Magnetic Mode Coupling in Hyperbolic Bowtie Meta-Antennas.

Hyperbolic metaparticles have emerged as the next step in metamaterial applications, providing tunable electromagnetic properties on demand. However, coupling of optical modes in hyperbolic meta-antennas has not been explored. Here, we present in detail the magnetic and electric dipolar modes supported by a hyperbolic bowtie meta-antenna and clearly demonstrate the existence of two magnetic coupling regimes in such hyperbolic systems. The coupling nature is shown to depend on the interplay of the magnetic dipole moments, controlled by the meta-antenna effective permittivity and nanogap size. In parallel, the meta-antenna effective permittivity offers fine control over the electrical field spatial distribution. Our work highlights new coupling mechanisms between hyperbolic systems that have not been reported before, with a detailed study of the magnetic coupling nature, as a function of the structural parameters of the hyperbolic meta-antenna, which opens the route toward a range of applications from magnetic nanolight sources to chiral quantum optics and quantum interfaces.

Lost world of complex life and the late rise of the eukaryotic crown.

Eukaryotic life appears to have flourished surprisingly late in the history of our planet. This view is based on the low diversity of diagnostic eukaryotic fossils in marine sediments of mid-Proterozoic age (around 1,600 to 800 million years ago) and an absence of steranes, the molecular fossils of eukaryotic membrane sterols1,2. This scarcity of eukaryotic remains is difficult to reconcile with molecular clocks that suggest that the last eukaryotic common ancestor (LECA) had already emerged between around 1,200 and more than 1,800 million years ago. LECA, in turn, must have been preceded by stem-group eukaryotic forms by several hundred million years3. Here we report the discovery of abundant protosteroids in sedimentary rocks of mid-Proterozoic age. These primordial compounds had previously remained unnoticed because their structures represent early intermediates of the modern sterol biosynthetic pathway, as predicted by Konrad Bloch4. The protosteroids reveal an ecologically prominent 'protosterol biota' that was widespread and abundant in aquatic environments from at least 1,640 to around 800 million years ago and that probably comprised ancient protosterol-producing bacteria and deep-branching stem-group eukaryotes. Modern eukaryotes started to appear in the Tonian period (1,000 to 720 million years ago), fuelled by the proliferation of red algae (rhodophytes) by around 800 million years ago. This 'Tonian transformation' emerges as one of the most profound ecological turning points in the Earth's history.

Trait-based approaches reveal that deep reef ecosystems in the Western Indian Ocean are functionally distinct.

Tropical deep reefs (>30 m) are biologically and ecologically unique ecosystems with a higher geographic reach to shallow (<30 m) reefs. Yet they are poorly understood and rarely considered in conservation practices. Here, we characterise benthic and fish communities across a depth gradient (10-350 m) in remote coral atolls in Seychelles, Western Indian Ocean. Using taxonomic and trait-based approaches we present the taxonomic and functional composition of shallow and deep reef communities, with distinct communities and traits dominating different depths. Depth-related changes in community metrics (taxa richness, abundance and biomass) and functional diversity metrics (richness, dispersion, and evenness) indicate complex relationships across different biological components (fish, benthos) that differ between shallow and deep reefs. These in turn translate into different patterns of reef resilience against disturbance or species invasions with depth. Notably, deep reefs host on average fewer and less abundant taxa but with higher functional contribution and originality scores, some of which are of conservation concern. Overall, the results highlight the unique nature of deep reefs that requires their explicit consideration in conservation and management activities.

Sulfurized diterpenoids in amber as diagenetic indicators of sulfate-reducing processes in past depositional environments.

Two novel compounds isolated from an amber sample from the Santonian of Piolenc (Vaucluse, SE France) were identified using nuclear magnetic resonance and high-resolution mass spectrometry as sulfurized analogues of diterpenic acids from the isopimaric series originating from ancient conifers possibly related to the Cupressaceae family. These two compounds are members of a diterpenoid series corresponding to early diagenetic transformation products of resin diterpenoids. They were likely formed once plant resin comes into contact with reduced sulfur species originating from bacterial sulfate reduction occurring in anaerobic settings such as mangroves or marshes. They represent the first evidence of sulfurization processes affecting plant resin prior to diagenetic transformation into amber. Given their mode of formation, these compounds may be used as indicators of sulfate-reducing processes in past depositional environments.

