Dimers of Plasmonic Nanocubes to Reach Single-Molecule Strong Coupling with High Emission Yields.

Reaching reproducible strong coupling between a quantum emitter and a plasmonic resonator at room temperature, while maintaining high emission yields, would make quantum information processing with light possible outside of cryogenic conditions. We theoretically propose to exploit the high local curvatures at the tips of plasmonic nanocubes to reach Purcell factors of >106 at visible frequencies, rendering single-molecule strong coupling more easily accessible than with the faceted spherical nanoparticles used in recent experimental demonstrations. In the case of gold nanocube dimers, we highlight a trade-off between coupling strength and emission yield that depends on the nanocube size. Electrodynamic simulations on silver nanostructures are performed using a realistic dielectric constant, as confirmed by scattering spectroscopy performed on single nanocubes. Dimers of silver nanocubes feature Purcell factors similar to those of gold while allowing emission yields of >60%, thus providing design rules for efficient strongly coupled hybrid nanostructures at room temperature.

Few-Molecule Strong Coupling with Dimers of Plasmonic Nanoparticles Assembled on DNA.

Hybrid nanostructures, in which a known number of quantum emitters are strongly coupled to a plasmonic resonator, should feature optical properties at room temperature such as few-photon nonlinearities or coherent superradiant emission. We demonstrate here that this coupling regime can only be reached with dimers of gold nanoparticles in stringent experimental conditions, when the interparticle spacing falls below 2 nm. Using a short transverse DNA double-strand, we introduce five dye molecules in the gap between two 40 nm gold particles and actively decrease its length down to sub-2 nm values by screening electrostatic repulsion between the particles at high ionic strengths. Single-nanostructure scattering spectroscopy then evidence the observation of a strong-coupling regime in excellent agreement with electrodynamic simulations. Furthermore, we highlight the influence of the planar facets of polycrystalline gold nanoparticles on the probability of observing strongly coupled hybrid nanostructures.

Far-Field Wavefront Control of Nonlinear Luminescence in Disordered Gold Metasurfaces.

We demonstrate the local optimization of nonlinear luminescence from disordered gold metasurfaces by shaping the phase of femtosecond excitation. This process is enabled by the far-field wavefront control of plasmonic modes delocalized over the sample surface, leading to a coherent enhancement of subwavelength electric fields. In practice, the increase in nonlinear luminescence is strongly sensitive to both the nanometer-scale morphology and the level of structural complexity of the gold metasurface. We typically observe a 2 orders of magnitude enhancement of the luminescence signal for an optimized excitation wavefront compared to a random one. These results demonstrate how disordered metasurfaces made of randomly coupled plasmonic resonators, together with wavefront shaping, provide numerous degrees of freedom to program locally optimized nonlinear responses and optical hotspots.

Temperature-Dependent Plasmonic Responses from Gold Nanoparticle Dimers Linked by Double-Stranded DNA.

DNA is a powerful tool to assemble gold nanoparticles into discrete structures with tunable plasmonic properties for photonic or biomedical applications. Because of their photothermal properties or their use in biological media, these nanostructures can experience drastic modifications of the local temperature that can affect their morphology and, therefore, their optical responses. Using single-nanostructure spectroscopy, we demonstrate that, even with a fully stable DNA linker, gold particle dimers can undergo substantial conformational changes at temperatures larger than 50 °C and aggregate irreversibly. Such temperature-dependent resonant optical properties could find applications in imaging and in the design of nonlinear photothermal sources. Inversely, to provide fully stable DNA-templated plasmonic nanostructures at biologically relevant temperatures, we show how passivating the gold nanoparticles using amphiphilic surface chemistries renders the longitudinal plasmon resonance of gold particle dimers nearly independent of the local temperature.

Enhancing Magnetic Light Emission with All-Dielectric Optical Nanoantennas.

