TiO2 Films with Macroscopic Chiral Nematic-Like Structure Stabilized by Copper Promoting Light-Harvesting Capability for Hydrogen Generation.

Cellulose nanocrystals (CNCs) have inspired the synthesis of various advanced nanomaterials, opening opportunities for different applications. However, a simple and robust approach for transferring the long-range chiral nematic nanostructures into TiO2 photocatalyst is still fancy. Herein, a successful fabrication of freestanding TiO2 films maintaining their macroscopic chiral nematic structures after removing the CNCs biotemplate is reported. It is demonstrated that including copper acetate in the sol avoids the epitaxial growth of the lamellar-like structure of TiO2 and stabilizes the chiral nematic structure instead. The experimental results and optical simulation demonstrate an enhancement at the blue and red edges of the Fabry-Pérot reflectance peak located in the visible range. This enhancement arises from the light scattering effect induced by the formation of the chiral nematic structure. The nanostructured films showed 5.3 times higher performance in the photocatalytic hydrogen generation, compared to lamellar TiO2, and benefited from the presence of copper species for charge carriers' separation. This work is therefore anticipated to provide a simple approach for the design of chiral nematic photocatalysts and also offers insights into the electron transfer mechanisms on TiO2/CuxO with variable oxidation states for photocatalytic hydrogen generation.

Unsupervised topological analysis of polarized light microscopy: application to quantitative birefringence imaging.

The determination of birefringence (magnitude and axis orientation) of optical materials is of significant interest in various fields. In the case of composite samples, this task becomes complicated and time-consuming; therefore, a partially automated procedure for reconstructing birefringence spatial distribution becomes valuable. Herein, we propose a procedure to reconstruct the spatial distributions of the retardance and optical axis orientation in a geological thin section from sparse quantitative birefringence measurements, using automatic boundary detection on cross-polarized light microscopy images. We examine two particular areas on the selected geological thin section: one that presents a uniaxial crystal with a circular cross-section of its refractive index ellipsoid and the other with grains of varying orientations. The measurement gives the orientation of the grain's optical axis both in and out of the plane of the thin section, which explains the qualitative observations with the cross-polarized light microscope. Future work will connect the measured orientation of the rock thin section with its 3D geological orientation in the field.

Genetic-algorithm-aided ultra-broadband perfect absorbers using plasmonic metamaterials.

Complete absorption of electromagnetic waves is paramount in today's applications, ranging from photovoltaics to cross-talk prevention into sensitive devices. In this context, we use a genetic algorithm (GA) strategy to optimize absorption properties of periodic arrays of truncated square-based pyramids made of alternating stacks of metal/dielectric layers. We target ultra-broadband quasi-perfect absorption of normally incident electromagnetic radiations in the visible and near-infrared ranges (wavelength comprised between 420 and 1600 nm). We compare the results one can obtain by considering one, two or three stacks of either Ni, Ti, Al, Cr, Ag, Cu, Au or W for the metal, and poly(methyl methacrylate) (PMMA) for the dielectric. More than 1017 configurations of geometrical parameters are explored and reduced to a few optimal ones. This extensive study shows that Ni/PMMA, Ti/PMMA, Cr/PMMA and W/PMMA provide high-quality solutions with an integrated absorptance higher than 99% over the considered wavelength range, when considering realistic implementation of these ultra-broadband perfect electromagnetic absorbers. Robustness of optimal solutions with respect to geometrical parameters is investigated and local absorption maps are provided. Moreover, we confirm that these optimal solutions maintain quasi-perfect broadband absorption properties over a broad angular range when changing the inclination of the incident radiation. The study also reveals that noble metals (Au, Ag, Cu) do not provide the highest performance for the present application.

Analysis of accuracy and ambiguities in spatial measurements of birefringence in uniaxial anisotropic media.

