2
1 Introduction
Despite the advances made in THz technology thus far, traditionally many tabletop sources of THz radiation have remained weaker than those attainable in other
regions of the electromagnetic spectrum. More recently, new THz generation techniques coupled with amplified femtosecond lasers have begun to bridge the gap to
high-power, tabletop sources of THz radiation, capable of producing pulses with
electric field strengths exceeding 1 MVcm
−1 and a duration of only a few hundred
femtoseconds [20]. This extreme THz radiation cannot only provide higher-power
sources for applications such as chemical sensing [21], but can also be utilised in the
coherent control of collective degrees of freedom in the THz range and nonlinear
spectroscopy [22, 23].
The focus of this thesis is on the development of techniques for the spectroscopy
of anisotropic materials in the THz region, particularly in terms of enhancing the
control of the polarisation state produced by photoconductive emitters, which are
perhaps the most common source of THz radiation in both commercially available
and custom-made THz spectroscopy and imaging systems. The rest of this chapter
will discuss the interaction between light and matter in the linear regime in anisotropic
materials, in order to provide the necessary background required to understand the
experimental results in future chapters of this thesis. Sections 1.1 and 1.2 will focus
on the effect that an anisotropic material structure has on the polarisation state of light
propagating through it, and how the polarisation state can be described. Section 1.3
will then discuss the inverse scenario, regarding the effects that the electromagnetic
field of light has on anisotropic media, with a particular focus on multiferroics.
Section 1.4 will then describe the structure of the rest of this thesis.
1.1 Crystal Optics
As a guide to the reader, the following section will provide a theoretical description
of the interaction of the electric field of light with an anisotropic dielectric medium.
This can then be used to describe the propagation of electromagnetic waves through
such an anisotropic medium, and its resulting effects on the polarisation state of the
probing light.
1.1.1 The Dielectric Tensor
In an optically isotropic, linear medium, such as a gas or an amorphous solid such as
glass, the components of the electric displacement field D are linearly proportional
to the corresponding components of the optical electric field E,
⎡
⎣
D x
D y
D z
⎤
⎦ =
⎡
⎣
E x
E y
E z
⎤
⎦ ,
(1.1)
1 Introduction
Despite the advances made in THz technology thus far, traditionally many tabletop sources of THz radiation have remained weaker than those attainable in other
regions of the electromagnetic spectrum. More recently, new THz generation techniques coupled with amplified femtosecond lasers have begun to bridge the gap to
high-power, tabletop sources of THz radiation, capable of producing pulses with
electric field strengths exceeding 1 MVcm
−1 and a duration of only a few hundred
femtoseconds [20]. This extreme THz radiation cannot only provide higher-power
sources for applications such as chemical sensing [21], but can also be utilised in the
coherent control of collective degrees of freedom in the THz range and nonlinear
spectroscopy [22, 23].
The focus of this thesis is on the development of techniques for the spectroscopy
of anisotropic materials in the THz region, particularly in terms of enhancing the
control of the polarisation state produced by photoconductive emitters, which are
perhaps the most common source of THz radiation in both commercially available
and custom-made THz spectroscopy and imaging systems. The rest of this chapter
will discuss the interaction between light and matter in the linear regime in anisotropic
materials, in order to provide the necessary background required to understand the
experimental results in future chapters of this thesis. Sections 1.1 and 1.2 will focus
on the effect that an anisotropic material structure has on the polarisation state of light
propagating through it, and how the polarisation state can be described. Section 1.3
will then discuss the inverse scenario, regarding the effects that the electromagnetic
field of light has on anisotropic media, with a particular focus on multiferroics.
Section 1.4 will then describe the structure of the rest of this thesis.
1.1 Crystal Optics
As a guide to the reader, the following section will provide a theoretical description
of the interaction of the electric field of light with an anisotropic dielectric medium.
This can then be used to describe the propagation of electromagnetic waves through
such an anisotropic medium, and its resulting effects on the polarisation state of the
probing light.
1.1.1 The Dielectric Tensor
In an optically isotropic, linear medium, such as a gas or an amorphous solid such as
glass, the components of the electric displacement field D are linearly proportional
to the corresponding components of the optical electric field E,
⎡
⎣
D x
D y
D z
⎤
⎦ =
⎡
⎣
E x
E y
E z
⎤
⎦ ,
(1.1)