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Nanoparticle manipulation requires careful analysis of the forces at play. Unfortunately, traditional force measurement techniques based on the particle velocity do not provide sufficient resolution, while balancing approaches involving counteracting forces are often cumbersome. Here, we demonstrate that a nanoparticle dielectrophoretic response can be quantitatively studied by a straightforward visual delineation of the dielectrophoretic trapping volume. We reveal this volume by detecting the width of the region depleted of gold nanoparticles by the dielectrophoretic force. Comparison of the measured widths for various nanoparticle sizes with numerical simulations obtained by solving the particle-conservation equation shows excellent agreement, thus providing access to the particle physical properties, such as polarizability and size. These findings can be further extended to investigate various types of nano-objects, including bio- and molecular aggregates, and offer a robust characterization tool that can enhance the control of matter at the nanoscale.

We demonstrate a nonlinear plasmonic metasurface that exhibits strongly asymmetric second-harmonic generation: nonlinear scattering is efficient upon excitation in one direction, and it is substantially suppressed when the excitation direction is reversed, thus enabling a diode-like functionality. A significant (approximately 10 dB) extinction ratio of SHG upon opposite excitations is measured experimentally, and those findings are substantiated with full-wave simulations. This effect is achieved by employing a combination of two commonly used metals─aluminum and silver─producing a material composition asymmetry that results in a bianisotropic response of the system, as confirmed by performing homogenization analysis and extracting an effective susceptibility tensor. Finally, we discuss the implications of our results from the more fundamental perspectives of reciprocity and time-reversal asymmetry.

Hybrid metal-dielectric nanostructures have recently gained prominence because they combine strong field enhancement of plasmonic metals and the several low-loss radiation channels of dielectric resonators, which are qualities pertaining to the best of both worlds. In this work, an array of such hybrid nanoantennas is successfully fabricated over a large area and utilized for bulk refractive index sensing with a sensitivity of 208 nm/RIU. Each nanoantenna combines a Si cylinder with an Al disk, separated by a SiO 2 spacer. Its optical response is analyzed in detail using the multipoles supported by its subparts and their mutual coupling. The nanoantenna is further modified experimentally with an undercut in the SiO 2 region to increase the interaction of the electric field with the background medium, which augments the sensitivity to 245 nm/RIU. A detailed multipole analysis of the hybrid nanoantenna supports our experimental findings.

The relationship between composition and plasmonic properties in noble metal nanoalloys is still largely unexplored. Yet, nanoalloys of noble metals, such as gold, with transition elements, such as iron, have unique properties and a number of potential applications, ranging from nanomedicine to magneto-plasmonics and plasmon-enhanced catalysis. Here, we investigate the localized surface plasmon resonance at the level of the single Au-Fe nanoparticle by applying a strategy that combines experimental measurements using near field electron energy loss spectroscopy with theoretical studies via a full wave numerical analysis and density functional theory calculations of electronic structure. We show that, as the iron fraction increases, the plasmon resonance is blue-shifted and significantly damped, as a consequence of the changes in the electronic band structure of the alloy. This allows the identification of three relevant phenomena to be considered in the design and realization of any plasmonic nanoalloy, specifically: the appearance of new states around the Fermi level; the change in the free electron density of the metal; and the blue shift of interband transitions. Overall, this study provides new opportunities for the control of the optical response in Au-Fe and other plasmonic nanoalloys, which are useful for the realization of magneto-plasmonic devices for molecular sensing, thermo-plasmonics, bioimaging, photocatalysis, and the amplification of spectroscopic signals by local field enhancement.

We investigate optical second-harmonic generation (SHG) from metasurfaces where noncentrosymmetric V-shaped gold nanoparticles are ordered into regular array configurations. In contrast to expectations, a substantial enhancement of the SHG signal is observed when the number density of the particles in the array is reduced. More specifically, by halving the number density, we obtain over 5-fold enhancement in SHG intensity. This striking result is attributed to favorable interparticle interactions mediated by the lattice, where surface-lattice resonances lead to spectral narrowing of the plasmon resonances. Importantly, however, the results cannot be explained by the improved quality of the plasmon resonance alone. Instead, the lattice interactions also lead to further enhancement of the local fields at the particles. The experimental observations agree very well with results obtained from numerical simulations including lattice interactions.

Electromagnetic metasurfaces with strong nonlinear responses and angular selectivity could offer many new avenues for designing ultrathin optics components. We investigated the optical second harmonic generation from plasmonic metasurfaces composed of aligned gold nanopillars with a pronounced out-of-plane tilt using a flexible nonlinear Fourier microscope. The experimental and computational results demonstrate that these samples function as wavevector-selective nonlinear metasurfaces, that is, the coherent second harmonic signal does not only depend on the polarization and wavelength of the excitation beam, but also of its direction of incidence, in spite of the subwavelength thickness of the active layer. Specifically, we observe that the nonlinear response can vary by almost two orders-of-magnitude when the incidence angle is changed from positive to negative values compared to the surface normal. Further, it is demonstrated that these metasurfaces act as a directional nonlinear mirrors, paving the way for new design of directional meta-mirrors in the nonlinear regime.