Accuracy and ambiguities in retardance and optical axis orientation spatial measurements are analyzed in detail in the context of the birefringence imaging method introduced by Shribak and Oldenbourg [Appl. Opt.42, 3009 (2003)APOPAI0003-693510.1364/AO.42.003009]. An alternative formula was derived in order to determine the optical axis orientation more accurately, and without indetermination in the case of a quarter-wave plate sample. Following Shribak and Oldenbourg's experimental configuration using two variable retarders, a linear polarizer, and five polarization probes, we examined the effect of the swing angle χ, which selected the ellipticity of each polarization state, on the accuracy of retardance (Δ) and axis orientation (ϕ) measurements. Using a quarter-wave plate, excellent agreement between measured and expected values was obtained for both the retardance and the axis orientation, as demonstrated by the statistical analysis of Δ and ϕ spatial distributions. The intrinsic ambiguity in the determination of Δ and ϕ for superimposed layers of transparent anisotropic cello-tape is discussed in detail, and solutions are provided to remove this ambiguity. An example of application of the method on geological samples is also presented. We believe our analysis will guide researchers willing to exploit this long-standing method in their laboratories.

Analysis of accuracy and ambiguities in spatial measurements of birefringence in uniaxial anisotropic media: publisher's note.

This publisher's note corrects an error in Appl. Opt.61, 8081 (2022)APOPAI0003-693510.1364/AO.463657.

Scattering of ultraviolet light by avian eggshells.

Eggshells are essential for the reproduction of birds since the optical properties of shells may have an impact on biological functions such as heating and UV protection, recognition by parents or camouflage. Whereas ultraviolet reflection by some bird eggshells has been recently described, its physical origin remains poorly understood. In this study, we identified a porous structure in eggshells. Using Mie scattering modelling, we found it was most likely responsible for reflectance peaks (intensities of ca. 20-50%) observed in the near-UV range. These peaks were observed by spectrophotometric measurements from eggshells of several breeds of hen, one breed of duck and one breed of quail. This optical response was interpreted in terms of the distinct visual perception of hens and humans: eggshells appearing achromatic for humans proved to be chromatic for hens. Fluorescence emission from these eggs was also characterised and attributed to the presence of protoporphyrin IX and biliverdin IXα in the shells. Electron microscopy observations revealed the presence of pores within the so-called calcified shell part (i.e., at depths between ca. 20 µm and ca. 240 µm from the eggshell's outer surface). Mercury intrusion porosimetry allowed us to quantify the pore size distribution. Simulations of the UV response of this porous structure using Mie scattering theory as well as an effective approach accounting for multiple scattering indicate that these pores are responsible for the backscattering peaks observed in the UV range, in the case of beige hen eggshells. Due to the similarities between the pore size distributions observed for beige hen eggshells and other investigated poultry eggshells, we expect Mie backscattering to be the origin of the UV response of the eggshells of many other bird species.

Efficiency enhancement of perovskite solar cells based on opal-like photonic crystals.

Perovskite solar cells have shown a tremendous interest for photovoltaics since the past decade. However, little is known on the influence of light management using photonic crystals inside such structures. We present here numerical simulations showing the effect of photonic crystal structuring on the integrated quantum efficiency of perovskite solar cells. The photo-active layer is made of an opal-like perovskite structure (monolayer, bilayer or trilayer of perovskite spheres) built in a T i O 2 matrix. Fano resonances are exploited in order to enhance the absorption, especially near the bandgap of perovskite material. The excitation of quasi-guided modes inside the absorbing spheres enhances the integrated quantum efficiency and the photonic enhancement factor. More specifically, a photonic enhancement factor as high as 6.4% is predicted in the case of spheres monolayer compared to an unstructured perovskite layer. The influences of sphere's radius and incident angle on the absorbing properties are also estimated. Those numerical results can be applied to the nascent field of photonic structuring inside perovskite solar cells.

Controlled fluorescence in a beetle's photonic structure and its sensitivity to environmentally induced changes.

