Electron iduced light emission in photonic crystals

The interaction of a fast electron with a photonic crystal is studied by solving the Maxwell equations exactly for the external field provided by the electron in the presence of the crystal. The polarization currents and charges produced by the passage of the electron give rise to the emission of the so-called Smith-Purcell radiation. The emitted light probability is obtained by integrating the Poynting vector over planes parallel to the crystal at a large distance from the latter. Both reflected and transmitted light components are analyzed and related to the photonic band structure of the crystal. Emission spectra are compared with the energy loss probability and also with the reflectance and transmittance of the crystal.Comment: 9 pages, 3 figures, nano-7/ecoss-21 proceedings, submitted to Surface Scienc

Elastic scattering of low-energy electrons by randomly oriented and aligned molecules: Influence of full non-spherical potentials

Elastic scattering of low (10–50 eV) kinetic energy electrons from free diatomic molecules is studied using a single-center expansion of the full molecular potential. Dynamic exchange and polarization are included in a local form. The calculated elastic differential scattering cross-sections (DCS) for electron impact on CO and N2 are in good agreement with available experimental data. The importance of using the full molecular potential instead of a two-center potential approach is pointed out. These corrections are small for energies above 50 eV, but they become increasingly important at lower energies. When discussing the angular distributions of elastically-scattered electrons from oriented molecules (like surface adsorbates), we show that these corrections are particularly significant. The results have implications for other electron scattering problems such as those encountered in low-energy photoelectron diffraction from both core and valence levels

Molecular Plasmon–Phonon Coupling

Charged polycyclic aromatic hydrocarbons (PAHs), ultrasmall analogs of hydrogen-terminated graphene consisting of only a few fused aromatic carbon rings, have been shown to possess molecular plasmon resonances in the visible region of the spectrum. Unlike larger nanostructures, the PAH absorption spectra reveal rich, highly structured spectral features due to the coupling of the molecular plasmons with the vibrations of the molecule. Here, we examine this molecular plasmon–phonon interaction using a quantum mechanical approach based on the Franck–Condon approximation. We show that an independent boson model can be used to describe the complex features of the PAH absorption spectra, yielding an analytical and semiquantitative description of their spectral features. This investigation provides an initial insight into the coupling of fundamental excitationsplasmons and phononsin molecules

Alternating Plasmonic Nanoparticle Heterochains Made by Polymerase Chain Reaction and Their Optical Properties

Organization of nanoparticles (NPs) of different materials into superstructures of higher complexity represents a key challenge in nanotechnology. Polymerase chain reaction (PCR) was used in this study to fabricate chains consisting of plasmonic NPs of different sizes, thus denoted heterochains. The NPs in such chains are connected by DNA oligomers, alternating in a sequence big–small–big–small–... and spanning lengths in the range of 40–300 nm by varying the number of PCR cycles. They display strong plasmonic chirality at 500–600 nm, the chiral activity revealing nonmonotonous dependence on the length of heterochains. We find the strength of surface-enhanced Raman scattering (SERS) to increase with chain length, while the chiral response initially increased and then decreased with the number of PCR cycles. The relationship between the optical properties of the heterochains and their structure/length is discussed. The length-dependent intense optical response of the plasmonic NP heterochains holds great potential for biosensing applications

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