Photonic-plasmonic-coupled nanoantennas for polarization-controlled multispectral nanofocusing.

We report on the design and experimental demonstration of array-enhanced nanoantennas for polarization-controlled multispectral nanofocusing in the near-IR spectral range. We design plasmonic double bow-tie nanoantennas-coupled to multiple-periodic nanoparticle arrays to harvest radiation of designed wavelengths from a large spatial area and to focus it into a targeted nanoscale region. Near-field calculations were performed on a gold nanoantenna array using three-dimensional finite difference time domain simulations. Cross-shaped optical nanoantennas were fabricated on glass substrates using electron beam lithography. The optical characterization of the fabricated nanoantennas was performed using second harmonic excitation spectroscopy that demonstrates multiwavelength photonic coupling in good agreement with the antenna modeling. The nanoantenna structures introduced in this Letter provide the ability to focus optical energy into deep subwavelength areas and to address multiple spectral regions with polarization control. Such attributes are highly desirable in optical biosensing, enhanced Raman scattering, and for nonlinear plasmonic applications.

Quantum confinement and electroluminescence in ultrathin silicon nanowires fabricated by a maskless etching technique.

We present a novel approach for the direct synthesis of ultrathin Si nanowires (NWs) exhibiting room temperature light emission. The synthesis is based on a wet etching process assisted by a metal thin film. The thickness-dependent morphology of the metal layer produces uncovered nanometer-size regions which act as precursor sites for NW formation. The process is cheap, fast, maskless and compatible with Si technology. Very dense arrays of long (several micrometers) and small (diameter of 5-9 nm) NWs have been synthesized. An efficient room temperature luminescence, visible with the naked eye, is observed when NWs are optically excited, exhibiting a blue-shift with decreasing NW size in agreement with quantum confinement effects. A prototype device based on Si NWs has been fabricated showing a strong and stable electroluminescence at low voltages. The relevance and the perspectives of the reported results are discussed, opening the route toward novel applications of Si NWs.

Generation and self-organization of bimetallic Pd/Au nanoparticles on SiO2 by sequential sputtering depositions and annealing processes.

PdAu nanoparticles have been grown on SiO2 by room-temperature sequential sputtering depositions. The nucleation and growth kinetics have been determined crossing atomic force and scanning electron microscopy measurements. From these measurements the mechanisms of the nucleation and growth have been determined. In particular: (1) during the deposition of the first metal (Pd), atoms adsorbed on the substrate are readily trapped on the substrate defects, forming stable nuclei which grow further. During the deposition of the second metal (Au), adsorbed atoms are captured by the clusters formed during the first deposition, before they have time to form a stable nucleus of pure second metal on the surface sites. So, the nucleation is mainly controlled by the Pd and the Au atoms are incorporated essentially by direct impingement of the vapour atoms on the already formed particles. (2) fixing the amount of Pd and Au, during post-deposition thermal treatments, a surface diffusion limited ripening of the NPs occurs. Applying the standard ripening growth theory several parameters characterizing the process were determined, in particular, the growth exponent n and the activation energy E(a). n was found to be dependent on temperature and amount of Au deposited. E(a) was found to linearly increase with the amount of Au deposited. Such a dependence is discussed separating E(a) in two terms: one describing the activation energy for atomic surface diffusion (independent on the amount of Au deposited), the other one describing the activation energy for the film clustering process (dependent on the amount of Au deposited).

Control of growth mechanisms and orientation in epitaxial Si nanowires grown by electron beam evaporation.

The growth mechanisms of epitaxial Si nanowires (NWs) grown by electron beam evaporation (EBE) and catalyzed through gold droplets are identified. NWs are seen to grow both from adsorbed Si atoms diffusing from the substrate and forming a dip around them, and from directly impinging atoms. The growth of a 2D planar layer competing with the axial growth of the NWs is also observed and the experimental parameters determining which of the two processes prevails are identified. NWs with (111), (100) and (110) orientation have been found and the growth rate is observed to have a strong orientation dependence, suggesting a microscopic growth mechanism based on the atomic ordering along (110) ledges onto (111)-oriented terraces. By properly changing the range of experimental conditions we demonstrate how it is possible to favor the axial growth of the NWs, define their length and control their crystallographic orientation.