The scales covering the elytra of the male Hoplia coerulea beetle contain fluorophores embedded within a porous photonic structure. The photonic structure controls both insect colour (reflected light) and fluorescence emission. Herein, the effects of water-induced changes on the fluorescence emission from the beetle were investigated. The fluorescence emission peak wavelength was observed to blue-shift on water immersion of the elytra whereas its reflectance peak wavelength was observed to red-shift. Time-resolved fluorescence measurements, together with optical simulations, confirmed that the radiative emission is controlled by a naturally engineered photonic bandgap while the elytra are in the dry state, whereas non-radiative relaxation pathways dominate the emission response of wet elytra.

Vapor sensing with a natural photonic cell.

Photonic structures encased by a permeable envelope give rise to iridescent blue color in the scales covering the male Hoplia coerulea beetle. This structure comprises a periodic porous multilayer. The color of these scales is known for changing from blue to green upon contact with water despite the presence of the envelope. This optical system has been referred to as a photonic cell due to the role of the envelope that mediates fluid exchanges with the surrounding environment. Following from previously studied liquid-induced changes in the color appearance of H. coerulea, we measured vapor-induced color changes in its appearance. This response to vapor exposure was marked by reflectance redshift and an increase in peak reflectance intensity. Different physico-chemical processes were investigated to explain the increase in reflectance intensity, a property not usually associated with vapor-induced optical signature changes. These simulations indicated the optical response arose from physisorption of a liquid film on the beetle scales followed by liquid penetration through the envelope and the filling of micropores within the body of the photonic structure.

A fair comparison between ultrathin crystalline-silicon solar cells with either periodic or correlated disorder inverted pyramid textures.

Fabrication of competitive solar cells based on nano-textured ultrathin silicon technology is challenging nowadays. Attention is paid to the optimization of this type of texture, with a lot of simulation and experimental results published in the last few years. While previous studies discussed mainly the local features of the surface texture, we highlight here the importance of their filling fraction. In this work, we focus on a fair comparison between a technologically realizable correlated disorder pattern of inverted nano-pyramids on an ultrathin crystalline-silicon layer, and its periodically patterned counterpart. A fair comparison is made possible by defining an equivalent periodic structure for each hole filling fraction. Moreover, in order to be as realistic as possible, we consider patterns that could be fabricated by standard patterning techniques: hole-mask colloidal lithography, nanoimprint lithography and wet chemical etching. Based on numerical simulations, we show that inverted nano-pyramid patterns with correlated disorder provide typically greater efficiency than their periodic counterparts. However, the hole filling fraction of the etched pattern plays a crucial role and may limit the benefits of the correlated disorder due to experimental restrictions on pattern fabrication.

Quantum vacuum photon modes and superhydrophobicity.

Nanostructures are commonly used for developing superhydrophobic surfaces. However, available wetting theoretical models ignore the effect of vacuum photon-mode alteration on van der Waals forces and thus on hydrophobicity. Using first-principles calculations, we show that superhydrophibicity of nanostructured surfaces is dramatically enhanced by vacuum photon-mode tuning. As a case study, wetting contact angles of a water droplet above a polyethylene nanostructured surface are obtained from the interaction potential energy calculated as a function of the droplet-surface separation distance. This new approach could pave the way for the design of novel superhydrophobic coatings.

Multi-objective genetic algorithm for the optimization of a flat-plate solar thermal collector.

We present a multi-objective genetic algorithm we developed for the optimization of a flat-plate solar thermal collector. This collector consists of a waffle-shaped Al substrate with NiCrOx cermet and SnO(2) anti-reflection conformal coatings. Optimal geometrical parameters are determined in order to (i) maximize the solar absorptance α and (ii) minimize the thermal emittance ε. The multi-objective genetic algorithm eventually provides a whole set of Pareto-optimal solutions for the optimization of α and ε, which turn out to be competitive with record values found in the literature. In particular, a solution that enables α = 97.8% and ε = 4.8% was found.

Plasmon hybridization in pyramidal metamaterials: a route towards ultra-broadband absorption.

