Fluorescence Enhancement in Topologically Optimized Gallium Phosphide All-Dielectric Nanoantennas

Nanoantennas capable of large fluorescence enhancement with minimal absorption are crucial for future optical technologies from single-photon sources to biosensing. Efficient dielectric nanoantennas have been designed, however, evaluating their performance at the individual emitter level is challenging due to the complexity of combining high-resolution nanofabrication, spectroscopy and nanoscale positioning of the emitter. Here, we study the fluorescence enhancement in infinity-shaped gallium phosphide (GaP) nanoantennas based on a topologically optimized design. Using fluorescence correlation spectroscopy (FCS), we probe the nanoantennas enhancement factor and observe an average of 63-fold fluorescence brightness enhancement with a maximum of 93-fold for dye molecules in nanogaps between 20 and 50 nm. The experimentally determined fluorescence enhancement of the nanoantennas is confirmed by numerical simulations of the local density of optical states (LDOS). Furthermore, we show that beyond design optimization of dielectric nanoantennas, increased performances can be achieved via tailoring of nanoantenna fabrication.

Nanophotonic Chirality Transfer to Dielectric Mie Resonators

Nanophotonics can boost the weak circular dichroism of chiral molecules. One mechanism for enhanced chiral sensing relies on using a resonator to create fields with high optical chirality at the molecular position. Here, we elucidate how the reverse interaction between molecules and the resonator, called chirality transfer, can produce stronger circular dichroism. The chiral analyte modifies the electric and magnetic dipole moments of the resonator, imprinting a chiral response on an otherwise achiral resonance. We demonstrate that silicon nanoparticles and metasurfaces tailored for chirality transfer generate chiroptical signals orders of magnitude higher than the contribution from optical chirality alone. We derive closed-form equations for the dependence of chirality transfer on molecular chirality, molecule-resonator distance, and Mie coefficients. We propose a dielectric metasurface for a 900-fold circular dichroism enhancement on the basis of these principles. Finally, we identify a fundamental limit to chirality transfer. Our findings thus establish key concepts for nanophotonic chiral sensing.

Impact of indirect transitions on valley polarization in WS2 and WSe2

Controlling the momentum of carriers in semiconductors, known as valley polarization, is a new resource for optoelectronics and information technologies. Materials exhibiting high polarization are needed for valley-based devices. Few-layer WS₂ shows a remarkable spin-valley polarization above 90%, even at room temperature. In stark contrast, polarization is absent for few-layer WSe₂ despite the expected material similarities. Here, we explain the origin of valley polarization in both materials based on the interplay between two indirect optical transitions. We show that the relative energy minima at the Λ- and K-valleys in the conduction band determine the spin-valley polarization of the direct K-K transition. Polarization appears as the energy of the K-valley rises above the Λ-valley as a function of temperature and number of layers. Our results advance the understanding of the high spin-valley polarization in WS₂. This insight will impact the design of both passive and tunable valleytronic devices operating at room temperature.

Fabrication of plasmonic structures with well-controlled nanometric features: a comparison between lift-off and ion beam etching

After providing a detailed overview of nanofabrication techniques for plasmonics, we discuss in detail two different approaches for the fabrication of metallic nanostructures based on e-beam lithography. The first approach relies on a negative e-beam resist, followed by ion beam milling, while the second uses a positive e-beam resist and lift-off. Overall, ion beam etching provides smaller and more regular features including tiny gaps between sub-parts, that can be controlled down to about 10 nm. In the lift-off process, the metal atoms are deposited within the resist mask and can diffuse on the substrate, giving rise to the formation of nanoclusters that render the nanostructure outline slightly fuzzy. Scattering cross sections computed for both approaches highlight some spectral differences, which are especially visible for structures that support complex resonances, such as Fano resonances. Both techniques can produce useful nanostructures and the results reported therein should guide the researcher to choose the best suited approach for a given application, depending on the available technology.

