364
M. Pal and M. Pradhan
narrow linewidth (≤0.0002 cm
−1 ), operation in both pulsed and continuous mode
operation along with modulation potential up to MHz-regime make this laser source
popular in the diverse fields of applications. Furthermore, the emergence of QCL
along with its key advancement in technological aspects has led to the development
of new measurement schemes in trace gas analysis for atmospheric and biomedical
applications, spectral analysis of bio-fluid and tissue imaging for non-invasive disease
diagnosis.
In trace gas sensing, QCL has played a promising role as an optical source for laser
absorption spectroscopy. Its operating wavelength region offers several windows of
atmospheric transmission, where many molecules (along with their isotopes) with
atmospheric interest have fundamental absorption bands. Moreover, the strength of
those bands is much intense than near-IR overtone and combinational bands. Also, its
high emission power enabled to couple laser light to high-finesse optical cavity based
spectroscopic techniques such as cavity ring-down spectroscopy, integrated cavity
output spectroscopy and photoacoustic spectroscopy. Therefore, such extraordinary
potentials of QCL have become beneficial for atmospheric trace gas sensing as well
as for exhaled breath trace constituents monitoring with unprecedented sensitivity.
However, the application of QCL spectroscopy in liquid samples (e.g. bio-fluids)
has also marked a success. The many biomedical samples such as glucose, proteins,
lipids, carbohydrates and deoxyribonucleic acids have their characteristics spectral
features containing reach information in the mid-IR spectral region. Moreover, QCL
based IR spectrometer has proved to be an equivalent or better performer in comparison with conventional FTIR spectrometer and due to its unique properties, several
expansions in the new modes of application have become feasible. Apart from that,
entry of QCL has also entered into the field of IR-imaging involving tissue and
bio-fluid imaging for disease diagnostics. Moreover, QCL has marked its excellence
in Terahertz spectroscopy. The emergence of THz QCL with its working temperature and high photon density enabled its applications in THz spectroscopy from
high-resolution gas sensing to imaging.
In this book chapter, we have presented a brief review of the applications of
quantum cascade laser spectroscopy in diverse fields. Following a short introduction,
the working principle of QCL along with its variants has been described. The main
attention has given to the applications of QCL coupled with some new-generation
spectroscopic methods in the field of atmospheric science, breath research, and tissue
imaging in the mid-IR spectral range. Moreover, a brief overview of the contribution
of QCL in THz spectroscopy applications has also been summarized here.
2 Basic Working Principle of QCL and Its Variants
The conventional semiconductor lasers emit wavelength-dependent radiation due
to radiative electron–hole recombination between the valence and conduction band
(inter-band transition) for different bandgaps of different materials [1, 2]. But in
Quantum Cascade Laser (QCL), the emission is achieved by utilizing intersubband
M. Pal and M. Pradhan
narrow linewidth (≤0.0002 cm
−1 ), operation in both pulsed and continuous mode
operation along with modulation potential up to MHz-regime make this laser source
popular in the diverse fields of applications. Furthermore, the emergence of QCL
along with its key advancement in technological aspects has led to the development
of new measurement schemes in trace gas analysis for atmospheric and biomedical
applications, spectral analysis of bio-fluid and tissue imaging for non-invasive disease
diagnosis.
In trace gas sensing, QCL has played a promising role as an optical source for laser
absorption spectroscopy. Its operating wavelength region offers several windows of
atmospheric transmission, where many molecules (along with their isotopes) with
atmospheric interest have fundamental absorption bands. Moreover, the strength of
those bands is much intense than near-IR overtone and combinational bands. Also, its
high emission power enabled to couple laser light to high-finesse optical cavity based
spectroscopic techniques such as cavity ring-down spectroscopy, integrated cavity
output spectroscopy and photoacoustic spectroscopy. Therefore, such extraordinary
potentials of QCL have become beneficial for atmospheric trace gas sensing as well
as for exhaled breath trace constituents monitoring with unprecedented sensitivity.
However, the application of QCL spectroscopy in liquid samples (e.g. bio-fluids)
has also marked a success. The many biomedical samples such as glucose, proteins,
lipids, carbohydrates and deoxyribonucleic acids have their characteristics spectral
features containing reach information in the mid-IR spectral region. Moreover, QCL
based IR spectrometer has proved to be an equivalent or better performer in comparison with conventional FTIR spectrometer and due to its unique properties, several
expansions in the new modes of application have become feasible. Apart from that,
entry of QCL has also entered into the field of IR-imaging involving tissue and
bio-fluid imaging for disease diagnostics. Moreover, QCL has marked its excellence
in Terahertz spectroscopy. The emergence of THz QCL with its working temperature and high photon density enabled its applications in THz spectroscopy from
high-resolution gas sensing to imaging.
In this book chapter, we have presented a brief review of the applications of
quantum cascade laser spectroscopy in diverse fields. Following a short introduction,
the working principle of QCL along with its variants has been described. The main
attention has given to the applications of QCL coupled with some new-generation
spectroscopic methods in the field of atmospheric science, breath research, and tissue
imaging in the mid-IR spectral range. Moreover, a brief overview of the contribution
of QCL in THz spectroscopy applications has also been summarized here.
2 Basic Working Principle of QCL and Its Variants
The conventional semiconductor lasers emit wavelength-dependent radiation due
to radiative electron–hole recombination between the valence and conduction band
(inter-band transition) for different bandgaps of different materials [1, 2]. But in
Quantum Cascade Laser (QCL), the emission is achieved by utilizing intersubband