Broadband Terahertz Spectroscopy
137
transport properties. Also, TRTS played a significant role in understanding the large
polarons formation that determines the modest mobility values in LHPs [101, 102].
Graphene is the most celebrated two-dimensional (2-D) material, a single layer of
hexagonally arranged covalently bonded carbon atoms. Graphene sheets are stacked
together by van der Waals forces to form graphite. The exotic properties of graphene
are dramatically different from that of graphite. There are several other 2D materials,
which are mostly few atomic layer sheets, such as transition metal dichalcogenides
(TMD), and MXenes that shows properties ranging from insulator to semiconductor
to metal. The properties of these 2D materials depend on several factors, including
the number of layers and level of defects. TRTS has been extensively used to study
the carrier dynamics and transport properties of graphene [103] and many TMDs
[104]. With many new 2D materials being designed and synthesized recently, TRTS
will play an essential role in understanding them [104].
4.3 Non-linear THz Spectroscopy
Recent advances in the generation of intense THz pulses have made it possible to
explore the light-matter interaction in the THz regime. With pulse duration corresponding to 1 ps, intense THz pulses can significantly distort the electronic potential
bringing about fundamental changes in the material properties. Because of its low
photon energies of the order of k B T or below (at room temperature), THz radiation
can access the molecular vibrational modes, which generally defines the equilibrium
physical and chemical properties of a system. Excitation using THz pulses have now
been utilized to study various systems using several experimental techniques like
the THz Kerr effect, THz-pump-X-ray-probe, THz-pump-optical probe, and THz
pump-THz-probe spectroscopy [105].
Understanding the intermolecular modes of vibrations in proteins and its local
rearrangement dynamics, which typically fall in the THz regime, is of utmost
importance because they strongly influence the protein structure. Whether quantum
mechanics play any part in the functioning of these biological systems has been a
matter of debate for a long time. Froehlich had proposed that within a protein, the
vibrational modes are ordered and can condense into a lowest-frequency vibrational
mode in a process similar to the Bose–Einstein condensation and thus, quantum
mechanical coherence could be observed in biological samples in the THz regime.
Structural changes induced by irradiation with 0.4 THz on a lysozyme crystal was
probed by X-ray diffraction showed non-thermal changes in the electron density.
These changes occurring in micro to millisecond timescale is much slower than
expected and can be explained only by Froehlich condensation [106].
Another non-linear experimental technique that is gaining popularity is the THz
Kerr effect (TKE). The advantage TKE has over the optical Kerr effect (OKE) is that
it can successfully determine α, where α T K E = α Z Z − ((α X X + α Y Y )/2,
with α Z Z being the polarizability along the permanent dipole moment [107]. The
sign of α is projected on the TKE signal, which gives new information on the
137
transport properties. Also, TRTS played a significant role in understanding the large
polarons formation that determines the modest mobility values in LHPs [101, 102].
Graphene is the most celebrated two-dimensional (2-D) material, a single layer of
hexagonally arranged covalently bonded carbon atoms. Graphene sheets are stacked
together by van der Waals forces to form graphite. The exotic properties of graphene
are dramatically different from that of graphite. There are several other 2D materials,
which are mostly few atomic layer sheets, such as transition metal dichalcogenides
(TMD), and MXenes that shows properties ranging from insulator to semiconductor
to metal. The properties of these 2D materials depend on several factors, including
the number of layers and level of defects. TRTS has been extensively used to study
the carrier dynamics and transport properties of graphene [103] and many TMDs
[104]. With many new 2D materials being designed and synthesized recently, TRTS
will play an essential role in understanding them [104].
4.3 Non-linear THz Spectroscopy
Recent advances in the generation of intense THz pulses have made it possible to
explore the light-matter interaction in the THz regime. With pulse duration corresponding to 1 ps, intense THz pulses can significantly distort the electronic potential
bringing about fundamental changes in the material properties. Because of its low
photon energies of the order of k B T or below (at room temperature), THz radiation
can access the molecular vibrational modes, which generally defines the equilibrium
physical and chemical properties of a system. Excitation using THz pulses have now
been utilized to study various systems using several experimental techniques like
the THz Kerr effect, THz-pump-X-ray-probe, THz-pump-optical probe, and THz
pump-THz-probe spectroscopy [105].
Understanding the intermolecular modes of vibrations in proteins and its local
rearrangement dynamics, which typically fall in the THz regime, is of utmost
importance because they strongly influence the protein structure. Whether quantum
mechanics play any part in the functioning of these biological systems has been a
matter of debate for a long time. Froehlich had proposed that within a protein, the
vibrational modes are ordered and can condense into a lowest-frequency vibrational
mode in a process similar to the Bose–Einstein condensation and thus, quantum
mechanical coherence could be observed in biological samples in the THz regime.
Structural changes induced by irradiation with 0.4 THz on a lysozyme crystal was
probed by X-ray diffraction showed non-thermal changes in the electron density.
These changes occurring in micro to millisecond timescale is much slower than
expected and can be explained only by Froehlich condensation [106].
Another non-linear experimental technique that is gaining popularity is the THz
Kerr effect (TKE). The advantage TKE has over the optical Kerr effect (OKE) is that
it can successfully determine α, where α T K E = α Z Z − ((α X X + α Y Y )/2,
with α Z Z being the polarizability along the permanent dipole moment [107]. The
sign of α is projected on the TKE signal, which gives new information on the