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S. Banerjee et al.
polarizability tensor of the system. TKE has been successfully used to study water,
methanol, and various other simple liquids [108–110].
Excitonic interactions in ZnSe/ZnMgSSe multiple quantum wells with THz radiation have been explored using the THz-pump-optical probe spectroscopy. At low THz
fields, the dependence of the excitonic absorption on the THz field was in accordance
to the perturbation theory (the Stark shift). In contrast, there was an apparent deviation of the experimental results from this theory at high THz fields, implying that
the interactions are now in the non-perturbative regime [111]. THz-pump-THz-probe
experiments have been utilized to explore the field of semiconductor nonlinearities in
the THz frequency range. Single-cycle intense THz pulses accelerate the free carriers
in doped semiconductors to high energies, while the THz probe beam can probe the
free carrier dynamics in picosecond timescales. THz pump/THz probe experiments
have been done to monitor the intervalley/intravalley scattering of hot electrons in
GaAs [112] and the impact ionization in InSb and InAs [112, 113].
5 Conclusion
Today THz spectroscopy has come a long way, starting from the first successful
generation and detection of pulsed THz radiation. Now, it is possible to perform THz
spectroscopy with intense broadband THz light on a tabletop. THz spectrometers
capable of performing THz-TDS and TRTS with reasonably large spectral bandwidth are commercially available for several years. In the coming years, with the
advances in laser technology and electronics, the performance and applicability of
such instruments will undoubtedly improve. This will encourage more and more
researchers to utilize THz spectroscopy in various fields that are not explored yet.
THz-TDS is a steady-state absorption spectroscopy in the far-far Infrared range.
For a sample to absorb THz radiation, it should have dipole moment oscillating in
THz frequency. So, this technique is useful for most polar systems. On the other hand,
for the centrosymmetric system, THz-TDS can probe only the IR-active (THz-active)
vibrational modes. The Raman active modes will remain silent while interacting with
THz photons. The use of optical Kerr effect (OKE) spectroscopy can conveniently
solve this issue [114, 115]. The Optical Kerr effect is a third-order non-linear process.
It measures the derivative of the time-correlation function of the anisotropic part of
the polarizability tensor, unlike THz-TDS, which measures the two-point correlation
function of the dipole moment. The advantage of doing an OKE and THz-TDS study
together is that it is possible to get complementary information on the same dynamics
of a system [116]. The experimental setup for OKE is relatively straight-forward
compared to other forms of pump-probe spectroscopies.
This era is the era of exotic materials for innovative applications. The non-contact
nature of THz spectroscopy to probe materials is one of the many advantages that will
be handy to study properties and dynamics in nanomaterials and sensitive materials of
a different kind. Because of its unique way of probing materials that are not possible
by other methods, THz spectroscopy will find more applications in materials science
S. Banerjee et al.
polarizability tensor of the system. TKE has been successfully used to study water,
methanol, and various other simple liquids [108–110].
Excitonic interactions in ZnSe/ZnMgSSe multiple quantum wells with THz radiation have been explored using the THz-pump-optical probe spectroscopy. At low THz
fields, the dependence of the excitonic absorption on the THz field was in accordance
to the perturbation theory (the Stark shift). In contrast, there was an apparent deviation of the experimental results from this theory at high THz fields, implying that
the interactions are now in the non-perturbative regime [111]. THz-pump-THz-probe
experiments have been utilized to explore the field of semiconductor nonlinearities in
the THz frequency range. Single-cycle intense THz pulses accelerate the free carriers
in doped semiconductors to high energies, while the THz probe beam can probe the
free carrier dynamics in picosecond timescales. THz pump/THz probe experiments
have been done to monitor the intervalley/intravalley scattering of hot electrons in
GaAs [112] and the impact ionization in InSb and InAs [112, 113].
5 Conclusion
Today THz spectroscopy has come a long way, starting from the first successful
generation and detection of pulsed THz radiation. Now, it is possible to perform THz
spectroscopy with intense broadband THz light on a tabletop. THz spectrometers
capable of performing THz-TDS and TRTS with reasonably large spectral bandwidth are commercially available for several years. In the coming years, with the
advances in laser technology and electronics, the performance and applicability of
such instruments will undoubtedly improve. This will encourage more and more
researchers to utilize THz spectroscopy in various fields that are not explored yet.
THz-TDS is a steady-state absorption spectroscopy in the far-far Infrared range.
For a sample to absorb THz radiation, it should have dipole moment oscillating in
THz frequency. So, this technique is useful for most polar systems. On the other hand,
for the centrosymmetric system, THz-TDS can probe only the IR-active (THz-active)
vibrational modes. The Raman active modes will remain silent while interacting with
THz photons. The use of optical Kerr effect (OKE) spectroscopy can conveniently
solve this issue [114, 115]. The Optical Kerr effect is a third-order non-linear process.
It measures the derivative of the time-correlation function of the anisotropic part of
the polarizability tensor, unlike THz-TDS, which measures the two-point correlation
function of the dipole moment. The advantage of doing an OKE and THz-TDS study
together is that it is possible to get complementary information on the same dynamics
of a system [116]. The experimental setup for OKE is relatively straight-forward
compared to other forms of pump-probe spectroscopies.
This era is the era of exotic materials for innovative applications. The non-contact
nature of THz spectroscopy to probe materials is one of the many advantages that will
be handy to study properties and dynamics in nanomaterials and sensitive materials of
a different kind. Because of its unique way of probing materials that are not possible
by other methods, THz spectroscopy will find more applications in materials science