Denitza Denkova

Mode parity-controlled fano- and lorentz-like line shapes arising in plasmonic nanorods

We present the experimental observation of spectral lines of distinctly different shapes in the optical extinction cross-section of metallic nanorod antennas under near-normal plane wave illumination. Surface plasmon resonances of odd mode parity present Fano interference in the scattering cross-section, resulting in asymmetric spectral lines. Contrarily, modes with even parity appear as symmetric Lorentzian lines. Finite element simulations are used to verify the experimental results. The emergence of either constructive or destructive mode interference is explained with a semianalytical 1D line current model. This simple model directly explains the mode-parity dependence of the Fano-like interference. Plasmonic nanorods are widely used as half-wave optical dipole antennas. Our findings offer a perspective and theoretical framework for operating these antennas at higher-order modes.

Mapping magnetic near-field distributions of plasmonic nanoantennas

We present direct experimental mapping of the lateral magnetic near-field distribution in plasmonic nanoantennas using aperture scanning near-field optical microscopy (SNOM). By means of full-field simulations it is demonstrated how the coupling of the hollow-pyramid aperture probe to the nanoantenna induces an effective magnetic dipole which efficiently excites surface plasmon resonances only at lateral magnetic field maxima. This excitation in turn affects the detected light intensity enabling the visualization of the lateral magnetic near-field distribution of multiple odd and even order plasmon modes with subwavelength spatial resolution.

Plasmon-Enhanced Sub-Wavelength Laser Ablation: Plasmonic Nanojets

In response to the incident lights electric field, the electron density oscillates in the plasmonic hotspots producing an electric current. Associated Ohmic losses raise the temperature of the material within the plasmonic hotspot above the melting point. A nanojet and nanosphere ejection can then be observed precisely from the plasmonic hotspots.

The role of chiral local field enhancements below the resolution limit of Second Harmonic Generation microscopy

While it has been demonstrated that, above its resolution limit, Second Harmonic Generation (SHG) microscopy can map chiral local field enhancements, below that limit, structural defects were found to play a major role. Here we show that, even below the resolution limit, the contributions from chiral local field enhancements to the SHG signal can dominate over those by structural defects. We report highly homogeneous SHG micrographs of star-shaped gold nanostructures, where the SHG circular dichroism effect is clearly visible from virtually every single nanostructure. Most likely, size and geometry determine the dominant contributions to the SHG signal in nanostructured systems.

Distributing the Optical Near‐Field for Efficient Field‐Enhancements in Nanostructures

We are grateful to Saloomeh Shariati from the crypto group in the Universite Catholique de Louvain, for helpful discussion on the measures of the uniformity in images. We acknowledge financial support from the fund for scientific research Flanders (FWO-V), the K. U. Leuven (CREA, GOA), Methusalem Funding by the Flemish government and the Belgian Inter-University Attraction Poles IAP Programmes. V. K. V. and S. V. are grateful for the support from the FWO-Vlaanderen. B. DC. is thankful to the IWT.

Lateral magnetic near-field imaging of plasmonic nanoantennas with increasing complexity

The design of many promising, newly emerging classes of photonic metamaterials and subwavelength confinement structures requires detailed knowledge and understanding of the electromagnetic near-field interactions between their building blocks. While the electric field distributions and, respectively, the electric interactions of different nanostructures can be routinely measured, for example, by scattering near-field microscopy, only recently experimental methods for imaging the magnetic field distributions became available. In this paper, we provide direct experimental maps of the lateral magnetic near-field distributions of variously shaped plasmonic nanoantennas by using hollow-pyramid aperture scanning near-field optical microscopy (SNOM). We study both simple plasmonic nanoresonators, such as bars, disks, rings and more complex antennas. For the studied structures, the magnetic near-field distributions of the complex resonators have been found to be a superposition of the magnetic near-fields of the individual constituting elements. These experimental results, explained and validated by numerical simulations, open new possibilities for engineering and characterization of complex plasmonic antennas with increased functionality.

Note: Simple vacuum feedthrough for optical fiber with SubMiniature version A connectors at both ends

We present a simple way to insert an optical fiber, with existing standard SubMiniature version A connectors on both ends into a vacuum system. The fitting is tested in scanning electron microscope, at working pressures down to 2 × 10(-5) mbar for cathodoluminescent measurements. With slight modifications this fitting could be successfully adapted for optical fiber insertion into pressurized systems.

Magnetic Near-Field Imaging of Increasingly Complex Plasmonic Antennas

In the previous two chapters we have verified that the aperture SNOM technique can be used for visualizing the lateral magnetic near-fields of metallic nanostructures. We have demonstrated the technique on geometrically simple structures—metallic bars as a proof of principle. Then we have discussed the underlying physics of this imaging concept. In this chapter we are expanding the method and applying it for imaging the magnetic near-fields of structures with geometries beyond simple bars. We first study other simple plasmonic nanoresonators, such as disks and rings and confirm that the technique is indeed imaging the lateral magnetic field for these geometries too. Then, we focus on more complex antennas, constructed from building blocks of different horizontal and vertical bars. For the studied structures, the magnetic near-field distributions of the complex resonators have been found to be a superposition of the magnetic near-fields of the individual constituting elements. These experimental results, explained and validated by numerical simulations, open new possibilities for engineering and characterization of complex plasmonic antennas with increased functionality.

Near-Field Mapping of Optical Fabry–Perot Modes in All-Dielectric Nanoantennas

Subwavelength optical resonators and scatterers are dramatically expanding the toolset of the optical sciences and photonics engineering. By offering the opportunity to control and shape light waves in nanoscale volumes, recent developments using high-refractive-index dielectric scatterers gave rise to efficient flat-optical components such as lenses, polarizers, phase plates, color routers, and nonlinear elements with a subwavelength thickness. In this work, we take a deeper look into the unique interaction of light with rod-shaped amorphous silicon scatterers by tapping into their resonant modes with a localized subwavelength light source-an aperture scanning near-field probe. Our experimental configuration essentially constitutes a dielectric antenna that is locally driven by the aperture probe. We show how leaky transverse electric and magnetic modes can selectively be excited and form specific near-field distribution depending on wavelength and antenna dimensions. The probes transmittance is furthermore enhanced upon coupling to the Fabry-Perot cavity modes, revealing all-dielectric nanorods as efficient transmitter antennas for the radiation of subwavelength emitters, in addition to constituting an elementary building block for all-dielectric metasurfaces and flat optics.

Imaging the Magnetic Near-Field of Plasmon Modes in Bar Antennas

In this chapter, we show how the scanning near-field optical microscopy (SNOM) technique can be used to visualize the lateral magnetic near-fields of metallic nanostructures, namely gold bars. We present direct experimental maps of these fields by using hollow-pyramid aperture probe SNOM. The results are supported by numerical simulations in which we first simulate the fields of the probe and the bars separately. Then we simulate and discuss in details how the probe-sample interaction results in the effective formation of a lateral magnetic dipole. This allows obtaining optical contrast in the SNOM images corresponding to the lateral magnetic near-fields of the structures. We verify the results for different bar lengths and wavelengths, respectively different plasmon modes. The obtained specific relation of the bar length versus resonant wavelength (so called dispersion relation), allows to unambiguously confirm that the observed optical contrast is related to plasmonic effects.