Electric and magnetic optical fields carry the same amount of energy. Nevertheless, the efficiency with which matter interacts with electric optical fields is commonly accepted to be at least 4 orders of magnitude higher than with magnetic optical fields. Here, we experimentally demonstrate that properly designed photonic nanoantennas can selectively manipulate the magnetic versus electric emission of luminescent nanocrystals. In particular, we show selective enhancement of magnetic emission from trivalent europium-doped nanoparticles in the vicinity of a nanoantenna tailored to exhibit a magnetic resonance. Specifically, by controlling the spatial coupling between emitters and an individual nanoresonator located at the edge of a near-field optical scanning tip, we record with nanoscale precision local distributions of both magnetic and electric radiative local densities of states (LDOS). The map of the radiative LDOS reveals the modification of both the magnetic and electric quantum environments induced by the presence of the nanoantenna. This manipulation and enhancement of magnetic light-matter interaction by means of nanoantennas opens up new possibilities for the research fields of optoelectronics, chiral optics, nonlinear and nano-optics, spintronics, and metamaterials, among others.

All-Dielectric Silicon Nanogap Antennas To Enhance the Fluorescence of Single Molecules.

Plasmonic antennas have a profound impact on nanophotonics as they provide efficient means to manipulate light and enhance light-matter interactions at the nanoscale. However, the large absorption losses found in metals can severely limit the plasmonic applications in the visible spectral range. Here, we demonstrate the effectiveness of an alternative approach using all-dielectric nanoantennas based on silicon dimers to enhance the fluorescence detection of single molecules. The silicon antenna design is optimized to confine the near-field intensity in the 20 nm nanogap and reach a 270-fold fluorescence enhancement in a nanoscale volume of λ(3)/1800 with dielectric materials only. Our conclusions are assessed by combining polarization resolved optical spectroscopy of individual antennas, scanning electron microscopy, numerical simulations, fluorescence lifetime measurements, fluorescence burst analysis, and fluorescence correlation spectroscopy. This work demonstrates that all-silicon nanoantennas are a valid alternative to plasmonic devices for enhanced single molecule fluorescence sensing, with the additional key advantages of reduced nonradiative quenching, negligible heat generation, cost-efficiency, and complementary metal-oxide-semiconductor (CMOS) compatibility.

Picosecond Lifetimes with High Quantum Yields from Single-Photon-Emitting Colloidal Nanostructures at Room Temperature.

Minimizing the luminescence lifetime while maintaining a high emission quantum yield is paramount in optimizing the excitation cross-section, radiative decay rate, and brightness of quantum solid-state light sources, particularly at room temperature, where nonradiative processes can dominate. We demonstrate here that DNA-templated 60 and 80 nm diameter gold nanoparticle dimers, featuring one fluorescent molecule, provide single-photon emission with lifetimes that can fall below 10 ps and typical quantum yields in a 45-70% range. Since these colloidal nanostructures are obtained as a purified aqueous suspension, fluorescence spectroscopy can be performed on both fixed and freely diffusing nanostructures to quantitatively estimate the distributions of decay rate and fluorescence intensity enhancements. These data are in excellent agreement with theoretical calculations and demonstrate that millions of bright fluorescent nanostructures, with radiative lifetimes below 100 ps, can be produced in parallel.

Correction: Fabrication of poly-crystalline Si-based Mie resonators via amorphous Si on SiO2 dewetting.

Correction for 'Fabrication of poly-crystalline Si-based Mie resonators via amorphous Si on SiO2 dewetting' by Meher Naffouti, et al., Nanoscale, 2016, 8, 2844-2849.

Fabrication of poly-crystalline Si-based Mie resonators via amorphous Si on SiO2 dewetting.

We report the fabrication of Si-based dielectric Mie resonators via a low cost process based on solid-state dewetting of ultra-thin amorphous Si on SiO2. We investigate the dewetting dynamics of a few nanometer sized layers annealed at high temperature to form submicrometric Si-particles. Morphological and structural characterization reveal the polycrystalline nature of the semiconductor matrix as well as rather irregular morphologies of the dewetted islands. Optical dark field imaging and spectroscopy measurements of the single islands reveal pronounced resonant scattering at visible frequencies. The linewidth of the low-order modes can be ∼20 nm in full width at half maximum, leading to a quality factor Q exceeding 25. These values reach the state-of-the-art ones obtained for monocrystalline Mie resonators. The simplicity of the dewetting process and its cost-effectiveness opens the route to exploiting it over large scales for applications in silicon-based photonics.

