High reflectance ta-C coatings in the extreme ultraviolet

Refractiveindex.info database of optical constants

We introduce the refractiveindex.info database, a comprehensive open-source repository containing optical constants for a wide array of materials, and describe in detail the underlying dataset. This collection, derived from a meticulous compilation of data sourced from peer-reviewed publications, manufacturers' datasheets, and authoritative texts, aims to advance research in optics and photonics. The data is stored using a YAML-based format, ensuring integrity, consistency, and ease of access. Each record is accompanied by detailed metadata, facilitating a comprehensive understanding and efficient utilization of the data. In this descriptor, we outline the data curation protocols and the file format used for data records, and briefly demonstrate how the data can be organized in a user-friendly fashion akin to the books in a traditional library.

Laminar-type gratings overcoated with carbon-based materials to enhance analytical sensitivity of flat-field emission spectrograph in the VUV region

Laminar-type spherical diffraction gratings overcoated with carbon-based materials were designed, fabricated, and evaluated for the purpose of enhancing the analytical sensitivity of the flat-field spectrograph in a vacuum ultraviolet region of 35-110 eV. As the design benchmark for numerical calculations, diffraction efficiency (DE) and spectral flux, which are defined by the product of the DE and numerical aperture and correlate with the analytical sensitivity of the spectrograph, were used. To simplify the feasibility study on the overcoating effects, we assumed a laminar-type grating having a grating constant of 1/1000 mm and coated with a Au layer of 30.0 nm thickness and an incidence angle of 84.0°. The optimized groove depth and duty ratio were 30.0 nm and 0.3, respectively. In addition, the optimum thicknesses of the overcoating layer were 44, 46, 24, and 30 nm for B4C, C, diamond-like-carbon, and SiC, respectively. Based on these results, we have fabricated a varied-line-spacing holographic grating overcoated with B4C with a thickness of 47 nm. For the experimental evaluation, we used the light source of Mg-L and Al-L emissions excited by the electron beam generated from an electron microscope, an objective flat-field spectrograph, and a CCD imaging detector. The experimental results showed that the spectrograph employing a new grating overcoated with the B4C layer indicated almost the same spectral resolution and 2.9-4.2 times higher analytical sensitivity compared with those obtained with a previously designed Au-coated grating having a grating constant of 1/1200 mm and used at an incidence of 86.0°.

Optical Properties of Amorphous Carbon Thin Films Fabricated Using a High-Energy-Impulse Magnetron-Sputtering Technique

This paper reports the results of amorphous carbon thin films fabricated by using the gas-impulse-injection magnetron-sputtering method and differing the accelerating voltage (1.0-1.4 kV). The obtained layers were investigated using Raman spectroscopy, X-ray photoelectron spectroscopy (XRD), and spectroscopic ellipsometry (SE). The analysis of the Raman and XPS spectra point to the significant content of sp3 hybridization in the synthesized materials (above 54-73%). The refractive index of the films is very high-above 2.45 in the infrared spectral range. The band-gap energy (determined using the inversed-logarithmic-derivative method) depends on the discharging voltage and is in the range from 1.58 eV (785 nm) to 2.45 eV (506 nm). Based on the obtained results, we have elaborated a model explaining the a-C layers' formation process.

In Operando Photoswitching of Cu Oxidation States in Cu-Based Plasmonic Heterogeneous Photocatalysis for Efficient H2 Evolution

Metal nanoparticles (NP) supported on TiO₂ are known to be efficient photocatalysts for solar-to-chemical energy conversion. While TiO₂ decorated with copper NPs has the potential to become an attractive system, the poor oxidative stability of Cu severely limits its applicability. In this work, we demonstrate that, when Cu NPs supported on TiO₂ nanobelts (NBs) are engaged in the photocatalytic generation of H₂ from water under light illumination, Cu is not only oxidized in CuO but also dissolved under the form of Cu⁺/Cu²⁺ ions, leading to a continuous reconstruction of nanoparticles via Ostwald ripening. By nanoencapsulating the CuO_x (Cu/CuO/Cu₂O) NPs by a few layers of carbon supported on TiO₂ (TC@C), Ostwald ripening can be suppressed. Simultaneously, the resulting CuO_x@C NPs are photoreduced under light illumination to generate Cu@C NPs. This photoswitching strategy allows the preparation of a Cu plasmonic photocatalyst with enhanced activity for H₂ production. Remarkably, the photocatalyst is even active when illuminated with visible light, indicating a clear plasmonic enhancement of photocatalytic activity from the surface plasmonic resonance (SPR) effect of Cu NPs. Three-dimensional electromagnetic wave-frequency domain (3D-EWFD) simulations were conducted to confirm the SPR enhancement. This advance bodes for the development of scalable multifunctional Cu-based plasmonic photocatalysts for solar energy transfer.

