Phase-Resolved Two-Dimensional Infrared Spectroscopy of Solution-Phase Vibrational Polaritons on Gold Antenna Meta-Surfaces.

Vibrational polaritons are quasiparticles that form when optically allowed molecular vibrations strongly couple to photonic resonances of infrared cavity. Understanding vibro-polariton properties is essential to harness their potential in fields from synthetic chemistry to quantum technologies. We studied vibro-polaritons generated with infrared meta-surfaces─high-optical-quality arrays of gold microantennas. Linear and third-order nonlinear two-dimensional infrared spectroscopy (2DIR) combined with electromagnetic analysis emphasized, on one hand, the surface-confined character of vibro-polariton waves and their quantum state nature on the other. Phase-resolved 2DIR line shapes highlighted the anharmonic character of vibro-polaritons and revealed coupling with the excitation of molecular-like reservoir states. Experimental data were qualitatively described by nonlinear response functions with signal phases obtained from electromagnetic calculations and anharmonic constants of few wavenumbers. Such agreement allows us to rule out alternative scenarios like selective signal enhancement from inhomogeneous distribution of molecular frequencies or signal amplification by weak coupling to photonic resonances.