Abstract
This thesis reports advances in terahertz time-domain spectroscopy, relating to the
development of new techniques and components that enhance the experimentalist’s
control over the terahertz polarisation state produced by photoconductive emitters. Two methods of controlling the terahertz polarisation state are reported: The
first method is based upon mechanical rotation of an interdigitated photoconductive emitter, and is implemented in a rotatable-polarisation terahertz time-domain
spectrometer; the calibration of which is demonstrated to produce a highly uniform
polarisation state at all angles. This method is then demonstrated experimentally to
identify the orientations of the normal modes of propagation in the plane of birefringent samples, to extract the full complex refractive index along these directions, and
to investigate the optical selection rules of an absorbing material. The second method
presents a new photoconductive emitter design, based upon separate interdigitated
pixel elements for the generation of the horizontally and vertically polarised components of the terahertz pulses, that permits rotation of the polarisation state solely by
electrical control. The design, fabrication and experimental verification of the device
is reported, demonstrating polarisation control on timescales orders of magnitude
faster than those achievable in mechanical rotation methods.
Additionally, preliminary investigations into the properties of materials exposed
to extreme terahertz optical electric fields are performed. Nonlinear terahertz transmission in single-walled carbon nanotube films and evidence for nonlinear behaviour
of electromagnons in CuO are observed.
Further to these advances, included in the original thesis (but not reproduced in
the Springer version), a method of using the electromagnon response in Cu 1−x Zn x O
alloys as a sensitive probe of a phase transition is also demonstrated. Using this
method, the phase transition is observed to broaden upon the introduction of spindisorder when alloying with non-magnetic zinc ions, and the first-order nature of the
transition is confirmed by the observation of thermal hysteresis.
vii
This thesis reports advances in terahertz time-domain spectroscopy, relating to the
development of new techniques and components that enhance the experimentalist’s
control over the terahertz polarisation state produced by photoconductive emitters. Two methods of controlling the terahertz polarisation state are reported: The
first method is based upon mechanical rotation of an interdigitated photoconductive emitter, and is implemented in a rotatable-polarisation terahertz time-domain
spectrometer; the calibration of which is demonstrated to produce a highly uniform
polarisation state at all angles. This method is then demonstrated experimentally to
identify the orientations of the normal modes of propagation in the plane of birefringent samples, to extract the full complex refractive index along these directions, and
to investigate the optical selection rules of an absorbing material. The second method
presents a new photoconductive emitter design, based upon separate interdigitated
pixel elements for the generation of the horizontally and vertically polarised components of the terahertz pulses, that permits rotation of the polarisation state solely by
electrical control. The design, fabrication and experimental verification of the device
is reported, demonstrating polarisation control on timescales orders of magnitude
faster than those achievable in mechanical rotation methods.
Additionally, preliminary investigations into the properties of materials exposed
to extreme terahertz optical electric fields are performed. Nonlinear terahertz transmission in single-walled carbon nanotube films and evidence for nonlinear behaviour
of electromagnons in CuO are observed.
Further to these advances, included in the original thesis (but not reproduced in
the Springer version), a method of using the electromagnon response in Cu 1−x Zn x O
alloys as a sensitive probe of a phase transition is also demonstrated. Using this
method, the phase transition is observed to broaden upon the introduction of spindisorder when alloying with non-magnetic zinc ions, and the first-order nature of the
transition is confirmed by the observation of thermal hysteresis.
vii