Spectral effects and enhancement quantification in healthy human saliva with surface-enhanced Raman spectroscopy using silver nanopillar substrates.

OBJECTIVES: Raman spectroscopy as a diagnostic tool for biofluid applications is limited by low inelastic scattering contributions compared to the fluorescence background from biomolecules. Surface-enhanced Raman spectroscopy (SERS) can increase Raman scattering signals, thereby offering the potential to reduce imaging times. We aimed to evaluate the enhancement related to the plasmonic effect and quantify the improvements in terms of spectral quality associated with SERS measurements in human saliva. METHODS: Dried human saliva was characterized using spontaneous Raman spectroscopy and SERS. A fabrication protocol was implemented leading to the production of silver (Ag) nanopillar substrates by glancing angle deposition. Two different imaging systems were used to interrogate saliva from 161 healthy donors: a custom single-point macroscopic system and a Raman micro-spectroscopy instrument. Quantitative metrics were established to compare spontaneous RS and SERS measurements: the Raman spectroscopy quality factor (QF), the photonic count rate (PR), the signal-to-background ratio (SBR). RESULTS: SERS measurements acquired with an excitation energy four times smaller than with spontaneous RS resulted in improved QF, PR values an order of magnitude larger and a SBR twice as large. The SERS enhancement reached 100×, depending on which Raman bands were considered. CONCLUSIONS: Single-point measurement of dried saliva with silver nanopillars substrates led to reproducible SERS measurements, paving the way to real-time tools of diagnosis in human biofluids.

Percolation threshold determination of sputtered silver films using Stokes parameters and in situ conductance measurements.

This work presents a straightforward approach to determine the percolation threshold of silver thin films deposited by magnetron sputtering on various oxide layers at room temperature. The proposed method is based on the observation of the coupling of p-polarized light with local surface plasmons. By measuring the first Stokes parameter in real time, one can determine the moment at which the nano-islands of silver begin to coalesce into a continuous film. We confirm the results by in situ and ex situ conductance measurements. The method is then used to assess the percolation threshold on different oxide seed layers such as ZnSnO, ZnO, TiO2, and SiO2.

Design and fabrication of stress-compensated optical coatings: Fabry-Perot filters for astronomical applications.

The performance of optical coatings may be negatively affected by the deleterious effects of mechanical stress. In this work, we propose an optimization tool for the design of optical filters taking into account both the optical and mechanical properties of the substrate and of the individual deposited layers. The proposed method has been implemented as a supplemental module in the OpenFilters open source design software. It has been experimentally validated by fabricating multilayer stacks using e-beam evaporation, in combination with their mechanical stress assessment performed as a function of temperature. Two different stress-compensation strategies were evaluated: (a) design of two complementary coatings on either side of the substrate and (b) implementing the mechanical properties of the individual materials in the design of the optical coating on one side only. This approach has been tested by the manufacture of a Fabry-Perot etalon used in astronomy while using evaporated SiO2 and TiO2 films. We found that the substrate curvature can be decreased by 85% and 49% for the first and second strategies, respectively.

Enhanced Durability and Antireflective Performance of Ag-Based Transparent Conductors Achieved via Controlled N-Doping.

Ag-based transparent conductors (TCs) are often proposed as an alternative to ITO coatings. However, while their performance has been widely demonstrated, their environmental durability is frequently overlooked or addressed with the use of highly specific encapsulating layers. In this work, the durability and antireflective performance of Ag-based TCs are simultaneously enhanced. To do so, a transfer matrix modeling approach is used to determine the general requirements for high performance antireflective properties as a function of Ag thickness and dielectric refractive indices, offering more widely applicable insight into stack optimization. Coating durability is investigated as a function of the Ag microstructure, which is modified by altering the N2 concentration used for doping of the Ag layer and the selection of the seed layer. Increasing N2 concentration during Ag deposition was found to decrease grain size and durability of Ag coatings deposited on Si3N4 whereas all coatings on ZnO(Al) showed higher stability. Significantly higher durability is found when specifically combining intermediate N2 concentrations in the sputtering gas mixture (Ag(N):5%, compared to 0% and 50%) and a ZnO(Al) seed layer, and a mechanism accounting for this increased durability is proposed. The addition of NiCrNx protective coatings increases the system durability without altering these trends. These findings are combined to fabricate a highly performant Ag-based TC (TV = 89.2%, RVFS = 0.23%, 21.4 Ω), which shows minimal property changes following corrosion testing by immersion in a heated and highly concentrated aqueous NaCl solution (200 g/L, 50 °C).

