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4 Scalable Interdigitated Photoconductive Emitters for the Electrical …
of overlapping the radiation generated by dipoles with different orientations to create
a coherent beam with a complex polarisation state in the far-field.
The ability to variably control the THz polarisation state has been demonstrated
via the rotation of PCEs; previously in the literature by rotating a wide-gap photoconductive emitter to a few fixed angles [7], whilst a rotatable-polarisation THz-TDS
system making use of the rotation of an interdigitated PCE to arbitrary angles was
reported in Chap. 3 of this thesis. Electrical modulation between two orthogonal
polarisation states has also been achieved [8], using a four-contact large area emitter.
When the entire central area of the device is illuminated by femtosecond optical
pulses, the THz polarisation state produced by the device can be altered by changing
which of the contacts are electrically biased and which are grounded. The electrical
nature of this polarisation switching allows rapid modulation of the THz polarisation
state. However, the polarisation state produced by this emitter demonstrated significant deviations from a pure linear state, with an ellipticity of up to 4.5
◦ at 1 THz,
which was attributed to asymmetries in the pump beam profile and in the bias fields
generated in the device. The authors of reference [8] also utilise this four-contact
PCE to produce and modulate between different-handed circularly polarised THz
pulses, which will be discussed in futher detail in Sect. 4.4.
4.2 Multi-Pixel Interdigitated Photoconductive Emitters
In this section I will report the design and fabrication of interdigitated photoconductive emitters consisting of separate pixels for the generation of horizontally and
vertically polarised THz radiation, making use of a combination of the electrical
modulation and overlapping dipole concepts discussed in the previous section. This
photoconductive emitter design has the benefit of allowing the direct control of the
polarisation angle of the emitted THz beam, without requiring any additional or
mechanically moving components, via the applied bias voltage on the pixels.
4.2.1 Emitter Concept and Design
In Chap. 3 of this thesis a method of rotating a linear polarisation state to arbitrary
angles has been demonstrated, using the mechanical rotation of an interdigitated PCE.
The results presented in Sect. 3.2.4 demonstrate that an interdigitated PCE produces
highly linearly polarised THz pulses with an intrinsic ellipticity of less than 1
◦ ; the
direction of the bias field applied between the electrodes defines the direction of
acceleration of the photoexcited charge carriers, and therefore the polarisation state
of the generated THz radiation. Hence the orientation of the interdigitated PCE itself
can be used to define the polarisation of the generated THz radiation.
The results presented in Sect. 3.2.3 highlight improvements that could be made
relative to this method of polarisation rotation: it is based upon mechanically rotating
4 Scalable Interdigitated Photoconductive Emitters for the Electrical …
of overlapping the radiation generated by dipoles with different orientations to create
a coherent beam with a complex polarisation state in the far-field.
The ability to variably control the THz polarisation state has been demonstrated
via the rotation of PCEs; previously in the literature by rotating a wide-gap photoconductive emitter to a few fixed angles [7], whilst a rotatable-polarisation THz-TDS
system making use of the rotation of an interdigitated PCE to arbitrary angles was
reported in Chap. 3 of this thesis. Electrical modulation between two orthogonal
polarisation states has also been achieved [8], using a four-contact large area emitter.
When the entire central area of the device is illuminated by femtosecond optical
pulses, the THz polarisation state produced by the device can be altered by changing
which of the contacts are electrically biased and which are grounded. The electrical
nature of this polarisation switching allows rapid modulation of the THz polarisation
state. However, the polarisation state produced by this emitter demonstrated significant deviations from a pure linear state, with an ellipticity of up to 4.5
◦ at 1 THz,
which was attributed to asymmetries in the pump beam profile and in the bias fields
generated in the device. The authors of reference [8] also utilise this four-contact
PCE to produce and modulate between different-handed circularly polarised THz
pulses, which will be discussed in futher detail in Sect. 4.4.
4.2 Multi-Pixel Interdigitated Photoconductive Emitters
In this section I will report the design and fabrication of interdigitated photoconductive emitters consisting of separate pixels for the generation of horizontally and
vertically polarised THz radiation, making use of a combination of the electrical
modulation and overlapping dipole concepts discussed in the previous section. This
photoconductive emitter design has the benefit of allowing the direct control of the
polarisation angle of the emitted THz beam, without requiring any additional or
mechanically moving components, via the applied bias voltage on the pixels.
4.2.1 Emitter Concept and Design
In Chap. 3 of this thesis a method of rotating a linear polarisation state to arbitrary
angles has been demonstrated, using the mechanical rotation of an interdigitated PCE.
The results presented in Sect. 3.2.4 demonstrate that an interdigitated PCE produces
highly linearly polarised THz pulses with an intrinsic ellipticity of less than 1
◦ ; the
direction of the bias field applied between the electrodes defines the direction of
acceleration of the photoexcited charge carriers, and therefore the polarisation state
of the generated THz radiation. Hence the orientation of the interdigitated PCE itself
can be used to define the polarisation of the generated THz radiation.
The results presented in Sect. 3.2.3 highlight improvements that could be made
relative to this method of polarisation rotation: it is based upon mechanically rotating