The thickness-dependent magnetic and electrical transport properties of nearly strain-free La0.7Sr0.3CoO3 (LSCO) films grown on (001)-oriented (LaAlO3)0.3 (Sr2AlTaO6)0.7 substrates were systematically studied. A crossover from ferromagnetic/metallic to non-magnetic/insulating behavior occurs at a critical thickness (∼8 nm) that is significantly smaller than LSCO films under larger strains in reported literature. X-ray absorption measurements revealed that the difference of functional properties at reduced film thicknesses was accompanied by changes in the valence state of Co ions at the film/substrate interface.

Surface plasmon polaritons (SPPs) are surface charge density oscillations localized to a metal-dielectric interface. In addition to being considered as promising candidates for a variety of applications, structures that support SPPs, including metal-insulator-metal (MIM) multilayers, are of fundamental interest because of the variety of collective plasmonic modes they support. Previously, a particular class of “forbidden” plasmon polariton modes (PPMs) was proposed that includes plasmon polariton modes confined to a region of dispersion space not typically accessible to surface-constructed collective excitations. Specifically, for these modes, known as Guided Wave PPMs (GW-PPMs), due to the dielectric asymmetry of the central layer, the solution to the wave equation in the center insulator layer is oscillatory while remaining surface bound both to the supporting substrate and the exposed surface. These modes are supported by a simple physical structure that results from a minor symmetry modification of the traditional MIM structure, specifically the use of a central insulator layer with a higher refractive index than the supporting substrate. However, they display fundamental properties that are distinctly different from those of standard SPPs and from recently reported hybrid plasmonic modes. While GW-PPMs have been explored theoretically, they have not yet been realized experimentally. In this article, we present the first experimental demonstration of GW-PPMs. Specifically, we excite and detect GW-PPMs at visible frequencies and match model predictions to experimental results with remarkable accuracy using minimal parameter fitting. In addition to the experimental detection, we calculate and report on other interesting and relevant features of the detected modes, including the associated electric field profiles, confinement values, and propagation lengths, and discuss in terms of the applications-relevance of GW-PPMs.