Reconfigurable chirality with achiral excitonic materials in the strong-coupling regime.

We introduce and theoretically analyze the concept of manipulating optical chirality via strong coupling of the optical modes of chiral nanostructures with excitonic transitions in molecular layers or semiconductors. With chirality being omnipresent in chemistry and biomedicine, and highly desirable for technological applications related to efficient light manipulation, the design of nanophotonic architectures that sense the handedness of molecules or generate the desired light polarization in an externally controllable manner is of major interdisciplinary importance. Here we propose that such capabilities can be provided by the mode splitting resulting from polaritonic hybridization. Starting with an object with well-known chiroptical response-here, for a proof of concept, a chiral sphere-we show that strong coupling with a nearby excitonic material generates two spectral branches that retain the object's high chirality density, which manifest most clearly through anticrossings in circular-dichroism or differential-scattering dispersion diagrams. These windows can be controlled by the intrinsic properties of the excitonic layer and the strength of the interaction, enabling thus the post-fabrication manipulation of optical chirality. Our findings are further verified via simulations of circular dichroism of a realistic chiral architecture, namely a helical assembly of plasmonic nanospheres embedded in a resonant matrix.

Quantum surface effects in the electromagnetic coupling between a quantum emitter and a plasmonic nanoantenna: time-dependent density functional theory vs. semiclassical Feibelman approach.

We use time-dependent density functional theory (TDDFT) within the jellium model to study the impact of quantum-mechanical effects on the self-interaction Green's function that governs the electromagnetic interaction between quantum emitters and plasmonic metallic nanoantennas. A semiclassical model based on the Feibelman parameters, which incorporates quantum surface-response corrections into an otherwise classical description, confirms surface-enabled Landau damping and the spill out of the induced charges as the dominant quantum mechanisms strongly affecting the nanoantenna-emitter interaction. These quantum effects produce a redshift and broadening of plasmonic resonances not present in classical theories that consider a local dielectric response of the metals. We show that the Feibelman approach correctly reproduces the nonlocal surface response obtained by full quantum TDDFT calculations for most nanoantenna-emitter configurations. However, when the emitter is located in very close proximity to the nanoantenna surface, we show that the standard Feibelman approach fails, requiring an implementation that explicitly accounts for the nonlocality of the surface response in the direction parallel to the surface. Our study thus provides a fundamental description of the electromagnetic coupling between plasmonic nanoantennas and quantum emitters at the nanoscale.

Disentangling Cathodoluminescence Spectra in Nanophotonics: Particle Eigenmodes vs Transition Radiation.

Cathodoluminescence spectroscopy performed in an electron microscope has proven a versatile tool for analyzing the near- and far-field optical response of plasmonic and dielectric nanostructures. Nevertheless, the transition radiation produced by electron impact is often disregarded in the interpretation of the spectra recorded from resonant nanoparticles. Here we show, experimentally and theoretically, that transition radiation can by itself generate distinct resonances that, depending on the time-of-flight of the electron beam inside the particle, can result from constructive or destructive interference in time. Superimposed on the eigenmodes of the investigated structures, these resonances can distort the recorded spectrum and lead to potentially erroneous assignment of modal characters to the spectral features. We develop an intuitive analogy that helps distinguish between the two contributions. As an example, we focus on the case of silicon nanospheres and show that our analysis facilitates the unambiguous interpretation of experimental measurements on Mie-resonant nanoparticles.

Detection of the Faraday Chiral Anisotropy.

The connection between chirality and electromagnetism has attracted much attention through the recent history of science, allowing the discovery of crucial nonreciprocal optical phenomena within the context of fundamental interactions between matter and light. A major phenomenon within this family is the so-called Faraday chiral anisotropy, the long-predicted but yet unobserved effect which arises due to the correlated coaction of both natural and magnetically induced optical activities at concurring wavelengths in chiral systems. Here, we report on the detection of the elusive anisotropic Faraday chiral phenomenon and demonstrate its enantioselectivity. The existence of this fundamental effect reveals the accomplishment of envisioned nonreciprocal electromagnetic metamaterials referred to as Faraday chiral media, systems where novel electromagnetic phenomena such as negative refraction of light at tunable wavelengths or even negative reflection can be realized. From a more comprehensive perspective, our findings have profound implications for the general understanding of parity-violating photon-particle interactions in magnetized media.

