Far-field e-beam detection of hybrid cavity-plasmonic modes in gold micro-holes

TL;DR

This paper demonstrates the use of focused electron-beams to directly excite and study hybrid cavity-plasmonic modes in gold micro-holes, providing insights into extraordinary optical transmission phenomena at subwavelength scales.

Contribution

It introduces a novel electron-beam excitation method for probing cavity and surface plasmon polariton modes in gold micro-holes, revealing their properties and interactions.

Findings

Identification of radiative cavity modes unaffected by SPs

Observation of SP polariton modes with waveguide characteristics

Long-range electron-beam interaction involving electromagnetic excitations

Abstract

Manipulation of light-beams with subwavelenth metallic devices has motivated intensive studies, following the discovery of extraordinary transmission of electromagnetic waves through sub-wavelength apertures in thin noble-metal films. The propagation of light in these holes can be investigated at greately improved spatial resolution by means of focused electron-beams. Here we demonstrate direct e-beam excitation of radiative cavity modes well below the surface plasmon (SP) frequency, of isolated rectangular holes in gold films, illuminating the hotly debated phenomenon of the extraordinary optical transmission through subwavelength holes. The exceptionally long range e-beam interaction with the metal through the vacuum, involving electromagnetic excitations within the light cone, is allowed by momentum conservation breakdown along the e-beam axis. Two types of lowlying excited modes are revealed: radiative cavity modes which are nearly unaffected by SPs, and SP polariton modes with waveguide character in the near field region of the slit walls, which in spite of the strong hybridization preserve the waveguide cutoff frequencies and symmetry characteristics.