Temporal and spectral multiplexing for EUV multibeam ptychography with a high harmonic light source

We demonstrate temporally multiplexed multibeam ptychography implemented for the first time in the EUV, by using a high harmonic based light source. This allows for simultaneous imaging of different sample areas, or of the same area at different times or incidence angles. Furthermore, we show that this technique is compatible with wavelength multiplexing for multibeam spectroscopic imaging, taking full advantage of the temporal and spectral characteristics of high harmonic light sources. This technique enables increased data throughput using a simple experimental implementation and with high photon efficiency.

Measuring the refractive index and sub-nanometre surface functionalisation of nanoparticles in suspension

Direct measurements to determine the degree of surface coverage of nanoparticles by functional moieties are rare, with current strategies requiring a high level of expertise and expensive equipment. Here, a practical method to determine the ratio of the volume of the functionalisation layer to the particle volume based on measuring the refractive index of nanoparticles in suspension is proposed. As a proof of concept, this technique is applied to poly(methyl methacrylate) (PMMA) nanoparticles and semicrystalline carbon dots functionalised with different surface moieties, yielding refractive indices that are commensurate to those from previous literature and Mie theory. In doing so, it is demonstrated that this technique is able to optically detect differences in surface functionalisation or composition of nanometre-sized particles. This non-destructive and rapid method is well-suited for in situ industrial particle characterisation and biological applications.

Facile fabrication of Ag@C@C8 nanoparticles as a SERS substrate and their environmental applications

Carbon-coated silver (Ag@C) nanoparticles were prepared by a one-step hydrothermal synthesis method, the surface was modified with chlorodimethyloctylsilane (C8) to generate C8 functionalized Ag@C@C8 nanoparticles with long-term stability and high sensitivity.

A 3D C@TiO2 multishell nanoframe for simultaneous photothermal catalytic hydrogen generation and organic pollutant degradation

Rapid heat loss and fast charge carrier recombination constitute two crucial issues that hinder the development of efficient solar energy utilization and conversion over the semiconductor in a photothermal catalytic system. Inspired by energy production from waste water, we designed an advanced 3D C@TiO₂ multishell nanoframe (MNF) photocatalyst. Its unique structural features of heat confinement and vibrant photocarrier kinetics lead to excellent photo-thermal conversion for synchronous superior photocatalytic H₂ evolution (503 μmol g^-1h^-1) and 98.2% RhB removal without the use of any co-catalyst and sacrificial reagent under simulated sunlight irradiation (AM 1.5G). Mechanism exploration reveals that the difference between the inner and outer gas pressure formed inside C@TiO₂ precursor facilitates the selective cleavage of outer TiO₂ layers at selected temperatures during calcination. Synergistic effects between residual carbon core and multi-shelled TiO₂ framework endow C@TiO₂ MNF with excellent heat confinement and vibrant photocarrier kinetics. Such MNF photo-thermocatalyst concept provides a novel strategy for effective utilization of solar energy, and this work may open a novel avenue towards advanced nanostructures for efficient waste-to-energy conversion.

Facile synthesis of Ag@C@Ag hybrid nanoparticles as SERS substrate

Ag@C core-shell nanoparticles (NPs) were first prepared by a low-temperature heating-stirring method and subsequently modified with polyethyleneimine (PEI) at different concentrations. Finally, Ag@C@Ag hybrid NPs were prepared by a simple self-assembly procedure, and 24-nm Ag NPs were attached onto the surface of the initially fabricated PEI-modified Ag@C NPs via interaction between the NH₂ groups of PEI and Ag. The results demonstrated that rhodamine 6G (R6G) could be detected at a concentration as low as 10^-10 M using the Ag@C@Ag NPs as a substrate. To further understand the signal enhancement mechanism, finite-difference time-domain (FDTD) simulations were performed to calculate the electromagnetic field distributions and illustrate the generated Raman hot spots. The FDTD indicated that this enhancement was attributed to the surface plasmon resonance effects of the core Ag NPs in the Ag@C NPs, hot spots between the Ag@C NPs, and external assembly of the 24-nm Ag NPs, as well as between the massive outlayer 24-nm Ag NPs themselves. These fabricated materials were further applied for the detection of folic acid as an actual sample. The outstanding performance of the Ag@C@Ag NPs can be attributed to both the excellent properties of this hybrid substrate and the absorption capability of the carbon layer. Thus, this Ag@C@Ag NP material demonstrates excellent and stable optical properties, and can be used as a surface-enhanced Raman scattering (SERS) substrate in the field of ultrasensitive spectral analysis. Graphical abstract Ag@C@Ag hybrid nanoparticles are prepared by a simple self-assembly method. Then the synthesized Ag@C@Ag hybrid nanoparticles are used as SERS substrate for folic acid detection. To further understand the signal enhancement mechanism, finite-difference time-domain simulations are performed to calculate the electromagnetic field distributions and illustrate the generated SERS hot spots.

