Broadband Terahertz Spectroscopy
135
water cages. The response obtained for hydration water was also found to correlate
well with the Hofmeister series for both the cations and anions [88].
Steady-state THz spectroscopy has also been utilized to explore the low-frequency
dynamics of various peptides and proteins. Due to the low energy of THz radiation, THz-TDS is sensitive to the low-frequency motions of these molecules. Also,
since most local conformational changes in proteins occur in picosecond timescales,
THz-TDS becomes an important tool to probe these dynamics. THz-TDS, along
with DFT calculations, have successfully identified various modes in the absorption spectra of L-carnosine, a dipeptide naturally occurring in muscles [89]. The
THz-TDS spectra of six different tetrameric peptides show that this technique is
sensitive to the differences in the primary and secondary structures of these peptides
[90]. Terahertz absorption spectroscopy, in combination with molecular dynamics
simulations, has also been used to understand the molecular mechanism behind the
antifreeze activity of antifreeze protein III [91].
THz spectroscopy has been used extensively to develop new medical techniques
to examine different biological tissues and organs. It has been used to determine the
water content in various biological samples. A 2018 study of the THz absorption
coefficient spectra of 70 human blood samples shows that the spectra are sensitive
to the blood glucose level and follow a linear relationship [92].
4.2 Application of Time-Resolved THz Spectroscopy
Time-resolved THz spectroscopy offers the unique advantages of monitoring
dynamics from sub-picosecond to nanosecond timescales of excited states of matter
that respond to optical field oscillating in THz frequency with photon energies in
the range of 10–400 cm
−1 . Molecular rotations, intra, and intermolecular vibrations, charge carriers in metals and semiconductors, lattice vibrations in any single
crystal or crystallites are some of the entities that can interact with THz light. Hence,
in principle, time-resolved THz spectroscopy can monitor the properties of matter
and their temporal evolution at the ultrafast timescale. There is no limitation in
terms of the physical state of the system to be evaluated. This technique has already
been established as a non-contact probe for ac-conductivity and has been utilized
to study semiconductors (bulk and nanoparticles), superconductors, and many other
composite materials. Here, we discuss some recent representative applications of
TRTS in semiconductors and 2D materials.
Lead halide perovskites (LHP) have garnered unprecedented attention in the last
six years due to their incredible improvement in solar cell efficiency (~25.5%) [93]
within a very short span. LHPs are synthesized using simple wet chemistry, yet
they exhibit properties that can be expected only from extremely pure expensive
semiconductors such as silicon and GaAs. Time-resolved THz spectroscopy has
played a significant role in understanding the fundamental physics responsible for
its remarkable showcase of properties [94–96]. The charge carrier dynamics and
important semiconductor properties, such as carrier mobility, diffusion length, have
135
water cages. The response obtained for hydration water was also found to correlate
well with the Hofmeister series for both the cations and anions [88].
Steady-state THz spectroscopy has also been utilized to explore the low-frequency
dynamics of various peptides and proteins. Due to the low energy of THz radiation, THz-TDS is sensitive to the low-frequency motions of these molecules. Also,
since most local conformational changes in proteins occur in picosecond timescales,
THz-TDS becomes an important tool to probe these dynamics. THz-TDS, along
with DFT calculations, have successfully identified various modes in the absorption spectra of L-carnosine, a dipeptide naturally occurring in muscles [89]. The
THz-TDS spectra of six different tetrameric peptides show that this technique is
sensitive to the differences in the primary and secondary structures of these peptides
[90]. Terahertz absorption spectroscopy, in combination with molecular dynamics
simulations, has also been used to understand the molecular mechanism behind the
antifreeze activity of antifreeze protein III [91].
THz spectroscopy has been used extensively to develop new medical techniques
to examine different biological tissues and organs. It has been used to determine the
water content in various biological samples. A 2018 study of the THz absorption
coefficient spectra of 70 human blood samples shows that the spectra are sensitive
to the blood glucose level and follow a linear relationship [92].
4.2 Application of Time-Resolved THz Spectroscopy
Time-resolved THz spectroscopy offers the unique advantages of monitoring
dynamics from sub-picosecond to nanosecond timescales of excited states of matter
that respond to optical field oscillating in THz frequency with photon energies in
the range of 10–400 cm
−1 . Molecular rotations, intra, and intermolecular vibrations, charge carriers in metals and semiconductors, lattice vibrations in any single
crystal or crystallites are some of the entities that can interact with THz light. Hence,
in principle, time-resolved THz spectroscopy can monitor the properties of matter
and their temporal evolution at the ultrafast timescale. There is no limitation in
terms of the physical state of the system to be evaluated. This technique has already
been established as a non-contact probe for ac-conductivity and has been utilized
to study semiconductors (bulk and nanoparticles), superconductors, and many other
composite materials. Here, we discuss some recent representative applications of
TRTS in semiconductors and 2D materials.
Lead halide perovskites (LHP) have garnered unprecedented attention in the last
six years due to their incredible improvement in solar cell efficiency (~25.5%) [93]
within a very short span. LHPs are synthesized using simple wet chemistry, yet
they exhibit properties that can be expected only from extremely pure expensive
semiconductors such as silicon and GaAs. Time-resolved THz spectroscopy has
played a significant role in understanding the fundamental physics responsible for
its remarkable showcase of properties [94–96]. The charge carrier dynamics and
important semiconductor properties, such as carrier mobility, diffusion length, have