Molecular evidence for improved tribological performances of MoDTC induced by methylene-bis(dithiocarbamates) in engine lubricants.

During engine tests, it has been observed that the combined use of molybdenum dithiocarbamates (MoDTC) and methylene-bis(dithiocarbamates) (MBDTC) in formulated engine oils resulted in better fuel efficiency, keeping the friction coefficient stable at low values for a longer period of time as compared to the same oil devoid of MBDTC. Therefore, the interactions between MBDTC and MoDTC have been investigated at the molecular level. The qualitative and quantitative evolution of MoDTC in two engine oils similarly formulated, but with and without MBDTC, were compared during engine tests using a specifically developed high performance liquid chromatography-mass spectrometry (HPLC-MS) analytical method. Parallel to the molecular study, the evolution of the friction coefficients of both lubricants as well as the evolution of the fuel consumption of the engine were determined. The combined use of MoDTC and MBDTC was shown to exhibit better fuel efficiency and to maintain a relatively low friction coefficient for longer periods of time as compared to the oil devoid of MBDTC. It could be determined that the enhanced performances observed were presumably related to an extension of the lifetime of MoDTC in the engine oil containing MBDTC. Since the MoDTC remaining at the end of the engine test in oil containing MBDTC exclusively bear ligands corresponding to the dithiocarbamate moieties of MBDTC, it can be concluded that the prolonged existence of MoDTC was due to the progressive replacement of the degraded dithiocarbamate ligands on MoDTC educts by those released from MBDTC during engine functioning. As a result, the concentrations of MoDTC could be maintained at a useful level for a longer period in the engine oil containing MBDTC, leading to better fuel consumption performances.

Gradient SERS Substrates with Multiple Resonances for Analyte Screening: Fabrication and SERS Applications.

Surface-enhanced Raman spectroscopy (SERS) provides a strong enhancement to an inherently weak Raman signal, which strongly depends on the material, design, and fabrication of the substrate. Here, we present a facile method of fabricating a non-uniform SERS substrate based on an annealed thin gold (Au) film that offers multiple resonances and gap sizes within the same sample. It is not only chemically stable, but also shows reproducible trends in terms of geometry and plasmonic response. Scanning electron microscopy (SEM) reveals particle-like and island-like morphology with different gap sizes at different lateral positions of the substrate. Extinction spectra show that the plasmonic resonance of the nanoparticles/metal islands can be continuously tuned across the substrate. We observed that for the analytes 1,2-bis(4-pyridyl) ethylene (BPE) and methylene blue (MB), the maximum SERS enhancement is achieved at different lateral positions, and the shape of the extinction spectra allows for the correlation of SERS enhancement with surface morphology. Such non-uniform SERS substrates with multiple nanoparticle sizes, shapes, and interparticle distances can be used for fast screening of analytes due to the lateral variation of the resonances within the same sample.

Revealing local structural properties of an atomically thin MoSe2 surface using optical microscopy.

Using a triangular molybdenum diselenide (MoSe2) flake as surface-enhanced Raman spectroscopy (SERS) platform, we demonstrate the dependency of the Raman enhancement on laser beam polarization and local structure using copper phthalocyanine (CuPc) as probe. Second harmonic generation (SHG) and photoluminescence spectroscopy and microscopy are used to reveal the structural irregularities of the MoSe2 flake. The Raman enhancement in the focus of an azimuthally polarized beam, which possesses exclusively an in-plane electric field component is stronger than the enhancement by a focused radially polarized beam, where the out-of-plane electric field component dominates. This phenomenon indicates that the face-on oriented CuPc molecules strongly interact with the MoSe2 flake via charge transfer and dipole-dipole interaction. Furthermore, the Raman scattering maps on the irregular MoSe2 surface show a distinct correlation with the SHG and photoluminescence optical images, indicating the relationship between local structure and optical properties of the MoSe2 flake. These results contribute to understand the impacts of local structural properties on the Raman enhancement at the surface of the 2D transition-metal dichalcogenide.