Fabrication of Plasmonic Indium Tin Oxide Nanoparticles by Means of a Gas Aggregation Cluster Source.

In this work, we demonstrate, for the first time, the possibility to fabricate indium tin oxide nanoparticles (ITO NPs) using a gas aggregation cluster source. A stable and reproducible deposition rate of ITO NPs has been achieved using magnetron sputtering of an In2O3/SnO2 target (90/10 wt %) at an elevated pressure of argon. Remarkably, most of the generated NPs possess a crystalline structure identical to the original target material, which, in combination with their average size of 17 nm, resulted in a localized surface plasmon resonance peak at 1580 nm in the near-infrared region.

WO3∕SiO2 composite optical films for the fabrication of electrochromic interference filters.

New security devices based on innovative technologies and ideas are essential in order to limit counterfeiting's profound impact on our economy and society. Interference security image structures have been in circulation for more than 20 years, but commercially available iridescent products now represent a potential threat. Therefore, the introduction of active materials, such as electrochromic WO3, to present-day optical security devices offers interesting possibilities. We have previously proposed electrochromic interference filters based on porous and dense WO3, which possessed an angle-dependent and voltage-driven color shift. However, the low index contrast required filters with a high number of layers. In this article, we increase the index contrast (0.61) by mixing WO3 with SiO2 and study the physical and electrochromic properties of mixtures. We next combine high and low index films in tandem configurations to observe the bleaching/coloration dynamics. To account for the film performance, we propose a simple explanation based on the differences in electron diffusion coefficients. An 11 layer electrochromic interference filter (EIF) based on the alternation of pure WO3 and (WO3)0.17(SiO2)0.83 films with a blue to purple angular color shift is then presented. Finally, we discuss possible applications of these EIFs for security.

Effect of postdeposition annealing on the structure, composition, and the mechanical and optical characteristics of niobium and tantalum oxide films.

Optical, mechanical, and thermal properties of optical thin films are very important for a reliable device performance. In the present work, the effect of annealing on the stability and the characteristics of niobium and tantalum oxide films grown at room temperature (RT) by dual ion beam sputtering were studied. The refractive index (n(λ)), extinction coefficient (k(λ)), hardness (H), reduced Young's modulus (E(r)), and film stress (σ) were investigated as a function of the annealing temperature (T(A)). X-ray diffraction analysis showed that all as-deposited films were amorphous, and crystallization was observed only after annealing at 700°C. Compositional analyses confirmed that the atomic ratio of oxygen to metal in as-deposited and annealed films was close to 2.5, indicating that the films were stoichiometric pentoxides of Nb and Ta. The properties of Nb(2)O(5) and Ta(2)O(5) films were, respectively, affected by postdeposition annealing: n(λ) values (at 550 nm) decreased from 2.30 to 2.20 and from 2.14 to 2.08, the average H and E(r) values increased from 5.6 to 7.4 GPa, and from 121 to 132 GPa for Nb(2)O(5), and from 6.5 to 8.3 GPa, and from 132 to 144 GPa for Ta(2)O(5), and the initial low compressive stress for both materials changed to tensile. We explain the variation of the coating material properties in terms of film stoichiometry, crystallinity, electronic structure, and possible reactions at the film-substrate interface.

