The well-mixed powder was compressed into a pellet of 13 mm diameter with a thickness of 2 mm to 3 mm using a hydraulic press (Specac Ltd., Kent, UK) at a load of 2 ton, and a blank polyethylene pellet prepared in the same way was used as a reference. During the THz-TDS transmission measurement, 1000 waveforms were acquired and averaged, with a resolution of 0.94 cm -1 .