Self-Organized Nanogratings for Large-Area Surface Plasmon Polariton Excitation and Surface-Enhanced Raman Spectroscopy Sensing

Surface plasmon polaritons (SPP) are exploited due to their intriguing properties for the fabrication and miniaturization of photonic circuits, for surface-enhanced spectroscopy and imaging beyond the diffraction limit. However, excitation of these plasmonic modes by direct illumination is forbidden by energy/momentum conservation rules. One strategy to overcome this limitation relies on diffraction gratings to match the wavevector of the incoming photons with that of propagating SPP excitations. The main limit of the approaches so far reported in the literature is that they rely on highly ordered diffraction gratings fabricated by means of demanding nanolithographic processes. In this work, we demonstrate that an innovative, fully self-organized method based on wrinkling-assisted ion-beam sputtering can be exploited to fabricate large-area (cm2 scale) nanorippled soda lime templates, which conformally support ultrathin Au films deposited by physical deposition. The self-organized patterns act as quasi-one-dimensional (1D) gratings characterized by a remarkably high spatial order, which properly matches the transverse photon coherence length. The gratings can thus enable the excitation of hybrid SPP modes confined at the Au/dielectric interfaces, with a resonant wavelength that can be tuned by modifying the grating period, photon incidence angle, or, potentially, the choice of the thin-film conductive material. Surface-enhanced Raman scattering experiments show promising gains in the range of 103, which are competitive, even before a systematic optimization of the sample fabrication parameters, with state-of-the art lithographic systems, demonstrating the potential of such templates for a broad range of optoelectronic applications aiming at plasmon-enhanced photon harvesting for molecular or biosensing

Second harmonic generation on self-assembled GaAs/Au nanowires with thickness gradient

Here we investigated the SH generation at the wavelength of 400 nm (pump laser at 800 nm, 120 fs pulses) of a "metasurface" composed by an alternation of GaAs nano-grooves and Au nanowires capping portions of flat GaAs. The nano-grooves depth and the Au nanowires thickness gradually vary across the sample. The samples are obtained by ion bombardment at glancing angle on a 150 nm Au mask evaporated on a GaAs plane wafer. The irradiation process erodes anisotropically the surface, creating Au nanowires and, at high ion dose, grooves in the underlying GaAs substrate (pattern transfer). The SHG measurements are performed for different pump linear polarization angle at different positions on the "metasurface" in order to explore the regions with optimal conditions for SHG efficiency. The pump polarization angle is scanned by rotating a half-wave retarder plate. While the output SH signal in reflection is analyzed by setting the polarizer in s or p configuration in front of the detector. The best polarization condition for SHG is obtained in the configuration where the pump and second harmonic fields are both p polarized, and the experiments show a SH polarization dependence of the same symmetry of bulk GaAs. Thus, the presence of gold contributes only as field localization effect, but do not contributes directly as SH generator

Self-Organized Nanorod Arrays for Large-Area Surface-Enhanced Infrared Absorption

Capabilities of highly sensitive surface-enhanced infrared absorption (SEIRA) spectroscopy are demonstrated by exploiting large-area templates (cm2) based on self-organized (SO) nanorod antennas. We engineered highly dense arrays of gold nanorod antennas featuring polarization-sensitive localized plasmon resonances, tunable over a broadband near- and mid-infrared (IR) spectrum, in overlap with the so-called "functional group" window. We demonstrate polarization-sensitive SEIRA activity, homogeneous over macroscopic areas and stable in time, by exploiting prototype self-assembled monolayers of IR-active octadecanthiol (ODT) molecules. The strong coupling between the plasmonic excitation and molecular stretching modes gives rise to characteristic Fano resonances in SEIRA. The SO engineering of the active hotspots in the arrays allows us to achieve signal amplitude improved up to 5.7%. This figure is competitive to the response of lithographic nanoantennas and is stable when the optical excitation spot varies from the micro- to macroscale, thus enabling highly sensitive SEIRA spectroscopy with cost-effective nanosensor devices

Asymmetric transmission and anomalous refraction in metal nanowires metasurface

Here we investigated the asymmetric transmission and the anomalous refraction introduced by a metasurface of bent gold nanowires. The refraction follows the generalized Snell's law that takes into account the resonant behavior of metallic nanostructures located at the interface between two dielectrics. Measurements performed in the linear optical regime reveal a large sensitivity to the subwavelength features of the gold nanostructures

Light absorption enhancement in thin film hydrgenated amorphus Si solar cells

In this paper, light absorption enhancement in thin film solar cell (SC) is reported and analyzed. The suggested design is based on a nanostructured pattern that increases the diffuse scattered component of radiation and hence the absorption through the active layer. An ion beam sputtering (lBS) approach is used to texture large areas of the glass substrate with high aspect-ratio ripples in order to increase light scattering. Then, thin film SC supported on the textured glass is simulated and analyzed using 3D finite difference time domain (FDTD) method. The suggested SC can offer an ultimate efficiency of 19.26% with short circuit current of 15.76 mA/cm2 with an enhancement of 31.435% over the SC without texturing surface

Trace Metals in Soot and PM2.5from Heavy-Fuel-Oil Combustion in a Marine Engine

Heavy fuel oil (HFO) particulate matter (PM) emitted by marine engines is known to contain toxic heavy metals, including vanadium (V) and nickel (Ni). The toxicity of such metals will depend on the their chemical state, size distribution, and mixing state. Using online soot-particle aerosol mass spectrometry (SP-AMS), we quantified the mass of five metals (V, Ni, Fe, Na, and Ba) in HFO-PM soot particles produced by a marine diesel research engine. The in-soot metal concentrations were compared to in-PM2.5measurements by inductively coupled plasma-optical emission spectroscopy (ICP-OES). We found that <3% of total PM2.5metals was associated with soot particles, which may still be sufficient to influence in-cylinder soot burnout rates. Since these metals were most likely present as oxides, whereas studies on lower-temperature boilers report a predominance of sulfates, this result implies that the toxicity of HFO PM depends on its combustion conditions. Finally, we observed a 4-to-25-fold enhancement in the ratio V:Ni in soot particles versus PM2.5, indicating an enrichment of V in soot due to its lower nucleation/condensation temperature. As this enrichment mechanism is not dependent on soot formation, V is expected to be generally enriched within smaller HFO-PM particles from marine engines, enhancing its toxicity

Adatom diffusion on vicinal surfaces with permeable steps

We study the behavior of single atoms on an infinite vicinal surface assuming certain degree of step permeability. Assuming complete lack of re-evaporation an ruling out nucleation the atoms will inevitably join kink sites at the steps but can do many attempts before that. Increasing the probability of step permeability or the kink spacing lead to increase of the number of steps crossed before incorporation of the atoms into kink sites. The asymmetry of the attachment-detachment kinetics (Ehrlich-Schwoebel effect) suppresses the step permeability and completely eliminates it in the extreme case of infinite Ehrlich-Schwoebel barrier. The average number of permeability events per atom scales with the average kink spacing. A negligibly small drift of the adatoms in a direction perpendicular to the steps leads to a significant asymmetry of the distribution of the permeability events the atoms thus visiting more distant steps in the direction of the drift.Comment: 12 pages, 6 figure