Search

Large surface area applications such as high efficiency >26% solar cells require surface patterning with 1–10 μm periodic patterns at high fidelity over 1–10 cm2$1 - \left(\text{cm}\right)^{2}$ areas (before up scaling to 1 m2$ \left(\text{m}\right)^{2}$) to perform at, or exceed, the Lambertian (ray optics) limit of light trapping. Herein, a pathway is shown to high‐resolution sub‐1 μm etch mask patterning by ablation using direct femtosecond laser writing performed at room conditions (without the need for a vacuum‐based lithography approach). A Bessel beam is used to alleviate the required high surface tracking tolerance for ablation of 0.3–0.8 μm diameter holes in 40 nm alumina Al2O3$\left(\text{Al}\right)_{2} \left(\text{O}\right)_{3}$–mask at high writing speed, 7.5 cm s−1; a patterning rate 1 cm2 per 20 min. Plasma etching protocol was optimized for a zero‐mesa formation of photonic‐crystal‐trapping structures and smooth surfaces at the nanoscale level. The maximum of minority carrier recombination time of 2.9 ms was achieved after the standard wafer passivation etch; resistivity of the wafer was 3.5 Ω cm. Scaling up in area and throughput of the demonstrated approach is outlined. [ABSTRACT FROM AUTHOR]

The performance and parameters of the online photon single‐shot spectrometer (PSSS) at the Aramis beamline of the SwissFEL free‐electron laser are presented. The device operates between the photon energies 4 and 13 keV and uses diamond transmission gratings and bent Si crystals for spectral measurements on the first diffraction order of the beam. The device has an energy window of 0.7% of the median photon energy of the free‐electron laser pulses and a spectral resolution (full width at half‐maximum) ΔE/E on the order of 10−5. The device was characterized by comparing its performance with reference data from synchrotron sources, and a parametric study investigated other effects that could affect the reliability of the spectral information. [ABSTRACT FROM AUTHOR]

Here a direct comparison is made between various X‐ray wavefront sensing methods with application to optics alignment and focus characterization at X‐ray free‐electron lasers (XFELs). Focus optimization at XFEL beamlines presents unique challenges due to high peak powers as well as beam pointing instability, meaning that techniques capable of single‐shot measurement and that probe the wavefront at an out‐of‐focus location are desirable. The techniques chosen for the comparison include single‐phase‐grating Talbot interferometry (shearing interferometry), dual‐grating Talbot interferometry (moiré deflectometry) and speckle tracking. All three methods were implemented during a single beam time at the Linac Coherent Light Source, at the X‐ray Pump Probe beamline, in order to make a direct comparison. Each method was used to characterize the wavefront resulting from a stack of beryllium compound refractive lenses followed by a corrective phase plate. In addition, difference wavefront measurements with and without the phase plate agreed with its design to within λ/20, which enabled a direct quantitative comparison between methods. Finally, a path toward automated alignment at XFEL beamlines using a wavefront sensor to close the loop is presented. [ABSTRACT FROM AUTHOR]

First principles electrodyanmics and quantum chemical simulations are performed to gain insights into the underlying mechanisms of the surface enhanced Raman spectra of 22BPY adsorbed on pure Au and Ag as well as on Au–Ag alloy nanodiscs. Experimental SERS spectra from Au and Ag nanodiscs show similar peaks, whereas those from Au–Ag alloy reveal new spectral features. The physical enhancement factors due to surface nano‐texture were considered by numerical FDTD simulations of light intensity distribution for the nano‐textured Au, Ag, and Au–Ag alloy and compared with experimental results. For the chemical insights of the enhancement, the DFT calculations with the dispersion interaction were performed using Au20, Ag20, and Au10Ag10 clusters of a pyramidal structure for SERS modeling. Binding of 22BPY to the clusters was simulated by considering possible arrangements of vertex and planar physical as well as chemical adsorption models. The DFT results indicate that 22BPY prefers a coplanar adsorption on a (111) face with trans‐conformation having close energy difference to cis‐conformation. Binding to pure Au cluster is stronger than to pure Ag or Au–Ag alloy clusters and adsorption onto the alloy surface can deform the surface. The computed Raman spectra are compared with experimental data and assignments for pure Au and Ag models are well matching, indicating the need of dispersion interaction to reproduce strong Raman signal at around 800 cm–1. This work provides insight into 3D character of SERS on nanorough surfaces due to different binding energies and bond length of nanoalloys. © 2018 Wiley Periodicals, Inc. The authors made a comparative study for experimental Au–Ag alloy nanostructure system for SERS measurements and density functional theory calculations as a model of 20 metal cluster model, as well as finite difference time‐domain calculations. [ABSTRACT FROM AUTHOR]

