Ultraconfined Plasmons
In article number 2302520, J. Enrique Ortega, F. Javier García de Abajo, and co‐workers present a new approach to fabricating ultrathin crystalline silver films consisting of a few atomic layers in thickness and featuring collective electron oscillations—plasmons—with an unprecedented combination of spatial confinement and long lifetimes. The image illustrates one of those structures, where bowtie pillars are carved from a silicon wafer, followed by a chemical and physical treatment that flattens the upper surface, and subsequent epitaxial deposition of a controlled number of silver (111) atomic layers. The resulting silver bowtie structures present confined plasmons at the gap region with a quality factor (spectral frequency‐to‐width ratio) approaching ten and a spatial footprint of a few tens of nanometers.