Design and experimental evaluation of enhanced diffraction efficiency of lanthanum-based material coated laminar-type gratings in the boron K-emission region

Numerical and experimental studies have been performed to evaluate the enhancement of diffraction efficiency of diffraction gratings around B $K$-emission by overcoating lanthanum series layers on conventional metal-coated laminar-type gratings. We propose an optical design method based on the concept of spectral flux given by collection efficiency and diffraction efficiency. A diffraction grating with a small angle of incidence provides an advantage to soft x-ray spectrographs because it collects the emission at a larger solid angle compared to that of conventional grazing incidence diffraction gratings. Numerical calculations indicated that La and ${\rm{La}}{{\rm{F}}_3}$ were promising as overcoating materials on a laminar-type Ni-coated diffraction grating, and we performed an experimental study using ${\rm{La}}{{\rm{F}}_3}$ and La/C overcoatings, considering their producibility and durability. The diffraction efficiencies were measured using a reflectometer at a synchrotron facility. The diffraction efficiencies observed at 183.4 eV were 29.4% and 34.3% at angles of incidence of 85.1° and 84.9° for ${\rm{Ni}}/{\rm{La}}{{\rm{F}}_3}$ and Ni/La/C gratings, respectively.

Simulation and optimization of a broadband reflective far ultraviolet polarimeter

Traditional transmissive polarimetric methods can be used for wavelengths above 123 nm where birefringent materials transmit light and create significant birefringence. Below 123 nm, no suitable solution is known to measure the four Stokes parameters on a large wavelength range. Therefore, we study here an innovative reflective (rather than transmissive) polarimeter working in the far ultraviolet (FUV) range from 90 to 130 nm. We take advantage of the phase shift created by reflections as well as the different reflectivities for p (orthogonal ⊥) and s (parallel ∥ to the plane of incidence) polarizations to design an FUV polarimeter. Simulation of the analyzer and modulator using Mueller matrices coupled to polarimetric efficiencies calculations allowed optimization of reflective polarimeters for the first time, to the best of our knowledge. This opens up a new perspective for FUV polarimetry below 123 nm.

Direct optical excitation of dark plasmons for hot electron generation

We demonstrate the excitation of dark modes and creation of hot electrons using linearly polarized light and scalable, cost-effective plasmonic surfaces.

Polycapillary-boosted instrument performance in the extreme ultraviolet regime for inverse photoemission spectroscopy

A collimating polycapillary half lens, traditionally used in the medium and hard X-ray band, is operated at a photon energy of 36 eV for the first time. While the transmission still exceeds 50%, the measured and simulated spatial resolution and angular divergence approach 0.4 mm or less and at most 20 mrad, respectively. This unexpected, superior performance of the polycapillary optic in the extreme Ultraviolet could enable the design of an efficient, versatile and compact spectrometer for inverse photoemission spectroscopy (IPES): Its wavelength-dispersive component, a customized reflection zone plate, can maintain an energy resolution of 0.3 eV, whereas the sensitivity may be enhanced by more than one order of magnitude, compared to conventional spectrometers. Furthermore, the overall length of 0.9 m would allow for an eased alignment and evacuation. We see a significant potential for numerous polycapillary-based XUV / soft X-ray instruments in the future, in particular after further optimization for this long wavelength regime.

Spectroscopic Ellipsometry of Nanocrystalline Diamond Film Growth

With the retention of many of the unrivaled properties of bulk diamond but in thin-film form, nanocrystalline diamond (NCD) has applications ranging from micro-/nano-electromechanical systems to tribological coatings. However, with Young's modulus, transparency, and thermal conductivity of films all dependent on the grain size and nondiamond content, compositional and structural analysis of the initial stages of diamond growth is required to optimize growth. Spectroscopic ellipsometry (SE) has therefore been applied to the characterization of 25-75 nm thick NCD samples atop nanodiamond-seeded silicon with a clear distinction between the nucleation and bulk growth regimes discernable. The resulting presence of an interfacial carbide and peak in nondiamond carbon content upon coalescence is correlated with Raman spectroscopy, whereas the surface roughness and microstructure are in accordance with values provided by atomic force microscopy. As such, SE is demonstrated to be a powerful technique for the characterization of the initial stages of growth and hence the optimization of seeding and nucleation within films to yield high-quality NCD.

Dramatic Modification of Coupled-Plasmon Resonances Following Exposure to Electron Beams

Studies of the plasmon resonances in individual and coupled metal nanoparticles often involve imaging of the nanostructures of interest in an electron microscope. We show that this process can dramatically modify the optical spectra of coupled plasmonic nanoparticles, illustrated here with the case of gold nanorod-nanosphere dimers. The spectral changes are due to the thin, partially conductive carbonaceous layer that deposits onto the particles during imaging. These changes are particularly significant for coupled nanoparticles with subnanometer interparticle gaps but have largely been neglected in previous studies of such structures, including studies intended to probe quantum-mechanical effects in plasmon coupling. Accounting for the effects of the carbonaceous layer will lead to a more accurate understanding of such systems.