Amorphous WO3 as transparent conductive oxide in the near-IR

The demand for transparent conductive films (TCFs) is dramatically increasing. In this work tungsten oxide (WO3-x) is studied as a possible option additional to the existed TCFs. We introduce WO3-x thin films fabricated by a non-reactive magnetron RF-sputtering process at room temperature, followed by thermal annealing in dry air. Films are characterized morphologically, structurally, electrically, optically, and dielectrically. Amorphous WO3-x thin films are shown to be ntype conductive while the transparency extends to the near-IR. By evaluating a figure of merit for transparent-conductive performance and comparing to some most-widely used TCFs, WO3-x turns out to outperform in the near-IR optical range

The Impact of Bacteria Exposure on the Plasmonic Response of Silver Nanostructured Surfaces

Silver, especially in the form of nanostructures, is widely employed as an antimicrobial agent in a large range of commercial products. The origin of the biocidal mechanism has been elucidated in the last decades, and most likely originates from silver cation release due to oxidative dissolution followed by cellular uptake of silver ions, a process that causes a severe disruption of bacterial metabolism and eventually leads to eradication. Despite the large number of works dealing with the effects of nanosilver shape/size on the antibacterial mechanism and on the (bio)physical chemistry pathways that drive bacterial eradication, little effort has been devoted to the investigation of the silver NPs plasmon response upon interaction with bacteria. Here we present a detailed investigation of the bacteria-induced changes of the plasmon spectral and dynamical features after exposure to one of the most studied bacterial models, Escherichia Coli. Ultrafast pump-probe measurements indicate that the dramatic changes on particle size/shape and crystallinity, which stem from a bacteria-induced oxidative dissolution process, translate into a clear modification of the plasmon spectral and dynamical features. This study may open innovative new avenues in the field of biophysics of bio-responsive materials, with the aim of providing new and reliable biophysical signatures of the interaction of these materials with complex biological environments

Imaging photoinduced surface potentials on hybrid perovskites by real-time Scanning Electron Microscopy

We introduce laser-assisted Time-Resolved SEM (TR-SEM), joining Scanning Electron Microscopy and laser light excitation, to probe the long-term temporal evolution of optically excited charge distributions at the surface of Metal Ammonium Lead Triiodide (MAPbI3) hybrid perovskite thin films. Laser-assisted TR-SEM relies on the optically induced local modification of Secondary Electron (SE) detection yield to provide mapping of photoexcited potentials and charge dynamics at surfaces, and qualifies as a complementary approach to near-field probe microscopies and nonlinear photoemission spectroscopies for photovoltage measurements. Real-time imaging of evolving field patterns are provided on timescales compatible with SEM scanning rates, so that temporal resolution in the millisecond range can be ultimately envisaged. MAPbI3 is an outstanding light-sensitive material candidate for applications in solar light harvesting and photovoltaics, also appealing as an active system for light generation. In this work, the real time temporal evolution of optically induced SE contrast patterns in MAPbI3 is experimentally recorded, both under illumination by a 405 nm blue laser and after light removal, showing the occurrence of modifications related to photoinduced positive charge fields at surface. The long term evolution of these surface fields are tentatively attributed to ion migration within the film, under the action of the illumination gradient and the hole collecting substrate. This optical excitation is fully reversible in MAPbI3 over timescales of hours and a complete recovery of the system occurs within days. Permanent irradiation damage of the material is avoided by operating the SEM at 5 keV of energy and 1–10 pA of primary current. Optical excitation is provided by intense above-bandgap illumination (up to 50 W/cm^2). TR-SEM patterns show a strong dependence on the geometry of SE collection. Measurements are taken at different axial orientations of the sample with respect to the entrance of the in-column detection system of the SEM and compared with numerical modeling of the SE detection process. This enables to single out the information regarding the local potential distribution. Results are interpreted by combining data about the spectral distribution of emitted SEs with the configuration of the electric and magnetic fields in the specimen chamber. The present modeling sets a robust basis for the understanding of photoinduced SE electron contrast

Direct Observation of Local Birefringence and Axis Rotation in Spun Fiber with Centimetric Resolution

By means of the centimetric resolution of the developed cut-back method, we have directly observed a rotation of the birefringence axis and a sinusoidal evolution of the local retardation along spun fiber samples, confirming theoretical predictions regarding how the spinning process acts in reducing the internal fiber birefringence

LR-SPP Mode Cutoff in Strip Waveguides as Affected by Technologically Induced Asymmetries: a Numerical Study

We conducted a numerical analysis on the cutoff conditions for Long-Range Surface Plasmon-Polariton in plasmonic strip waveguides at =1.55 um. Several aspects, related to the technological process of realization of the waveguides have been considered. The joint effects of dielectric asymmetries between upper and lower claddings and layering of the strip metallic core in affecting propagation parameters are highlighted

UHV Ultrafast Scanning Electron Microscopy

Abstract|Ultrafast Scanning Electron Microscopy (USEM) merges photonics and electron microscopy, combining the temporal resolution of femtosecond laser spectroscopy with the nanoscale spatial resolution and surface selectivity typical of scanning electron microscopy, and provides imaging of dynamical photoinduced processes at surfaces and in ultra-thin films. Our USEM apparatus is based on a UHV (10^-9 - 10^-10 Torr) Field-E®ect SEM, modified with optical windows at the tip of the electron gun and at the specimen chamber and optically coupled to an ultrafast pulsed fs laser source (300 fs, 1030 nm) operated at 10MHz repetition rate. The UHV controlled environment guarantees elevated sensitivity to the physical chemistry of sample surfaces. Time-resolved operation is achieved by means of a pump-probe configuration, where the UV third harmonic (TH) beam of the laser source works as the optical pump, while the fourth harmonic (FH) beam promotes the emission of an ultrafast pulsed electron probe beam from the SEM tip. Pump and probe pulses are relatively delayed by an optical delay stage set on the optical path of the pump beam and the pump-probe delay is tuned over 4 ns with sub-ps resolution. SE detection is provided by an Everhart-Thorley detector, either directly operated in current mode for time resolved imaging or lock-in demodulated for time spectroscopy on selected areas A signal rise time of about 10 ps has been demonstrated