Engineering of the Photon Local Density of States: Strong Inhibition of Spontaneous Emission near the Resonant and High-Refractive Index Dielectric Nano-objects.

Metallic or dielectric nano-objects change the photon local density of states of closely placed emitters, particularly when plasmon or Mie resonances are present. Depending on the shape and material of these nano-objects, they may induce either a decrease or an increase in decay rates of the excited states of the emitter. In this work, we consider the reduction of the probability of optical transitions in emitters near high-refractive index dielectric (silicon and zinc selenide) nanoparticles. We tune the spectral positions of magnetic and electric modes of nanocylinders to obtain the largest overlap of the valleys in the total decay rate spectra for differently oriented dipoles and, in this way, find the highest inhibition of about 80% for randomly oriented emitters. The spectral positions of these valleys are easy to control since the wavelengths of the modes depend on the height and diameter of nanocylinders. The inhibition value is robust to the distance between the emitter and the nanoparticle in the range of nearly 50 nm, which is crucially important for the applications, such as selective optical transition engineering and photovoltaics.

Molecular evidence for sulfurization of molybdenum dithiocarbamates (MoDTC) by zinc dithiophosphates: a key process in their synergetic interactions and the enhanced preservation of MoDTC in formulated lubricants?

Molybdenum dithiocarbamates (MoDTC) are widely used in automotive industries as lubricant additives to reduce friction and to enhance fuel economy. Sulfur-containing additives such as zinc dithiophosphates (ZnDTP) are proposed to play a key role in the improvement of friction reducing properties of MoDTC in formulated lubricants by facilitating the formation of MoS2 tribofilm at the rubbing contacts. This study focuses on the interactions between MoDTC and ZnDTP under conditions comparable with those prevailing in operating engines. The capacity of ZnDTP to sulfurize MoDTC in solution in a hydrocarbon base oil could be demonstrated. Sulfurized Mo complexes bearing one or two additional sulfur atoms (1S-MoDTC and 2S-MoDTC, respectively) which have replaced the genuine oxygen atom(s) from the MoDTC core were detected and quantified using a specifically developed HPLC-MS analytical method. A possible sulfurization mechanism relying on the higher affinity of phosphorus from ZnDTP for oxygen could be proposed. In parallel, the evolution and molecular transformation of the prepared 2S-MoDTC in hydrocarbon base oil under thermal and thermo-oxidative conditions were followed using HPLC-MS and compared with the evolution of their friction coefficients. 2S-MoDTC complexes were shown to exhibit a better retention of friction reducing capability under oxidative conditions than the "classical" MoDTC, although they did not seem to significantly reduce the friction coefficients of lubricants as compared to the "classical" MoDTC. Therefore, sulfurization of MoDTC by ZnDTP might contribute to delaying the progressive consumption of MoDTC and the loss of their friction-reducing efficiency in lubricants under thermo-oxidative conditions.

Inhomogeneous defect distribution of triangular WS2 monolayer revealed by surface-enhanced and tip-enhanced Raman and photoluminescence spectroscopy.

Confocal optical microscopy and tip-enhanced optical microscopy are applied to characterize the defect distributions in chemical vapor deposition-grown WS2 monolayer triangles qualitatively and quantitatively. The presence of defects in individual monolayer WS2 triangles is revealed with diffraction-limited spatial resolution in their photoluminescence (PL) images, from which the inhomogeneous defect density distribution is calculated, showing an inverse relationship to the PL intensity. The defect-related surface-enhanced Raman spectroscopy (SERS) effect is investigated by depositing a thin copper phthalocyanine layer (5 nm) as the probe molecule on the monolayer WS2 triangles surface. Higher SERS enhancement effects are observed at the defect-rich areas. Furthermore, tip-enhanced optical measurements are performed, which can reveal morphologically defected areas invisible in the confocal optical measurements. Furthermore, the area with high defect density appears brighter than the low-defected area in the tip-enhanced optical measurements, which are different from the observation in the confocal optical measurements. The underlying reasons are attributed to the near-field enhancement of the defect exciton emission induced by the optically excited tip and to an improved coupling efficiency between the tip-generated near-field with the altered dipole moment orientation at the local defect.

