Selection rules for structured light in nanooligomers and other nanosystems

TL;DR

This paper derives optical selection rules for nanostructures excited by structured light, revealing forbidden transitions with longer lifetimes, and demonstrates how structured light enables access to new optical excitations in nanosystems.

Contribution

It introduces a group theory-based framework for understanding how structured light interacts with nanosystems, including selection rules for complex beam profiles and multi-photon processes.

Findings

Structured light excites forbidden optical transitions in nanooligomers.

Forbidden modes have longer lifetimes and narrower resonances.

Selection rules are provided for various structured light beams.

Abstract

Structured light are custom light fields where the phase, polarization, and intensity vary with position. It has been used for nanotweezers, nanoscale imaging, and quantum information technology, but its role in exciting optical transitions in materials has been little examined so far. Here we use group theory to derive the optical selection rules for nanosystems that get excited by structured light. If the size of the nanostructure is comparable to the light wavelength, it will sample the full beam profile during excitation with profound consequences on optical excitations. Using nanooligomers as model nanosystems, we show that structured light excites optical transitions that are forbidden for linearly polarized or unpolarized light. Such dipole forbidden modes have longer lifetimes and narrower resonances than dipole allowed transitions. We derive symmetry-adapted eigenmodes for nanooligomers containing up to six monomers. Our study includes tables with selection rules for cylindrical vector beams, for beams with orbital angular momentum, and for field retardation along the propagation direction. We discuss multi-photon processes of nonlinear optics in addition to one-photon absorption. Structured light will unlock a broad range of excitations in nanooligomers and other nanostructures that are currently inaccessible to optical studies.