The spot of terahertz radiation probing the centre of the cell was about 2 mm in diametre (as highlighted in Figure ). Before the crystals had grown into the centre of the cell, the sample volume probed with terahertz radiation was entirely filled with liquid, and the terahertz spectra were hence completely featureless. However, as crystal growth continued toward the centre of the cell, the absorption below 1.6 THz decreased and a peak emerged at 1.6 THz. This indicated the existence of crystals in the field of view of the spectrometer (see Three frequencies were chosen to illustrate the changes of the spectrum over the course of the experiment: 1.6 THz, i.e. the peak maximum, 1.0 THz, the frequency where the spectrometer has the highest signal to noise ratio, and 0.5 THz, which was a sufficiently low frequency that it should not directly be influenced by the crystalline spectral feature. At each of those three frequencies, the absorption coefficient was extracted and plotted as a function of time, as illustrated in Figure .