Pyramidal metamaterials are currently developed for ultra-broadband absorbers. They consist of periodic arrays of alternating metal/dielectric layers forming truncated square-based pyramids. The metallic layers of increasing lengths play the role of vertically and, to a less extent, laterally coupled plasmonic resonators. Based on detailed numerical simulations, we demonstrate that plasmon hybridization between such resonators helps in achieving ultra-broadband absorption. The dipolar modes of individual resonators are shown to be prominent in the electromagnetic coupling mechanism. Lateral coupling between adjacent pyramids and vertical coupling between alternating layers are proven to be key parameters for tuning of plasmon hybridization. Following optimization, the operational bandwidth of Au/Ge pyramids, i.e. the bandwidth within which absorption is higher than 90%, extends over a 0.2-5.8 µm wavelength range, i.e. from UV-visible to mid-infrared, and total absorption (integrated over the operational bandwidth) amounts to 98.0%. The omni-directional and polarization-independent high-absorption properties of the device are verified. Moreover, we show that the choice of the dielectric layer material (Si versus Ge) is not critical for achieving ultra-broadband characteristics, which confers versatility for both design and fabrication. Realistic fabrication scenarios are briefly discussed. This plasmon hybridization route could be useful in developing photothermal devices, thermal emitters or shielding devices that dissimulate objects from near infrared detectors.

First-principle calculation of solar cell efficiency under incoherent illumination.

Because of the temporal incoherence of sunlight, solar cells efficiency should depend on the degree of coherence of the incident light. However, numerical computation methods, which are used to optimize these devices, fundamentally consider fully coherent light. Hereafter, we show that the incoherent efficiency of solar cells can be easily analytically calculated. The incoherent efficiency is simply derived from the coherent one thanks to a convolution product with a function characterizing the incoherent light. Our approach is neither heuristic nor empiric but is deduced from first-principle, i.e. Maxwell's equations. Usually, in order to reproduce the incoherent behavior, statistical methods requiring a high number of numerical simulations are used. With our method, such approaches are not required. Our results are compared with those from previous works and good agreement is found.

Method for modeling additive color effect in photonic polycrystals with form anisotropic elements: the case of Entimus imperialis weevil.

The calculation of the reflectance of photonic crystals having form-birefringent anisotropic elements in the crystal unit cell, such as cylinders, often turns out to be problematic, especially when the reflectance spectrum has to be computed according to different crystal orientations as in polycrystals for instance. The method we propose here solves this problem in the specific case of photonic crystals whose periodicities are such that there are no diffraction orders except Bragg reflection in the visible range. For a given crystal orientation, the crystal is sliced into layers and the periodic spatial variations of the dielectric function ε are homogenized. Thanks to that homogenization, the calculation can be performed using standard thin film computation codes. In order to demonstrate the usefulness of our method, we applied it to the case of a natural photonic polycrystal found on the cuticle of Entimus imperialis weevil which is a remarkable example of additive color effect. Although each photonic crystal grain of the polycrystal produces a single bright iridescent color, a non-iridescent green matt coloration is perceived by the human eye due to multiscale averaging effects.

Tuning and transferring slow photons from TiO2 photonic crystals to BiVO4 nanoparticles for unprecedented visible light photocatalysis.

Periodic structures with alternating refractive indices such as inverse opal photonic crystals are capable of reducing the group velocity of light such that this slowed light can be more efficiently harvested for highly enhanced solar energy conversion. However, the generation, the manipulation and, in particular, the practical applications of these slow photons remain highly challenging. Here, we report the first proof of concept on the ability to control, in an inverse opal TiO2-BiVO4 hetero-composite, the transfer of slow photons generated from the inverse opal photonic structure to the photocatalytically active BiVO4 nanoparticles for highly enhanced visible light photoconversion. Tuning the slow photon frequencies, in order to accommodate the electronic band gap of BiVO4 for slow photon transfer and for significantly improved light harvesting, was successfully achieved by varying the structural periodicity (pore size) of inverse opal and the light incidence angle. The photocatalytic activity of BiVO4 in all inverse opal structures, promoted by slow photon effect, reached up to 7 times higher than those in the non-structured compact films. This work opens new avenues for the practical utilization of slow photon effect under visible light in photocatalytic energy-related applications like water splitting and carbon dioxide reduction and in photovoltaics.