Dual Nanoresonators for Ultrasensitive Chiral Detection

The discrimination of enantiomers is crucial in biochemistry. However, chiral sensing faces significant limitations due to inherently weak chiroptical signals. Nanophotonics is a promising solution to enhance sensitivity thanks to increased optical chirality maximized by strong electric and magnetic fields. Metallic and dielectric nanoparticles can separately provide electric and magnetic resonances. Here we propose their synergistic combination in hybrid metal-dielectric nanostructures to exploit their dual character for superchiral fields beyond the limits of single particles. For optimal optical chirality, in addition to maximization of the resonance strength, the resonances must spectrally coincide. Simultaneously, their electric and magnetic fields must be parallel and π/2 out of phase and spatially overlap. We demonstrate that the interplay between the strength of the resonances and these optimal conditions constrains the attainable optical chirality in resonant systems. Starting from a simple symmetric nanodimer, we derive closed-form expressions elucidating its fundamental limits of optical chirality. Building on the trade-offs of different classes of dimers, we then suggest an asymmetric dual dimer based on realistic materials. These dual nanoresonators provide strong and decoupled electric and magnetic resonances together with optimal conditions for chiral fields. Finally, we introduce more complex dual building blocks for a metasurface with a record 300-fold enhancement of local optical chirality in nanoscale gaps, enabling circular dichroism enhancement by a factor of 20. By combining analytical insight and practical designs, our results put forward hybrid resonators to increase chiral sensitivity, particularly for small molecular quantities.

Effective Negative Diffusion of Singlet Excitons in Organic Semiconductors

Using diffraction-limited ultrafast imaging techniques, we investigate the propagation of singlet and triplet excitons in single-crystal tetracene. Instead of an expected broadening, the distribution of singlet excitons narrows on a nanosecond time scale after photoexcitation. This narrowing results in an effective negative diffusion in which singlet excitons migrate toward the high-density region, eventually leading to a singlet exciton distribution that is smaller than the laser excitation spot. Modeling the excited-state dynamics demonstrates that the origin of the anomalous diffusion is rooted in nonlinear triplet-triplet annihilation (TTA). We anticipate that this is a general phenomenon that can be used to study exciton diffusion and nonlinear TTA rates in semiconductors relevant for organic optoelectronics.

Hybrid Metal-Dielectric Metasurfaces for Refractive Index Sensing

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 SiO₂ 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 SiO₂ 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.

Correlated Exciton Fluctuations in a Two-Dimensional Semiconductor on a Metal

Excitons in nanoscale materials can exhibit fluorescence fluctuations. Intermittency is pervasive in zero-dimensional emitters such as single molecules and quantum dots. In contrast, two-dimensional semiconductors are generally regarded as stable light sources. Noise contains, however, valuable information about a material. Here, we demonstrate fluorescence fluctuations in a monolayer semiconductor due to sensitivity to its nanoscopic environment focusing on the case of a metal film. The fluctuations are spatially correlated over tens of micrometers and follow power-law statistics, with simultaneous changes in emission intensity and lifetime. At low temperatures, an additional spectral contribution from interface trap states emerges with fluctuations that are correlated with neutral excitons and anticorrelated with trions. Mastering exciton fluctuations has implications for light-emitting devices such as single-photon sources and could lead to novel excitonic sensors. The quantification of fluorescence fluctuations, including imaging, unlocks a set of promising tools to characterize and exploit two-dimensional semiconductors and their interfaces.

Origin of enhancement in Raman scattering from Ag-dressed carbon-nanotube antennas: experiment and modelling

The D- and G-band Raman signals from random arrays of vertically aligned, multi-walled carbon nanotubes are significantly enhanced (up to ∼14×) while the signal from the underlying Si substrate is simultaneously attenuated (up to ∼6×) when the nanotubes are dressed, either capped or coated, with Ag. These Ag-induced counter-changes originate with the difference in geometry of the nanotubes and planar Si substrate and contrast in the Ag depositions on the substrate (essentially thin film) and the nanotube (nano-particulate). The surface integral equation technique is used to perform detailed modelling of the electromagnetic response of the system in a computationally efficient manner. Within the modelling the overall antenna response of the Ag-dressed nanotubes is shown to underpin the main contribution to enhancement of the nanotube Raman signal with hot-spots between the Ag nanoparticles making a subsidiary contribution on account of their relatively weak penetration into the nanotube walls. Although additional hot-spot activity likely accounts for a shortfall in modelling relative to experiment it is nonetheless the case that the significant antenna-driven enhancement stands in marked contrast to the hot-spot dominated enhancement of the Raman spectra from molecules adsorbed on the same Ag-dressed structures. The Ag-dressing procedure for amplifying the nanotube Raman output not only allows for ready characterisation of individual nanotubes, but also evidences a small peak at ∼1150 cm^-1 (not visible for the bare, undressed nanotube) which is suggested to be due to the presence of trans-polyacetylene in the structures.

