Alejandro Manjavacas

Graphene plasmon waveguiding and hybridization in individual and paired nanoribbons.

Plasmons in doped graphene exhibit relatively large confinement and long lifetime compared to noble-metal plasmons. Here, we study the propagation properties of plasmons guided along individual and interacting graphene nanoribbons. Besides their tunability via electrostatic gating, an additional handle to control these excitations is provided by the dielectric environment and the relative arrangement of the interacting waveguides. Plasmon interaction and hybridization in pairs of neighboring aligned ribbons are shown to be strong enough to produce dramatic modifications in the plasmon field profiles. We introduce a universal scaling law that considerably simplifies the analysis an understanding of these plasmons. Our work provides the building blocks to construct graphene plasmon circuits for future compact plasmon devices with potential application to optical signal processing, infrared sensing, and quantum information technology.

Quantum plexcitonics: strongly interacting plasmons and excitons.

We present a fully quantum mechanical approach to describe the coupling between plasmons and excitonic systems such as molecules or quantum dots. The formalism relies on Zubarevs Green functions, which allow us to go beyond the perturbative regime within the internal evolution of a plasmonic nanostructure and to fully account for quantum aspects of the optical response and Fano resonances in plasmon-excition (plexcitonic) systems. We illustrate this method with two examples consisting of an exciton-supporting quantum emitter placed either in the vicinity of a single metal nanoparticle or in the gap of a nanoparticle dimer. The optical absorption of the combined emitter-dimer structure is shown to undergo dramatic changes when the emitter excitation level is tuned across the gap-plasmon resonance. Our work opens a new avenue to deal with strongly interacting plasmon-excition hybrid systems.

Quantum Finite-Size Effects in Graphene Plasmons

Graphene plasmons are emerging as an alternative solution to noble metal plasmons, adding the advantages of tunability via electrostatic doping and long lifetimes. These excitations have been so far described using classical electrodynamics, with the carbon layer represented by a local conductivity. However, the question remains, how accurately is such a classical description representing graphene? What is the minimum size for which nonlocal and quantum finite-size effects can be ignored in the plasmons of small graphene structures? Here, we provide a clear answer to these questions by performing first-principles calculations of the optical response of doped nanostructured graphene obtained from a tight-binding model for the electronic structure and the random-phase approximation for the dielectric response. The resulting plasmon energies are in good agreement with classical local electromagnetic theory down to ∼10 nm sizes, below which plasmons split into several resonances that emphasize the molecular character of the carbon structures and the quantum nature of their optical excitations. Additionally, finite-size effects produce substantial plasmon broadening compared to homogeneous graphene up to sizes well above 20 nm in nanodisks and 10 nm in nanoribbons. The atomic structure of edge terminations is shown to be critical, with zigzag edges contributing to plasmon broadening significantly more than armchair edges. This study demonstrates the ability of graphene nanostructures to host well-defined plasmons down to sizes below 10 nm, and it delineates a roadmap for understanding their main characteristics, including the role of finite size and nonlocality, thus providing a solid background for the emerging field of graphene nanoplasmonics.

Vivid, full-color aluminum plasmonic pixels.

Significance Full-color displays are typically fabricated using a combination of chromatic materials of various types, introduced into an addressable pixel-based electronic device. Here we show that brightly colored, highly vivid pixels, directly suitable for RGB displays, can be fabricated using periodic areas of Al nanorods in each pixel. Both nanorod length and spacing are critically important in achieving strong and spectrally distinct scattering signatures across the visible spectrum. This use of a low-cost, abundant metal for achieving full-spectrum coloration is compatible with complementary metal-oxide semiconductor manufacturing methods and directly applicable to current liquid crystal display technology. Aluminum is abundant, low in cost, compatible with complementary metal-oxide semiconductor manufacturing methods, and capable of supporting tunable plasmon resonance structures that span the entire visible spectrum. However, the use of Al for color displays has been limited by its intrinsically broad spectral features. Here we show that vivid, highly polarized, and broadly tunable color pixels can be produced from periodic patterns of oriented Al nanorods. Whereas the nanorod longitudinal plasmon resonance is largely responsible for pixel color, far-field diffractive coupling is used to narrow the plasmon linewidth, enabling monochromatic coloration and significantly enhancing the far-field scattering intensity of the individual nanorod elements. The bright coloration can be observed with p-polarized white light excitation, consistent with the use of this approach in display devices. The resulting color pixels are constructed with a simple design, are compatible with scalable fabrication methods, and provide contrast ratios exceeding 100:1.

Distinguishing between plasmon-induced and photoexcited carriers in a device geometry.

The use of surface plasmons, charge density oscillations of conduction electrons of metallic nanostructures, to boost the efficiency of light-harvesting devices through increased light-matter interactions could drastically alter how sunlight is converted into electricity or fuels. These excitations can decay directly into energetic electron–hole pairs, useful for photocurrent generation or photocatalysis. However, the mechanisms behind plasmonic carrier generation remain poorly understood. Here we use nanowire-based hot-carrier devices on a wide-bandgap semiconductor to show that plasmonic carrier generation is proportional to internal field-intensity enhancement and occurs independently of bulk absorption. We also show that plasmon-induced hot electrons have higher energies than carriers generated by direct excitation and that reducing the barrier height allows for the collection of carriers from plasmons and direct photoexcitation. Our results provide a route to increasing the efficiency of plasmonic hot-carrier devices, which could lead to more efficient devices for converting sunlight into usable energy.

