Silver nanoparticle arrays placed on top of a high-refractive index substrate enhance the coupling of light into the substrate over a broad spectral range. We perform a systematic numerical and experimental study of the light incoupling by arrays of Ag nanoparticle arrays in order to achieve the best impedance matching between light propagating in air and in the substrate. We identify the parameters that determine the incoupling efficiency, including the effect of Fano resonances in the scattering, interparticle coupling, as well as resonance shifts due to variations in the near-field coupling to the substrate and spacer layer. The optimal configuration studied is a square array of 200 nm wide, 125 nm high spheroidal Ag particles, at a pitch of 450 nm on a 50 nm thick Si(3)N(4) spacer layer on a Si substrate. When integrated over the AM1.5 solar spectral range from 300 to 1100 nm, this particle array shows 50% enhanced incoupling compared to a bare Si wafer, 8% higher than a standard interference antireflection coating. Experimental data show that the enhancement occurs mostly in the spectral range near the Si band gap. This study opens new perspectives for antireflection coating applications in optical devices and for light management in Si solar cells.

We demonstrate that polarization conversion in coupled dimer antennas, used in phase discontinuity metasurfaces, can be tuned by careful design. By controlling the gap width, a strong variation of the coupling strength and polarization conversion is found between capacitively and conductively coupled antennas. A theoretical two-oscillator model is proposed which shows a universal scaling of the degree of polarization conversion with the energy splitting of the symmetric and anti-symmetric modes supported by the antennas.Using single antenna spectroscopy, we find good agreement for the scaling of mode splitting and polarization conversion with gap width over the range from capacitive to conductive

We investigate the efficiency of secondharmonic generation (SHG) over the transition from capacitive to conductive coupling in orthogonal L-shaped dimer gold antennas. By tuning both the gap and antenna length, the bonding and antibonding resonances are individually addressed. Results on the intensity and polarization of SHG are compared quantitatively with microscopic numerical simulations taking into account the nanoscale nonlinear surface dipole distribution, elucidating the interplay between symmetry at macroscopic and microscopic levels and optical resonance effects. Microscopic modeling reveals strong cancellations of nonlinear dipoles by capacitive coupling in plasmonic nanogaps, resulting in only small changes in SHG efficiency despite large local field enhancement in the gap. Experimentally, irreproducible polarization properties are obtained in a range of parameters associated with strong optical near fields in the gap of the antennas, which is interpreted as a consequence of nanoscopic asymmetries inherited from the fabrication process. Our results demonstrate that nanoscopic defects can either strongly impact the nonlinear optical emission or have a barely detectable influence depending on the excited optical resonance and associated optical near-field distribution. These results provide useful design rules to optimize the design of nonlinear plasmonic nanostructures.

Polarization control using single plasmonic nanoantennas is of interest for subwavelength optical components in nano-optical circuits and metasurfaces. Here, we investigate the role of two mechanisms for polarization conversion by plasmonic antennas: Structural asymmetry and plasmon hybridization through strong coupling. As a model system we investigate L-shaped antennas consisting of two orthogonal nanorods which lengths and coupling strength can be independently controlled. An analytical model based on field susceptibilities is developed to extract key parameters and to address the influence of antenna morphology and excitation wavelength on polarization conversion efficiency and scattering intensities. Optical spectroscopy experiments performed on individual antennas, further supported by electrodynamical simulations based on the Green Dyadic Method, confirm the trends extracted from the analytical model. Mode hybridization and structural asymmetry allow address-ing different input polarizations and wavelengths, providing additional degrees of freedom for agile polarization conversion in nanophotonic devices.

With progress in nanofabrication, new strategies have become available that allow precise control of nanoscale optical fields using metallic nanostructures. Here we review recent progress in the control of optical resonances in metal nanostructures for applications in sensing and spectroscopy. We discuss the use of new techniques, such as helium-ion beam milling, which allow precise sculpting of nanometer-scale gaps; new materials such as metal oxides, which have a response somewhere inbetween that of conventional dielectrics and noble metals; and new designs such as L-shaped gap antennas which allow controlling the polarization state of light through near-field interactions between closely spaced antennas.

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