Viktor Myroshnychenko

Modelling the optical response of gold nanoparticles

This tutorial review presents an overview of theoretical methods for predicting and understanding the optical response of gold nanoparticles. A critical comparison is provided, assisting the reader in making a rational choice for each particular problem, while analytical models provide insights into the effects of retardation in large particles and non-locality in small particles. Far- and near-field spectra are discussed, and the relevance of the latter in surface-enhanced Raman spectroscopy and electron energy-loss spectroscopy is emphasized.

Nano-optical Trapping of Rayleigh Particles and Escherichia coli Bacteria with Resonant Optical Antennas

Immobilizing individual living microorganisms at designated positions in space is important to study their metabolism and to initiate an in situ scrutiny of the complexity of life at the nanoscale. While optical tweezers enable the trapping of large cells at the focus of a laser beam, they face difficulties in maintaining them steady and can become invasive and produce substantial damage that prevents preserving the organisms intact for sufficient time to be studied. Here we demonstrate a novel optical trapping scheme that allows us to hold living Escherichia coli bacteria for several hours using moderate light intensities. We pattern metallic nanoantennas on a glass substrate to produce strong light intensity gradients responsible for the trapping mechanism. Several individual bacteria are trapped simultaneously with their orientation fixed by the asymmetry of the antennas. This unprecedented immobilization of bacteria opens an avenue toward observing nanoscopic processes associated with cell metabolism, as well as the response of individual live microorganisms to external stimuli, much in the same way as pluricellular organisms are studied in biology.

Probing bright and dark surface-plasmon modes in individual and coupled noble metal nanoparticles using an electron beam.

The rich structure of bright and dark surface-plasmon modes localized in individual and coupled gold nanoparticles is unveiled by electron-energy-loss spectroscopy performed in a scanning transmission electron microscope. Spatially resolved maps of surface-plasmon modes in the approximately 1.5-2.5 eV range (wavelengths approximately 500-800 nm), collected for individual nanorods, coupled nanorod dimers, and touching nanosphere dimers, are in excellent agreement with theory. Surface-plasmon maps constructed from the spatially and spectrally resolved energy-loss signals are shown to mimic rather well the near fields calculated for external illumination in the case of bright surface-plasmon modes (i.e., those coupling to external light). Dark surface-plasmon modes that cannot be excited by optical means are also found, and our electron probing technique provides further insight into their corresponding spatial distribution and symmetry, which are not accessible to any other existing techniques. Our results initiate the study of a whole set of new dark surface-plasmon modes that should become a source of new applications in sensing and microscopy but have escaped experimental scrutiny so far.

Plasmon Spectroscopy and Imaging of Individual Gold Nanodecahedra: A Combined Optical Microscopy, Cathodoluminescence, and Electron Energy-Loss Spectroscopy Study

Imaging localized plasmon modes in noble-metal nanoparticles is of fundamental importance for applications such as ultrasensitive molecular detection. Here, we demonstrate the combined use of optical dark-field microscopy (DFM), cathodoluminescence (CL), and electron energy-loss spectroscopy (EELS) to study localized surface plasmons on individual gold nanodecahedra. By exciting surface plasmons with either external light or an electron beam, we experimentally resolve a prominent dipole-active plasmon band in the far-field radiation acquired via DFM and CL, whereas EELS reveals an additional plasmon mode associated with a weak dipole moment. We present measured spectra and intensity maps of plasmon modes in individual nanodecahedra in excellent agreement with boundary-element method simulations, including the effect of the substrate. A simple tight-binding model is formulated to successfully explain the rich plasmon structure in these particles encompasing bright and dark modes, which we predict to be fully observable in less lossy silver decahedra. Our work provides useful insight into the complex nature of plasmon resonances in nanoparticles with pentagonal symmetry.

Finite-element modeling method for the prediction of the complex effective permittivity of two-phase random statistically isotropic heterostructures

This article is devoted to the study of the complex permittivity of two-dimensional two-phase statistically isotropic heterostructures on a small scale such that the quasistatic limit is applicable. Even though several analytical approximation techniques have been developed in the past, today it is desirable to be able to simulate these media via computer, which necessitates the development of efficient numerical techniques for the solution of the resulting equations. The simulation data concern the effective permittivity of continuum composites consisting of distributions of hard disks of a dielectric phase randomly dispersed in a continuous matrix of another dielectric phase. The three-dimensional equivalent of this model would be a composite with cylindrical symmetry, i.e., all interfaces are parallel to a fixed direction. The two constituents are assumed to be isotropic and homogeneous materials with scalar permittivities. Ab initio calculations are accomplished self-consistently with a computer code....

