A case study: The negative THz time shift observed for Cu nanoparticle colloids

We report a case study of the experimentally observed negative shift for the Copper nanoparticle colloids (NPs) when they are kept in contact with a quartz interface. This investigation is based on the previously reported measurements on various volumetric concentrations of Cu NPs using the Terahertz Time-Domain Spectroscopy (THz-TDS) technique. A gradually increasing negative time shift reminiscent of a superluminal propagation was observed for the Cu NPs in contrast with Ag NPs. In this work, we explain the possible reason for such observation by quantifying the recorded negative delays with high precision and by modelling the THz peak profiles under the assumption of a thin film formation at the dielectric colloid interface. Cu nanoparticle colloids with the same specifications were produced under the same conditions to verify the effect of the quartz interfacial layer. Quartz slides were kept immersed inside the colloids for up to 3 days and periodically monitored using UV-visible spectroscopy technique. While the Ag NPs did not show any evidence of thin film formation, the Surface Plasmon resonance absorption peaks were measured for Cu NPs, proving a thin layer formation on the slide surface. Our findings explain the advanced phase shift occurring at the dielectric-conductor interface which is valid only when the electromagnetic field transmits into the absorbing medium in the presence of a thin conducting layer.