108
5 High-Field Terahertz Time-Domain Spectroscopy …
where is the oscillator strength, ω 0 is the oscillator frequency and is the
linewidth, and the subscripts a and b denote the two different oscillators in the fit.
An example fit at 265 kV cm
−1 is shown in Fig. 5.10e, where the black points are the
experimental data, the black line is the best fit, the blue curve represents the electromagnon mode and the green curve represents the shoulder mode. The fit parameters
of oscillator strength, mode frequency and linewidth are presented in Fig. 5.10b, c
and d, respectively. To produce these fits the resonant frequency of the oscillator
corresponding to the shoulder feature was fixed at 1.2 THz and its linewidth was
fixed at 10 THz, which are values consistent with those previously reported in the
literature [45]. Errors in Fig. 5.10b–d are taken from errors in the fit. All values of the
fit parameters below E THz = 265 kV cm
−1 remain around a constant value (averages
of the parameters between E THz = 38 kV cm
−1 and 265 kV cm
−1 are represented by
the dashed lines in Fig. 5.10). However above E THz = 265 kV cm
−1 , while the resonant frequency of the electromagnon remains the same, an increase in the oscillator
strength of the main electromagnon can be observed, accompanied by an increase
in linewidth, along with a corresponding decrease in the oscillator strength of the
shoulder mode.
The results presented here could be indicative of a nonlinear enhancement in
the electromagnon absorption strength in CuO at high THz electric field strengths.
However the results are not conclusive, as they consist of only one dataset, with
only a single time-domain scan used for each sample and reference measurement
at each field strength; as such, it is difficult to provide an accurate estimation of
the uncertainty in the absorption data and fit parameters, which are required to conclusively prove a field-dependent change in the electromagnon absorption strength.
Further experimental investigations are required to prove that these observations are
reproducible, and to extend them to higher electric field strengths.
5.6 Summary
This chapter has presented the design and operation of a THz-TDS system capable
of producing pulses of high electric field strength THz radiation, for use in the
spectroscopic study of material properties in the nonlinear regime. The spectrometer
performance was characterised, and was found to be capable of producing THz
pulses with electric fields of up to 370 kV cm
−1 . The nonlinear THz transmission of
InSb was used as a test material for the spectrometer, and the observed behaviour
was consistent with that reported previously in the literature, demonstrating that the
spectrometer works as intended.
Preliminary investigations into the nonlinear behaviour of SWCNT films and electromagnons in CuO were also reported. A nonlinear increase in the THz transmission
with increasing field strength was observed in the SWCNT film, which also exhibited evidence of saturation for electric fields above ∼400 kV cm
−1 . The nonlinear
increase in transmission can be explained by similar processes of electron heating
and intervalley scattering as observed in InSb, whilst the saturation may arise due
5 High-Field Terahertz Time-Domain Spectroscopy …
where is the oscillator strength, ω 0 is the oscillator frequency and is the
linewidth, and the subscripts a and b denote the two different oscillators in the fit.
An example fit at 265 kV cm
−1 is shown in Fig. 5.10e, where the black points are the
experimental data, the black line is the best fit, the blue curve represents the electromagnon mode and the green curve represents the shoulder mode. The fit parameters
of oscillator strength, mode frequency and linewidth are presented in Fig. 5.10b, c
and d, respectively. To produce these fits the resonant frequency of the oscillator
corresponding to the shoulder feature was fixed at 1.2 THz and its linewidth was
fixed at 10 THz, which are values consistent with those previously reported in the
literature [45]. Errors in Fig. 5.10b–d are taken from errors in the fit. All values of the
fit parameters below E THz = 265 kV cm
−1 remain around a constant value (averages
of the parameters between E THz = 38 kV cm
−1 and 265 kV cm
−1 are represented by
the dashed lines in Fig. 5.10). However above E THz = 265 kV cm
−1 , while the resonant frequency of the electromagnon remains the same, an increase in the oscillator
strength of the main electromagnon can be observed, accompanied by an increase
in linewidth, along with a corresponding decrease in the oscillator strength of the
shoulder mode.
The results presented here could be indicative of a nonlinear enhancement in
the electromagnon absorption strength in CuO at high THz electric field strengths.
However the results are not conclusive, as they consist of only one dataset, with
only a single time-domain scan used for each sample and reference measurement
at each field strength; as such, it is difficult to provide an accurate estimation of
the uncertainty in the absorption data and fit parameters, which are required to conclusively prove a field-dependent change in the electromagnon absorption strength.
Further experimental investigations are required to prove that these observations are
reproducible, and to extend them to higher electric field strengths.
5.6 Summary
This chapter has presented the design and operation of a THz-TDS system capable
of producing pulses of high electric field strength THz radiation, for use in the
spectroscopic study of material properties in the nonlinear regime. The spectrometer
performance was characterised, and was found to be capable of producing THz
pulses with electric fields of up to 370 kV cm
−1 . The nonlinear THz transmission of
InSb was used as a test material for the spectrometer, and the observed behaviour
was consistent with that reported previously in the literature, demonstrating that the
spectrometer works as intended.
Preliminary investigations into the nonlinear behaviour of SWCNT films and electromagnons in CuO were also reported. A nonlinear increase in the THz transmission
with increasing field strength was observed in the SWCNT film, which also exhibited evidence of saturation for electric fields above ∼400 kV cm
−1 . The nonlinear
increase in transmission can be explained by similar processes of electron heating
and intervalley scattering as observed in InSb, whilst the saturation may arise due