Abstract
Abstract Plasmonic nanostructures have considerable applicability in light–matter interactions owing to their capacity for strong field confinement and enhancement. Nanogap structures allow us to tailor electric field distributions at the nanoscale, bridging the differences in size and shape of atomic and light structures. In this study, we demonstrated that a plasmonic tetramer structure can squeeze structured light into a nanoscale area, in which a strong field gradient allows access to forbidden transitions. Numerical simulations showed that the gold tetramer structure on a glass substrate possesses a plasmonic eigenmode, which forms structured light with a quadrupole profile in the nanogap region at the center of the tetramer. The top–down technique employed using electron-beam lithography allows us to produce a gap size of approximately 50 nm, which supports plasmonic resonance in the near-infrared regime. In addition, we demonstrated an array architecture in which a collective lattice resonance enhances the intensity of the quadrupole field in multiple lattice units. This study highlights the possibility of accessing multipolar transitions in a combined system of structured light and plasmonic nanostructures. Our findings may lead to new platforms for spectroscopy, sensing, and light sources that take advantage of the full electronic spectrum of an emitter.
Highlights
Spontaneous emission, a fundamental process in the field of light–matter interaction, is responsible for the characteristic emission spectrum of an emitter
We demonstrated that a plasmonic tetramer structure can squeeze structured light into a nanoscale area, in which a strong field gradient allows access to forbidden transitions
The plasmonic nanosystem consists of a gold tetramer constructed from nanotriangles The gold tetramer structure was designed such that plasmonic resonance occurs in the near-infrared region (∼911 nm)
Summary
Spontaneous emission, a fundamental process in the field of light–matter interaction, is responsible for the characteristic emission spectrum of an emitter. The transitions are slow because the wavelength of emitted light (∼103–105 Å) is typically far larger than the size of the atomic or molecular orbitals participating in the transition (∼1–10 Å) Because of this difference in the length scales, the rate of electronic transitions as a function of AM varies by many orders of magnitude, making high-AM transitions invisible in the absorption and emission spectra of an emitter. Recent theoretical and experimental studies have shown that plasmonic nanosystems can squeeze the wavelength of light by a few orders of magnitude, which allows us to access multipolar transitions [7,8]. When rubidium atoms absorb two units of AM from the squeezed structured light in the plasmonic nanosystem, they exhibit an electric quadrupole transition at a wavelength of 911 nm [14]. The concept presented of shrinking structured light to allow forbidden transitions provides the possibility of accessing the full electronic spectrum of an emitter, which is critical for a broad spectrum of nanophotonics applications
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