On the applicability of quantum-optical concepts in strong-coupling nanophotonics.

Rooted in quantum optics and benefiting from its well-established foundations, strong coupling in nanophotonics has experienced increasing popularity in recent years. With nanophotonics being an experiment-driven field, the absence of appropriate theoretical methods to describe ground-breaking advances has often emerged as an important issue. To address this problem, the temptation to directly transfer and extend concepts already available from quantum optics is strong, even if a rigorous justification is not always available. In this review we discuss situations where, in our view, this strategy has indeed overstepped its bounds. We focus on exciton-plasmon interactions, and particularly on the idea of calculating the number of excitons involved in the coupling. We analyse how, starting from an unfounded interpretation of the term N/V that appears in theoretical descriptions at different levels of complexity, one might be tempted to make independent assumptions for what the number N and the volume V are, and attempt to calculate them separately. Such an approach can lead to different, often contradictory results, depending on the initial assumptions (e.g. through different treatments of V as the-ambiguous in plasmonics-mode volume). We argue that the source of such contradictions is the question itself-How many excitons are coupled?, which disregards the true nature of the coupled components of the system, has no meaning and often not even any practical importance. If one is interested in validating the quantum nature of the system-which appears to be the motivation driving the pursuit of strong coupling with small N-one could instead focus on quantities such as the photon emission rate or the second-order correlation function. While many of the issues discussed here may appear straightforward to specialists, our target audience is predominantly newcomers to the field, either students or scientists specialised in different disciplines. We have thus tried to minimise the occurrence of proofs and overly-technical details, and instead provide a qualitative discussion of analyses that should be avoided, hoping to facilitate further growth of this promising area.

Strong Coupling of Carbon Quantum Dots in Plasmonic Nanocavities.

Confining light in extremely small cavities is crucial in nanophotonics, central to many applications. Employing a unique nanoparticle-on-mirror plasmonic structure and using a graphene film as a spacer, we create nanoscale cavities with volumes of only a few tens of cubic nanometers. The ultracompact cavity produces extremely strong optical near-fields, which facilitate the formation of single carbon quantum dots in the cavity and simultaneously empower the strong coupling between the excitons of the formed carbon quantum dot and the localized surface plasmons. This is manifested in the optical scattering spectra, showing a magnificent Rabi splitting of up to 200 meV under ambient conditions. In addition, we demonstrate that the strong coupling is tuneable with light irradiation. This opens new paradigms for investigating the fundamental light emission properties of carbon quantum dots in the quantum regime and paves the way for many significant applications.

Atomically Thin Boron Nitride as an Ideal Spacer for Metal-Enhanced Fluorescence.

Metal-enhanced fluorescence (MEF) considerably enhances the luminescence for various applications, but its performance largely depends on the dielectric spacer between the fluorophore and plasmonic system. It is still challenging to produce a defect-free spacer having an optimized thickness with a sub-nanometer accuracy that enables reusability without affecting the enhancement. In this study, we demonstrate the use of atomically thin hexagonal boron nitride (BN) as an ideal MEF spacer owing to its multifold advantages over the traditional dielectric thin films. With rhodamine 6G as a representative fluorophore, it largely improves the enhancement factor (up to ∼95 ± 5), sensitivity (10-8 M), reproducibility, and reusability (∼90% of the plasmonic activity is retained after 30 cycles of heating at 350 °C in air) of MEF. This can be attributed to its two-dimensional structure, thickness control at the atomic level, defect-free quality, high affinities to aromatic fluorophores, good thermal stability, and excellent impermeability. The atomically thin BN spacers could increase the use of MEF in different fields and industries.

