Exploring electron induced photon radiation in plasmonic nanostructures by angle and polarization-resolved cathodoluminescence spectroscopy

IntroductionLocalized surface plasmon (SP) resonances of plasmonic nanoparticles have been deeply investigated due to their major role in a wide range of applications such as single-molecule sensing and surface-enhanced Raman... [ view full abstract ]

Introduction

Localized surface plasmon (SP) resonances of plasmonic nanoparticles have been deeply investigated due to their major role in a wide range of applications such as single-molecule sensing and surface-enhanced Raman scattering. Experimental access to the electromagnetic field distribution associated with SP excitations with high spatial resolution is of fundamental importance. In recent years an electron based cathodoluminescence (CL) spectroscopy has attracted significant attention due to its nanometer spatial resolution and broad spectral range. The CL probes only radiative processes, however, it offers a large set of measurements including spectral, spatial, polarization, and angle dependent properties of the SP modes.

Methods

We use electron-beam lithography to pattern high quality gold nanoprisms and nanoprisms pairs in the bowtie antenna alignment on SiO2 layers. Angle- and polarization-resolved CL measurements of the optical properties of these nanostructures are performed in a scanning transmission electron microscope equipped with a parabolic mirror and combined with a light detection system (Fig. 1a,b). To interpret the experimental results, numerical simulations are performed using the retarded boundary-element method.

Results and discussion

The optical properties of individual and coupled Au nanoprisms are systematically investigated using the CL with special emphasis on angle and polarization dependences. By exciting nanostructures with an electron beam, we measure spectral features and spatially resolved maps of SPs in the far-field radiation (Fig. 1c,d). By resolving polarization and angular distribution patterns of the far-field emission we access to the symmetry of the modes and interpret them within a group theory description. In particular, the degeneracy of the modes and plasmon hybridization in bow-tie triangles are explored. We demonstrate that combination of the angle and polarization-resolved CL provides a powerful technique with the ability of efficiently detecting and mapping weakly radiative dipole plasmons, higher-order and dark modes. Furthermore, we discuss a link between the CL and the electromagnetic local density of states associated to the SP modes. All our experimental results are supported by numerical simulations showing excellent agreement.