Exploring Phase Topology of THz Vibro-Polaritons in Metasurfaces

Strong coupling between molecules and the vacuum field in optical cavities gives rise to hybrid quantum states known as polaritons, a topic of growing interest in recent years. While most research has focused on intensity spectra, the phase dynamics of polaritons remain relatively unexplored. In this study, we used metasurfaces designed to support optical modes, which allow strong interactions with intermolecular vibrations in α-lactose at terahertz (THz) frequencies, leading to the formation of vibropolaritons. Using terahertz time-domain spectroscopy (THz-TDS), we analyze both amplitude and phase, using the phase topology as an unique signature of the coupling strength. The results reveal previously hidden aspects of the temporal evolution and enhanced light–matter interactions in these systems, emphasizing the fundamental importance of the phase in their characterization.