Giant Spin-splitting in the Bi/Ag(111) Surface Alloy

Surface alloying is shown to produce electronic states with a very large spin-splitting. We discuss the long range ordered bismuth/silver(111) surface alloy where an energy bands separation of up to one eV is achieved. Such strong spin-splitting enables angular resolved photoemission spectroscopy to directly observe the region close to the band edge, where the density of states shows quasi-one dimensional behavior. The associated singularity in the local density of states has been measured by low temperature scanning tunneling spectroscopy. The implications of this new class of materials for potential spintronics applications as well as fundamental issues are discussed.Comment: 4 pages, 4 figure

Relating localized nanoparticle resonances to an associated antenna problem

We conceptually unify the description of resonances existing at metallic nanoparticles and optical nanowire antennas. To this end the nanoantenna is treated as a Fabry-Perot resonator with arbitrary semi-nanoparticles forming the terminations. We show that the frequencies of the quasi-static dipolar resonances of these nanoparticles coincide with the frequency where the phase of the complex reflection coefficient of the fundamental propagating plasmon polariton mode at the wire termination amounts to $\pi$. The lowest order Fabry-Perot resonance of the optical wire antenna occurs therefore even for a negligible wire length. This approach can be used either to easily calculate resonance frequencies for arbitrarily shaped nanoparticles or for tuning the resonance of nanoantennas by varying their termination.Comment: Submitted to Phys. Rev.

Long-Distance Indirect Excitation of Nanoplasmonic Resonances

In nanoscopic systems, size, geometry, and arrangement are the crucial determinants of the light-matter interaction and resulting nanoparticles excitation. At optical frequencies, one of the most prominent examples is the excitation of localized surface plasmon polaritons, where the electromagnetic radiation is coupled to the confined charge density oscillations. Here, we show that beyond direct near- and far-field excitation, a long-range, indirect mode of particle excitation is available in nanoplasmonic systems. In particular, in amorphous arrays of plasmonic nanodiscs we find strong collective and coherent influence on each particle from its entire active neighborhood. This dependency of the local field response on excitation conditions at distant areas brings exciting possibilities to engineer enhanced electromagnetic fields through controlled, spatially configured illumination

k-space Imaging of the Eigenmodes of Sharp Gold Tapers for Scanning Near-Field Optical Microscopy

We investigate the radiation patterns of sharp conical gold tapers, designed as adiabatic nanofocusing probes for scanning near-field optical microscopy (SNOM). Field calculations show that only the lowest order eigenmode of such a taper can reach the very apex and thus induce the generation of strongly enhanced near-field signals. Higher order modes are coupled into the far field at finite distances from the apex. Here, we demonstrate experimentally how to distinguish and separate between the lowest and higher order eigenmodes of such a metallic taper by filtering in the spatial frequency domain. Our approach has the potential to considerably improve the signal-to-background ratio in spectroscopic experiments on the nanoscale

Interplay between Strong Coupling and Radiative Damping of Excitons and Surface Plasmon Polaritons in Hybrid Nanostructures

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Breaking the Mode Degeneracy of Surface-Plasmon Resonances in a Triangular System

In this paper, we present a systematic investigation of symmetry-breaking in the plasmonic modes of triangular gold nanoprisms. Their geometrical C3 symmetry is one of the simplest possible that allows degeneracy in the particle's mode spectrum. It is reduced to the non-degenerate symmetries Cv or E by positioning additional, smaller gold nanoprisms in close proximity, either in a lateral or a vertical configuration. Corresponding to the lower symmetry of the system, its eigenmodes also feature lower symmetries (Cv), or preserve only the identity (E) as symmetry. We discuss how breaking the symmetry of the plasmonic system not only breaks the degeneracy of some lower order modes, but also how it alters the damping and eigenenergies of the observed Fano-type resonances

Raman, Brillouin and photoluminescence spectroscopy of elemental and compound semiconductors

