Chapter 4
Scalable Interdigitated Photoconductive
Emitters for the Electrical Modulation of
Terahertz Beams with Arbitrary Linear
Polarisation
Photoconductive emitters are a crucial component for terahertz (THz) radiation
sources and detectors, being the most commonly used in commercially available
and custom-built spectroscopy and imaging systems based on oscillator and fibre
lasers [1, 2]. A recent development in THz generation from photoconductive emitters is the creation of complex polarisation states [3–6], and the ability to control the
polarisation state of the emitted radiation [7–9]. These advances in THz polarisation
control may permit otherwise inaccessible excitations, and the anisotropic properties
of materials, to be probed in the THz region.
In this chapter, a multi-element interdigitated photoconductive emitter for broadband THz polarisation rotation, consisting of separate pixels for the emission of
horizontally and vertically polarised THz radiation, is proposed and experimentally
verified. The device is scalable in design, and with its simple method of polarisation
rotation it allows the modulation of the generated THz polarisation at rates significantly faster than those achievable in ellipsometry systems based on mechanically
rotating components. Sect. 4.1 gives a short overview of the generation of electromagnetic radiation by electric dipoles, and of the generation of THz pulses with
various polarisation states via photoconductive emitters with novel geometries. The
design and fabrication of the multi-element device proposed in this thesis is outlined in Sect. 4.2, accompanied by simulations of the radiation pattern the device is
expected to produce. Section 4.3 details the experimental verification of the multipixel photoconductive emitters, which demonstrates the broadband nature of the
device and the control of the polarisation angle of the generated far-field THz radiation by varying the relative bias voltage applied to the horizontally and vertically
emitting pixels. Finally, Sect. 4.4 details an experimental setup for the investigation
of circular dichroic properties of materials, based on a silicon prism converting the
linear polarisation produced by the multi-pixel emitters into left- and right-handed
circularly polarised states, which allows for rapid modulation between the two chiralities. Sections 4.2 and 4.3 of this chapter have been published as C. D. W. Mosley
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
C. D. W. Mosley et al., Enhanced Polarisation Control and Extreme
Electric Fields, Springer Theses,
https://doi.org/10.1007/978-3-030-66902-7_4
69
Scalable Interdigitated Photoconductive
Emitters for the Electrical Modulation of
Terahertz Beams with Arbitrary Linear
Polarisation
Photoconductive emitters are a crucial component for terahertz (THz) radiation
sources and detectors, being the most commonly used in commercially available
and custom-built spectroscopy and imaging systems based on oscillator and fibre
lasers [1, 2]. A recent development in THz generation from photoconductive emitters is the creation of complex polarisation states [3–6], and the ability to control the
polarisation state of the emitted radiation [7–9]. These advances in THz polarisation
control may permit otherwise inaccessible excitations, and the anisotropic properties
of materials, to be probed in the THz region.
In this chapter, a multi-element interdigitated photoconductive emitter for broadband THz polarisation rotation, consisting of separate pixels for the emission of
horizontally and vertically polarised THz radiation, is proposed and experimentally
verified. The device is scalable in design, and with its simple method of polarisation
rotation it allows the modulation of the generated THz polarisation at rates significantly faster than those achievable in ellipsometry systems based on mechanically
rotating components. Sect. 4.1 gives a short overview of the generation of electromagnetic radiation by electric dipoles, and of the generation of THz pulses with
various polarisation states via photoconductive emitters with novel geometries. The
design and fabrication of the multi-element device proposed in this thesis is outlined in Sect. 4.2, accompanied by simulations of the radiation pattern the device is
expected to produce. Section 4.3 details the experimental verification of the multipixel photoconductive emitters, which demonstrates the broadband nature of the
device and the control of the polarisation angle of the generated far-field THz radiation by varying the relative bias voltage applied to the horizontally and vertically
emitting pixels. Finally, Sect. 4.4 details an experimental setup for the investigation
of circular dichroic properties of materials, based on a silicon prism converting the
linear polarisation produced by the multi-pixel emitters into left- and right-handed
circularly polarised states, which allows for rapid modulation between the two chiralities. Sections 4.2 and 4.3 of this chapter have been published as C. D. W. Mosley
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
C. D. W. Mosley et al., Enhanced Polarisation Control and Extreme
Electric Fields, Springer Theses,
https://doi.org/10.1007/978-3-030-66902-7_4
69