Plasmon‐Enhanced Vibrational Upconversion Pumped by Continuous Wave: Photothermal Effect Elucidated by Colocalized Complementary Vibrational Spectroscopy

ABSTRACT The integration of infrared (IR) and Raman spectroscopy within a single optical system has long been a research goal, owing to the rich chemical information contained within the vibrational spectra. Recent works have demonstrated that IR‐active molecular vibrations can be efficiently upconverted into fluorescence under continuous‐wave (CW) excitation in the plasmonic nanocavity. This approach can achieve rigorous colocalization of complementary vibrational information, including IR and Raman modes. However, the mechanism responsible for such high efficiency under CW excitation remains unclear, and the potential contribution of the photothermal effect in the plasmonic cavity is ambiguous. In this study, we leverage the vibrational‐encoded fluorescence (VEF) technique to probe colocalized complementary vibrational modes within the plasmon‐enhanced nanocavities. Through simultaneous measurements of colocalized Raman and fluorescence intensities, we calculate the equivalent temperature based on the Bose‐Einstein statistics. The results confirm that the photothermal effect is pronounced to pump the IR vibrations and thereby contributes to upconverted fluorescence. The modulation of IR power also validates the photothermal effect originating from the relaxation of vibrational states. In summary, this technique establishes a robust platform for investigating physicochemical reactions of molecules from a multimodal spectroscopic perspective. VEF exhibits great potential to interpret the comprehensive vibrational information in future physical chemistry research.