Supervisor’s Foreword
Connor’s Ph.D. research was focused upon understanding how we can use terahertz
spectroscopy to study the spin-wave excitations of helical magnetic structures in
CuO, an improper multiferroic that exhibits ferroelectricity as a result of its magnetic
order. Early in his Ph.D. project, he identified limitations in the current technique
of terahertz time-domain spectroscopy as applied to anisotropic materials (including
the improper multiferroics under study). The principle challenge was combining
precise measurements of the anisotropic THz absorbance along different crystalline
directions, with the cryogenic cooling required to access the helical magnetic states.
As such, Connor embarked on an ambitious programme to improve the accuracy
and precision of THz polarisation measurements, building a polarisation-resolved
THz spectrometer based on electro-optic sampling. He used this to test different
concepts for the rotation of the polarisation state of linearly polarised THz radiation, using either mechanical rotation (as described in Chap. 3) or electrical control
(Chap. 4). He used his new methods to return to the study of improper multiferroics,
making precise measurements of the hysteretic nature of a magnetic-to-magnetic
phase transition for the first time (Appendix A). He also established a high-field
THz spectrometer and demonstrated its use in two proof-of-concept experiments on
conductive nanomaterials—carbon nanotubes and epitaxial InSb—and further on the
multiferroic CuO.
The work presented in this thesis may have immediate impacts on the study of
anisotropic media at THz frequencies, with photoconductive emitters and detectors
being the most commonly used components for commercially available terahertz
spectroscopy and imaging systems, and 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
terahertz time-domain spectroscopy, may help develop new magnetoelectric and
multiferroic materials for applications such as spintronics.
Coventry, England
August 2020
Dr. James Lloyd-Hughes
v
Connor’s Ph.D. research was focused upon understanding how we can use terahertz
spectroscopy to study the spin-wave excitations of helical magnetic structures in
CuO, an improper multiferroic that exhibits ferroelectricity as a result of its magnetic
order. Early in his Ph.D. project, he identified limitations in the current technique
of terahertz time-domain spectroscopy as applied to anisotropic materials (including
the improper multiferroics under study). The principle challenge was combining
precise measurements of the anisotropic THz absorbance along different crystalline
directions, with the cryogenic cooling required to access the helical magnetic states.
As such, Connor embarked on an ambitious programme to improve the accuracy
and precision of THz polarisation measurements, building a polarisation-resolved
THz spectrometer based on electro-optic sampling. He used this to test different
concepts for the rotation of the polarisation state of linearly polarised THz radiation, using either mechanical rotation (as described in Chap. 3) or electrical control
(Chap. 4). He used his new methods to return to the study of improper multiferroics,
making precise measurements of the hysteretic nature of a magnetic-to-magnetic
phase transition for the first time (Appendix A). He also established a high-field
THz spectrometer and demonstrated its use in two proof-of-concept experiments on
conductive nanomaterials—carbon nanotubes and epitaxial InSb—and further on the
multiferroic CuO.
The work presented in this thesis may have immediate impacts on the study of
anisotropic media at THz frequencies, with photoconductive emitters and detectors
being the most commonly used components for commercially available terahertz
spectroscopy and imaging systems, and 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
terahertz time-domain spectroscopy, may help develop new magnetoelectric and
multiferroic materials for applications such as spintronics.
Coventry, England
August 2020
Dr. James Lloyd-Hughes
v