Broadband Terahertz Spectroscopy
133
4 Applications of Broadband Terahertz Spectroscopy
4.1 Application of THz-TDS
THz spectroscopy gained popularity because of its broad bandwidth, sub-ps temporal
resolution, and the ability to simultaneously obtain the amplitude and phase information, making it a better alternative to FTIR. The complex dielectric spectrum can
be obtained in all three phases, solids, liquids, and gases utilizing either transmission
or reflection geometries.
One of the most studied systems using THz-TDS is water vapor [31] and liquid
water, along with water mixtures at these low frequencies [75, 76]. Formation and
melting of solid phases of water like ice and clathrate hydrates have also been studied
using THz absorption spectroscopy. The solid phases have a distinct hydrogen bond
network, which might also be present in supercooled water and the hydration shell
around a hydrophobe. A temperature-dependent THz/far-infrared absorption spectroscopy reveals the similarity of the hydrophobic hydration of the aqueous alcohol
solution with the structure of both supercooled water and ice [77]. THz spectroscopic
studies of ices in molecular clouds are essential to understand the mechanism behind
the evolution of stars and planetary systems. One of the significant difficulties in
probing molecular clouds is to estimate the amount of gases present in the cloud.
THz-TDS provides accurate measurements of the dielectric properties of ice without
any edge distortions that come with the Kramer-Kronig analysis in the THz frequencies. The dielectric spectrum of ice has information about its intermolecular modes,
which gives unique information about the structure of ice. THz-TDS also makes
possible the study of other ice analogs, like CO, CO 2 , and CH 3 OH, present in these
atmospheric clouds [78, 79].
THz-TDS has been used to measure the complex dielectric properties of atmospheric pressure plasma jets. Plasma can be envisioned as a quasi-neutral gas made up
of charged and neutral particles that exhibit collective properties. The electron density
of the plasma has been calculated from the complex refractive index. A comparison
of the results shows a linear increase of electron density with the increase of discharge
voltage. Helium has been found to perform lower than neon at the same discharge
voltage due to its higher ionization potential [80].
THz time-domain spectroscopy has been used as a powerful probe to understand
the ionic conduction mechanism in solids. It has been used to characterize solid
electrolytes, like zirconia, to improve the fuel cells and solid-state batteries. The
THz response obtained has been identified as vacancy hopping to adjacent sites. A
temperature-dependent steady-state study gives information on the intrinsic energy
of migration of these vacancies [81].
Metal–organic frameworks (MOFs) have emerged as an important class of functional materials due to their intrinsic structural and mechanical properties, microporosity, and large surface areas. Identifying the low-frequency vibrational modes of
these MOFs is essential as these modes play a crucial role in enzymatic catalysis
and ligand binding. THz-TDS has been used extensively to study the low-frequency
133
4 Applications of Broadband Terahertz Spectroscopy
4.1 Application of THz-TDS
THz spectroscopy gained popularity because of its broad bandwidth, sub-ps temporal
resolution, and the ability to simultaneously obtain the amplitude and phase information, making it a better alternative to FTIR. The complex dielectric spectrum can
be obtained in all three phases, solids, liquids, and gases utilizing either transmission
or reflection geometries.
One of the most studied systems using THz-TDS is water vapor [31] and liquid
water, along with water mixtures at these low frequencies [75, 76]. Formation and
melting of solid phases of water like ice and clathrate hydrates have also been studied
using THz absorption spectroscopy. The solid phases have a distinct hydrogen bond
network, which might also be present in supercooled water and the hydration shell
around a hydrophobe. A temperature-dependent THz/far-infrared absorption spectroscopy reveals the similarity of the hydrophobic hydration of the aqueous alcohol
solution with the structure of both supercooled water and ice [77]. THz spectroscopic
studies of ices in molecular clouds are essential to understand the mechanism behind
the evolution of stars and planetary systems. One of the significant difficulties in
probing molecular clouds is to estimate the amount of gases present in the cloud.
THz-TDS provides accurate measurements of the dielectric properties of ice without
any edge distortions that come with the Kramer-Kronig analysis in the THz frequencies. The dielectric spectrum of ice has information about its intermolecular modes,
which gives unique information about the structure of ice. THz-TDS also makes
possible the study of other ice analogs, like CO, CO 2 , and CH 3 OH, present in these
atmospheric clouds [78, 79].
THz-TDS has been used to measure the complex dielectric properties of atmospheric pressure plasma jets. Plasma can be envisioned as a quasi-neutral gas made up
of charged and neutral particles that exhibit collective properties. The electron density
of the plasma has been calculated from the complex refractive index. A comparison
of the results shows a linear increase of electron density with the increase of discharge
voltage. Helium has been found to perform lower than neon at the same discharge
voltage due to its higher ionization potential [80].
THz time-domain spectroscopy has been used as a powerful probe to understand
the ionic conduction mechanism in solids. It has been used to characterize solid
electrolytes, like zirconia, to improve the fuel cells and solid-state batteries. The
THz response obtained has been identified as vacancy hopping to adjacent sites. A
temperature-dependent steady-state study gives information on the intrinsic energy
of migration of these vacancies [81].
Metal–organic frameworks (MOFs) have emerged as an important class of functional materials due to their intrinsic structural and mechanical properties, microporosity, and large surface areas. Identifying the low-frequency vibrational modes of
these MOFs is essential as these modes play a crucial role in enzymatic catalysis
and ligand binding. THz-TDS has been used extensively to study the low-frequency