4.3 Experimental Device Performance
81
exciting the device closer to the anode increases the THz emission strength [18–20].
In the case of negative bias on the multi-pixel emitter, excitation over the entire device
acts as being close to the anode, producing more efficient THz emission than with a
positive bias. The difference in response between the horizontal and vertical emission
may occur due to the polarisation of the pump pulse relative to the geometry of the
pixels; it has been previously shown that orienting the polarisation state of the pump
pulse parallel to the wires of an interdigitated photoconductive emitter increases the
THz emission strength [9]. In the setup used in this work, the polarisation of the pump
was parallel to the wires in the vertically emitting pixels, enhancing their emission
strength relative to the horizontally emitting pixels. The amplitude of the THz pulses
emitted by the device presented here may therefore be made more uniform during
rotation of the polarisation state by: using a half-wave plate to rotate the polarisation
of the pump pulses to 45
◦ , to ensure a similar response from both sets of pixels; and
by using only negative bias voltages and swapping the biased and grounded contacts
to change the polarity of the generated THz pulses.
The ellipticities of the generated THz radiation at each orientation angle are
reported in Fig. 4.5d. The red points represent the ellipticity averaged over the
0.3−5.0 THz bandwidth of the pulses, whilst the blue points show the ellipticity
at 1 THz. At 1 THz, the ellipticity remains small at all angles, varying less than 4
◦
over the full 360
◦ rotation. When the higher frequency components are taken into
account, the variation in ellipticity becomes larger, particularly when the polarisation
state is between a purely horizontal or vertical state. One reason for the increase in
ellipticity at these angles may be due to the different response of the pixels due to their
geometry relative to the polarisation of the pump pulse. The grating created by the
interdigitated wires of the emitter has a different reflectivity for an s- or p-polarised
pump beam, and hence different pump beam powers are coupled to the photoconductor, giving a different carrier density in each set of pixels. Such an effect may be
observed in Fig. 4.4d, where the horizontal and vertical components of the electric
field in the ψ T = 45
◦ case have slightly different spectral shapes, as a result of small
differences in the time-domain waveforms for the two components. Another reason
for the larger variation at higher frequencies may be due to the higher frequencies
having a smaller beam divergence, and as such creating a slightly asymmetrical beam
profile in the far-field if the high frequency components generated from each pixel
do not overlap fully; an example of this is demonstrated by the simulations of the
expected radiation pattern at 3 THz in Fig. 4.3e and f. One possible solution would
be to use a smaller pixel size, which would position the pixels closer together and
increase the overlap of the high frequency components in the far-field.
81
exciting the device closer to the anode increases the THz emission strength [18–20].
In the case of negative bias on the multi-pixel emitter, excitation over the entire device
acts as being close to the anode, producing more efficient THz emission than with a
positive bias. The difference in response between the horizontal and vertical emission
may occur due to the polarisation of the pump pulse relative to the geometry of the
pixels; it has been previously shown that orienting the polarisation state of the pump
pulse parallel to the wires of an interdigitated photoconductive emitter increases the
THz emission strength [9]. In the setup used in this work, the polarisation of the pump
was parallel to the wires in the vertically emitting pixels, enhancing their emission
strength relative to the horizontally emitting pixels. The amplitude of the THz pulses
emitted by the device presented here may therefore be made more uniform during
rotation of the polarisation state by: using a half-wave plate to rotate the polarisation
of the pump pulses to 45
◦ , to ensure a similar response from both sets of pixels; and
by using only negative bias voltages and swapping the biased and grounded contacts
to change the polarity of the generated THz pulses.
The ellipticities of the generated THz radiation at each orientation angle are
reported in Fig. 4.5d. The red points represent the ellipticity averaged over the
0.3−5.0 THz bandwidth of the pulses, whilst the blue points show the ellipticity
at 1 THz. At 1 THz, the ellipticity remains small at all angles, varying less than 4
◦
over the full 360
◦ rotation. When the higher frequency components are taken into
account, the variation in ellipticity becomes larger, particularly when the polarisation
state is between a purely horizontal or vertical state. One reason for the increase in
ellipticity at these angles may be due to the different response of the pixels due to their
geometry relative to the polarisation of the pump pulse. The grating created by the
interdigitated wires of the emitter has a different reflectivity for an s- or p-polarised
pump beam, and hence different pump beam powers are coupled to the photoconductor, giving a different carrier density in each set of pixels. Such an effect may be
observed in Fig. 4.4d, where the horizontal and vertical components of the electric
field in the ψ T = 45
◦ case have slightly different spectral shapes, as a result of small
differences in the time-domain waveforms for the two components. Another reason
for the larger variation at higher frequencies may be due to the higher frequencies
having a smaller beam divergence, and as such creating a slightly asymmetrical beam
profile in the far-field if the high frequency components generated from each pixel
do not overlap fully; an example of this is demonstrated by the simulations of the
expected radiation pattern at 3 THz in Fig. 4.3e and f. One possible solution would
be to use a smaller pixel size, which would position the pixels closer together and
increase the overlap of the high frequency components in the far-field.