2.3 THz-TDS Experimental Setup
33
beam. This beam is focussed onto a photoconductive emitter, which generates singlecycle pulses of THz radiation. The THz is generated as a diverging beam, and so is
collected and collimated by a f = 50.8 mm gold off-axis parabolic (OAP) mirror,
positioned the focal length away from the PCE. OAP mirrors are used to steer and
focus the THz pulses to avoid the effects of dispersion on the pulses, which would
be introduced by transmissive optics such as lenses. The THz beam is focussed
onto the sample position, and then collected and collimated after the focus, by two
OAP mirrors each with a focal length f = 76.2 mm. This focal length is required
in order to accommodate the size of the optical cryostat the samples will be held
in, to facilitate low-temperature measurements. These two mirrors are oriented such
as to create a “z-shape” spectrometer, as orienting the mirrors in a “u-shape" (i.e.
by rotating the collecting OAP by 180
◦ ) can introduce an ellipticity of around 5 -
10
◦ to the THz beam after collimation. The precise degree of ellipticity induced will
depend on the alignment of the OAP mirrors. A final OAP mirror with a focal length
f = 101.6 mm focuses the THz beam onto the detection crystal. Atmospheric water
vapour has a number of strong absorption features at THz frequencies [22]; to avoid
their influence on the measured THz radiation and increase the signal-to-noise of the
experiment, the full THz beam path is contained within a sealed box which can be
purged with dry nitrogen gas.
The low-power portion of the beam after the beamsplitter is used to detect the
THz pulses via electro-optic sampling (EOS). The gate beam is passed through a
hole in the final OAP mirror in order for it to propagate collinearly with the THz
beam through the detection crystal. The detection crystal used was a [111]-oriented
zinc-blende crystal, in order to perform polarisation-resolved EOS; a 0.5 mm-thick
ZnTe crystal was used in Chap. 3, whilst a 0.3 mm-thick GaP crystal was used in
Chap. 4. A large EOS signal can be obtained using thick detection crystals with
large nonlinear coefficients, while a larger detection bandwidth is generally found
for thinner crystals in which the lowest optical phonon frequency is higher. Thus
there is a balance to be struck between signal size and bandwidth, and the optimal
choice of detection crystal will vary depending on the experimental requirements.
2.3.1 Performing a THz-TDS Experiment
A THz-TDS experiment is performed by varying the relative arrival times of the gate
and THz pulses at the detection crystal, via the mechanical delay stage incorporated
into the THz generation beam. The duration of the gate pulse (80 fs) is much shorter
than that of the THz pulse (typically a few ps), and as such the gate pulse samples
the birefringence created by only a small portion of the THz pulse; thus the temporal
profile of the THz electric field can be recorded, by varying the length of the delay line
and sampling the instantaneous electric field at each step. To increase the signal-tonoise level of the measurement the THz beam is modulated, by either mechanically
chopping the THz generation beam or electrically modulating the PCE bias voltage,
and a lock-in amplifier locked to this modulation frequency is used to measure V .
33
beam. This beam is focussed onto a photoconductive emitter, which generates singlecycle pulses of THz radiation. The THz is generated as a diverging beam, and so is
collected and collimated by a f = 50.8 mm gold off-axis parabolic (OAP) mirror,
positioned the focal length away from the PCE. OAP mirrors are used to steer and
focus the THz pulses to avoid the effects of dispersion on the pulses, which would
be introduced by transmissive optics such as lenses. The THz beam is focussed
onto the sample position, and then collected and collimated after the focus, by two
OAP mirrors each with a focal length f = 76.2 mm. This focal length is required
in order to accommodate the size of the optical cryostat the samples will be held
in, to facilitate low-temperature measurements. These two mirrors are oriented such
as to create a “z-shape” spectrometer, as orienting the mirrors in a “u-shape" (i.e.
by rotating the collecting OAP by 180
◦ ) can introduce an ellipticity of around 5 -
10
◦ to the THz beam after collimation. The precise degree of ellipticity induced will
depend on the alignment of the OAP mirrors. A final OAP mirror with a focal length
f = 101.6 mm focuses the THz beam onto the detection crystal. Atmospheric water
vapour has a number of strong absorption features at THz frequencies [22]; to avoid
their influence on the measured THz radiation and increase the signal-to-noise of the
experiment, the full THz beam path is contained within a sealed box which can be
purged with dry nitrogen gas.
The low-power portion of the beam after the beamsplitter is used to detect the
THz pulses via electro-optic sampling (EOS). The gate beam is passed through a
hole in the final OAP mirror in order for it to propagate collinearly with the THz
beam through the detection crystal. The detection crystal used was a [111]-oriented
zinc-blende crystal, in order to perform polarisation-resolved EOS; a 0.5 mm-thick
ZnTe crystal was used in Chap. 3, whilst a 0.3 mm-thick GaP crystal was used in
Chap. 4. A large EOS signal can be obtained using thick detection crystals with
large nonlinear coefficients, while a larger detection bandwidth is generally found
for thinner crystals in which the lowest optical phonon frequency is higher. Thus
there is a balance to be struck between signal size and bandwidth, and the optimal
choice of detection crystal will vary depending on the experimental requirements.
2.3.1 Performing a THz-TDS Experiment
A THz-TDS experiment is performed by varying the relative arrival times of the gate
and THz pulses at the detection crystal, via the mechanical delay stage incorporated
into the THz generation beam. The duration of the gate pulse (80 fs) is much shorter
than that of the THz pulse (typically a few ps), and as such the gate pulse samples
the birefringence created by only a small portion of the THz pulse; thus the temporal
profile of the THz electric field can be recorded, by varying the length of the delay line
and sampling the instantaneous electric field at each step. To increase the signal-tonoise level of the measurement the THz beam is modulated, by either mechanically
chopping the THz generation beam or electrically modulating the PCE bias voltage,
and a lock-in amplifier locked to this modulation frequency is used to measure V .