1.4 Structure of This Thesis
21
1.4 Structure of This Thesis
The overarching theme of this thesis is the development of new techniques and
components to be employed in terahertz time-domain spectroscopy, and how they
may be applied in the study of anisotropic materials. In particular, the work presented
here focuses on enhancing the experimentalist’s control over the polarisation state of
the generated THz radiation for time-domain spectroscopy experiments, while also
exploring the applicability of using resonant excitations to probe material properties
at a phase transition, and presenting preliminary investigations into the spectrocopy
of various condensed matter systems in the nonlinear regime.
Chapter 2 will describe the technique of terahertz time-domain spectroscopy,
detailing the methods used in the generation and detection of broadband pulses of
THz radiation, and how they may be combined to form the spectrometer constructed
as part of this work. The process of extracting the full complex refractive index of a
sample under study, and quantitative information about the polarisation state of the
THz pulses from the experimental data, will also be described.
Chapter 3 will then describe the development of a rotatable-polarisation terahertz time-domain spectrometer, detailing the adaptations made to the spectrometer
described in Chap. 2 which allow the generation and detection of an arbitrarily rotatable, linear THz polarisation state, based upon the mechanical rotation of an interdigitated photoconductive emitter. The optimisation of the setup will be described,
then this method of polarisation rotation will be compared to a competing technique
- projection of the polarisation state by a wire-grid polariser. Finally, the rotatablepolarisation spectroscopy technique developed in this chapter will be implemented
experimentally: demonstrating how the in-plane polarisation eigenvectors of a birefringent sample can be identified, and how the full complex refractive index along
each eigenvector can be extracted, then demonstrating how the selection rules of an
anisotropic absorption feature can be determined.
Inspired by the work presented in the previous chapter, Chap. 4 will describe the
development, fabrication and experimental verification of a novel photoconductive
emitter design, which allows the arbitrary rotation of a linear THz polarisation state
using electrical control. As such, this method does not rely on any moving parts, and
rotation of the polarisation state may be performed on timescales orders of magnitude
faster than those achievable with methods based on rotating emitters or WGPs. The
characterisation of the devices will be described, before exploring how the emitters
may be utilised to modulate between two circular polarisation states for rapid circular
dichroism spectroscopy.
Lastly, Chap. 5 will detail the construction and operation of a spectrometer utilising pulses of intense THz radiation to explore nonlinear optical properties of materials at THz frequencies. Nonlinear THz transmission in n-type indium antimonide
is used as a test case to explore the performance of the spectrometer. Preliminary
investigations are then carried out on two material systems; nonlinear transmission is
investigated in single-walled carbon nanotube films, and tantalising hints of nonlinear
behaviour of electromagnons are observed in CuO.
21
1.4 Structure of This Thesis
The overarching theme of this thesis is the development of new techniques and
components to be employed in terahertz time-domain spectroscopy, and how they
may be applied in the study of anisotropic materials. In particular, the work presented
here focuses on enhancing the experimentalist’s control over the polarisation state of
the generated THz radiation for time-domain spectroscopy experiments, while also
exploring the applicability of using resonant excitations to probe material properties
at a phase transition, and presenting preliminary investigations into the spectrocopy
of various condensed matter systems in the nonlinear regime.
Chapter 2 will describe the technique of terahertz time-domain spectroscopy,
detailing the methods used in the generation and detection of broadband pulses of
THz radiation, and how they may be combined to form the spectrometer constructed
as part of this work. The process of extracting the full complex refractive index of a
sample under study, and quantitative information about the polarisation state of the
THz pulses from the experimental data, will also be described.
Chapter 3 will then describe the development of a rotatable-polarisation terahertz time-domain spectrometer, detailing the adaptations made to the spectrometer
described in Chap. 2 which allow the generation and detection of an arbitrarily rotatable, linear THz polarisation state, based upon the mechanical rotation of an interdigitated photoconductive emitter. The optimisation of the setup will be described,
then this method of polarisation rotation will be compared to a competing technique
- projection of the polarisation state by a wire-grid polariser. Finally, the rotatablepolarisation spectroscopy technique developed in this chapter will be implemented
experimentally: demonstrating how the in-plane polarisation eigenvectors of a birefringent sample can be identified, and how the full complex refractive index along
each eigenvector can be extracted, then demonstrating how the selection rules of an
anisotropic absorption feature can be determined.
Inspired by the work presented in the previous chapter, Chap. 4 will describe the
development, fabrication and experimental verification of a novel photoconductive
emitter design, which allows the arbitrary rotation of a linear THz polarisation state
using electrical control. As such, this method does not rely on any moving parts, and
rotation of the polarisation state may be performed on timescales orders of magnitude
faster than those achievable with methods based on rotating emitters or WGPs. The
characterisation of the devices will be described, before exploring how the emitters
may be utilised to modulate between two circular polarisation states for rapid circular
dichroism spectroscopy.
Lastly, Chap. 5 will detail the construction and operation of a spectrometer utilising pulses of intense THz radiation to explore nonlinear optical properties of materials at THz frequencies. Nonlinear THz transmission in n-type indium antimonide
is used as a test case to explore the performance of the spectrometer. Preliminary
investigations are then carried out on two material systems; nonlinear transmission is
investigated in single-walled carbon nanotube films, and tantalising hints of nonlinear
behaviour of electromagnons are observed in CuO.