Monitoring tautomerization of single hypericin molecules in a tunable optical λ/2 microcavity.

Hypericin tautomerization that involves the migration of the labile protons is believed to be the primary photophysical process relevant to its light-activated antiviral activity. Despite the difficulty in isolating individual tautomers, it can be directly observed in single-molecule experiments. We show that the tautomerization of single hypericin molecules in free space is observed as an abrupt flipping of the image pattern accompanied with fluorescence intensity fluctuations, which are not correlated with lifetime changes. Moreover, the study can be extended to a λ/2 Fabry-Pérot microcavity. The modification of the local photonic environment by a microcavity is well simulated with a theoretical model that shows good agreement with the experimental data. Inside a microcavity, the excited state lifetime and fluorescence intensity of single hypericin molecules are correlated, and a distinct jump of the lifetime and fluorescence intensity reveals the temporal behavior of the tautomerization with high sensitivity and high temporal resolution. The observed changes are also consistent with time-dependent density functional theory calculations. Our approach paves the way to monitor and even control reactions for a wider range of molecules at the single molecule level.

Engineering Electromagnetic Hot-Spots in Nanoparticle Cluster Arrays on Reflective Substrates for Highly Sensitive Detection of (Bio)molecular Analytes.

Intense electromagnetic (EM) hot-spots arising at the junctions or gaps in plasmonic nanoparticle assemblies can drive ultrahigh sensitivity in molecular detection by surface-enhanced spectroscopies. Harnessing this potential however requires access to the confined physical space at the EM hot-spots, which is a challenge for larger analytes such as biomolecules. Here, we demonstrate self-assembly derived gold nanoparticle cluster arrays (NCAs) on gold substrates exhibiting controlled interparticle (<1 nm wide) and intercluster (<10 nm wide) hot-spots as highly promising in this direction. Sensitivity of the NCAs toward detection of small (<1 nm) or large (protein-receptor interactions) analytes in surface-enhanced Raman and metal-enhanced fluorescence assays is found to be strongly impacted by the size of the cluster and the presence of reflective substrates. Experiments supported by numerical simulations attribute the higher sensitivity to higher EM field enhancements at the hot-spots, as well as greater analyte leverage over EM hot-spots. The best-performing arrays could push the sensitivity down to picomolar detection limits for sub-nanometric organic analytes as well as large protein analytes. The investigation paves the way for rational design of plasmonic biosensors and highlights the unique capabilities of a molecular self-assembly approach toward catering to this objective.

Evidence of the retardation effect on the plasmonic resonances of aluminum nanodisks in the symmetric/asymmetric environment.

A single metallic nanodisk is the simplest plasmonic nanostructure, but it is robust enough to generate a Fano resonance in the forward and backward scattering spectra by the increment of nanodisk height in the symmetric and asymmetric dielectric environment. Thanks to the phase retardation effect, the non-uniform distribution of electric field along the height of aluminum (Al) nanodisk generates the out-of-plane higher-order modes, which interfere with the dipolar mode and subsequently result in the Fano-lineshape scattering spectra. Meanwhile, the symmetry-breaking effect by the dielectric substrate and the increment of refractive index of the symmetric dielectric environment further accelerate the phase retardation effect and contribute to the appearance of out-of-plane modes. The experimental results on the periodic Al nanodisk arrays with different heights confirm the retardation-induced higher modes in the asymmetric and symmetric environment. The appearance of higher modes and blueshifted main dips in the transmission spectra prove the dominant role of out-of-plane higher modes on the plasmonic resonances of the taller Al nanodisk.

