Plasmon resonances in silicon nanowires: geometry effects on the trade-off between dielectric and metallic behaviour

Surface plasmons (SP) arising from nanometer silicon objects allow control and manipulation of light at the nanoscale exhibiting significant advantages in a plethora of applied research areas such as nanophotonic, environment, energy, biology, and medicine. These SP can achieve more significant potential, thanks to the industrial scalability and low cost offered by silicon compared with other metals and semiconductor nanosized materials. However, as they have not yet been fully understood and exploited, silicon's plasmon mechanisms need to be thoroughly studied. In particular, the influence of nanowire shape on surface plasmon behavior and the existence of physical constraints for surface plasmon excitation remains to be fully understood. In a previous study, we have demonstrated that thanks to their anisotropic onedimensional shape, silicon nanowires sustain two types of plasmon resonances, the longitudinal ones along the main nanowire axis, with harmonic behavior and the transversal resonance, which takes place along the diameter. We demonstrated our data on a particular set of sizes, 30 nm for the diameter and about 400 nm for the length. Here we show how the resonances change when the diameter is smaller than 30 nm and the length is smaller than 400 nm. We use electron energy loss spectroscopy to map the several plasmonic modes from the fundamental one to the higher orders, with the goal of understanding how the SP resonances change when the diameter and length are smaller than 30 nm and 400 nm, respectively. We then use modeling to support the experimental findings. According to the mode order, the study illustrates the various locations inside the nanowires where discrete resonance spots can be found. Another important finding of this work is the disappearance of the surface plasmon modes for nanowires shorter than a predetermined threshold for any diameter in the range investigated, showing that the nanowire length is a key factor in maintaining electron oscillations. With this finding, a crucial physical limit for this phenomenon in silicon is established.