COupling Ultraquickly and Ultrastrongly Plasmonic and Photonic modes for Largely Efficient Sensing (COUUPPLES). Optical sensors allow contactless interrogation and rely on the availability of numerous sources and detectors. Metal nanoparticles (NPs) are largely used as their localized surface plasmon resonance is altered by small perturbations of their environment, enabling high detection sensitivity. We already demonstrated a spectacular enhancement of the ultrafast optical response of gold NPs once coupled with a resonant photonic mode of a 1D microcavity in the weak-coupling regime, together with a reduction of the resonance linewidth. It is even possible to reach the ultrastrong coupling regime. A laser pulse can induce switching from the strong to the weak coupling in less than a picosecond. Our project will first demonstrate this experimentally by inserting an array of aligned gold nanorods at the photonic antinode of a multi-layered cavity. The anticrossing behaviour of the polariton mode dispersion curve will be shown and the proof for ultrastrong coupling regime will be established. The high susceptibility to the NP environment of the polariton modes and their ultrafast dynamics will then be exploited to realize new plasmon-based sensors with high sensitivity and large effective volume. Hybrid cavities will be elaborated by mixed nanofabrication techniques and their optical response assessed and modelled. The near-field dynamics will be determined by via an original pump-probe fluorescence investigation. The cavities will then be integrated in a microfluidic environment and their potential for sensing will be tested through six different configurations with growing complexity, from the simple continuous monochromatic light interrogation to the exploitation of the spectral and temporal signatures of the device’s ultrafast transient optical response. The sensitivity of these sensing configurations to changes in the refractive index of the gold nanorod environment will be first determined. Then, a DNA aptamer will be grafted on the nanorod surface, able to bind with both large and small biomolecules. In order to establish the proof of concept of our new localized plasmon-based sensing pathways, we will chose as the analyte thrombin, a protein involved in several cardiovascular diseases, as well as small drug molecules possessing known anticancer and anti-viral activities. The project will be carried out through an interdisciplinary approach gathering three academic laboratories: LuMIn for the theoretical and experimental ultrafast optical response assessment, L2n for nanofabrication and optical characterization, and LBPA for biofunctionalization.