Imaging-based quantification of sub-25 nm coking layer on Au by dielectric-loaded plasmonic azimuthally chirped gratings

Noble metals are commonly used in heterogeneous catalysis due to their active catalytic sites. However, carbon layer deposition, or coking, can block these sites and reduce performance, even at thicknesses below 25 nm. Rapid monitoring of ultrathin coking layers is therefore essential for timely surface renewal and sustained catalytic efficiency. Conventional analysis methods are expensive, destructive, require bulky instruments, and lack in-situ, real-time capabilities. In this work, we present a simple, image-based optical method for non-destructive monitoring of sub-25 nm coking layers. This approach uses direct imaging of dielectric-loaded plasmonic azimuthally chirped gratings (DL-pACGs) on the coked gold surface, where reflection images reveal azimuthal dark bands from plasmon coupling. The position of these bands correlates with carbon thickness, enabling quantitative analysis. Ellipsometry data from thin carbon films were incorporated into simulations, showing strong agreement with experimental results. This method is simple, effective, and non-destructive for quantifying thin coke films.