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Squeezing optical fields into nanometer scale is the key step to perform spatially resolved near-field optics. In scattering-type near-field optical microscopy, this task is accomplished by nanoantennas that convert propagating radiation to local near-fields and vice versa. The usual nanoantenna is composed by an elongated metal structure whose longitudinal dimension is scaled to support dipole modes of localized surface plasmon resonances. However, monopole modes can also be explored if the elongated metal nanoparticle is electrically grounded on a flat metallic plateau that acts like a mirror providing the monopole's image that closes the dipole system. Here, a method for batch production of monopole nanoantennas for scattering-type near-field scanning optical microscopy is presented. The nanoantennas are composed of a micropyramidal body with a nanopyramidal end whose lateral dimension can be scaled to fine-tune localized surface plasmon resonance modes. The monopole character of the nanoantennas is revealed by electron energy loss spectroscopy, and their efficiency and reproducibility are tested in tip-enhanced Raman spectroscopy experiments performed on single-layer graphene and single-walled carbon nanotubes.

Squeezing optical fields into nanometer scale is the key step to perform spatially resolved near-field optics. In scattering-type near-field optical microscopy, this task is accomplished by nanoantennas that convert propagating radiation to local near-fields and vice versa. The usual nanoantenna is composed by an elongated metal structure whose longitudinal dimension is scaled to support dipole modes of localized surface plasmon resonances. However, monopole modes can also be explored if the elongated metal nanoparticle is electrically grounded on a flat metallic plateau that acts like a mirror providing the monopole's image that closes the dipole system. Here, a method for batch production of monopole nanoantennas for scattering-type near-field scanning optical microscopy is presented. The nanoantennas are composed of a micropyramidal body with a nanopyramidal end whose lateral dimension can be scaled to fine-tune localized surface plasmon resonance modes. The monopole character of the nanoantennas is revealed by electron energy loss spectroscopy, and their efficiency and reproducibility are tested in tip-enhanced Raman spectroscopy experiments performed on single-layer graphene and single-walled carbon nanotubes.

Plasmonic nanoantennas are pushing the limits of optical imaging resolution capabilities in near-field scanning optical microscopy (NSOM). Accordingly, these techniques are driving the basic understanding of photonic and optoelectronic nanoscale devices with applications in sensing, energy conversion, solid-state lighting, and information technology. Imaging the localized surface plasmon resonance (LSPR) at the nanoscale is a key to understanding the optical responses of a given tip geometry in order to engineer better plasmonic nanoantennas for near-field experiments. In recent years the advancement of focused ion beam technology provides the ability to directly modify plasmonic structures with nanometer resolution. Also, scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS) is an established technique allowing imaging of LSPR. Specifically, the combination of these two techniques provides spectrally sensitive two-dimensional (2D) image information to better visualize and understand LSPR on the nanometer scale. This can be combined with electron tomography to provide the three-dimensional LSPR distribution. Here we demonstrate the fabrication of Au nanopyramids using helium ion microscopy, and analyze the LSPR in 3D reconstructions produced by total variation (TV)-norm minimization of a set of 2D STEM-EELS maps. Additionally, a boundary element simulation method was used to verify the experimentally observed nanopyramid LSPR modes. Finally, we show that the point-spread-functions (PSF) of LSPR mode hot spots in nanopyramids differ to local electric-field enhancement under optical excitation making direct comparison to NSOM experimental resolution difficult. However, the STEM-EELS results show how LSPR modes are influenced by the tip characteristics, which can inform the development of new nanoantenna designs.

Plasmonic nanoantennas are pushing the limits of optical imaging resolution capabilities in near-field scanning optical microscopy (NSOM). Accordingly, these techniques are driving the basic understanding of photonic and optoelectronic nanoscale devices with applications in sensing, energy conversion, solid-state lighting, and information technology. Imaging the localized surface plasmon resonance (LSPR) at the nanoscale is a key to understanding the optical responses of a given tip geometry in order to engineer better plasmonic nanoantennas for near-field experiments. In recent years the advancement of focused ion beam technology provides the ability to directly modify plasmonic structures with nanometer resolution. Also, scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS) is an established technique allowing imaging of LSPR. Specifically, the combination of these two techniques provides spectrally sensitive two-dimensional (2D) image information to better visualize and understand LSPR on the nanometer scale. This can be combined with electron tomography to provide the three-dimensional LSPR distribution. Here we demonstrate the fabrication of Au nanopyramids using helium ion microscopy, and analyze the LSPR in 3D reconstructions produced by total variation (TV)-norm minimization of a set of 2D STEM-EELS maps. Additionally, a boundary element simulation method was used to verify the experimentally observed nanopyramid LSPR modes. Finally, we show that the point-spread-functions (PSF) of LSPR mode hot spots in nanopyramids differ to local electric-field enhancement under optical excitation making direct comparison to NSOM experimental resolution difficult. However, the STEM-EELS results show how LSPR modes are influenced by the tip characteristics, which can inform the development of new nanoantenna designs.