Active metameric security devices using an electrochromic material.

In order to increase the anticounterfeiting performance of interference security image structures, we propose to implement an active component using an electrochromic material. This novel device, based on metamerism, offers the possibility of creating various surprising optical effects, it is more challenging to duplicate due to its complexity, and it adds a second level of authentication. By designing optical filters that match the bleached and colored states of the electrochromic device, one can obtain two hidden images-one appearing when the device is tilted, and the other one disappearing when the device is colored under an applied potential. Specifically, we present an example of a filter that is metameric with the colored state of the electrochromic device, demonstrate how the dynamic nature of the device offers more fabrication flexibility, and discuss its performance. We also describe a design methodology for metameric filters based on the luminous efficiency curve of the human eye: this approach results in filters with a lower number of layers and hence lower fabrication costs, and with a lower color difference sensitivity under various illuminants and for nonstandard observers.

Optical and tribomechanical stability of optically variable interference security devices prepared by dual ion beam sputtering.

Optical security devices applied to banknotes and other documents are exposed to different types of harsh environments involving the cycling of temperature, humidity, chemical agents, and tribomechanical intrusion. In the present work, we study the stability of optically variable devices, namely metameric interference filters, prepared by dual ion beam sputtering onto polycarbonate and glass substrates. Specifically, we assess the color difference as well as the changes in the mechanical properties and integrity of all-dielectric and metal-dielectric systems due to exposure to bleach, detergent and acetone agents, and heat and humidity. The results underline a significant role of the substrate material, of the interfaces, and of the nature and microstructure of the deposited films in long term stability under everyday application conditions.

Mechanical and thermoelastic characteristics of optical thin films deposited by dual ion beam sputtering.

Mechanical and thermoelastic properties of optical films are very important to ensure the performance of optical interference filters and optical coating systems. We systematically study the growth and the mechanical and thermoelastic characteristics of niobium oxide (Nb(2)O(5)), tantalum oxide (Ta(2)O(5)), and silicon dioxide (SiO(2)) thin films prepared by dual ion beam sputtering. First, we investigate the stress (sigma), hardness (H), reduced Young's modulus (E(r)), and scratch resistance. Second, we focus on the methodology and assessment of the coefficient of thermal expansion (CTE) and Poisson's ratio (nu) using the two-substrate method. For the high refractive index films, namely, Nb(2)O(5) (n at 550 nm=2.30) and Ta(2)O(5) (n at 550 nm=2.13), we obtained H approximately 6 GPa, E(r) approximately 125 GPa, CTE=4.9x10(-6) degrees C(-1), nu=0.22, and H approximately 7 GPa, E(r) approximately 133 GPa, CTE=4.4x10(-6) degrees C(-1), and nu=0.27, respectively. In comparison, for SiO(2) (n at 550 nm=1.48), these values are H approximately 9.5 GPa, E(r) approximately 87 GPa, CTE=2.1x10(-6) degrees C(-1), and nu=0.11. Correlations between the growth conditions (secondary beam ion energy and ion current), the microstructure, and the film properties are discussed.

Optical filters with prescribed optical thickness and refined refractive indices.

We propose to refine the refractive index of the layers composing optical filters while keeping their optical thicknesses constant. Using this technique, one can optimize filters made of quarter-wave layers using conventional optimization techniques, while preserving the possibility to use turning-point monitoring during their fabrication. Application of this method to the design of a dual narrowband filter and a tilted edge filter demonstrates its effectiveness.

Step method: a new synthesis method for the design of optical filters with intermediate refractive indices.

We propose a new synthesis method for the design of multilayer optical filters with intermediate refractive indices, the step method. This method consists in adding infinitesimally small index steps in the index profile at optimal positions and then reoptimizing the thickness and the refractive index of the layers. Application of the method to the design of an antireflective coating, a low-pass edge filter, and an immersed polarizing beam splitter shows that it provides interesting solutions, even in the absence of a proper starting design. The formalism developed for the method also serves to demonstrate that the optimal filter consists of either homogeneous layers that maximize the effective refractive index contrast, or of graded-index layers.