The SwissFEL Aramis beamline, covering the photon energies between 1.77 keV and 12.7 keV, features a suite of online photon diagnostics tools to help both users and FEL operators in analysing data and optimizing experimental and beamline performance. Scientists will be able to obtain information about the flux, spectrum, position, pulse length, and arrival time jitter versus the experimental laser for every photon pulse, with further information about beam shape and size available through the use of destructive screens. This manuscript is an overview of the diagnostics tools available at SwissFEL and presents their design, working principles and capabilities. It also features new developments like the first implementation of a THz‐streaking based temporal diagnostics for a hard X‐ray FEL, capable of measuring pulse lengths to 5 fs r.m.s. or better. [ABSTRACT FROM AUTHOR]

Submicron‐thick gold films endowed with subwavelength patterning allow on‐demand topological shaping of light, hence the precise delivery of optical orbital angular momentum. Several kinds of metallic metasurfaces enabling the generation of optical vortices with arbitrary topological charges in the visible domain are fabricated following a design based on space‐variant subwavelength grating with discrete orientational distribution. The optical performances of obtained flat spin–orbit optical elements designed to operate at 532 nm wavelength are experimentally assessed and satisfactorily compared to expectations. About 80% of optical vortex generation purity for predetermined topological charge up to | ℓ |   =   10 is achieved. Other designs that allow continuous orientational distribution of nanogratings as well as the superposition of orbital angular momentum states are also proposed. [ABSTRACT FROM AUTHOR]

Reactive ion etching was used to fabricate black-Si over the entire surface area of 4-inch Si wafers. After 20 min of the plasma treatment, surface reflection well below 2% was achieved over the 300-1000 nm spectral range. The spikes of the black-Si substrates were coated by gold, resulting in an island film for surface-enhanced Raman scattering (SERS) sensing. A detection limit of 1 × 10−6 M (at count rate > 102 s−1 . mW−1) was achieved for rhodamine 6G in aqueous solution when drop cast onto a ∼ 100-nm-thick Au coating. The sensitivity increases for thicker coatings. A mixed mobile-on-immobile platform for SERS sensing is introduced by using dog-bone Au nanoparticles on the Au/black-Si substrate. The SERS intensity shows a non-linear dependence on the solid angle (numerical aperture of excitation/collection optics) for a thick gold coating that exhibits a 10 times higher enhancement. This shows promise for augmented sensitivity in SERS applications. [ABSTRACT FROM AUTHOR]

The universal problem of surface charging during focused ion milling has been fully resolved using a flood-gun approach based on simultaneous co-illumination with a UV light-emitting diode (LED). Non-distorted as-designed nano-patterns were milled using Ga+ ions on dielectric materials which charge up strongly. Deep-UV (250-280 nm) LED co-illumination during the ion beam milling fully discharges optically the surface under standard Ga+ ion-milling conditions. Photo-ionization of electrons trapped at the sub-surface defects to the free vacuum state is a key to the phenomenon ( [ABSTRACT FROM AUTHOR]

A simple solution to lithographically write down to 20–30 nm features over micrometer‐sized nanoparticle arrays with high fidelity of pattern transfer from the designed to fabricated architectures is shown. It is achieved via a two‐step approach: (i) fabrication of basic shape nanoparticles by electron beam lithography, gold deposition via sputtering and lift‐off, then (ii) nano‐patterning by focused ion beam lithography. Application potential of 3D tailored nanoparticles for nanotweezers is discussed on the basis of numerical modeling and experimental measurements of extinction. [ABSTRACT FROM AUTHOR]

The list of authors in the paper by Juranić et al. (2018) [J. Synchrotron Rad.25, 1238–1248] is corrected. [ABSTRACT FROM AUTHOR]