Nerea Zabala

Optical Spectroscopy of Conductive Junctions in Plasmonic Cavities

The optical properties of a nanoparticle dimer bridged by a conductive junction depend strongly on the junction conductivity. As the conductivity increases, the bonding dimer plasmon blueshifts and broadens. For large conductance, a low energy charge transfer plasmon also appears in the spectra with a line width that decreases with increasing conductance. A simple physical model for the understanding of the spectral feature is presented. Our finding of a strong influence of junction conductivity on the optical spectrum suggests that plasmonic cavities might serve as probes of molecular conductance at elevated frequencies not accessible through electrical measurements.

Image potential in scanning transmission electron microscopy

In the framework of the classical dielectric theory, the role of the image potential in electron energy loss spectroscopy (EELS) of fast electrons commonly used in scanning transmission electron microscopy travelling near a surface is studied. Relativistic and dispersive corrections are evaluated to establish the range of validity of this theory. The spatial resolution of the EELS technique is discussed for valence and core electron excitations. The effect of the quantal nature of the probe is also discussed. Finally, several problems involving planar surfaces, small particles, cylinders and truncated targets of interest in nanotechnology are studied.

Self-consistent study of electron confinement to metallic thin films on solid surfaces

We present a method for density-functional modeling of metallic overlayers grown on metallic supports. It offers a tool to study nanostructures and combines the power of self-consistent pseudopotential calculations with the simplicity of a one-dimensional approach. The model is applied to Pb layers grown on the Cu(111) surface. More specifically, Pb is modeled as stabilized jellium and the Cu(111) substrate is represented by a one-dimensional pseudopotential that reproduces experimental positions of both the Cu Fermi level and the energy gap of the band structure projected along the (111) direction. The model is used to study the quantum well states in the Pb overlayer. Their analysis gives the strength of the electron confinement barriers at the interface and at the surface facing the vacuum. Our results and analysis support the interpretation of the quantum well state spectra measured by the scanning tunneling spectroscopy.

Energy loss of electrons travelling through cylindrical holes

Abstract Expressions for the energy loss due to interaction with surfaces experienced by fast electrons passing through holes drilled in a coated medium are derived within the framework of the classical dielectric theory. These expressions are evaluated for Al coated with alumina using experimental values of the dielectric constant ϵ(ω) of the different media. Retarded energy loss probabilities are derived and some applications are discussed. Deflection effects due to the image force acting on the particle are also analysed.

Optical characterization of charge transfer and bonding dimer plasmons in linked interparticle gaps

We present a theoretical study of the optical properties of nanoparticle dimers connected by conductive gap linkers. The geometrical and conductive properties of the linker modify strongly the optical response of the linked metallic cavity. Two plasmonic modes are responsible for the main spectral features of the cavity: a bonding dimer plasmon (BDP) and a charge transfer plasmon (CTP). We first explore how these two modes are modified as a function of the geometry and the conductance through the cavity, identifying the spatial distribution of the linking current densities. Furthermore, we introduce a resonant feature in the conductivity of the linker, where we observe a complex splitting of the plasmon modes. We also study the capabilities of the BDP and CTP modes in localized surface plasmon resonance (LSPR) sensing.

Antenna-assisted picosecond control of nanoscale phase transition in vanadium dioxide