Interacting polariton fluids in a monolayer of tungsten disulfide.

Atomically thin transition metal dichalcogenides (TMDs) possess a number of properties that make them attractive for realizing room-temperature polariton devices1. An ideal platform for manipulating polariton fluids within monolayer TMDs is that of Bloch surface waves, which confine the electric field to a small volume near the surface of a dielectric mirror2-4. Here we demonstrate that monolayer tungsten disulfide can sustain Bloch surface wave polaritons (BSWPs) with a Rabi splitting of 43 meV and propagation lengths reaching 33 µm. In addition, we show strong polariton-polariton nonlinearities within BSWPs, which manifest themselves as a reversible blueshift of the lower polariton resonance. Such nonlinearities are at the heart of polariton devices5-11 and have not yet been demonstrated in TMD polaritons. As a proof of concept, we use the nonlinearity to implement a nonlinear polariton source. Our results demonstrate that BSWPs using TMDs can support long-range propagation combined with strong nonlinearities, enabling potential applications in integrated optical processing and polaritonic circuits.

Mechanical and microstructural properties of hybrid poly(ethylene glycol)-soy protein hydrogels for wound dressing applications.

Biomimetic hydrogel made of poly(ethylene glycol) and soy protein with a water content of 96% has been developed for moist wound dressing applications. In this study, such hybrid hydrogels were investigated by both tensile and unconfined compression measurements in order to understand the relationships between structural parameters of the network, its mechanical properties and protein absorption in vitro. Elastic moduli were found to vary from 1 to 17 kPa depending on the composition, while the Poisson's ratio (approximately 0.18) and deformation at break (approximately 300%) showed no dependence on this parameter. Further calculations yielded the crosslinking concentration, the average molecular weight between crosslinks (M(C)) and the mesh size. The results show that reactions between PEG and protein create polymeric chains comprising molecules of PEG and protein fragments between crosslinks. M(C) is three times higher than that expected for a "theoretical network." On the basis of this data, we propose a model for the 3D network of the hydrogel, which is found to be useful for understanding drug release properties and biomedical potential of the studied material.

Galvanostatic Rejuvenation of Electrochromic WO3 Thin Films: Ion Trapping and Detrapping Observed by Optical Measurements and by Time-of-Flight Secondary Ion Mass Spectrometry.

Electrochromic (EC) smart windows are able to decrease our energy footprint while enhancing indoor comfort and convenience. However, the limited durability of these windows, as well as their cost, result in hampered market introduction. Here, we investigate thin films of the most widely studied EC material, WO3. Specifically, we combine optical measurements (using spectrophotometry in conjunction with variable-angle spectroscopic ellipsometry) with time-of-flight secondary ion mass spectrometry and atomic force microscopy. Data were taken on films in their as-deposited state, after immersion in a Li-ion-conducting electrolyte, after severe degradation by harsh voltammetric cycling and after galvanostatic rejuvenation to regain the original EC performance. Unambiguous evidence was found for the trapping and detrapping of Li ions in the films, along with a thickness increase or decrease during degradation and rejuvenation, respectively. It was discovered that (i) the trapped ions exhibited a depth gradient; (ii) following the rejuvenation procedure, a small fraction of the Li ions remained trapped in the film and gave rise to a weak short-wavelength residual absorption; and (iii) the surface roughness of the film was larger in the degraded state than in its virgin and rejuvenated states. These data provide important insights into the degradation mechanisms of EC devices and into means of achieving improved durability.

Alkali Metal Halide Salts as Interface Additives to Fabricate Hysteresis-Free Hybrid Perovskite-Based Photovoltaic Devices.

