Chapter 2
Terahertz Time-Domain Spectroscopy
This chapter aims to serve as an overview of the basic technology and experimental
techniques that the work presented in later chapters of this thesis expands on and
develops. Section 2.1 will introduce and provide background to the main methods
of generating broadband pulses of THz radiation used throughout this thesis, while
Sect. 2.2 will describe the detection of these THz pulses. Section 2.3 will then describe
how these two aspects are brought together to create a spectrometer, and describe the
process of performing a typical THz-TDS experiment. Finally, Sect. 2.4 will outline
the process by which material properties are extracted from the experimental data.
2.1 Generation of Broadband Terahertz Radiation
Any THz generation or detection methods used in this work are based upon the use
of femtosecond infrared laser pulses, produced by a mode-locked Ti:Sapphire laser.
The basic principle used to produce these ultrashort laser pulses is a lasing medium
with a broad gain bandwidth: as the time-domain and frequency-domain are inversely
related by the Fourier transform, a broad frequency spectrum will produce a pulse
with a short temporal profile.
The femtosecond laser used to drive the spectrometer used in the majority of
this thesis is a Spectra-Physics Mai Tai Ti:Sapphire oscillator, which produces 80 fs
duration, 800 nm central wavelength pulses at a repetition rate of 80 MHz, resulting
in an average optical power of ∼1.5 W. In the Mai Tai, a semiconductor laser array
pumps a Nd:YVO 4 laser, the 1064 nm output of which is frequency-doubled to 532 nm
and used to pump the Ti:Sapphire crystal, which produces the output 800 nm laser
pulses. The electronic ground states of the Ti
3+ ions are split into two vibrationally
broadened energy levels by the sapphire matrix; hence stimulated emission can occur
© 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_2
25
Terahertz Time-Domain Spectroscopy
This chapter aims to serve as an overview of the basic technology and experimental
techniques that the work presented in later chapters of this thesis expands on and
develops. Section 2.1 will introduce and provide background to the main methods
of generating broadband pulses of THz radiation used throughout this thesis, while
Sect. 2.2 will describe the detection of these THz pulses. Section 2.3 will then describe
how these two aspects are brought together to create a spectrometer, and describe the
process of performing a typical THz-TDS experiment. Finally, Sect. 2.4 will outline
the process by which material properties are extracted from the experimental data.
2.1 Generation of Broadband Terahertz Radiation
Any THz generation or detection methods used in this work are based upon the use
of femtosecond infrared laser pulses, produced by a mode-locked Ti:Sapphire laser.
The basic principle used to produce these ultrashort laser pulses is a lasing medium
with a broad gain bandwidth: as the time-domain and frequency-domain are inversely
related by the Fourier transform, a broad frequency spectrum will produce a pulse
with a short temporal profile.
The femtosecond laser used to drive the spectrometer used in the majority of
this thesis is a Spectra-Physics Mai Tai Ti:Sapphire oscillator, which produces 80 fs
duration, 800 nm central wavelength pulses at a repetition rate of 80 MHz, resulting
in an average optical power of ∼1.5 W. In the Mai Tai, a semiconductor laser array
pumps a Nd:YVO 4 laser, the 1064 nm output of which is frequency-doubled to 532 nm
and used to pump the Ti:Sapphire crystal, which produces the output 800 nm laser
pulses. The electronic ground states of the Ti
3+ ions are split into two vibrationally
broadened energy levels by the sapphire matrix; hence stimulated emission can occur
© 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_2
25