Broad-Band Near-Infrared Plasmonic Nanoantennas for Higher Harmonic Generation

We propose a broad-band near-infrared trapezoidal plasmonic nanoantenna, analyze it numerically using finite integration and difference time domain techniques, and explain qualitatively its performance <i>via</i> a multidipolar scenario as well as a conformal transformation. The plasmonic nanoantenna reported here intercepts the incoming light as if it were of cross-sectional area larger than double its actual physical size for a 1500 nm bandwidth expanding from the near-infrared to the visible spectrum. Within this bandwidth, it also confines the incoming light to its center with more than 1 order of magnitude field enhancement. This wide-band operation is achieved due to the overlapping of the different dipole resonances excited across the nanoantenna. We further demonstrate that the broad-band field enhancement leads to efficient third harmonic generation in a simplified wire trapezoidal geometry when a Kerr medium is introduced, due to the lightning rod effect at the fundamental and the Purcell effect at the induced third harmonic

Plasmonic Control of Radiative Properties of Semiconductor Quantum Dots Coupled to Plasmonic Ring Cavities

In recent years, a lot of effort has been made to achieve controlled delivery of target particles to the hotspots of plasmonic nanoantennas, in order to probe and/or exploit the extremely large field enhancements produced by such structures. While in many cases such high fields are advantageous, there are instances where they should be avoided. In this work, we consider the implications of using the standard nanoantenna geometries when colloidal quantum dots are employed as target entities. We show that in this case, and for various reasons, dimer antennas are not the optimum choice. Plasmonic ring cavities are a better option despite low field enhancements, as they allow collective coupling of many quantum dots in a reproducible and predictable manner. In cases where larger field enhancements are required, or for larger quantum dots, nonconcentric ring-disk cavities can be employed instead

Metal–Dielectric Parabolic Antenna for Directing Single Photons

Quantum emitters radiate light omni-directionally, making it hard to collect and use the generated photons. Here, we propose a three-dimensional metal–dielectric parabolic antenna surrounding an individual quantum dot as a source of collimated single photons, which can then be easily extracted and manipulated. Our fabrication method relies on a single optically induced polymerization step once the selected emitter has been localized by confocal microscopy. Compared to conventional nanoantennas, our geometry does not require near-field coupling, and it is, therefore, very robust against misalignment issues and minimally affected by absorption in the metal. The parabolic antenna provides one of the largest reported experimental directivities (<i>D</i> = 106) and the lowest beam divergences (Θ_1/2 = 13.5°) and a broadband operation over all of the visible and near-infrared range together with extraction efficiency of more than 96%, offering a practical advantage for quantum technological applications

Imaging Plasmon Hybridization of Fano Resonances via Hot-Electron-Mediated Absorption Mapping

The inhibition of radiative losses in dark plasmon modes allows storing electromagnetic energy more efficiently than in far-field excitable bright-plasmon modes. As such, processes benefiting from the enhanced absorption of light in plasmonic materials could also take profit of dark plasmon modes to boost and control nanoscale energy collection, storage, and transfer. We experimentally probe this process by imaging with nanoscale precision the hot-electron driven desorption of thiolated molecules from the surface of gold Fano nanostructures, investigating the effect of wavelength and polarization of the incident light. Spatially resolved absorption maps allow us to show the contribution of each element of the nanoantenna in the hot-electron driven process and their interplay in exciting a dark plasmon mode. Plasmon-mode engineering allows control of nanoscale reactivity and offers a route to further enhance and manipulate hot-electron driven chemical reactions and energy-conversion and transfer at the nanoscale

Continuous spectral and coupling-strength encoding with dual-gradient metasurfaces

Optical metasurfaces excel at enhancing and controlling light-matter interactions, which are primarily dictated by two factors: the spectral overlap of the resonances with target excitations in the material and the coupling-strength between them, where resonance linewidth and localized field enhancement are the governing influences. Current metasurface designs are limited to sampling a few discrete points within this vast 2D interaction parameter space or have varied only a single parameter. Symmetry-protected bound states in the continuum (BICs) allow precise control over the wavelength and linewidth of individual resonances, but rely on large arrangements of identical unit cells, limiting the continuous mapping of the parameter space. Therefore, optical platforms that concurrently probe the spectral and coupling parameters, so far, remained elusive. Here, we introduce the concept of dual-gradient metasurfaces for the continuous and simultaneous encoding of the spectral and coupling-strength of light-matter interactions, enabled by smooth local variations of the unit cell parameters. Contrary to conventional understanding, we demonstrate that BICs can be excited in such non-periodic systems provided the parameter variations are sufficiently small. Our dual-gradient metasurface exhibits an extraordinary resonance density, with each unit cell supporting a unique mode. This results in up to 27,500 distinct modes, all contained within a compact footprint. We apply this technology to surface-enhanced molecular sensing, capturing not only the spectral fingerprint of molecules but also unveiling an additional coupling-based dimension of spectroscopic data. This advancement in metasurface design paves the way for generalized light-matter coupling with metasurfaces, with applications ranging from on-chip spectrometer, to chirality encoding and AI-driven biochemical spectroscopy

