Understanding how the plasmonic response of a metallic nanoparticle is modified by its coupling with a metallic film is a fundamental research problem relevant for many applications including sensing, solar energy harvesting, spectroscopy, and photochemistry. Despite significant research effort on this topic, the nature of the interaction between colloidal nanoparticles and metallic films is not fully understood. Here, we investigate, both experimentally and theoretically, the optical response of surface ligand-coated gold nanorods interacting with gold films. We find that the scattering cross section of these systems is dominated by a charge transfer plasmon mode, for which charge flows between the particle and the film. The properties of this mode are dictated by the characteristics of the particle–film junction, which makes the frequency of this charge transfer plasmon far less sensitive to the nanoparticle size and geometry than a typical dipolar plasmon mode excited in similar nanorods placed directly on a purely dielectric substrate. The results of this work serve to advance our understanding of the interaction between metallic nanoparticles and metallic films, as well as provide a method for creating more robust plasmonic platforms that are less affected by changes in the size of individual nanoparticles.

Plasmons, the collective oscillations of mobile electrons in metallic nanostructures, interact strongly with light and produce vivid colors, thus offering a new route to develop color printing technologies with improved durability and material simplicity compared with conventional pigments. Over the last decades, researchers in plasmonics have been devoted to manipulating the characteristics of metallic nanostructures to achieve unique and controlled optical effects. However, before plasmonic nanostructures became a science, they were an art. The invention of the daguerreotype was publicly announced in 1839 and is recognized as the earliest photographic technology that successfully captured an image from a camera, with resolution and clarity that remain impressive even by today’s standards. Here, using a unique combination of daguerreotype artistry and expertise, experimental nanoscale surface analysis, and electromagnetic simulations, we perform a comprehensive analysis of the plasmonic properties of these early photographs, which can be recognized as an example of plasmonic color printing. Despite the large variability in size, morphology, and material composition of the nanostructures on the surface of a daguerreotype, we are able to identify and characterize the general mechanisms that give rise to the optical response of daguerreotypes. Therefore, our results provide valuable knowledge to develop preservation protocols and color printing technologies inspired by past ones.

Nanostructures made of metallic materials support collective oscillations of their conduction electrons, commonly known as surface plasmons. These modes, whose characteristics are determined by the material and morphology of the nanostructure, couple strongly to light and confine it into subwavelength volumes. Of particular interest are metallic nanostructures for which the size along one dimension approaches the nanometer or even the subnanometer scale, since such morphologies can lead to stronger light–matter interactions and higher degrees of confinement than regular three‐dimensional nanostructures. Here, the plasmonic response of metallic nanodisks of varying thicknesses and aspect ratios is investigated under far‐ and near‐field excitation conditions. It is found that, for far‐field excitation, the strength of the plasmonic response of the nanodisk increases with its thickness, as expected from the increase in the number of conduction electrons in the system. However, for near‐field excitation, the plasmonic response becomes stronger as the thickness of the nanodisk is reduced. This behavior is attributed to the higher efficiency with which a near‐field source couples to the plasmons supported by thinner nanodisks. The results of this work advance the understanding of the plasmonic response of thin metallic nanostructures, thus increasing their potential for the development of novel applications.

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