26
2 Terahertz Time-Domain Spectroscopy
at a wide range of energies, producing the characteristic broadband pulses output by
these lasers, with a typical gain bandwidth of 700–900 nm.
2.1.1 Photoconductive Emitters
The schematic layout of a typical simple photoconductive emitter (PCE) is shown in
Fig. 2.1a. The PCE consists of a metal electrode structure on top of a semiconducting
substrate, acting as a fast optically-activated switch [1–3]. The active region of the
PCE is a small gap between the metal electrodes, typically around 10 µm in size.
To generate THz radiation from this device, a femtosecond laser pulse is focused
onto the gap, which photoexcites charge carriers in this region of the semiconductor.
Applying a biasing electric field across the gap causes the electrons and holes to
be accelerated in opposite directions, parallel to the applied electric field, and this
separation of charges generates a transient photocurrent in the gap. The time-varying
photocurrent across the PCE causes the device to act as a transient electric dipole,
which generates far-field radiation that can be described by
E (t) ∝
d J (t)
dt
,
(2.1)
where E(t) is the far-field THz electric field produced and J (t) is the photocurrent
density in the gap, both as a function of time t. The typical behaviour of the charge
carrier density and the resultant photocurrent in the PCE as a function of time is
demonstrated by Fig. 2.1b–e. The femtosecond pulse photoexcites charge carriers in
the semiconductor at a rate proportional to its temporal intensity envelope, which
causes a corresponding rise in the photocurrent on a similar timescale. A short time
later the applied field is screened by the dipole formed, and momentum scattering
reduces the photocurrent and THz emission. The duration of THz pulses produced
by the PCE, hence the bandwidth of the generated radiation, therefore depends on
the pump pulse duration and charge carrier dynamics in the substrate material.
A major limitation of this simple PCE design is a saturation of the THz power with
increasing pump power due to the screening of the bias voltage by the photoexcited
charges [4, 5]. One method used to reduce this effect is to use a larger gap size, on the
order of ∼100 µm; however an order of magnitude increase in gap size also requires
a corresponding increase in the bias voltage, and as such requiring the use of highpower voltage supplies. An alternative method of increasing the active area of the
device is to employ an interdigitated electrode structure [6, 7]. Such a device is shown
schematically in Fig. 2.1f. This design consists of alternating electrodes connected to
the bias and ground terminals of the device, with a semiconductor gap between each
pair of electrodes. Each gap acts as an individual dipole antenna, and the overlap of all
the individual dipole radiation in the far-field is the resultant output of the device. The
THz radiation produced by the bare electrode geometry in Fig. 2.1f would interfere
destructively in the far-field, as the bias field switches direction in adjacent gaps, so a
2 Terahertz Time-Domain Spectroscopy
at a wide range of energies, producing the characteristic broadband pulses output by
these lasers, with a typical gain bandwidth of 700–900 nm.
2.1.1 Photoconductive Emitters
The schematic layout of a typical simple photoconductive emitter (PCE) is shown in
Fig. 2.1a. The PCE consists of a metal electrode structure on top of a semiconducting
substrate, acting as a fast optically-activated switch [1–3]. The active region of the
PCE is a small gap between the metal electrodes, typically around 10 µm in size.
To generate THz radiation from this device, a femtosecond laser pulse is focused
onto the gap, which photoexcites charge carriers in this region of the semiconductor.
Applying a biasing electric field across the gap causes the electrons and holes to
be accelerated in opposite directions, parallel to the applied electric field, and this
separation of charges generates a transient photocurrent in the gap. The time-varying
photocurrent across the PCE causes the device to act as a transient electric dipole,
which generates far-field radiation that can be described by
E (t) ∝
d J (t)
dt
,
(2.1)
where E(t) is the far-field THz electric field produced and J (t) is the photocurrent
density in the gap, both as a function of time t. The typical behaviour of the charge
carrier density and the resultant photocurrent in the PCE as a function of time is
demonstrated by Fig. 2.1b–e. The femtosecond pulse photoexcites charge carriers in
the semiconductor at a rate proportional to its temporal intensity envelope, which
causes a corresponding rise in the photocurrent on a similar timescale. A short time
later the applied field is screened by the dipole formed, and momentum scattering
reduces the photocurrent and THz emission. The duration of THz pulses produced
by the PCE, hence the bandwidth of the generated radiation, therefore depends on
the pump pulse duration and charge carrier dynamics in the substrate material.
A major limitation of this simple PCE design is a saturation of the THz power with
increasing pump power due to the screening of the bias voltage by the photoexcited
charges [4, 5]. One method used to reduce this effect is to use a larger gap size, on the
order of ∼100 µm; however an order of magnitude increase in gap size also requires
a corresponding increase in the bias voltage, and as such requiring the use of highpower voltage supplies. An alternative method of increasing the active area of the
device is to employ an interdigitated electrode structure [6, 7]. Such a device is shown
schematically in Fig. 2.1f. This design consists of alternating electrodes connected to
the bias and ground terminals of the device, with a semiconductor gap between each
pair of electrodes. Each gap acts as an individual dipole antenna, and the overlap of all
the individual dipole radiation in the far-field is the resultant output of the device. The
THz radiation produced by the bare electrode geometry in Fig. 2.1f would interfere
destructively in the far-field, as the bias field switches direction in adjacent gaps, so a