4.2. THz measurements
Fig. 4(a) and (c) shows temporal waveforms of the THz electric field pulses transmitted through the two solutions measured by THz-TDS at 295 K and 277 K, respectively. Eight sets of data are superimposed in the figure, and the waveforms are found to be reproduced in the repeated measurement. This is valid at all the temperatures examined, except at Tc. All temporal waveforms are reproduced for the reference solution at Tc in Fig. 4(b), whereas the waveform for the sample solution abruptly changes during the repeated measurement. This indicates that the condition of the sample solution changes at Tc. Here, we note that the solution was homogeneously opaque after the aggregation process began during the THz measurement, and that the opaque solution for visible light does not affect the transmission of the THz wave because the wavelength of the THz wave is around 0.1 mm and much longer than that of visible light. Furthermore, water was observed at the bottom of the sample chamber more than several hours after the THz measurement, that is, a biphasic water–oil system was formed. At this stage, the sample temperature returned to room temperature.