Ultraviolet Interband Plasmonics With Si Nanostructures.

Although Si acts as an electrical semiconductor, it has properties of an optical dielectric. Here, we revisit the behavior of Si as a plasmonic metal. This behavior was previously shown to arise from strong interband transitions that lead to negative permittivity of Si across the ultraviolet spectral range. However, few have studied the plasmonic characteristics of Si, particularly in its nanostructures. In this paper, we report localized plasmon resonances of Si nanostructures and the observation of plasmon hybridization in the UV (∼250 nm wavelength). In addition, simulation results show that Si nanodisk dimers can achieve a local intensity enhancement greater than ∼500-fold in a 1 nm gap. Lastly, we investigate hybrid Si-Al nanostructures to achieve sharp resonances in the UV, due to the coupling between plasmon resonances supported by Si and Al nanostructures. These results will have potential applications in the UV range, such as nanostructured devices for spectral filtering, plasmon-enhanced Si photodetectors, interrogation of molecular chirality, and catalysis. It could have significant impact on UV photolithography on patterned Si structures.

Field enhancement and spectral features of hexagonal necklaces of silver nanoparticles for enhanced nonlinear optical processes.

The nonlinear properties of hybrid metallic-dielectric systems are attracting great interest due to their potential for the enhancement of frequency conversion processes at nanoscale dimensions. In this work, we theoretically and experimentally address the correlation between the near field distribution of hexagonal plasmonic necklaces of silver nanoparticles formed on the surface of a LiNbO3 crystal and the second harmonic generation (SHG) produced by this nonlinear crystal in the vicinities of the necklaces. The spectral response of the hexagonal necklaces does not depend on the polarization direction and is characterized by two main modes, the absorptive high-energy mode located in the UV spectral region and the lower energy mode, which is strongly radiant and extends from the visible to the near infrared region. We show that the spatial distribution of the enhanced SHG is consistent with the local field related to the low energy plasmon mode, which spectrally overlaps the fundamental beam. The results are in agreement with the low absorption losses of this mode and the two-photon character of the nonlinear process and provide deeper insight in the connection between the linear and nonlinear optical properties of the hybrid plasmonic-ferroelectric system. The study also highlights the potential of hexagonal necklaces as useful plasmonic platforms for enhanced optical processes at the nanoscale.

2D Arrays of Hexagonal Plasmonic Necklaces for Enhanced Second Harmonic Generation.

Hexagonal plasmonic necklaces of silver nanoparticles organized in 2D superlattices on functional ferroelectric templates are fabricated in large-scale spatial regions by using a surfactant-free photo-deposition process. The plasmonic necklaces support broad radiative plasmonic resonances allowing the enhancement of second harmonic generation (SHG) at the ferroelectric domain boundaries. A 400-fold SHG enhancement is achieved at the near-UV spectral region with subsequent interest for technological applications.

Molecular fluorescence enhancement in plasmonic environments: exploring the role of nonlocal effects.

Molecular spontaneous emission and fluorescence depend strongly on the emitter local environment. Plasmonic nanoparticles provide excellent templates for tailoring fluorophore emission, as they exhibit potential for both fluorescence enhancement and quenching, depending on emitter positioning in the nanoparticle vicinity. Here we explore the influence of hitherto disregarded nonclassical effects on the description of emitter-plasmon hybrids, focusing on the roles of the metal nonlocal response and especially size-dependent plasmon damping. Through extensive modelling of metallic nanospheres and nanoshells coupled to dipole emitters, we show that within a purely classical description a remarkable fluorescence enhancement can be achieved. However, once departing from the local-response approximation, and particularly by implementing the recent generalised nonlocal optical response theory, which provides a more complete physical description combining electron convection and diffusion, we show that not only are fluorescence rates dramatically reduced compared to the predictions of the local description and the common hydrodynamic Drude model, but the optimum emitter-nanoparticle distance is also strongly affected. In this respect, experimental measurements of fluorescence, the theoretical description of which requires a precise concurrent evaluation of far- and near-field properties of the system, constitute a novel, more sensitive probe for assessing the validity of state-of-the-art nonclassical theories.