Increasing the Morphological Stability of DNA-Templated Nanostructures with Surface Hydrophobicity.

DNA has been extensively used as a versatile template to assemble inorganic nanoparticles into complex architectures; thanks to its programmability, stability, and long persistence length. But the geometry of self-assembled nanostructures depends on a complex combination of attractive and repulsive forces that can override the shape of a molecular scaffold. In this report, an approach to increase the morphological stability of DNA-templated gold nanoparticle (AuNP) groupings against electrostatic interactions is demonstrated by introducing hydrophobicity on the particle surface. Using single nanostructure spectroscopy, the nanometer-scale distortions of 40 nm diameter AuNP dimers are compared with different hydrophilic, amphiphilic, neutral, and negatively charged surface chemistries, when modifying the local ionic strength. It is observed that, with most ligands, a majority of studied nanostructures deform freely from a stretched geometry to touching particles when increasing the salt concentration while hydrophobicity strongly limits the dimer distortions. Furthermore, an amphiphilic surface chemistry provides DNA-linked AuNP dimers with a high long-term stability against internal aggregation.

Widefield spectral monitoring of nanometer distance changes in DNA-templated plasmon rulers.

The nanometer-scale sensitivity of electromagnetic plasmon coupling allows the translation of minute morphological changes in nanostructures into macroscopic optical signals. We demonstrate here a widefield spectral analysis of 40 nm diameter gold nanoparticle (AuNP) dimers, linked by a short DNA double strand, using a low-cost color CCD camera and allowing a quantitative estimation of interparticle distances in a 3-20 nm range. This analysis can be extended to lower spacings and a parallel monitoring of dimer orientations by performing a simple polarization analysis. Our measurement approach is calibrated against confocal scattering spectroscopy using AuNP dimers that are distorted from a stretched geometry at low ionic strength to touching particles at high salt concentrations. We then apply it to identify dimers featuring two different conformations of the same DNA template and discuss the parallel colorimetric sensing of short sequence-specific DNA single strands using dynamic plasmon rulers.

Wafer scale formation of monocrystalline silicon-based Mie resonators via silicon-on-insulator dewetting.

Subwavelength-sized dielectric Mie resonators have recently emerged as a promising photonic platform, as they combine the advantages of dielectric microstructures and metallic nanoparticles supporting surface plasmon polaritons. Here, we report the capabilities of a dewetting-based process, independent of the sample size, to fabricate Si-based resonators over large scales starting from commercial silicon-on-insulator (SOI) substrates. Spontaneous dewetting is shown to allow the production of monocrystalline Mie-resonators that feature two resonant modes in the visible spectrum, as observed in confocal scattering spectroscopy. Homogeneous scattering responses and improved spatial ordering of the Si-based resonators are observed when dewetting is assisted by electron beam lithography. Finally, exploiting different thermal agglomeration regimes, we highlight the versatility of this technique, which, when assisted by focused ion beam nanopatterning, produces monocrystalline nanocrystals with ad hoc size, position, and organization in complex multimers.

Selective excitation of single molecules coupled to the bright mode of a plasmonic cavity.

Plasmon-based optical antennas featuring a nanometer-sized gap can enhance the photophysical properties of solid-state quantum emitters by several orders of magnitude at room temperature. However, controlling the position and orientation of an isolated emitter in a metallic resonator, at the nanometer scale, has only been achieved in scanning probe geometries. Using radially polarized cylindrical vector beams and DNA-assembled gold nanoparticle dimers, we demonstrate the reproducible interaction of single dye molecules with the bright longitudinal mode of a plasmonic cavity, achieving decay rate enhancements of 2 orders of magnitude. These results demonstrate that interfacing efficiently isolated quantum emitters and optical nanoantennas is possible on a large scale.

Reversible switching of the interparticle distance in DNA-templated gold nanoparticle dimers.

We produce gold nanoparticle dimers with a surface-to-surface distance that varies reversibly by a factor of 3 when hybridizing or removing a single target DNA strand. The dimers are built on one DNA template that features a stem-loop enabling the interparticle distance change. Using electrophoresis, we reach 90% sample purities and demonstrate that this chemical process is reversible in solution at room temperature for a low molar excess of the target DNA strand. The kinetics of the reaction is asymmetric due to steric hindrance in the stem-loop opening process. Furthermore, a statistical analysis of cryo-electron microscopy measurements allows us to provide the first quantitative analysis of distance changes in chemically switchable nanoparticle assemblies.