Double Blind Ultrafast Pulse Characterization by Mixed Frequency Generation in a Gold Antenna

Ultrafast pulse characterization requires the analysis of correlation functions generated by frequency mixing of optical pulses in a nonlinear medium. In this work, we use a gold optical nanoantenna to generate simultaneously Four Wave Mixing and Sum Frequency Generation across the tuning range of a Ti:Sapphire and Optical Parametric Oscillator (OPO) system. Since metal nanoparticles create remarkably strong nonlinear responses for their size without the need for phase matching, this allows us to simultaneously characterize the unknown OPO pulse and its pump pulse using a single spectrogram. The nonlinear mixing is efficient enough to retrieve pulses with energies in the picojoule range

Emergent resonances in a thin film tailored by optically-induced small permittivity asymmetries

Resonances are usually associated with finite systems - the vibrations of clamped strings in a guitar or the optical modes in a cavity defined by mirrors. In optics, resonances may be induced in infinite continuous media via periodic modulations of their optical properties. Here we demonstrate that periodic modulations of the permittivity of a featureless thin film can also act as a symmetry breaking mechanism, allowing the excitation of photonic $\textit{quasi}$-bound states in the continuum ($\textit{q}$BICs). By interfering two ultrashort laser pulses in the unbounded film, transient resonances can be tailored through different parameters of the pump beams. We show that the system offers resonances tunable in wavelength and quality-factor, and spectrally selective enhancement of third harmonic generation. Due to a fast decay of the permittivity asymmetry, we observe ultrafast dynamics, enabling time-selective near-field enhancement with picosecond precision. Optically-induced permittivity asymmetries may be exploited in on-demand weak to ultrastrong light-matter interaction regimes and light manipulation at dynamically chosen wavelengths in lithography-free metasurfaces

Surface-Enhanced Spectroscopies of a Molecular Monolayer in an All-Dielectric Nanoantenna

A reduction in the number of loss decay channels present in optical nanoantennas could help enhance an emitter’s radiation efficiency. These losses get amplified for emitters in close proximity to metallic surfaces, such as for self-assembled monolayers, reducing the fluorescence rate. However, such a proximity strongly enhances Raman scattering. A dual-sensing scheme should bypass this shortcoming, and switching from metals to high refractive index dielectrics could aid in that direction. In order to show this, we fabricated silicon nanodimers and coated them with a β-carotenal monolayer for detecting surface-enhanced Raman scattering and fluorescence emission of the same probe. We obtained a surface-enhanced Raman scattering (SERS) factor of 1720 ± 300 for the C–C bond stretching of the polyene chain and a surface fluorescence enhancement (SEF) factor of 470 ± 90. Furthermore, our theoretical studies of different materials and emitters located on the surface of nanostructures demonstrate that low-loss dielectric materials provide a robust architecture for enhancing the response of efficient emitters. These results could have a direct impact on the development of deterministic high-rate single-photon sources

Dynamical Instability of a Nonequilibrium Exciton-Polariton Condensate

By imaging single-shot realizations of an organic polariton quantum fluid, we observe the long-sought dynamical instability of nonequilibrium condensates. We find an excellent agreement between the experimental data and a numerical simulation of the open-dissipative Gross-Pitaevskii equation, without performing any parameter fitting, which allows us to draw several important conclusions about the physics of the system. We find that the reservoir dynamics are in the strongly nonadiabatic regime, which renders the complex Ginzburg–Landau description invalid. The observed transition from stable to unstable fluid can only be explained by taking into account the specific form of reservoir-mediated instability as well as particle currents induced by the finite extent of the pump spot

Theory of Three-Dimensional Nanocrescent Light Harvesters

The optical properties of three-dimensional crescent-shaped gold nanoparticles are studied using a transformation optics methodology. The polarization insensitive, highly efficient, and tunable light harvesting ability of singular nanocrescents is demonstrated. We extend our approach to more realistic blunt nanostructures, showing the robustness of their plasmonic performance against geometric imperfections. Finally, we provide analytical and numerical insights into the sensitivity of the device to radiative losses and nonlocal effects. Our theoretical findings reveal an underlying relation between structural bluntness and spatial dispersion in this particular nanoparticle configuration