Chapter 1
Introduction
The terahertz (THz) region of the electromagnetic spectrum can be approximately
defined as the frequency range 0.1–10 THz: a frequency of 1 THz corresponds to a
photon energy of ∼ 4 meV, and therefore THz radiation enables a wide variety of fascinating low energy excitations and intriguing material properties in the far-infrared
region of the electromagnetic spectrum to be investigated. Developments in THz
generation and detection technology over the past few decades have precipitated an
explosion in both research output and potential applications; the advent of femtosecond lasers and novel spectroscopic techniques have widely increased the availability
and applicability of THz sources. In particular, terahertz time-domain spectroscopy
(THz-TDS) has matured into a powerful tool for characterizing the optical properties
of materials at THz frequencies [1–3]. THz-TDS permits the direct determination
of the full complex refractive index ˜
n = n + iκ =
˜
˜
μ of the material under study,
where n is the real part of the refractive index, κ is the extinction coefficient, ˜
is the
complex permeability and ˜
μ is the complex permittivity of the material, and as such
does not require the use of the Kramers-Kronig relations [4].
Many materials demonstrate anisotropic behaviour at THz frequencies, such as
birefringence created by anisotropy in the vibrational or electronic response [5, 6],
and electro- and magneto-optical effects [7–9]. One particular class of materials
which has attracted much attention in recent years are multiferroics, due to a variety
of potential technological applications [10, 11], such as in spintronics [12–14] and
novel electronic components [15, 16]. In the case of ferroelectric multiferroics in
particular the material must be anisotropic, due to the requirement that the material
lacks an inversion centre for ferroelectricity to occur. In addition, designs of optical
components for polarisation control in the THz region, such as wire-grid polarisers
(WGPs) [17] and wave plates [18], depend critically on the optical anisotropy. Therefore the accurate determination of the optical properties of anisotropic materials is
crucial for both optical component design and fundamental physical research [19].
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
C. D. W. Mosley et al., Enhanced Polarisation Control and Extreme
Electric Fields, Springer Theses,
https://doi.org/10.1007/978-3-030-66902-7_1
1
Introduction
The terahertz (THz) region of the electromagnetic spectrum can be approximately
defined as the frequency range 0.1–10 THz: a frequency of 1 THz corresponds to a
photon energy of ∼ 4 meV, and therefore THz radiation enables a wide variety of fascinating low energy excitations and intriguing material properties in the far-infrared
region of the electromagnetic spectrum to be investigated. Developments in THz
generation and detection technology over the past few decades have precipitated an
explosion in both research output and potential applications; the advent of femtosecond lasers and novel spectroscopic techniques have widely increased the availability
and applicability of THz sources. In particular, terahertz time-domain spectroscopy
(THz-TDS) has matured into a powerful tool for characterizing the optical properties
of materials at THz frequencies [1–3]. THz-TDS permits the direct determination
of the full complex refractive index ˜
n = n + iκ =
˜
˜
μ of the material under study,
where n is the real part of the refractive index, κ is the extinction coefficient, ˜
is the
complex permeability and ˜
μ is the complex permittivity of the material, and as such
does not require the use of the Kramers-Kronig relations [4].
Many materials demonstrate anisotropic behaviour at THz frequencies, such as
birefringence created by anisotropy in the vibrational or electronic response [5, 6],
and electro- and magneto-optical effects [7–9]. One particular class of materials
which has attracted much attention in recent years are multiferroics, due to a variety
of potential technological applications [10, 11], such as in spintronics [12–14] and
novel electronic components [15, 16]. In the case of ferroelectric multiferroics in
particular the material must be anisotropic, due to the requirement that the material
lacks an inversion centre for ferroelectricity to occur. In addition, designs of optical
components for polarisation control in the THz region, such as wire-grid polarisers
(WGPs) [17] and wave plates [18], depend critically on the optical anisotropy. Therefore the accurate determination of the optical properties of anisotropic materials is
crucial for both optical component design and fundamental physical research [19].
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
C. D. W. Mosley et al., Enhanced Polarisation Control and Extreme
Electric Fields, Springer Theses,
https://doi.org/10.1007/978-3-030-66902-7_1
1