112
6 Conclusions
in this device permits control of the THz polarisation state on timescales orders of
magnitude faster than those achievable by mechanical rotation methods. An experimental technique permitting rapid modulation between two circular polarisation
states was also reported, utlising these pixel emitters in combination with a silicon
prism.
Chapter 5 described the design and operation of a high-field THz-TDS system.
The spectrometer performance was characterised by the use of nonlinear THz transmission in InSb as a test case; similar behaviour to that reported previously in the
literature was observed, confirming the effectiveness of the spectrometer. Preliminary investigations into the behaviour of SWCNTs and electromagnons in CuO in
the high-field regime were reported, suggesting promising areas of future work.
In the original version of this thesis, an additional chapter (see Appendix A)
demonstrated a method of using the dynamic magnetoelectric response at THz frequencies, in the form of an electromagnon, to probe a phase transition between
two magnetically ordered phases. The oscillator strength of the electromagnon in
Cu 1−x Zn x O alloys was used to track the relative amounts of the multiferroic AF2
phase and antiferromagnetic, paraelectric AF1 phase present in the sample. Using
this method, the phase transition was observed to broaden under spin-disorder introduced by the non-magnetic Zn ions, and the first-order nature of the transition was
confirmed by the observation of thermal hysteresis upon heating and cooling through
the transition.
6.1 Outlook and Future Work
The results presented in Chap. 5 represent the very early stages of the investigations
into the nonlinear behaviour of these material systems. Firstly, the results must be
proven to be reproducible, and then may be extended to higher field strengths. The
spectrometer described in Sect. 5.2 may be adapted to include an additional delay line
and low-field THz generation optics, to create a THz-pump-THz-probe system. This
would permit the dynamics of these nonlinear processes to be studied, which may
help to elucidate their nature. The work presented in the chapter not present in this
version of this thesis (see Appendix A) may have an immediate impact by providing
a new way to study the nature of magnetic phase transitions in multiferroics. In
the longer term the increased understanding of multiferroics yielded by ultrafast
spectroscopic methods, including THz-TDS, may help develop new magnetoelectric
and multiferroic materials for applications such as spintronics.
There is much scope to investigate the optimisation of the devices presented
in Chap. 4, and the implementation of the design in THz detectors. Future work,
both theoretical and experimental, may explore the effects of the number, size and
arragement of pixels in the device on the resultant far-field radiation produced; this
may lead to improvements in the polarisation purity and a more uniform electric field
amplitude from these devices.
The results presented in Chaps. 3 and 4 may have immediate impacts on the study
of anisotropic media at THz frequencies. Photoconductive emitters and detectors are
6 Conclusions
in this device permits control of the THz polarisation state on timescales orders of
magnitude faster than those achievable by mechanical rotation methods. An experimental technique permitting rapid modulation between two circular polarisation
states was also reported, utlising these pixel emitters in combination with a silicon
prism.
Chapter 5 described the design and operation of a high-field THz-TDS system.
The spectrometer performance was characterised by the use of nonlinear THz transmission in InSb as a test case; similar behaviour to that reported previously in the
literature was observed, confirming the effectiveness of the spectrometer. Preliminary investigations into the behaviour of SWCNTs and electromagnons in CuO in
the high-field regime were reported, suggesting promising areas of future work.
In the original version of this thesis, an additional chapter (see Appendix A)
demonstrated a method of using the dynamic magnetoelectric response at THz frequencies, in the form of an electromagnon, to probe a phase transition between
two magnetically ordered phases. The oscillator strength of the electromagnon in
Cu 1−x Zn x O alloys was used to track the relative amounts of the multiferroic AF2
phase and antiferromagnetic, paraelectric AF1 phase present in the sample. Using
this method, the phase transition was observed to broaden under spin-disorder introduced by the non-magnetic Zn ions, and the first-order nature of the transition was
confirmed by the observation of thermal hysteresis upon heating and cooling through
the transition.
6.1 Outlook and Future Work
The results presented in Chap. 5 represent the very early stages of the investigations
into the nonlinear behaviour of these material systems. Firstly, the results must be
proven to be reproducible, and then may be extended to higher field strengths. The
spectrometer described in Sect. 5.2 may be adapted to include an additional delay line
and low-field THz generation optics, to create a THz-pump-THz-probe system. This
would permit the dynamics of these nonlinear processes to be studied, which may
help to elucidate their nature. The work presented in the chapter not present in this
version of this thesis (see Appendix A) may have an immediate impact by providing
a new way to study the nature of magnetic phase transitions in multiferroics. In
the longer term the increased understanding of multiferroics yielded by ultrafast
spectroscopic methods, including THz-TDS, may help develop new magnetoelectric
and multiferroic materials for applications such as spintronics.
There is much scope to investigate the optimisation of the devices presented
in Chap. 4, and the implementation of the design in THz detectors. Future work,
both theoretical and experimental, may explore the effects of the number, size and
arragement of pixels in the device on the resultant far-field radiation produced; this
may lead to improvements in the polarisation purity and a more uniform electric field
amplitude from these devices.
The results presented in Chaps. 3 and 4 may have immediate impacts on the study
of anisotropic media at THz frequencies. Photoconductive emitters and detectors are