Chapter 6
Conclusions
This thesis has reported advances in terahertz spectroscopy relating to new techniques and components that enhance the control of THz polarisation produced by
photoconductive emitters, and the use of electromagnons to probe material properties
at a phase transition. In addition, preliminary investigations have been opened into
the nonlinear behaviour of SWCNTs and electromagnons in CuO under exposure to
extreme electric fields. This chapter will summarise the key results and findings presented in this thesis, and Sect. 6.1 will put these results into context with the current
state-of-the-art in THz-TDS technology and suggest future avenues of research.
Chapter 3 presented a method of rotatable-polarisation-THz-TDS, in which an
arbitrarily rotatable THz polarisation state was produced by the mechanical rotation
of an interdigitated photoconductive emitter. The optimisation of the system was
reported, resulting in a highly uniform, broadband electric field over a 180
◦ rotation of the polarisation state, with only a variation of <7% in amplitude and <1
◦
in ellipticity. This technique was implemented experimentally, investigating both
birefringent and absorbing anisotropic media. The orientations of the polarisation
eigenvectors were identified in the uniaxial materials ZnO and LaAlO 3 , and the full
complex refractive index along each eigenvector was extracted. The optical selection
rule of the electromagnon absorption in CuO was also precisely mapped using this
technique.
The layout, fabrication and experimental verification of a new design of photoconductive emitter, based on separate interdigitated pixel elements for the emission
of horizontally and vertically polarised THz radiation, is presented in Chap. 4. Both
theoretical and experimental results demonstrate that the radiation produced by the
device interacts constructively in the far-field; the THz pulses generated by the device
have been shown to be linear, and the orientation angle of the THz polarisation state
has been demonstrated to be arbitrarily rotatable simply by changing the bias voltage
applied to each set of pixels. The purely electrical nature of the polarisation rotation
© 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_6
111
Conclusions
This thesis has reported advances in terahertz spectroscopy relating to new techniques and components that enhance the control of THz polarisation produced by
photoconductive emitters, and the use of electromagnons to probe material properties
at a phase transition. In addition, preliminary investigations have been opened into
the nonlinear behaviour of SWCNTs and electromagnons in CuO under exposure to
extreme electric fields. This chapter will summarise the key results and findings presented in this thesis, and Sect. 6.1 will put these results into context with the current
state-of-the-art in THz-TDS technology and suggest future avenues of research.
Chapter 3 presented a method of rotatable-polarisation-THz-TDS, in which an
arbitrarily rotatable THz polarisation state was produced by the mechanical rotation
of an interdigitated photoconductive emitter. The optimisation of the system was
reported, resulting in a highly uniform, broadband electric field over a 180
◦ rotation of the polarisation state, with only a variation of <7% in amplitude and <1
◦
in ellipticity. This technique was implemented experimentally, investigating both
birefringent and absorbing anisotropic media. The orientations of the polarisation
eigenvectors were identified in the uniaxial materials ZnO and LaAlO 3 , and the full
complex refractive index along each eigenvector was extracted. The optical selection
rule of the electromagnon absorption in CuO was also precisely mapped using this
technique.
The layout, fabrication and experimental verification of a new design of photoconductive emitter, based on separate interdigitated pixel elements for the emission
of horizontally and vertically polarised THz radiation, is presented in Chap. 4. Both
theoretical and experimental results demonstrate that the radiation produced by the
device interacts constructively in the far-field; the THz pulses generated by the device
have been shown to be linear, and the orientation angle of the THz polarisation state
has been demonstrated to be arbitrarily rotatable simply by changing the bias voltage
applied to each set of pixels. The purely electrical nature of the polarisation rotation
© 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_6
111