2.1 Generation of Broadband Terahertz Radiation
29
An important consideration when generating THz radiation by OR is the phase
matching condition [13], which can be written as
k = k
ω opt + ω T Hz
− k
ω opt
− k (ω T Hz ) = 0,
(2.3)
where k is the wavenumber and ω opt and ω T Hz are the optical pump and THz frequencies respectively [14]. Taking into account optical dispersion in the material,
the phase matching condition becomes
k (ω T Hz )
ω T Hz
≈
∂k
∂ω
opt
.
(2.4)
Equation 2.4 states that the best pump-to-THz conversion efficiency is achieved when
the group velocity of the optical pump pulse, (∂ω/∂k) opt = v g,opt , and the phase
velocity of the THz pulse, ω T Hz /k = v p,T Hz , are the same. This allows the generated
THz radiation to interfere constructively as the pump pulse moves through the crystal.
In order to achieve phase matching the nonlinear crystal must be chosen such that its
properties match well with the pump laser pulse, or vice versa. Important properties
to consider are the nonlinear coefficient of the crystal, the THz refractive index, the
pump frequency group index and the absorption coefficient of the crystal at THz
frequencies. The most widely used materials for THz generation via OR are either
ZnTe or GaP crystals, as phase matching is approximately satisfied for the 800 nm
peak output wavelength of Ti:Sapphire lasers [14, 15].
2.2 Electro-optic Sampling
Detection of THz radiation in all experiments reported in this thesis was performed
by electro-optic sampling (EOS). EOS makes use of the Pockels effect [16], whereby
an electric field induces birefringence in an electro-optic detection crystal, which is
typically ZnTe or GaP. A standard EOS detection scheme is outlined in Fig. 2.2,
in which a THz pulse propagates through the detection crystal collinearly with a
femtosecond IR pulse. The birefringence created in the detection crystal by the THz
electric field induces a change in the polarisation state of the IR pulse. This change
in polarisation state can be converted into an intensity difference I between the
orthogonal components of the IR pulse via the use of a quarter-wave plate (QWP)
and a Wollaston prism (WP), and I is detected via the change in voltage V across
a pair of balanced photodiodes. All EOS measurements presented in this thesis were
performed using [111] surface normal zinc-blende detection crystals, in which V
is related to the THz electric field E THz by [17]
V
V tot
=
√
24ωLn
3 r 41 E THz
3c
,
(2.5)
29
An important consideration when generating THz radiation by OR is the phase
matching condition [13], which can be written as
k = k
ω opt + ω T Hz
− k
ω opt
− k (ω T Hz ) = 0,
(2.3)
where k is the wavenumber and ω opt and ω T Hz are the optical pump and THz frequencies respectively [14]. Taking into account optical dispersion in the material,
the phase matching condition becomes
k (ω T Hz )
ω T Hz
≈
∂k
∂ω
opt
.
(2.4)
Equation 2.4 states that the best pump-to-THz conversion efficiency is achieved when
the group velocity of the optical pump pulse, (∂ω/∂k) opt = v g,opt , and the phase
velocity of the THz pulse, ω T Hz /k = v p,T Hz , are the same. This allows the generated
THz radiation to interfere constructively as the pump pulse moves through the crystal.
In order to achieve phase matching the nonlinear crystal must be chosen such that its
properties match well with the pump laser pulse, or vice versa. Important properties
to consider are the nonlinear coefficient of the crystal, the THz refractive index, the
pump frequency group index and the absorption coefficient of the crystal at THz
frequencies. The most widely used materials for THz generation via OR are either
ZnTe or GaP crystals, as phase matching is approximately satisfied for the 800 nm
peak output wavelength of Ti:Sapphire lasers [14, 15].
2.2 Electro-optic Sampling
Detection of THz radiation in all experiments reported in this thesis was performed
by electro-optic sampling (EOS). EOS makes use of the Pockels effect [16], whereby
an electric field induces birefringence in an electro-optic detection crystal, which is
typically ZnTe or GaP. A standard EOS detection scheme is outlined in Fig. 2.2,
in which a THz pulse propagates through the detection crystal collinearly with a
femtosecond IR pulse. The birefringence created in the detection crystal by the THz
electric field induces a change in the polarisation state of the IR pulse. This change
in polarisation state can be converted into an intensity difference I between the
orthogonal components of the IR pulse via the use of a quarter-wave plate (QWP)
and a Wollaston prism (WP), and I is detected via the change in voltage V across
a pair of balanced photodiodes. All EOS measurements presented in this thesis were
performed using [111] surface normal zinc-blende detection crystals, in which V
is related to the THz electric field E THz by [17]
V
V tot
=
√
24ωLn
3 r 41 E THz
3c
,
(2.5)