A new method was developed for doping and fabricating hysteresis-free hybrid perovskite-based photovoltaic devices by using alkali metal halide salts as interface layer additives. Such salt layers introduced at the perovskite interface can provide excessive halide ions to fill vacancies formed during the deposition and annealing process. A range of solution-processed halide salts were investigated. The highest performance of methylammonium lead mixed-halide perovskite device was achieved with a NaI interlayer and showed a power conversion efficiency of 12.6% and a hysteresis of less than 2%. This represents a 90% improvement compared to control devices without this salt layer. Through depth-resolved mass spectrometry, optical modeling, and photoluminescence spectroscopy, this enhancement is attributed to the reduction of iodide vacancies, passivation of grain boundaries, and improved hole extraction. Our approach ultimately provides an alternative and facile route to high-performance and hysteresis-free perovskite solar cells.

Galvanostatic Ion Detrapping Rejuvenates Oxide Thin Films.

Ion trapping under charge insertion-extraction is well-known to degrade the electrochemical performance of oxides. Galvanostatic treatment was recently shown capable to rejuvenate the oxide, but the detailed mechanism remained uncertain. Here we report on amorphous electrochromic (EC) WO3 thin films prepared by sputtering and electrochemically cycled in a lithium-containing electrolyte under conditions leading to severe loss of charge exchange capacity and optical modulation span. Time-of-flight elastic recoil detection analysis (ToF-ERDA) documented pronounced Li(+) trapping associated with the degradation of the EC properties and, importantly, that Li(+) detrapping, caused by a weak constant current drawn through the film for some time, could recover the original EC performance. Thus, ToF-ERDA provided direct and unambiguous evidence for Li(+) detrapping.

Metameric interference security image structures.

We study innovative interference security image structures based on metamerism. We have designed, fabricated, and evaluated different structures that can be used in transmission or in reflection. These metameric structures are either a combination of two different interference filters or of an interference filter and a noniridescent colored material. In the latter case, by closely matching the spectra, the sensitivity of the device to changes in light sources and observers is minimized. Because of the intrinsic color shift of interference filters, one can create a hidden image that appears at a specific observation angle. The presence of the hidden image, as well as in some cases of the noniridescent material, which serves as a color reference, increases the complexity of such devices while facilitating the user's authentication process as well as automatized detection by using a laser at a specific angle. We present the design approach, analyze the filters' sensitivity to deposition errors, and evaluate the performance of prototype devices prepared by dual ion beam sputtering.

OpenFilters: open-source software for the design, optimization, and synthesis of optical filters.

The design of optical filters relies on powerful computer-assisted methods. Many of these methods are provided by commercial programs, but, in order to adapt and improve them, or to develop new methods, one needs to create his own software. To help people interested in such a process, we decided to release our in-house software, called OpenFilters, under the GNU General Public License, an open-source license. It is programmed in Python and C++, and the graphical user interface is implemented with wxPython. It allows creation of multilayer and graded-index filters and calculation of reflection, transmission, absorption, phase, group delay, group delay dispersion, color, ellipsometric variables, admittance diagram, circle diagram, electric field distribution, and generation of reflection, transmission, and ellipsometric monitoring curves. It also provides the refinement, needle, step, and Fourier transform methods.

Dispersion implementation in optical filter design by the Fourier transform method using correction factors.

The Fourier transform method to design graded-index optical filters, that relates the desired reflection spectrum and the index profile through the use of a Q function, has two important drawbacks: (1) It relies on approximate Q functions, and (2) it does not account for the dispersion of the index of refraction. The former is usually addressed by an iterative correction process. We propose to address the latter by scaling the wavelength in the Fourier transform by the optical thickness of the filter and to multiply the Q function by a wavelength-dependent correction factor. We demonstrate the high effectiveness of this approach by the performance of optical filters designed with such correction factors using the optical properties of SiO2/TiO2 mixtures.