Exploring Phase Topology and Group Index Modifications of THz Vibro-Polaritons in Metasurfaces

Strong coupling between molecules and the vacuum field in optical cavities creates hybrid light–matter quantum states known as polaritons, a phenomenon that has garnered significant interest in recent years. Although most studies of strong coupling have focused on the characteristics of intensity spectra, the behavior of the phase of polaritons remains comparatively unexplored. In this work, we employ metasurfaces that support optical modes to couple them strongly with intermolecular vibrations in α-lactose at THz frequencies, forming vibro-polaritons. Using terahertz time-domain spectroscopy (THz-TDS), we examine this strongly coupled system in both amplitude and phase and use the phase topology as an indicator for such coupling. In addition, we explore the group index in the weak and strong coupling regimes. We observe the decrease in the negative group index associated with the anomalous dispersion of the resonant modes as a function of the coupling strength. This work reveals previously hidden insights into temporal and enhanced light–matter interactions in strongly coupled systems and highlights the relevance of the phase in the analysis of these systems.