Photonic engineering of hybrid metal-organic chromophores.

An aureate dye: Confined electromagnetic fields in DNA-templated gold nanoparticle dimers were tuned to engineer the fluorescence properties of organic dyes in water (see picture). Purified suspensions of hybrid metal-organic chromophores featured unprecedented photophysical properties, such as a short lifetime and low quantum yield but high brightness.

Accelerated single photon emission from dye molecule-driven nanoantennas assembled on DNA.

A photon interacts efficiently with an atom when its frequency corresponds exactly to the energy between two eigenstates. But at the nanoscale, homogeneous and inhomogeneous broadenings strongly hinder the ability of solid-state systems to absorb, scatter or emit light. By compensating the impedance mismatch between visible wavelengths and nanometre-sized objects, optical antennas can enhance light-matter interactions over a broad frequency range. Here we use a DNA template to introduce a single dye molecule in gold particle dimers that act as antennas for light with spontaneous emission rates enhanced by up to two orders of magnitude and single photon emission statistics. Quantitative agreement between measured rate enhancements and theoretical calculations indicate a nanometre control over the emitter-particle position while 10 billion copies of the target geometry are synthesized in parallel. Optical antennas can thus tune efficiently the photo-physical properties of nano-objects by precisely engineering their electromagnetic environment.

Optical and topological characterization of gold nanoparticle dimers linked by a single DNA double strand.

We demonstrate that symmetric or asymmetric gold nanoparticle dimers with substantial scattering cross sections and plasmon coupling can be produced with a perfectly controlled chemical environment and a high purity using a single DNA linker as short as 7 nm. A statistical analysis of the optical properties and morphology of single dimers is performed using darkfield and cryo-electron microscopies. These results, correlated to Mie theory calculations, indicate that the particle dimers are stretched in water by electrostatic interactions.

Crucial role of the emitter-particle distance on the directivity of optical antennas.

We demonstrate that the reflecting properties of a single particle nanoantenna can be extremely sensitive to its distance from a quantum emitter at frequencies lower than the plasmon resonance. The phenomenon is shown to arise from rapid phase variations of the emitter field at short distances associated with a phase of the antenna particle polarizability lower than π/4.

Plasmon-based nanolenses assembled on a well-defined DNA template.

The controlled and reproducible synthesis of closely spaced noble metal nanoparticle groupings is an essential step toward the rational design of nanostructures for surface enhanced Raman scattering with single-molecule sensitivity. In this communication, we demonstrate the facile synthesis of 5, 8, and 18 nm gold particle groupings on a well-defined DNA template by hybridizing monoconjugated gold-DNA building blocks. The obtained nanometer interparticle gaps should yield local intensity enhancements up to 4 orders of magnitude as estimated by Generalized Mie Theory. By tuning the particle size and relative curvatures, we calculate that the maximum enhancement can be optimized and localized on a specific part of the nanostructure. These groupings act as plasmon-based nanolenses assembled on a biomolecular backbone that can be functionalized in proximity to the region of highest field.

Role of spatial distortions on the quadratic nonlinear optical properties of octupolar organic and metallo-organic molecules.

Following on the recent experimental demonstration of a discrepancy between the nonlinear optical (NLO) behavior of several pi-conjugated chromophores and their assumed octupolar symmetry, the authors investigate how geometrical distortions influence the NLO response of multipolar push-pull molecules. Their analytical model is set on a basis of valence-bond and charge-transfer states to estimate the hyperpolarizability of organic and metallo-organic chromophores using the lowest possible number of variables. Since symmetry breakdown changes the definition of the molecular Cartesian framework, tensorial spherical coordinates are implemented. The evolution of the nonlinear molecular anisotropy with possible rotational deviations is then evaluated for two recently studied chromophores. Zero-frequency calculations show that, outside optical resonance, weak geometrical distortions lead to strong anisotropy variations in agreement with experimental data. Their goal is to underscore which molecular engineering strategies should be applied when designing a photoisomerizable nonlinear octupole.