Robustness of the far-field response of nonlocal plasmonic ensembles.

Contrary to classical predictions, the optical response of few-nm plasmonic particles depends on particle size due to effects such as nonlocality and electron spill-out. Ensembles of such nanoparticles are therefore expected to exhibit a nonclassical inhomogeneous spectral broadening due to size distribution. For a normal distribution of free-electron nanoparticles, and within the simple nonlocal hydrodynamic Drude model, both the nonlocal blueshift and the plasmon linewidth are shown to be considerably affected by ensemble averaging. Size-variance effects tend however to conceal nonlocality to a lesser extent when the homogeneous size-dependent broadening of individual nanoparticles is taken into account, either through a local size-dependent damping model or through the Generalized Nonlocal Optical Response theory. The role of ensemble averaging is further explored in realistic distributions of isolated or weakly-interacting noble-metal nanoparticles, as encountered in experiments, while an analytical expression to evaluate the importance of inhomogeneous broadening through measurable quantities is developed. Our findings are independent of the specific nonclassical theory used, thus providing important insight into a large range of experiments on nanoscale and quantum plasmonics.

Plasmonic enhancement of second harmonic generation from nonlinear RbTiOPO4 crystals by aggregates of silver nanostructures.

We demonstrate a 60-fold enhancement of the second harmonic generation (SHG) response at the nanoscale in a hybrid metal-dielectric system. By using complex silver nanostructures photochemically deposited on the polar surface of a ferroelectric crystal, we tune the plasmonic resonances from the visible to the near-infrared (NIR) spectral region, matching either the SH or the fundamental frequency. In both cases the SHG signal at the metal-dielectric interface is enhanced, although with substantially different enhancement values: around 5 times when the plasmonic resonance is at the SH frequency or up to 60 times when it matches the fundamental NIR radiation. The results are consistent with the more spatially-extended near-field response of complex metallic nanostructures and can be well explained by taking into account the quadratic character of the SHG process. The work points out the potential of aggregates of silver nanostructures for enhancing optical nonlinearities at the nanoscale and provides an alternative approach for the development of nanometric nonlinear photonic devices in a scalable way.

Plasmon-Assisted Nd(3+)-Based Solid-State Nanolaser.

Solid-state lasers constitute essential tools in a variety of scientific and technological areas, being available in many different designs. However, although nanolasing has been successfully achieved for dyes and semiconductor gain media associated with plasmonic structures, the operation of solid-state lasers beyond the diffraction limit has not been reported yet. Here, we demonstrate room temperature laser action with subwavelength confinement in a Nd(3+)-based solid-state laser by means of the localized surface plasmon resonances supported by chains of metallic nanoparticles. We show a 50% reduction of the pump power at threshold and a remarkable 15-fold improvement of the slope efficiency with respect to the bulk laser operation. The results can be extended to the large diversity of solid-state lasers with the subsequent impact on their applications.

Controlling solid state gain media by deposition of silver nanoparticles: from thermally- quenched to plasmon-enhanced Nd(3+) luminescence.

We show the possibility of controlling the optical properties of Nd(3+) laser ions by using different configurations of metallic nanoparticles (NPs) deposited on a solid state gain medium. In particular, we analyze the effect of two different silver NP arrangements on the optical properties of Nd(3+) ions in LiNbO(3): a two-dimensional (2D) high density and disordered Ag NP distribution and a one-dimensional (1D) long single chain of Ag NPs. We demonstrate that while the 2D disordered distribution produces a thermal quenching of the Nd(3+) luminescence, the 1D single chain leads to the enhancement of the fluorescence from the (4)F(3/2) metastable state. The experimental data are theoretically interpreted by taking into account the different character, radiative or non-radiative, of the localized surface plasmonic modes supported by the Ag nanoparticle distributions at the excitation wavelength. The results point out the capabilities of rare earth ions as optical tools to probe the local plasmonic fields and are relevant to determine the optimal configuration of metallic arrays to improve the performance of potential rare earth ion based sub-micrometer lasers.