Analyte Co-localization at Electromagnetic Gap Hot-Spots for Highly Sensitive (Bio)molecular Detection by Plasmon Enhanced Spectroscopies.

Electromagnetic hot-spots at ultranarrow plasmonic nanogaps carry immense potential to drive detection limits down to few molecules in sensors based on surface-enhanced Raman or fluorescence spectroscopies. However, leveraging the EM hot-spots requires access to the gaps, which in turn depends on the size of the analyte in relation to gap distances. Herein, we leverage a well-calibrated process based on self-assembly of block copolymer colloids on a full-wafer level to produce high-density plasmonic nanopillar arrays exhibiting a large number (>1010 cm-2) of uniform interpillar EM hot-spots. The approach allows convenient handles to systematically vary the interpillar gap distances down to a sub-10 nm regime. The results show compelling trends of the impact of analyte dimensions in relation to the gap distances toward their leverage over interpillar hot-spots and the resulting sensitivity in SERS-based molecular assays. Comparing the detection of labeled proteins in surface-enhanced Raman and metal-enhanced fluorescence configurations further reveal the relative advantage of fluorescence over Raman detection while encountering the spatial limitations imposed by the gaps. Quantitative assays with limits of detection down to picomolar concentrations are realized for both small organic molecules and proteins. The well-defined geometries delivered by a nanofabrication approach are critical to arriving at realistic geometric models to establish meaningful correlation between the structure, optical properties, and sensitivity of nanopillar arrays in plasmonic assays. The findings emphasize the need for the rational design of EM hot-spots that takes into account the analyte dimensions to drive ultrahigh sensitivity in plasmon-enhanced spectroscopies.

Hot carrier-mediated avalanche multiphoton photoluminescence from coupled Au-Al nanoantennas.

Avalanche multiphoton photoluminescence (AMPL) is observed from coupled Au-Al nanoantennas under intense laser pumping, which shows more than one order of magnitude emission intensity enhancement and distinct spectral features compared with ordinary metallic photoluminescence. The experiments are conducted by altering the incident laser intensity and polarization using a home-built scanning confocal optical microscope. The results show that AMPL originates from the recombination of avalanche hot carriers that are seeded by multiphoton ionization. Notably, at the excitation stage, multiphoton ionization is shown to be assisted by the local electromagnetic field enhancement produced by coupled plasmonic modes. At the emission step, the giant AMPL intensity can be evaluated as a function of the local field environment and the thermal factor for hot carriers, in accordance with a linear relationship between the power law exponent coefficient and the emitted photon energy. The dramatic change in the spectral profile is explained by spectral linewidth broadening mechanisms. This study offers nanospectroscopic evidence of both the potential optical damages for plasmonic nanostructures and the underlying physical nature of light-matter interactions under a strong laser field; it illustrates the significance of the emerging topics of plasmonic-enhanced spectroscopy and laser-induced breakdown spectroscopy.

Direct Observation of Structural Heterogeneity and Tautomerization of Single Hypericin Molecules.

Tautomerization is a fundamental chemical reaction which involves the relocation of a proton in the reactants. Studying the optical properties of tautomeric species is challenging because of ensemble averaging. Many molecules, such as porphines, porphycenes, or phenanthroperylene quinones, exhibit a reorientation of the transition dipole moment (TDM) during tautomerization, which can be directly observed in single-molecule experiments. Here, we study single hypericin molecules, which is a prominent phenanthroperylene quinone showing antiviral, antidepressive, and photodynamical properties. Observing abrupt flipping of the image pattern combined with time-dependent density functional theory calculations allows drawing conclusions about the coexistence of four tautomers and their conversion path. This approach allows the unambiguous assignment of a TDM orientation to a specific tautomer and enables the determination of the chemical structure in situ. Our approach can be applied to other molecules showing TDM reorientation during tautomerization, helping to gain a deeper understanding of this important process.

