5.1 Terahertz Spectroscopy Using Extreme Electric Fields
91
radiation [29, 30]. THz generation by gas plasmas may be used to produce extremely
broad bandwidth pulses, as there is no absorption of the generated radiation in the
target material. More in-depth reviews of the generation of THz radiation from gas
plasmas, including generation mechanisms, can be found in references [25, 31].
Recently, an upscaling to amplified laser systems has been demonstrated for metallic spintronic emitters [32], in which electric fields of 300 kVcm
−1 were achieved,
with a peak frequency around 2 THz and a broad bandwidth between 0.1 and 10 THz.
The potential for creating longitudinally-polarised high-field THz radiation at a focus
has also been demonstrated using spintronic emitters, by patterning the magnetic
field applied to the device [33]. Photoconductive emitters also have the potential for
upscaling to produce higher field strengths, by increasing the size of the emitting
area and exciting it using amplified laser pulses. THz electric fields of 36 kVcm
−1
have been achieved using a pump laser fluence of 20 µJ/cm, bias electric field of 70
kV/cm and a large active emitter region of 1 mm
2 [34].
High-field THz radiation can be generated by optical rectification in materials
that possess a large nonlinear coefficient, such as organic crystals like DAST and
DSTMS [35, 36]. Emission strength from these organic crystals is limited by their
low damage threshold. An alternative method to produce THz radiation by optical
rectification is by using the tilted pulse-front pumping (TPFP) technique. The TPFP
technique was first demonstrated in a proof-of-principle experiment using (111)oriented GaP [37], and has since been utilised with materials such as lithium niobate
(LiNbO 3 ), in which THz electric fields of up to 1.2 MVcm
−1 have been achieved at a
diffraction-limited focus [38]. The TPFP technique forms the basis of the high-field
generation technique utilised in the spectrometer described in Sect. 5.2, and as such
will be discussed in further detail in Sect. 5.2.1.
5.1.2 Choosing the Right High-Field THz Source for You
An important consideration when choosing which method of high-field THz generation to employ is the spectral bandwidth it covers, and the material systems one is
interested in. Figure 5.1 presents a comparison of different sources and their typical
frequency spectra. Examining the sources, we see that plasma generation produces
ultrabroadband, gapless THz emission over a broad range of frequencies. However,
its spectrum is peaked around 8 THz, so whilst this technique provides emission over
a broad range of frequencies, the spectral weight around 1 THz is quite low. By comparison, optical rectification in LiNbO 3 and organic crystals have a lot of spectral
weight at low frequencies, with a narrower bandwidth. As such optical rectification
will be the best choice for investigating material properties at low THz frequencies,
and plasma generation for higher THz frequencies.
The choice of high-field source used in the spectrometer presented in this chapter
was motivated by the research interests of the THz group at the University of Warwick. Much of the fundamental physics of interest, such as charge transport dynamics
in carbon nanotubes [39, 40], metal halide perovskites [41, 42], semiconductors [43],
91
radiation [29, 30]. THz generation by gas plasmas may be used to produce extremely
broad bandwidth pulses, as there is no absorption of the generated radiation in the
target material. More in-depth reviews of the generation of THz radiation from gas
plasmas, including generation mechanisms, can be found in references [25, 31].
Recently, an upscaling to amplified laser systems has been demonstrated for metallic spintronic emitters [32], in which electric fields of 300 kVcm
−1 were achieved,
with a peak frequency around 2 THz and a broad bandwidth between 0.1 and 10 THz.
The potential for creating longitudinally-polarised high-field THz radiation at a focus
has also been demonstrated using spintronic emitters, by patterning the magnetic
field applied to the device [33]. Photoconductive emitters also have the potential for
upscaling to produce higher field strengths, by increasing the size of the emitting
area and exciting it using amplified laser pulses. THz electric fields of 36 kVcm
−1
have been achieved using a pump laser fluence of 20 µJ/cm, bias electric field of 70
kV/cm and a large active emitter region of 1 mm
2 [34].
High-field THz radiation can be generated by optical rectification in materials
that possess a large nonlinear coefficient, such as organic crystals like DAST and
DSTMS [35, 36]. Emission strength from these organic crystals is limited by their
low damage threshold. An alternative method to produce THz radiation by optical
rectification is by using the tilted pulse-front pumping (TPFP) technique. The TPFP
technique was first demonstrated in a proof-of-principle experiment using (111)oriented GaP [37], and has since been utilised with materials such as lithium niobate
(LiNbO 3 ), in which THz electric fields of up to 1.2 MVcm
−1 have been achieved at a
diffraction-limited focus [38]. The TPFP technique forms the basis of the high-field
generation technique utilised in the spectrometer described in Sect. 5.2, and as such
will be discussed in further detail in Sect. 5.2.1.
5.1.2 Choosing the Right High-Field THz Source for You
An important consideration when choosing which method of high-field THz generation to employ is the spectral bandwidth it covers, and the material systems one is
interested in. Figure 5.1 presents a comparison of different sources and their typical
frequency spectra. Examining the sources, we see that plasma generation produces
ultrabroadband, gapless THz emission over a broad range of frequencies. However,
its spectrum is peaked around 8 THz, so whilst this technique provides emission over
a broad range of frequencies, the spectral weight around 1 THz is quite low. By comparison, optical rectification in LiNbO 3 and organic crystals have a lot of spectral
weight at low frequencies, with a narrower bandwidth. As such optical rectification
will be the best choice for investigating material properties at low THz frequencies,
and plasma generation for higher THz frequencies.
The choice of high-field source used in the spectrometer presented in this chapter
was motivated by the research interests of the THz group at the University of Warwick. Much of the fundamental physics of interest, such as charge transport dynamics
in carbon nanotubes [39, 40], metal halide perovskites [41, 42], semiconductors [43],