Generalized circuit model for coupled plasmonic systems.

We develop an analytic circuit model for coupled plasmonic dimers separated by small gaps that provides a complete account of the optical resonance wavelength. Using a suitable equivalent circuit, it shows how partially conducting links can be treated and provides quantitative agreement with both experiment and full electromagnetic simulations. The model highlights how in the conducting regime, the kinetic inductance of the linkers set the spectral blue-shifts of the coupled plasmon.

Nanooptics of molecular-shunted plasmonic nanojunctions.

Gold nanoparticles are separated above a planar gold film by 1.1 nm thick self-assembled molecular monolayers of different conductivities. Incremental replacement of the nonconductive molecules with a chemically equivalent conductive version differing by only one atom produces a strong 50 nm blue-shift of the coupled plasmon. With modeling this gives a conductance of 0.17G(0) per biphenyl-4,4'-dithiol molecule and a total conductance across the plasmonic junction of 30G(0). Our approach provides a reliable tool quantifying the number of molecules in each plasmonic hotspot, here <200.

Monitoring morphological changes in 2D monolayer semiconductors using atom-thick plasmonic nanocavities.

Nanometer-sized gaps between plasmonically coupled adjacent metal nanoparticles enclose extremely localized optical fields, which are strongly enhanced. This enables the dynamic investigation of nanoscopic amounts of material in the gap using optical interrogation. Here we use impinging light to directly tune the optical resonances inside the plasmonic nanocavity formed between single gold nanoparticles and a gold surface, filled with only yoctograms of semiconductor. The gold faces are separated by either monolayers of molybdenum disulfide (MoS2) or two-unit-cell thick cadmium selenide (CdSe) nanoplatelets. This extreme confinement produces modes with 100-fold compressed wavelength, which are exquisitely sensitive to morphology. Infrared scattering spectroscopy reveals how such nanoparticle-on-mirror modes directly trace atomic-scale changes in real time. Instabilities observed in the facets are crucial for applications such as heat-assisted magnetic recording that demand long-lifetime nanoscale plasmonic structures, but the spectral sensitivity also allows directly tracking photochemical reactions in these 2-dimensional solids.

Optical response of threaded chain plasmons: from capacitive chains to continuous nanorods.

We present a detailed theoretical analysis of the optical response of threaded plasmonic nanoparticle strings, chains of metallic nanoparticles connected by cylindrical metallic bridges (threads), based on full-electrodynamic calculations. The extinction spectra of these complex metallic nanostructures are dominated by large resonances in the near infrared, which are associated with charge transfer along the entire string. By analysing contour plots of the electric field amplitude and phase we show that such strings can be interpreted as an intermediate situation between metallic nanoparticle chains and metallic nanorods, exhibiting characteristics of both. Modifying the dielectric environment, the number of nanoparticles within the strings, and the dimensions of the threads, allows for tuning the optical response of the strings within a very broad region in the visible and near infrared.

Threading plasmonic nanoparticle strings with light.

Nanomaterials find increasing application in communications, renewable energies, electronics and sensing. Because of its unsurpassed speed and highly tuneable interaction with matter, using light to guide the self-assembly of nanomaterials can open up novel technological frontiers. However, large-scale light-induced assembly remains challenging. Here we demonstrate an efficient route to nano-assembly through plasmon-induced laser threading of gold nanoparticle strings, producing conducting threads 12±2 nm wide. This precision is achieved because the nanoparticles are first chemically assembled into chains with rigidly controlled separations of 0.9 nm primed for re-sculpting. Laser-induced threading occurs on a large scale in water, tracked via a new optical resonance in the near-infrared corresponding to a hybrid chain/rod-like charge transfer plasmon. The nano-thread width depends on the chain mode resonances, the nanoparticle size, the chain length and the peak laser power, enabling nanometre-scale tuning of the optical and conducting properties of such nanomaterials.