Robust plasmon waveguides in strongly interacting nanowire arrays.

Arrays of parallel metallic nanowires are shown to provide a tunable, robust, and versatile platform for plasmon interconnects, including high-curvature turns with minimum signal loss. The proposed guiding mechanism relies on gap plasmons existing in the region between adjacent nanowire pairs and multiwire arrays. We focus on square and circular silver nanowires in silica, which we demonstrate to perform much better than previous schemes in terms of a relevant figure of merit measuring the degree of allowed integration. Our work provides the tools for designing plasmon-based interconnects and achieving a high degree of integration with minimum cross talk between adjacent plasmon guides, which should be relevant ingredients for future multiplexed biosensors.

Aluminum Nanocrystals as a Plasmonic Photocatalyst for Hydrogen Dissociation

Hydrogen dissociation is a critical step in many hydrogenation reactions central to industrial chemical production and pollutant removal. This step typically utilizes the favorable band structure of precious metal catalysts like platinum and palladium to achieve high efficiency under mild conditions. Here we demonstrate that aluminum nanocrystals (Al NCs), when illuminated, can be used as a photocatalyst for hydrogen dissociation at room temperature and atmospheric pressure, despite the high activation barrier toward hydrogen adsorption and dissociation. We show that hot electron transfer from Al NCs to the antibonding orbitals of hydrogen molecules facilitates their dissociation. Hot electrons generated from surface plasmon decay and from direct photoexcitation of the interband transitions of Al both contribute to this process. Our results pave the way for the use of aluminum, an earth-abundant, nonprecious metal, for photocatalysis.

Pronounced Linewidth Narrowing of an Aluminum Nanoparticle Plasmon Resonance by Interaction with an Aluminum Metallic Film

Aluminum nanocrystals and fabricated nanostructures are emerging as highly promising building blocks for plasmonics in the visible region of the spectrum. Even at the individual nanocrystal level, however, the localized plasmons supported by Al nanostructures possess a surprisingly broad spectral response. We have observed that when an Al nanocrystal is coupled to an underlying Al film, its dipolar plasmon resonance linewidth narrows remarkably and shows an enhanced scattering efficiency. This behavior is observable in other plasmonic metals, such as gold; however, it is far more dramatic in the aluminum nanoparticle-film system, reducing the dipolar plasmon linewidth by more than half. A substrate-mediated hybridization of the dipolar and quadrupolar plasmons of the nanoparticle reduces the radiative losses of the dipolar plasmon. While this is a general effect that applies to all metallic nanoparticle-film systems, this finding specifically provides a new mechanism for narrowing plasmon resonances in aluminum-based systems, quite possibly expanding the potential of Al-based plasmonics in real-world applications.

Tunable Molecular Plasmons in Polycyclic Aromatic Hydrocarbons

We show that chemically synthesized polycyclic aromatic hydrocarbons (PAHs) exhibit molecular plasmon resonances that are remarkably sensitive to the net charge state of the molecule and the atomic structure of the edges. These molecules can be regarded as nanometer-sized forms of graphene, from which they inherit their high electrical tunability. Specifically, the addition or removal of a single electron switches on/off these molecular plasmons. Our first-principles time-dependent density-functional theory (TDDFT) calculations are in good agreement with a simpler tight-binding approach that can be easily extended to much larger systems. These fundamental insights enable the development of novel plasmonic devices based upon chemically available molecules, which, unlike colloidal or lithographic nanostructures, are free from structural imperfections. We further show a strong interaction between plasmons in neighboring molecules, quantified in significant energy shifts and field enhancement, and enabling molecular-based plasmonic designs. Our findings suggest new paradigms for electro-optical modulation and switching, single-electron detection, and sensing using individual molecules.

High Chromaticity Aluminum Plasmonic Pixels for Active Liquid Crystal Displays.

Chromatic devices such as flat panel displays could, in principle, be substantially improved by incorporating aluminum plasmonic nanostructures instead of conventional chromophores that are susceptible to photobleaching. In nanostructure form, aluminum is capable of producing colors that span the visible region of the spectrum while contributing exceptional robustness, low cost, and streamlined manufacturability compatible with semiconductor manufacturing technology. However, individual aluminum nanostructures alone lack the vivid chromaticity of currently available chromophores because of the strong damping of the aluminum plasmon resonance in the visible region of the spectrum. In recent work, we showed that pixels formed by periodic arrays of Al nanostructures yield far more vivid coloration than the individual nanostructures. This progress was achieved by exploiting far-field diffractive coupling, which significantly suppresses the scattering response on the long-wavelength side of plasmonic pixel resonances. In the present work, we show that by utilizing another collective coupling effect, Fano interference, it is possible to substantially narrow the short-wavelength side of the pixel spectral response. Together, these two complementary effects provide unprecedented control of plasmonic pixel spectral line shape, resulting in aluminum pixels with far more vivid, monochromatic coloration across the entire RGB color gamut than previously attainable. We further demonstrate that pixels designed in this manner can be used directly as switchable elements in liquid crystal displays and determine the minimum and optimal numbers of nanorods required in an array to achieve good color quality and intensity.