4.1 Photoconductive Emitter Geometry and Terahertz Polarisation State
71
As such, the electric field generated by a small dipole oscillates only in the polar
direction in the far-field regime. A similar expression for the far-field magnetic field
H ff can be obtained, with its only component in the azimuthal φ direction; this
demonstrates that a small dipole radiates transverse electromagnetic (TEM) waves,
as the electric and magnetic fields are orthogonal to each other and the direction of
propagation.
An important concept in the physics of antennas is that of the dipole array. A single
dipole generates radiation over a wide range of angles. By positioning a number of
dipoles in close proximity, the radiation they produce may be designed to interfere
constructively in the far-field; this increases the efficiency and power output of the
array compared with a lone dipole, as well as improving the directionality of the
generated radiation. Some PCE designs, such as arrays of individual dipole antennas
[11, 12] and interdigitated photoconductive emitters [13, 14], utilise this emission
geometry to improve the generation efficiency of linearly polarised THz radiation.
4.1.2 Controlling the Terahertz Polarisation State with the
Emitter Geometry
The generation of complex polarisation states from PCEs can be performed by utilising emitter geometries that create a pattern of individual dipoles with different orientations, which then interact in the far-field to create the desired polarisation state.
Radially polarised THz beams have been generated by circularly-shaped large-gap
emitters [5, 6], whilst both radially- and azimuthally-polarised beams have been
produced using interdigitated PCEs with suitable electrode geometries [3, 4].
A design for PCEs that emit radially-polarised THz pulses has been demonstrated
by Winnerl et al., in Reference [3]. The electrode layout in these devices cause
the photoexcited charge carriers to be accelerated radially outwards or inwards,
depending on the polarity of the the bias voltage applied; this causes the orientation
of the THz electric field generated by a small segment of the device to vary depending
on the position of that segment around the device circumference. After interaction
in the far-field, the THz radiated by the device produces a radially symmetric pulse.
The time-domain waveforms observed by the authors of reference [3] demonstrate
the radial symmetry of the emission from their device: as a detector sensitive only
to horizontally polarised THz radiation is scanned in the x-direction (orthogonally
to the direction of propagation), the THz time-domain waveform flips in polarity
about the beam axis, while showing a similar decrease in strength as the detector
is moved in either the positive or negative x-direction. An analogous design for an
interdigitated PCE producing azimuthally polarised THz pulses is also presented by
the authors of reference [3], with the acceleration of carriers and hence the THz
polarisation of a small section of the device this time oriented azimuthally, with a
strength depending on the radial position of that particular section. These two device
designs, amongst the others reported in the literature, demonstrate the applicability
71
As such, the electric field generated by a small dipole oscillates only in the polar
direction in the far-field regime. A similar expression for the far-field magnetic field
H ff can be obtained, with its only component in the azimuthal φ direction; this
demonstrates that a small dipole radiates transverse electromagnetic (TEM) waves,
as the electric and magnetic fields are orthogonal to each other and the direction of
propagation.
An important concept in the physics of antennas is that of the dipole array. A single
dipole generates radiation over a wide range of angles. By positioning a number of
dipoles in close proximity, the radiation they produce may be designed to interfere
constructively in the far-field; this increases the efficiency and power output of the
array compared with a lone dipole, as well as improving the directionality of the
generated radiation. Some PCE designs, such as arrays of individual dipole antennas
[11, 12] and interdigitated photoconductive emitters [13, 14], utilise this emission
geometry to improve the generation efficiency of linearly polarised THz radiation.
4.1.2 Controlling the Terahertz Polarisation State with the
Emitter Geometry
The generation of complex polarisation states from PCEs can be performed by utilising emitter geometries that create a pattern of individual dipoles with different orientations, which then interact in the far-field to create the desired polarisation state.
Radially polarised THz beams have been generated by circularly-shaped large-gap
emitters [5, 6], whilst both radially- and azimuthally-polarised beams have been
produced using interdigitated PCEs with suitable electrode geometries [3, 4].
A design for PCEs that emit radially-polarised THz pulses has been demonstrated
by Winnerl et al., in Reference [3]. The electrode layout in these devices cause
the photoexcited charge carriers to be accelerated radially outwards or inwards,
depending on the polarity of the the bias voltage applied; this causes the orientation
of the THz electric field generated by a small segment of the device to vary depending
on the position of that segment around the device circumference. After interaction
in the far-field, the THz radiated by the device produces a radially symmetric pulse.
The time-domain waveforms observed by the authors of reference [3] demonstrate
the radial symmetry of the emission from their device: as a detector sensitive only
to horizontally polarised THz radiation is scanned in the x-direction (orthogonally
to the direction of propagation), the THz time-domain waveform flips in polarity
about the beam axis, while showing a similar decrease in strength as the detector
is moved in either the positive or negative x-direction. An analogous design for an
interdigitated PCE producing azimuthally polarised THz pulses is also presented by
the authors of reference [3], with the acceleration of carriers and hence the THz
polarisation of a small section of the device this time oriented azimuthally, with a
strength depending on the radial position of that particular section. These two device
designs, amongst the others reported in the literature, demonstrate the applicability