Nanoscale devices in which the interaction with light can be configured using external control signals hold great interest for next-generation optoelectronic circuits. Materials exhibiting a structural or electronic phase transition offer a large modulation contrast with multi-level optical switching and memory functionalities. In addition, plasmonic nanoantennas can provide an efficient enhancement mechanism for both the optically induced excitation and the readout of materials strategically positioned in their local environment. Here, we demonstrate picosecond all-optical switching of the local phase transition in plasmonic antenna-vanadium dioxide (VO2) hybrids, exploiting strong resonant field enhancement and selective optical pumping in plasmonic hotspots. Polarization- and wavelength-dependent pump–probe spectroscopy of multifrequency crossed antenna arrays shows that nanoscale optical switching in plasmonic hotspots does not affect neighboring antennas placed within 100 nm of the excited antennas. The antenna-assisted pumping mechanism is confirmed by numerical model calculations of the resonant, antenna-mediated local heating on a picosecond time scale. The hybrid, nanoscale excitation mechanism results in 20 times reduced switching energies and 5 times faster recovery times than a VO2 film without antennas, enabling fully reversible switching at over two million cycles per second and at local switching energies in the picojoule range. The hybrid solution of antennas and VO2 provides a conceptual framework to merge the field localization and phase-transition response, enabling precise, nanoscale optical memory functionalities.

Optical transport and sensing in plexcitonic nanocavities

We present a theoretical study of the optical properties of a strongly coupled metallic dimer when an ensemble of molecules is placed in the inter-particle cavity. The linking molecules are characterized by an excitonic transition which couples to the Bonding Dimer Plasmon (BDP) and the Bonding Quadrupolar Plasmon (BQP) resonances, arising from the hybridization of the dipolar and quadrupolar modes of the individual nanoparticles, respectively. As a consequence, both modes split into two coupled plasmon-exciton modes, so called plexcitons. The Charge Transfer Plasmon (CTP) resonance, involving plasmonic oscillations of the dimer as a whole, arises when the conductance of the excitonic junction is above a threshold value. The possibility of exploiting plexcitonic resonances for sensing is explored in detail. We find high sensitivity to the environment when different dielectric embedding media are considered. Contrary to standard methods, we propose a new framework for effective sensing based on the relative intensity of plexcitonic peaks.

Energy loss probability of STEM electrons in cylindrical surfaces

Abstract In the frame of the self-energy formalism, the interaction of STEM electrons with a cylindrical surface is studied. The surface modes of cylindrical interfaces so obtained are proven to fulfil certain sum rules. A general expression for the energy loss probability valid for any beam direction nonparallel to the cylinder axis is presented. In all the cases the so-called begrenzung effect is found.

Anisotropic Nanoantenna-Based Magnetoplasmonic Crystals for Highly Enhanced and Tunable Magneto-Optical Activity

We present a novel concept of a magnetically tunable plasmonic crystal based on the excitation of Fano lattice surface modes in periodic arrays of magnetic and optically anisotropic nanoantennas. We show how coherent diffractive far-field coupling between elliptical nickel nanoantennas is governed by the two in-plane, orthogonal and spectrally detuned plasmonic responses of the individual building block, one directly induced by the incident radiation and the other induced by the application of an external magnetic field. The consequent excitation of magnetic field-induced Fano lattice surface modes leads to highly tunable and amplified magneto-optical effects as compared to a continuous film or metasurfaces made of disordered noninteracting magnetoplasmonic anisotropic nanoantennas. The concepts presented here can be exploited to design novel magnetoplasmonic sensors based on coupled localized plasmonic resonances, and nanoscale metamaterials for precise control and magnetically driven tunability of light polarization states.

Optical properties and sensing in plexcitonic nanocavities: from simple molecular linkers to molecular aggregate layers

We present a theoretical study of a metal-molecular aggregate hybrid system consisting of a strongly coupled dimer connected by molecules characterized by an excitonic transition. The plasmonic resonances of the metallic dimer interact with the molecular excitations giving rise to coupled plasmon-exciton states, so called plexcitons. We compare the differences in the optical response when the excitonic material is placed only as a linker in the plasmonic gap of the dimer and when the material is distributed as an aggregate layer covering the dimer entirely. We also explore the efficiency of plexcitons for localized surface plamon resonance (LSPR) sensing in both situations. The ordinary shift-based sensing is more efficient for dimers connected through molecular linkers, whereas intensity-based sensing is more effective when the molecular aggregate covers the entire nanostructure. These results can serve to design the chemistry of excitons around metallic nanoparticles.