92
5 High-Field Terahertz Time-Domain Spectroscopy …
5
1 0
15
20
25
30
35
Frequency (THz)
Relative field amplitude
LiNbO 3
Organic
crystals
Plasma
DFG
Fig. 5.1 Schematic comparison of typical spectra produced by different high-field THz generation
techniques
and nanomaterial heterostructures [44], alongside low-energy collective excitations
such as electromagnons [45–47], all occur at low THz frequencies. As such, optical
rectification in LiNbO 3 was the generation scheme of choice, rather than an organic
crystal; the lower spectral peak frequency of ∼1 THz makes it the more applicable
method for much of the interesting physics, particularly the Drude and plasmon conductivity peaks in SWCNTs [40] and the resonant frequency of the electromagnon
in CuO.
5.2 High-Field Terahertz Time-Domain Spectrometer
This section will describe the high-field THz-TDS system used to perform the experiments described in Sects. 5.3, 5.4 and 5.5 of this chapter, starting with a description of
the high-field generation technique of choice, tilted pulse-front pumping in LiNbO 3 ,
in Sect. 5.2.1, before moving on to describe the optical setup of the spectrometer
in Sect. 5.2.2, and finally characterising the performance of the spectrometer and
describing the method of varying the THz electric field strength in Sect. 5.2.3.
5.2.1 Tilted Pulse-Front Pumping in LiNbO 3
As described in Sect. 2.1.2, the greatest efficiency is obtained for THz generation
via optical rectification in a nonlinear material where v p,THz = v g,opt . In materials in
5 High-Field Terahertz Time-Domain Spectroscopy …
5
1 0
15
20
25
30
35
Frequency (THz)
Relative field amplitude
LiNbO 3
Organic
crystals
Plasma
DFG
Fig. 5.1 Schematic comparison of typical spectra produced by different high-field THz generation
techniques
and nanomaterial heterostructures [44], alongside low-energy collective excitations
such as electromagnons [45–47], all occur at low THz frequencies. As such, optical
rectification in LiNbO 3 was the generation scheme of choice, rather than an organic
crystal; the lower spectral peak frequency of ∼1 THz makes it the more applicable
method for much of the interesting physics, particularly the Drude and plasmon conductivity peaks in SWCNTs [40] and the resonant frequency of the electromagnon
in CuO.
5.2 High-Field Terahertz Time-Domain Spectrometer
This section will describe the high-field THz-TDS system used to perform the experiments described in Sects. 5.3, 5.4 and 5.5 of this chapter, starting with a description of
the high-field generation technique of choice, tilted pulse-front pumping in LiNbO 3 ,
in Sect. 5.2.1, before moving on to describe the optical setup of the spectrometer
in Sect. 5.2.2, and finally characterising the performance of the spectrometer and
describing the method of varying the THz electric field strength in Sect. 5.2.3.
5.2.1 Tilted Pulse-Front Pumping in LiNbO 3
As described in Sect. 2.1.2, the greatest efficiency is obtained for THz generation
via optical rectification in a nonlinear material where v p,THz = v g,opt . In materials in