvantageous properties of THz QCL such as
381
8 Application of QCL Spectroscopy in Terahertz Spectral
Range
The invention of quantum cascade lasers (QCL) in terahertz (THz) spectral region (in
2002) [107] has opened the opportunity to explore the high-resolution spectroscopy
of molecules, atoms and ions by investigating their characteristic rotational and
fine-structure transitions. Moreover, the advent of THz-QCL coupled spectrometer
has become a popular analytical tool for analysis of structure and energy levels
of the molecules and atoms. The extraordinary high-resolution THz spectra also
provide important information about Doppler effect and pressure-broadening effect
of the molecules without affected by the instrumental resolution. The key features
of the THz-QCLs, such as compact sizes, broad spectral-coverage from 1 to 5 THz
[108], high-output powers (several mW), coherent-emission with quantum noiselimited linewidths make it most admired optical source among the researcher. Earlier
studies mainly focused on the development of THz-QCLs coupled high-resolution
spectroscopic methods such as frequency and wavelength modulation spectroscopy
[109], differential spectroscopy [110], saturation spectroscopy [111], photo-acoustic
spectroscopy [112] and frequency-comb assisted spectroscopy [113] for molecular
spectroscopic studies.
Recently, THz-QCLs has been served in Heterodyne spectrometer as a local oscillator (LO) for atmospheric sciences applications and remote sensing in astronomy
[114]. In 2015, THz QCL was exploited as a LO in the German Receiver for
Astronomy at Terahertz Frequencies (GREAT), which was placed on board Stratospheric Observatory for Infrared Astronomy (SOFIA) to detect the interstellar atomic
oxygen by probing fine-structure transition at 4.7 THz [115, 116]. However, others
spaceborne missions such as the Galactic/Extra galactic Ultra long Duration Balloon
Spectroscopic-Stratospheric Terahertz Observatory (GUSTO); Low-Cost UpperAtmosphere Sounder (LOCUS); the Far Infrared Spectroscopic Explorer for probing
the lifecycle of the interstellar medium (FIRSPEX) and the Heterodyne Receiver for
the Origins Space Telescope (HERO) exploited the QCL as a LO [117–121]. Moreover, the high output power emission with frequencies, which have low atmospheric
attenuation, has enabled THz QCL suitable for imaging spectroscopy. Different THz
imagining strategies have exploited QCL for chemical applications [122] (Fig. 10).
For example, in Paul dean et al., they utilised a continuum THz QCL source
operating at 2.8 THz in absorption-sensitive diffuse reflection imaging of powdered
admixtures of polystyrene and polymethyl methacrylate (PMMA) [123]. In another
study, they used the dual-frequency imaging using a tunable terahertz quantum
cascade laser for imaging organic explosive material pentaerythritol tetranitrate
(PETN) around 3THz spectral region [124].
Furthermore, high-spectral-resolution terahertz imaging system coupled with a
multi-mode quantum-cascade laser (QCL) was utilised to investigate the dynamics
of the CH 3 OH gas leaking from a terahertz-transparent tube into the evacuated cell
at 3.38 THz [125]. However, few disadvantageous properties of THz QCL such as