5.4 Nonlinear THz Transmission in Single-Walled Carbon Nanotube Films
105
deromotive energies on the order of a few hundred meV (easily achievable at the field
strengths of the THz pulses used in this study), the effective mass will experience a
significant change; as such, as the THz electric field strength increases, we can expect
that the effective mass also increases and thus conductivity decreases. For the highest THz electric field strength used in this study of 373 kV cm
−1 , the ponderomotive
potential calculated from Eq. 5.1 applied to electrons at the bottom of the conduction
band would be 0.7 eV, assuming an effective mass of m
∗
= 0.2 m 0 from the lowest
energy in Fig. 5.9d. From Fig. 5.9d an energy of 0.7 eV would result in an increase in
effective mass to 1.5 m 0 . Therefore the increasing transmission with increasing electric field strength observed in Fig. 5.8 can be explained as the higher electric fields
giving more ponderomotive energy to the charge carriers, which moves them higher
in the conduction band where they have an increased effective mass, leading to lower
mobility and conductivity which is observed as an increase in the transmission at
THz frequencies. A similar effect may occur in the metallic nanotubes, although due
to their Dirac cone-shaped lowest conduction band a more in-depth treatment than
the one used here for the semiconducting nanotubes is required.
The potential saturation observed in Fig. 5.8b may be due to the conducting
behaviour exhibited by the metallic nanotubes, and due to interband scattering and
hot carrier dynamics in both the semiconducting and metallic nanotubes that make up
the thin film. At higher energies the charge carriers may scatter into adjacent conduction bands, which changes their effective mass. Additionally, as the charge carriers
gain energy from the THz pulse they thermalise, which produces a non-equilibrium
carrier distribution; as such there are always some carriers with low energy, which
in turn means low mass and high conductivity, and hence the transmission in the
THz region remains at a finite value less than one, as seen at high field strengths in
Fig. 5.8b.
5.5 Electric Field-Dependent THz Transmission of CuO
Multiferroic materials are of great interest for new applications in data storage and
sensor devices [74, 75], and direct optical switching of magnetic or electric domains
by intense pulses of electromagnetic radiation may allow the dynamics of the domain
reorientation process in such devices to occur on faster timescales [76]. Direct manipulation of the magnetic order, using direct excitation via the magnetic field of electromagnetic pulses, has been demonstrated in a number of magnetic materials [18–20,
77]. However, since many of these excitations occur at THz frequencies, the field
strengths achievable in the currently available experimental techniques are not yet
sufficient to achieve domain switching.
An alternate route to domain control may be found in multiferroic materials,
particularly those which exhibit strong magnetoelectric coupling, such as improper
ferroelectrics. The magnetic origin of the ferroelectricity in these materials, along
with the dynamic magnetoelectric coupling exemplified by electromagnons, suggests
that the magnetic order and domain switching may instead be controlled by a strong
105
deromotive energies on the order of a few hundred meV (easily achievable at the field
strengths of the THz pulses used in this study), the effective mass will experience a
significant change; as such, as the THz electric field strength increases, we can expect
that the effective mass also increases and thus conductivity decreases. For the highest THz electric field strength used in this study of 373 kV cm
−1 , the ponderomotive
potential calculated from Eq. 5.1 applied to electrons at the bottom of the conduction
band would be 0.7 eV, assuming an effective mass of m
∗
= 0.2 m 0 from the lowest
energy in Fig. 5.9d. From Fig. 5.9d an energy of 0.7 eV would result in an increase in
effective mass to 1.5 m 0 . Therefore the increasing transmission with increasing electric field strength observed in Fig. 5.8 can be explained as the higher electric fields
giving more ponderomotive energy to the charge carriers, which moves them higher
in the conduction band where they have an increased effective mass, leading to lower
mobility and conductivity which is observed as an increase in the transmission at
THz frequencies. A similar effect may occur in the metallic nanotubes, although due
to their Dirac cone-shaped lowest conduction band a more in-depth treatment than
the one used here for the semiconducting nanotubes is required.
The potential saturation observed in Fig. 5.8b may be due to the conducting
behaviour exhibited by the metallic nanotubes, and due to interband scattering and
hot carrier dynamics in both the semiconducting and metallic nanotubes that make up
the thin film. At higher energies the charge carriers may scatter into adjacent conduction bands, which changes their effective mass. Additionally, as the charge carriers
gain energy from the THz pulse they thermalise, which produces a non-equilibrium
carrier distribution; as such there are always some carriers with low energy, which
in turn means low mass and high conductivity, and hence the transmission in the
THz region remains at a finite value less than one, as seen at high field strengths in
Fig. 5.8b.
5.5 Electric Field-Dependent THz Transmission of CuO
Multiferroic materials are of great interest for new applications in data storage and
sensor devices [74, 75], and direct optical switching of magnetic or electric domains
by intense pulses of electromagnetic radiation may allow the dynamics of the domain
reorientation process in such devices to occur on faster timescales [76]. Direct manipulation of the magnetic order, using direct excitation via the magnetic field of electromagnetic pulses, has been demonstrated in a number of magnetic materials [18–20,
77]. However, since many of these excitations occur at THz frequencies, the field
strengths achievable in the currently available experimental techniques are not yet
sufficient to achieve domain switching.
An alternate route to domain control may be found in multiferroic materials,
particularly those which exhibit strong magnetoelectric coupling, such as improper
ferroelectrics. The magnetic origin of the ferroelectricity in these materials, along
with the dynamic magnetoelectric coupling exemplified by electromagnons, suggests
that the magnetic order and domain switching may instead be controlled by a strong