Geological alteration of Precambrian steroids mimics early animal signatures.

The absence of unambiguous animal body fossils in rocks older than the late Ediacaran has rendered fossil lipids the most promising tracers of early organismic complexity. Yet much debate surrounds the various potential biological sources of putative metazoan steroids found in Precambrian rocks. Here we show that 26-methylated steranes-hydrocarbon structures currently attributed to the earliest animals-can form via geological alteration of common algal sterols, which carries important implications for palaeo-ecological interpretations and inhibits the use of such unconventional 'sponge' steranes for reconstructing early animal evolution.

Influence of order-to-disorder transitions on the optical properties of the aluminum plasmonic metasurface.

To mimic the optical influence of disorder in condensed matter, the effect of uniform disorder on plasmonic resonances were investigated numerically and experimentally on aluminum (Al) nanoparticle arrays. Resorting to the analogue of a plasmonic periodic array to a crystal on the sharp optical spectrum and its anisotropy, the disorder in the transition from crystal to glass (with broadened spectrum and isotropy) is imitated by three kinds of Al plasmonic metasurfaces: varying the displacement, size and rotation of each Al nanoparticle in the periodic array. The random variation on the location or size of each Al nanodisk in the plasmonic crystal induces broadening and reduction of their plasmonic resonances without significantly shifting its wavelength. Moreover, by rotating each Al nanorod in the plasmonic crystal by a random angle, the polarization dependence of plasmonic resonances is progressively decreased by increasing the rotation disorder. Thanks to these three kinds of Al metasurfaces, an enlightened understanding of the random physics in the solid state and the influence of manufacturing deviation in nanophotonics is supported.

Surface Enhanced Raman Scattering on Regular Arrays of Gold Nanostructures: Impact of Long-Range Interactions and the Surrounding Medium.

Long-range interaction in regular metallic nanostructure arrays can provide the possibility to manipulate their optical properties, governed by the excitation of localized surface plasmon (LSP) resonances. When assembling the nanoparticles in an array, interactions between nanoparticles can result in a strong electromagnetic coupling for specific grating constants. Such a grating effect leads to narrow LSP peaks due to the emergence of new radiative orders in the plane of the substrate, and thus, an important improvement of the intensity of the local electric field. In this work, we report on the optical study of LSP modes supported by square arrays of gold nanodiscs deposited on an indium tin oxyde (ITO) coated glass substrate, and its impact on the surface enhanced Raman scattering (SERS) of a molecular adsorbate, the mercapto benzoic acid (4-MBA). We estimated the Raman gain of these molecules, by varying the grating constant and the refractive index of the surrounding medium of the superstrate, from an asymmetric medium (air) to a symmetric one (oil). We show that the Raman gain can be improved with one order of magnitude in a symmetric medium compared to SERS experiments in air, by considering the appropriate grating constant. Our experimental results are supported by FDTD calculations, and confirm the importance of the grating effect in the design of SERS substrates.

Hybrid nanostructured plasmonic electrodes for flexible organic light-emitting diodes.

Improved performance in flexible organic light-emitting diodes (OLEDs) is demonstrated by using a hybrid nanostructured plasmonic electrode consisting of silver nanowires (AgNWs) decorated with silver nanoparticles (AgNPs) and covered by exfoliated graphene sheets. Such all-solution processed electrodes show high optical transparency and electrical conductivity. When integrated in an OLED with super yellow polyphenylene vinylene as the emissive layer, the plasmon coupling of the NW-NP hybrid plasmonic system is found to significantly enhance the fluorescence, demonstrated by both simulations and photoluminescence measurements, leading to a current efficiency of 11.61 cd A-1 and a maximum luminance of 20 008 cd m-2 in OLEDs. Stress studies reveal a superior mechanical flexibility to the commercial indium-tin-oxide (ITO) counterparts, due to the incorporation of exfoliated graphene sheets. Our results show that these hybrid nanostructured plasmonic electrodes can be applied as an effective alternative to ITO for use in high-performance flexible OLEDs.