Toroidal plasmonic eigenmodes in oligomer nanocavities for the visible

Area of Science:

• Plasmonics and Nanophotonics
• Electromagnetic Theory

Background:

• Plasmonic nanostructures exhibit electric and magnetic multipole resonances crucial for applications.
• Toroidal eigenmodes, a class of electromagnetic resonances, have been historically overlooked due to their unique properties.
• These modes are strongly confined to material structures and difficult to observe with conventional optical methods.

Purpose of the Study:

• To demonstrate and characterize toroidal eigenmodes in a specific plasmonic nanostructure.
• To highlight the utility of energy-filtering transmission electron microscopy for studying optically invisible modes.
• To explore the potential applications of toroidal moments in nanophotonics.

Main Methods:

• Fabrication of a metal ring nanostructure composed of an oligomer of holes.
• Utilizing combined energy-filtering transmission electron microscopy (EFTEM) and 3D finite-difference time-domain (FDTD) simulations.
• Employing EFTEM as the primary method for exciting and observing toroidal modes, which are not accessible via optical far-field spectroscopy.

Main Results:

• Successful demonstration of toroidal eigenmodes within the designed metal ring nanostructure.
• Distinctive features of toroidal modes were revealed through the synergistic application of EFTEM and FDTD analysis.
• EFTEM was confirmed as an effective technique for studying these modes, overcoming limitations of optical spectroscopy.

Conclusions:

• Toroidal eigenmodes exist in plasmonic nanostructures and possess unique characteristics.
• Energy-filtering transmission electron microscopy is a powerful tool for investigating these elusive electromagnetic phenomena.
• Toroidal moments offer promising avenues for future applications, particularly in enhancing quantum-optical emitters.