Extraordinary Enhancement of Quadrupolar Transitions Using Nanostructured Graphene

Surface plasmons supported by metallic nanostructures interact strongly with light and confine it into subwavelength volumes, thus forcing the corresponding electric field to vary within nanoscale distances. This results in exceedingly large field gradients that can be exploited to enhance the quadrupolar transitions of quantum emitters located in the vicinity of the nanostructure. Graphene nanostructures are ideally suited for this task, since their plasmons can confine light into substantially smaller volumes than equivalent excitations sustained by conventional plasmonic nanostructures. Furthermore, in addition to their geometric tunability, graphene plasmons can also be efficiently tuned by controlling the doping level of the nanostructure, which can be accomplished either chemically or electrostatically. Here, we provide a detailed investigation of the enhancement of the field gradient in the vicinity of different graphene nanostructures. Using rigorous solutions of Maxwell’s equations, as well as an analytic electrostatic approach, we analyze how this quantity is affected by the size, shape, doping level, and quality of the nanostructure. We investigate, as well, the performance of arrays of nanoribbons, which constitute a suitable platform for the experimental verification of our predictions. The results of this work bring new possibilities to enhance and control quadrupolar transitions of quantum emitters, which can find application in the detection of relevant chemical species, as well as in the design of novel light-emitting devices

Fast Topology Optimization for Near-Field Focusing All-Dielectric Metasurfaces Using the Discrete Dipole Approximation

Using an efficient implementation of the discrete dipole approximation and topology optimization, we design all-dielectric metasurfaces capable of focusing light into intense deep subwavelength hotspots. The light focusing of these metasurfaces far outweighs conventional lenses and can provide dramatic enhancements of processes that depend superlinearly on light intensity, such as light-powered membrane distillation and photocatalysis. Our approach can easily be generalized to optimize metasurfaces for other functionalities, such as nonlinear optics or photothermal conversion

Quantifying Efficiency of Remote Excitation for Surface-Enhanced Raman Spectroscopy in Molecular Junctions

Surface-enhanced Raman spectroscopy (SERS) is enabled by local surface plasmon resonances (LSPRs) in metallic nanogaps. When SERS is excited by direct illumination of the nanogap, the background heating of the lattice and electrons can prevent further manipulation of the molecules. To overcome this issue, we report SERS in electromigrated gold molecular junctions excited remotely: surface plasmon polaritons (SPPs) are excited at nearby gratings, propagate to the junction, and couple to the local nanogap plasmon modes. Like direct excitation, remote excitation of the nanogap can generate both SERS emission and an open-circuit photovoltage (OCPV). We compare the SERS intensity and the OCPV in both direct and remote illumination configurations. SERS spectra obtained by remote excitation are much more stable than those obtained through direct excitation when the photon count rates are comparable. By statistical analysis of 33 devices, the coupling efficiency of remote excitation is calculated to be around 10%, consistent with the simulated energy flow

Challenges in Plasmonic Catalysis

The use of nanoplasmonics to control light and heat close to the thermodynamic limit enables exciting opportunities in the field of plasmonic catalysis. The decay of plasmonic excitations creates highly nonequilibrium distributions of hot carriers that can initiate or catalyze reactions through both thermal and nonthermal pathways. In this Perspective, we present the current understanding in the field of plasmonic catalysis, capturing vibrant debates in the literature, and discuss future avenues of exploration to overcome critical bottlenecks. Our Perspective spans first-principles theory and computation of correlated and far-from-equilibrium light-matter interactions, synthesis of new nanoplasmonic hybrids, and new steady-state and ultrafast spectroscopic probes of interactions in plasmonic catalysis, recognizing the key contributions of each discipline in realizing the promise of plasmonic catalysis. We conclude with our vision for fundamental and technological advances in the field of plasmon-driven chemical reactions in the coming years

All-optical Reconfiguration of Ultrafast Dichroism in Gold Metasurfaces

Optical metasurfaces have come into the spotlight as a promising platform for light manipulation at the nanoscale, including ultrafast all-optical control via excitation with femtosecond laser pulses. Recently, dichroic metasurfaces have been exploited to modulate the polarization state of light with unprecedented speed. Here, we theoretically predict and experimentally demonstrate by pump-probe spectroscopy the capability to reconfigure the ultrafast dichroic signal of a gold metasurface by simply acting on the polarization of the pump pulse, which is shown to reshape the spatio-temporal distribution of the optical perturbation. The photoinduced anisotropic response, driven by out-of-equilibrium carriers and extinguished in a sub-picosecond temporal window, is readily controlled in intensity by tuning the polarization direction of the excitation up to a full sign reversal. This work proves that nonlinear metasurfaces offer the flexibility to tailor their ultrafast optical response in a fully all-optically reconfigurable platform

Thermoplasmonic Effect of Surface-Enhanced Infrared Absorption in Vertical Nanoantenna Arrays