Twisting Fluorescence through Extrinsic Chiral Antennas

Plasmonic antennas and planar structures have been undergoing intensive developments in order to control the scattering and absorption of light. One specific class, extrinsic chiral surfaces, that does not possess 2-fold rotational symmetry exhibits strong asymmetric transmission for different circular polarizations under obliquely incident illumination. In this work, we show that the design of those surfaces can be optimized with complex multipolar resonances in order to twist the fluorescence emission from nearby molecules. While this emission is usually dipolar and linearly polarized, the interaction with these resonances twists it into a multipolar radiation pattern with opposite helicity in different directions. The proposed structure maximizes this effect and provides control over the polarization of light. Splitting of left- and right-handed circularly polarized light is experimentally obtained in the backward direction. These results highlight the intricate interplay between the near-field absorption and the far-field scattering of a plasmonic nanostructure and are further used for modifying the emission of incoherent quantum sources. Our finding can potentially lead to the development of polarization- and angle-resolved ultracompact optical devices.

Controlling the nonlinear optical properties of plasmonic nanoparticles with the phase of their linear response

We numerically investigate the second harmonic generation from different plasmonic systems and evidence the key role played in their nonlinear response by the phase at the fundamental wavelength. In the case of a single plasmonic nanorod, the interference between the second harmonic dipolar and quadrupolar emission modes depends on their relative phase, which is deeply related to the excitation wavelength. The knowledge obtained in this simple case is then used to describe and understand the nonlinear response from a more complex structure, namely a gold nanodolmen. The complex phase evolution associated with a Fano resonance arising at the fundamental wavelength enables dramatically modifying the second harmonic emission patterns from plasmonic metamolecules within minute wavelength shifts. These results emphasize the importance of the phase in the nonlinear optical processes arising in plasmonic nanostructures, in addition to the increase in conversion yield associated with the excitation of localized surface plasmon resonances.

Internal optical forces in plasmonic nanostructures

We present a computational study of the internal optical forces arising in plasmonic gap antennas, dolmen structures and split rings. We find that very strong internal forces perpendicular to the propagation direction appear in these systems. These internal forces show a rich behaviour with varying wavelength, incident polarisation and geometrical parameters, which we explain in terms of the polarisation charges induced on the structures. Various interesting and anomalous features arise such as lateral force reversal, optical pulling force, and circular polarisation-induced forces and torques along directions symmetry-forbidden for orthogonal linear polarisations. Understanding these effects and mastering internal forces in plasmonic nanostructures will be instrumental in implementing new functionalities in these nanophotonic systems.

Optical forces in nanoplasmonic systems: how do they work, what can they be useful for?

In this article, we share our vision for a future nanofactory, where plasmonic trapping is used to control the different manufacturing steps associated with the transformation of initial nanostructures to produce complex compounds. All the different functions existing in a traditional factory can be translated at the nanoscale using the optical forces produced by plasmonic nanostructures. A detailed knowledge of optical forces in plasmonic nanostructures is however essential to design such a nanofactory. To this end, we review the numerical techniques for computing optical forces on nanostructures immersed in a strong optical field and show under which conditions approximate solutions, like the dipole approximation, can be used in a satisfactory manner. Internal optical forces on realistic plasmonic antennas are investigated and the reconfiguration of a Fano-resonant plasmonic system using such internal forces is also studied in detail.

Optical forces and torques on realistic plasmonic nanostructures: a surface integral approach

We develop a novel formalism to calculate the optical forces and torques on complex and realistic nanostructures by combining the surface integral equation (SIE) technique with Maxwell's stress tensor. The optical force is calculated directly on the scatterer surface from the currents obtained from the SIE, which does not require an additional surface to evaluate Maxwell's stress tensor; this is especially useful for intricate geometries such as plasmonic antennas. SIE enables direct evaluation of forces from the surface currents very efficiently and accurately for complex systems. As a proof of concept, we establish the accuracy of the model by comparing the results with the calculations from the Mie theory. The flexibility of the method is demonstrated by simulating a realistic plasmonic system with intricate geometry.

Polarisation charges and scattering behaviour of realistically rounded plasmonic nanostructures

We study the effect of realistically rounding nanorod antennae and gap antennae on their far field and near field properties. The simulations show that both scattering behaviour and polarisation charge distribution depend significantly on rounding. Rounding is also seen to have a major effect on coupling between nanostructures. The results suggest that it is important to incorporate the effect of rounding to be able to design plasmonic nanostructures with desired properties.