Unveiling Nanometer Scale Extinction and Scattering Phenomena through Combined Electron Energy Loss Spectroscopy and Cathodoluminescence Measurements

Plasmon modes of the exact same individual gold nanoprisms are investigated through combined nanometer-resolved electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) measurements. We show that CL only probes the radiative modes, in contrast to EELS, which additionally reveals dark modes. The combination of both techniques on the same particles thus provides complementary information and also demonstrates that although the radiative modes give rise to very similar spatial distributions when probed by EELS or CL, their resonant energies appear to be different. We trace this phenomenon back to plasmon dissipation, which affects in different ways the plasmon signatures probed by these techniques. Our experiments are in agreement with electromagnetic numerical simulations and can be further interpreted within the framework of a quasistatic analytical model. We therefore demonstrate that CL and EELS are closely related to optical scattering and extinction, respectively, with the addition of nanometer spatial resolution.

Transmitting hertzian optical nanoantenna with free-electron feed.

A pair of coupled gold nanorods excited by a beam of free electrons acts as a transmitting Hertzian antenna in the optical part of the spectrum. Significantly enhanced resonant emission is observed from the antenna when the electron beam is injected around the junction between the rods, where the local density of electromagnetic states is elevated.

Surface plasmon mapping of dumbbell-shaped gold nanorods: the effect of silver coating.

We report on the identification of surface plasmons in individual gold dumbbell-shaped nanoparticles (AuDBs), as well as AuDBs coated with silver. We use spatially resolved electron energy-loss spectroscopy in a scanning electron microscope, which allows us to map plasmon-energy and intensity spatial distributions. Two dominant plasmon resonances are experimentally resolved in both AuDBs and silver-coated AuDBs. The intensity of these features is peaked either at the tips or at the sides of the nanoparticles. We present boundary element method simulations in good agreement with the experiment, allowing us to elucidate the nature of such modes. While the lower-energy, tip-focused plasmon is of longitudinal character for all dumbbells under consideration, the second side-bound plasmon has a more involved symmetry, starting as a longitudinal quadrupole in homogeneous AuDBs and picking up transversal components when silver coating is added. The longitudinal dipolar mode energy is found to blue-shift upon coating with silver. We find that the substrate produces sizable shifts in the plasmons of silver-coated AuDBs. Our analysis portraits a complex plasmonic scenario in metal nanoparticles coated with silver, including a transition from the original homogeneous gold dumbbell plasmons to the modes of homogeneous silver rods. We believe that these findings can have potential application to plasmon engineering.

Interacting plasmon and phonon polaritons in aligned nano- and microwires

The availability of macroscopic, nearly periodic structures known as eutectics opens a new path for controlling light at wavelength scales determined by the geometrical parameters of these materials and the intrinsic properties of their component phases. Here, we analyze the optical waveguiding properties of eutectic mixtures of alkali halides, formed by close-packed arrangements of aligned cylindrical inclusions. The wavelengths of phonon polaritons in these constituents are conveniently situated in the infrared and are slightly larger than the diameter and separation of the inclusions, typically consisting on single-crystal wires down to submicrometer diameter. We first discuss the gap mode and the guiding properties of metallic cylindrical waveguides in the visible and near-infrared, and in particular we investigate the transition between cylinder touching and non-touching regimes. Then, we demonstrate that these properties can be extended to the mid infrared by means of phonon polaritons. Finally, we analyze the guiding properties of an actual eutectic. For typical eutectic dimensions, we conclude that crosstalk between neighboring cylindrical wires is small, thus providing a promising platform for signal propagation and image analysis in the mid infrared.

Magnetic polarization in the optical absorption of metallic nanoparticles.

We find remarkably strong absorption due to magnetic polarization in common colloidal and lithographic metallic nanoparticles. Our analysis is based upon a thorough examination of the dipolar electric and magnetic polarizabilities for representative combinations of nanoparticle composition, size, and morphology. We illustrate this concept by first discussing absorption in metallic spheres and then exploring ellipsoids, disks, and rings. Magnetic polarization reaches ~ 90% of the total absorption in 100 nm disks and rings for wavelengths above 1 μm under co-linear electric and magnetic irradiation. Our results demonstrate that the magnetic contribution to absorption cannot be naively overlooked, as it can largely exceed the contribution of electric polarization.