Thermoplasmonics is a method for increasing temperature remotely using focused visible or infrared laser beams interacting with plasmonic nanoparticles. Here, local heating induced by mid-infrared quantum cascade laser illumination of vertical gold-coated nanoantenna arrays embedded into polymer layers is investigated by infrared nanospectroscopy and electromagnetic/thermal simulations. Nanoscale thermal hotspot images are obtained by a photothermal scanning probe microscopy technique with laser illumination wavelength tuned at the different plasmonic resonances of the arrays. Spectral analysis indicates that both Joule heating by the metal antennas and surface-enhanced infrared absorption (SEIRA) by the polymer molecules located in the apical hotspots of the antennas are responsible for thermoplasmonic resonances, that is, for strong local temperature increase. At odds with more conventional planar nanoantennas, the vertical antenna structure enables thermal decoupling of the hotspot at the antenna apex from the heat sink constituted by the solid substrate. The temperature increase was evaluated by quantitative comparison of data obtained with the photothermal expansion technique to the results of electromagnetic/thermal simulations. In the case of strong SEIRA by the CO bond of poly-methylmethacrylate at 1730 cm^–1, for focused mid-infrared laser power of about 20 mW, the evaluated order of magnitude of the nanoscale temperature increase is of 10 K. This result indicates that temperature increases of the order of hundreds of K may be attainable with full mid-infrared laser power tuned at specific molecule vibrational fingerprints

Decentralized Solar-Driven Photothermal Desalination: An Interdisciplinary Challenge to Transition Lab-Scale Research to Off-Grid Applications

Sunlight can power thermal desalination as a carrier of electromagnetic energy if efficiently turned into heat. In the search for technologies to relieve global water scarcity, thermal desalination has key advantages in terms of sustainability, robustness, and limited salinity dependence. Solar-driven photothermal desalination (SDPD) can enable decentralized water purification, improved accessibility, and reduced environmental impact overcoming limitations of conventional, infrastructure-heavy desalination practices. However, there remains a lack of consensus on how to best evaluate the efficiency of diverse light-driven systems. While developing advanced absorbers, evaporators, and materials for desalination is essential, we have concluded that more efforts should focus on system-wide optimization, with particular attention paid to thermal energy recovery and loss mitigation. This Perspective offers a blueprint for achieving efficient and scalable SDPD under varying solar irradiation, emphasizing the need for interdisciplinary approaches to accomplish this goal

Chiral Plasmonic Pinwheels Exhibit Orientation-Independent Linear Differential Scattering under Asymmetric Illumination

Plasmonic nanoantennas have considerably stronger polarization-dependent optical properties than their molecular counterparts, inspiring photonic platforms for enhancing molecular dichroism and providing fundamental insight into light-matter interactions. One such insight is that even achiral nanoparticles can yield strong optical activity when they are asymmetrically illuminated from a single oblique angle instead of evenly illuminated. This effect, called extrinsic chirality, results from the overall chirality of the experimental geometry and strongly depends on the orientation of the incident light. Although extrinsic chirality has been well-characterized, an analogous effect involving linear polarization sensitivity has not yet been discussed. In this study, we investigate the differential scattering of rotationally symmetric chiral plasmonic pinwheels when asymmetrically irradiated with linearly polarized light. Despite their high rotational symmetry, we observe substantial linear differential scattering that is maintained over all pinwheel orientations. We demonstrate that this orientation-independent linear differential scattering arises from the broken mirror and rotational symmetries of our overall experimental geometry. Our results underscore the necessity of considering both the rotational symmetry of the nanoantenna and the experimental setup, including illumination direction and angle, when performing plasmon-enhanced chiroptical characterizations. Our results demonstrate spectroscopic signatures of an effect analogous to extrinsic chirality for linear polarizations

Photoinduced Temperature Gradients in Sub-wavelength Plasmonic Structures: The Thermoplasmonics of Nanocones

Plasmonic structures are renowned for their capability to efficiently convert light into heat at the nanoscale. However, despite the possibility to generate deep sub-wavelength electromagnetic hot spots, the formation of extremely localized thermal hot spots is an open challenge of research, simply because of the diffusive spread of heat along the whole metallic nanostructure. Here we tackle this challenge by exploiting single gold nanocones. We theoretically show how these structures can indeed realize extremely high temperature gradients within the metal, leading to deep sub-wavelength thermal hot spots, owing to their capability of concentrating light at the apex under resonant conditions even under continuous wave illumination. A three-dimensional Finite Element Method model is employed to study the electromagnetic field in the structure and subsequent thermoplasmonic behaviour, in terms of the three-dimensional temperature distribution. We show how the latter is affected by nanocone size, shape, and composition of the surrounding environment. Finally, we anticipate the use of photoinduced temperature gradients in nanocones for applications in optofluidics and thermoelectrics or for thermally induced nanofabrication