The effects of the zero-point motion and the anharmonicity of the lattice vibrations of diamond are explored theoretically employing a valence force model explicitly incorporating the isotopic composition \sp{12}C\sb{1-x}\sp{13}C\sb{x}\ (0 \leq x \leq 1). The predictions are tested in a study of the elastic moduli (c\sb{ij}) deduced from Brillouin spectra and the zone center optical mode frequency (\omega\sb0) from Raman spectra. It is predicted that the bulk modulus of \sp{13}C diamond exceeds that for \sp{12}C diamond by one part in a thousand, just below the experimental sensitivity accessible with Brillouin measurements; \omega\sb0 is found to exceed the value expected from the M\sp{-1/2} dependence, M = (1 - x)M\sb{12} + xM\sb{13}, by $\sim$0.3 cm\sp{-1}, consistent with observation. The elastic moduli for natural diamond determined in the present study, viz., c\sb{11} = 10.804(5), c\sb{12} = 1.270(10) and c\sb{44} = 5.766(5) in units of 10\sp{12}(dyn/cm\sp2) are the most accurate yet obtained. From a new study of the two-phonon Raman and infrared spectra of diamond, along with the polarization characteristics of the former, and exploiting space group selection rules, the frequencies of all the symmetry-related critical points of the Brillouin Zone are deduced with significant precision. Indirect transitions, free and impurity-bound excitons in gallium phosphide are revisited with photoluminescence spectroscopy. From the observed signatures of sulfur- and nitrogen-bound excitons and their phonon-sidebands, zone-center and zone-boundary phonon frequencies are deduced. They agree with the values for the first order Raman lines and the zone boundary phonons from the signatures of the indirect transitions mediated by phonons at the X-point, which appear in the spectrum of piezo-modulated transmission. The multi-mode behavior of the zone center optical phonons in bulk crystals of the tetrahedrally coordinated Cd\sb{1-x-y}Zn\sb{x}Mg\sb{y}Te ($0 \leq x,\ y \leq 1$) semiconductor alloys is investigated using Raman spectroscopy. Two mode behavior in Zn\sb{1-x}Mg\sb{x}Te and three mode behavior in Cd\sb{1-x-y}Zn\sb{x}Mg\sb{y}Te are observed in Raman scattering. The dependence of the MgTe-like , ZnTe-like and CdTe-like TO and LO frequencies on composition (x,y) is delineated and interpreted with the modified random isodisplacement model

Surface plasmon coupling to nanoscale Schottky-type electrical detectors

We have investigated the near-field coupling of surface plasmons to a titanium/CdS nanowire interface for two different device configurations. A bare aluminum grating on an underlying aluminum layer exhibited the expected stronger electrical signal for perpendicular versus parallel light polarization. An opposite intensity ratio was detected when the grating and the Schottky contact are connected via an aluminum-silica-aluminum sandwich structure. Based upon finite difference time domain device simulations, the enhanced coupling for parallel polarization is attributed to the emergence of a transversal electric wave within the metal-insulator-metal structure. (C) 2010 American Institute of Physics. [doi:10.1063/1.3503534

Plasmonic Nanowire Antennas: Experiment, Simulation, and Theory

Recent advances in nanolithography have allowed shifting of the resonance frequency of antennas into the optical and visible wavelength range with potential applications, for example, in single molecule spectroscopy by fluorescence and directionality enhancement of molecules. Despite such great promise, the analytical means to describe the properties of optical antennas is still lacking. As the phase velocity of currents at optical frequencies in metals is much below the speed of light, standard radio frequency (RF) antenna theory does not apply directly. For the fundamental linear wire antenna, we present an analytical description that overcomes this shortage and reveals profound differences between RF and plasmonic antennas. It is' fully supported by apertureless scanning near-field optical microscope measurements and finite-difference time-domain simulations. This theory is a starting point for the development of analytical models of more complex antenna structures