Abstract
The spatial structure of an electromagnetic field can determine the characteristics of light-matter interactions. A strong gradient of light in the near field can excite dipole-forbidden atomic transitions, e.g., electric quadrupole transitions, which are rarely observed under plane-wave far-field illumination. Structured light with a higher-order orbital angular momentum state may also modulate the selection rules in which an atom can absorb two quanta of angular momentum: one from the spin and another from the spatial structure of the beam. Here, we numerically demonstrate a strong focusing of structured light with a higher-order orbital angular momentum state in the near field. A quadrupole field was confined within a gap region of several tens of nanometres in a plasmonic tetramer structure. A plasmonic crystal surrounding the tetramer structure provides a robust antenna effect, where the incident structured light can be strongly coupled to the quadrupole field in the gap region with a larger alignment tolerance. The proposed system is expected to provide a platform for light-matter interactions with strong multipolar effects.
Highlights
The interactions of light and matter are of fundamental interest, and with the advent of photonic technologies, these interactions have become a flourishing field, stimulating basic science and consequent applications
Another scenario for selection rule modification has been argued in the field of structured light since the pioneering work of optical orbital angular momentum (OAM)[7]
The selection rules are interpreted for light with linear or circular polarization that possesses an angular momentum of ±ħ, but the total angular momentum can be augmented by OAM
Summary
The interactions of light and matter are of fundamental interest, and with the advent of photonic technologies, these interactions have become a flourishing field, stimulating basic science and consequent applications. Many works have reported that plasmonic systems can significantly enhance the multipolar effects because their k are much larger than that of light in the free space[14,15,16,17,18,19] Another scenario for selection rule modification has been argued in the field of structured light since the pioneering work of optical orbital angular momentum (OAM)[7]. The selection rules are interpreted for light with linear or circular polarization that possesses an angular momentum of ±ħ, but the total angular momentum can be augmented by OAM This augmentation becomes relevant for electric quadrupole transitions that require a change of two units of angular momentum (±2ħ) in the atom[20]. Strongly modified selection rules for a quadrupole transition have been experimentally observed, showing that an atom can absorb two quanta of angular momentum from a single photon: one from spin (circular polarization) and another from OAM24
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