90
5 High-Field Terahertz Time-Domain Spectroscopy …
are the field-ionsiation of impurities [1] and excitons [2], ballistic electron transport
[3, 4], intervalley scattering [5–7], and impact ionisation [8, 9] in semiconductors.
The hot-carrier dynamics induced by strong THz pulses have also been studied in
graphene [10, 11] and carbon nanotube films [12].
Intense THz pulses can also be used to exert coherent control over materials,
by directly coupling to a resonant excitation such as a phonon mode. Examples of
this have been demonstrated by driving the soft mode of SrTiO 3 [13, 14]. Resonant
control over electronic degrees of freedom have also been demonstrated, over Cooper
pairs in superconductors by ultrafast supression of superconductivity [15] and by the
excitation of Josephson plasmons [16].
In addition to the electric field of the intense THz pulses, the magnetic field of the
THz pulse can effect the spins in materials by the application of a Zeeman torque
T = m × B, where m is the magnetic dipole moment. The time-varying magnetic
field of the THz pulse deflects the spins from their equilibrium position, and induces
a precession of the spins about their equilibrium position. The high-field dynamics of
magnetic excitations in ferromagnets have been explored in Fe [17] and Co [18] thin
films, the results of which indicate that the magnetic dynamics are phase-locked to the
incident THz pulse [18]. THz control over spin dynamics can also be performed on
antiferromagnets, by making use of resonant excitation of magnons. Kampfrath et al.
used intense THz transients of maximum magnetic field amplitudes 0.13 T to excite
the 1 THz magnon in NiO [19]. By using 2 pulses with a suitable time delay between
them, it was found that the magnon could either be excited further by a factor of
two by an in-phase pulse, or the spin precession could be almost completely stopped
by an out-of-phase pulse. Subsequently coherent spin control over THz frequency
magnons has been demonstrated in YFeO 3 [20, 21].
5.1.1 Generating High-Field Terahertz Radiation
The intense optical pulses produced by amplified femtosecond lasers offer new routes
to efficient generation of high-field THz pulses, both by a scaling-up of established techniques such as photoconductive emission and optical rectification, and
by enabling new THz generation techniques to be developed.
By focusing femtosecond laser pulses with energies greater than ∼10 µJ onto a gas
target the ionisation intensity of the gas molecules (∼10
14 W/cm
2 ) can be exceeded,
creating a plasma. Because the femtosecond pulse is short enough to inertially confine
the ions, an ultrafast charge density separation is created in the plasma due to the
ponderomotive motion of the electrons, causing broadband THz emission from the
plasma [22–25]. Subsequently, a number of other methods of THz generation from
gas plasmas have been devised in an attempt to increase the THz field strength.
Single-cycle THz pulses can be generated by electrically biasing the gaseous target
as it is photoionized by the amplified laser pulses [26, 27] or by using few-cycle
optical pump pulses [28], however the most common method uses a superposition
of the fundamental and second harmonic of the femtosecond laser to produce THz
5 High-Field Terahertz Time-Domain Spectroscopy …
are the field-ionsiation of impurities [1] and excitons [2], ballistic electron transport
[3, 4], intervalley scattering [5–7], and impact ionisation [8, 9] in semiconductors.
The hot-carrier dynamics induced by strong THz pulses have also been studied in
graphene [10, 11] and carbon nanotube films [12].
Intense THz pulses can also be used to exert coherent control over materials,
by directly coupling to a resonant excitation such as a phonon mode. Examples of
this have been demonstrated by driving the soft mode of SrTiO 3 [13, 14]. Resonant
control over electronic degrees of freedom have also been demonstrated, over Cooper
pairs in superconductors by ultrafast supression of superconductivity [15] and by the
excitation of Josephson plasmons [16].
In addition to the electric field of the intense THz pulses, the magnetic field of the
THz pulse can effect the spins in materials by the application of a Zeeman torque
T = m × B, where m is the magnetic dipole moment. The time-varying magnetic
field of the THz pulse deflects the spins from their equilibrium position, and induces
a precession of the spins about their equilibrium position. The high-field dynamics of
magnetic excitations in ferromagnets have been explored in Fe [17] and Co [18] thin
films, the results of which indicate that the magnetic dynamics are phase-locked to the
incident THz pulse [18]. THz control over spin dynamics can also be performed on
antiferromagnets, by making use of resonant excitation of magnons. Kampfrath et al.
used intense THz transients of maximum magnetic field amplitudes 0.13 T to excite
the 1 THz magnon in NiO [19]. By using 2 pulses with a suitable time delay between
them, it was found that the magnon could either be excited further by a factor of
two by an in-phase pulse, or the spin precession could be almost completely stopped
by an out-of-phase pulse. Subsequently coherent spin control over THz frequency
magnons has been demonstrated in YFeO 3 [20, 21].
5.1.1 Generating High-Field Terahertz Radiation
The intense optical pulses produced by amplified femtosecond lasers offer new routes
to efficient generation of high-field THz pulses, both by a scaling-up of established techniques such as photoconductive emission and optical rectification, and
by enabling new THz generation techniques to be developed.
By focusing femtosecond laser pulses with energies greater than ∼10 µJ onto a gas
target the ionisation intensity of the gas molecules (∼10
14 W/cm
2 ) can be exceeded,
creating a plasma. Because the femtosecond pulse is short enough to inertially confine
the ions, an ultrafast charge density separation is created in the plasma due to the
ponderomotive motion of the electrons, causing broadband THz emission from the
plasma [22–25]. Subsequently, a number of other methods of THz generation from
gas plasmas have been devised in an attempt to increase the THz field strength.
Single-cycle THz pulses can be generated by electrically biasing the gaseous target
as it is photoionized by the amplified laser pulses [26, 27] or by using few-cycle
optical pump pulses [28], however the most common method uses a superposition
of the fundamental and second harmonic of the femtosecond laser to produce THz