34
2 Terahertz Time-Domain Spectroscopy
Fig. 2.5 a Time-domain waveforms of THz pulses generated and detected in the spectrometer
presented in this thesis, without (blue trace) and after (red trace) transmission through a THz lowpass filter (TYDEX LPF8.8-30). b The corresponding Fourier transform spectra of the data in the
shaded area in panel (a)
A typical time-domain waveform generated and detected in the spectrometer
described here, and its corresponding Fourier transformed spectrum, are presented
as the blue curves in Fig. 2.5a and b, respectively. The red curves represent the same
pulse after transmission through a sample, which in this example was a THz low-pass
filter (TYDEX LPF8.8-30). The sample introduces a phase delay in the transmitted
pulse, causing it to arrive later in time, and a reduction in amplitude, due to reflection
losses at the surfaces or absorptive properties of the material. Sample properties are
extracted from the data by comparison of the spectra with and without transmission
through the sample, which will be described in Sect. 2.4.
An important consideration when setting the experimental parameters is the resolution and bandwidth of the measurement. The resolution in the frequency-domain
f is related to the width of the time-domain by f = 1//t; similarly the
maximum measureable frequency in the measurement is proportional to the reciprocal of the spacing between data points in the time-domain. As such, to gain the
best frequency resolution it is advantageous to measure over the widest time-domain
possible. In addition to the main THz pulse, internal reflections of either the THz or
gate pulses in the emitter, detection crystal or sample may create artifacts in the EOS
signal; two such examples may be observed in the blue trace in Fig. 2.5a, around
5 ps for the first internal reflection and 9 ps for the next. Internal reflections in the
time-domain introduce periodic oscillations in the frequency-domain, so typically
in optically thick samples the time window that the Fourier transform is performed
over may be restricted to exclude the internal reflections, as demonstrated in Fig. 2.5.
In any THz-TDS experiment, a practical balance must be struck between the desired
resolution and bandwidth and the time taken to perform the measurement.
2 Terahertz Time-Domain Spectroscopy
Fig. 2.5 a Time-domain waveforms of THz pulses generated and detected in the spectrometer
presented in this thesis, without (blue trace) and after (red trace) transmission through a THz lowpass filter (TYDEX LPF8.8-30). b The corresponding Fourier transform spectra of the data in the
shaded area in panel (a)
A typical time-domain waveform generated and detected in the spectrometer
described here, and its corresponding Fourier transformed spectrum, are presented
as the blue curves in Fig. 2.5a and b, respectively. The red curves represent the same
pulse after transmission through a sample, which in this example was a THz low-pass
filter (TYDEX LPF8.8-30). The sample introduces a phase delay in the transmitted
pulse, causing it to arrive later in time, and a reduction in amplitude, due to reflection
losses at the surfaces or absorptive properties of the material. Sample properties are
extracted from the data by comparison of the spectra with and without transmission
through the sample, which will be described in Sect. 2.4.
An important consideration when setting the experimental parameters is the resolution and bandwidth of the measurement. The resolution in the frequency-domain
f is related to the width of the time-domain by f = 1//t; similarly the
maximum measureable frequency in the measurement is proportional to the reciprocal of the spacing between data points in the time-domain. As such, to gain the
best frequency resolution it is advantageous to measure over the widest time-domain
possible. In addition to the main THz pulse, internal reflections of either the THz or
gate pulses in the emitter, detection crystal or sample may create artifacts in the EOS
signal; two such examples may be observed in the blue trace in Fig. 2.5a, around
5 ps for the first internal reflection and 9 ps for the next. Internal reflections in the
time-domain introduce periodic oscillations in the frequency-domain, so typically
in optically thick samples the time window that the Fourier transform is performed
over may be restricted to exclude the internal reflections, as demonstrated in Fig. 2.5.
In any THz-TDS experiment, a practical balance must be struck between the desired
resolution and bandwidth and the time taken to perform the measurement.