4.2 Multi-Pixel Interdigitated Photoconductive Emitters
77
circular when both horizontal and vertical pixels are excited (panel (c)). In contrast,
in panel (d) the near-field E(x, y, z = 5 µm) at 3 THz and for vertical emission is
shown, where each photoexcited dipole line can be seen. As the radiation evolves
into the far-field, the individual lines are no longer visible, and the radiation pattern
becomes more beam-like for vertical (panel (e)) and combined (panel (f)) emission,
with a beam width below 40 mm at z = 50 mm. However as the divergence of a
higher frequency beam is lower, the beam profiles at 3 THz are more non-uniform
than at 300 GHz, and exhibit some destructive interference. Further calculations may
suggest design improvements to mitigate against these effects, for instance using a
smaller pixel size.
4.3 Experimental Device Performance
The performance of the fabricated device was investigated using terahertz timedomain spectroscopy, by using the multi-pixel photoconductive emitter as the THz
generation source in the spectrometer described in Sect. 2.3. The emitter was photoexcited by optical pulses with a peak wavelength of 800 nm with an average optical
power of 350 mW at the emitter. The photoexcitation beam was found to have a fullwidth at half-maximum (FWHM) spot size of 320 µm by a knife-edge measurement,
resulting in photoexcitation of the entire 300 µm×300 µm active area of the device.
The electrical contacts produced by photolithography were connected to the voltage
source and ground as displayed in Fig. 4.1. The bias voltages applied to the horizontal
and vertical contacts were varied independently from zero to a maximum voltage of
±10 V, with the voltage source modulated at a frequency of 50 kHz.
The generated THz pulses were detected via polarisation-resolved electro-optic
sampling [16, 17] in a 200 µm-thick, [111]-surface normal GaP crystal. Measurements were performed under a dry nitrogen purge, in order to avoid atmospheric absorption of the THz radiation. The polarisation state of the generated
THz pulses was parameterised by their ellipticity, χ, and orientation angle, ψ, which
were obtained from the experimental data by the method described previously in
Sect. 2.4.2. As described previously in Sect. 3.2.2, the orientation angle is the relative angle between the polarisation state and the x-axis of the lab reference frame,
which is defined by the [211] axis of the detection crystal using an identical calibration procedure to the one described in Sect. 3.2.2.
4.3.1 Initial Electrical Biasing Tests
To initially verify that the pixel emitter works as intended, three cases of emitter bias
were tested: biasing the horizontally emitting pixels only, to produce a THz pulse
with a target polarisation angle ψ T = 0
◦ ; applying the same bias voltage to both sets
of pixels, for ψ T = 45
◦ ; and biasing the vertically emitting pixels only, for ψ T = 90
◦ .
77
circular when both horizontal and vertical pixels are excited (panel (c)). In contrast,
in panel (d) the near-field E(x, y, z = 5 µm) at 3 THz and for vertical emission is
shown, where each photoexcited dipole line can be seen. As the radiation evolves
into the far-field, the individual lines are no longer visible, and the radiation pattern
becomes more beam-like for vertical (panel (e)) and combined (panel (f)) emission,
with a beam width below 40 mm at z = 50 mm. However as the divergence of a
higher frequency beam is lower, the beam profiles at 3 THz are more non-uniform
than at 300 GHz, and exhibit some destructive interference. Further calculations may
suggest design improvements to mitigate against these effects, for instance using a
smaller pixel size.
4.3 Experimental Device Performance
The performance of the fabricated device was investigated using terahertz timedomain spectroscopy, by using the multi-pixel photoconductive emitter as the THz
generation source in the spectrometer described in Sect. 2.3. The emitter was photoexcited by optical pulses with a peak wavelength of 800 nm with an average optical
power of 350 mW at the emitter. The photoexcitation beam was found to have a fullwidth at half-maximum (FWHM) spot size of 320 µm by a knife-edge measurement,
resulting in photoexcitation of the entire 300 µm×300 µm active area of the device.
The electrical contacts produced by photolithography were connected to the voltage
source and ground as displayed in Fig. 4.1. The bias voltages applied to the horizontal
and vertical contacts were varied independently from zero to a maximum voltage of
±10 V, with the voltage source modulated at a frequency of 50 kHz.
The generated THz pulses were detected via polarisation-resolved electro-optic
sampling [16, 17] in a 200 µm-thick, [111]-surface normal GaP crystal. Measurements were performed under a dry nitrogen purge, in order to avoid atmospheric absorption of the THz radiation. The polarisation state of the generated
THz pulses was parameterised by their ellipticity, χ, and orientation angle, ψ, which
were obtained from the experimental data by the method described previously in
Sect. 2.4.2. As described previously in Sect. 3.2.2, the orientation angle is the relative angle between the polarisation state and the x-axis of the lab reference frame,
which is defined by the [211] axis of the detection crystal using an identical calibration procedure to the one described in Sect. 3.2.2.
4.3.1 Initial Electrical Biasing Tests
To initially verify that the pixel emitter works as intended, three cases of emitter bias
were tested: biasing the horizontally emitting pixels only, to produce a THz pulse
with a target polarisation angle ψ T = 0
◦ ; applying the same bias voltage to both sets
of pixels, for ψ T = 45
◦ ; and biasing the vertically emitting pixels only, for ψ T = 90
◦ .