Manipulating multi-spectral slow photons in bilayer inverse opal TiO2@BiVO4 composites for highly enhanced visible light photocatalysis.

Manipulation of light has been proved to be a promising strategy to increase light harvesting in solar-to-chemical energy conversion, especially in photocatalysis. Inverse opal (IO) photonic structures are highly promising for light manipulation as their periodic dielectric structures enable them to slow down light and localize it within the structure, thereby improving light harvesting and photocatalytic efficiency. However, slow photons are confined to narrow wavelength ranges and hence limit the amount of energy that can be captured through light manipulation. To address this challenge, we synthesized bilayer IO TiO2@BiVO4 structures that manifested two distinct stop band gap (SBG) peaks, arising from different pore sizes in each layer, with slow photons available at either edge of each SBG. In addition, we achieved precise control over the frequencies of these multi-spectral slow photons through pore size and incidence angle variations, that enabled us to tune their wavelengths to the electronic absorption of the photocatalyst for optimal light utilization in aqueous phase visible light photocatalysis. This first proof of concept involving multi-spectral slow photon utilization enabled us to achieve up to 8.5 times and 2.2 times higher photocatalytic efficiencies than the corresponding non-structured and monolayer IO photocatalysts respectively. Through this work, we have successfully and significantly improved light harvesting efficiency in slow photon-assisted photocatalysis, the principles of which can be extended to other light harvesting applications.

Secondary ion mass spectrometry, a powerful tool for revealing ink formulations and animal skins in medieval manuscripts.

Book production by medieval scriptoria have gained growing interest in recent studies. In this context, identifying ink compositions and parchment animal species from illuminated manuscripts is of great importance. Here, we introduce time-of-flight secondary ion mass spectrometry (ToF-SIMS) as a non-invasive tool to identify both inks and animal skins in manuscripts, at the same time. For this purpose, both positive and negative ion spectra in inked and non-inked areas were recorded. Chemical compositions of pigments (decoration) or black inks (text) were determined by searching for characteristic ion mass peaks. Animal skins were identified by data processing of raw ToF-SIMS spectra using principal component analysis (PCA). In illuminated manuscripts from the fifteenth to sixteenth century, malachite (green), azurite (blue), cinnabar (red) inorganic pigments, as well as iron-gall black ink, were identified. Carbon black and indigo (blue) organic pigments were also identified. Animal skins were identified in modern parchments of known animal species by a two-step PCA procedure. We believe the proposed method will find extensive application in material studies of medieval manuscripts, as it is non-invasive, highly sensitive and able to identify both inks and animal skins at the same time, even from traces of pigments and tiny scanned areas.

Optimization of slow light one-dimensional Bragg structures for photocurrent enhancement in solar cells.

In 1D photonic crystal Bragg structures, strong localization of the light occurs in the high refractive index layers at wavelengths on the red edge of the photonic bandgap. We exploit this slow light effect for thin film solar cells in order to increase the absorption of light in silicon, as the latter has a high refractive index. Amorphous silicon and a transparent conductive oxide are chosen as high-index and low-index materials, respectively. Reference thin film cells have equivalent total thickness of amorphous silicon, plus antireflection coating and optional metallic back mirror. Through transfer-matrix calculations, we demonstrate that the spectrally integrated photon flux absorbed in active layers, hence the photocurrent, is enhanced by at least 10% with respect to reference using only a few periods. The enhancement is robust with respect to the light incidence angle. The key of such an enhancement is the tuning of the red edge to both the solar irradiance spectrum and the intrinsic material absorption spectrum, which is achieved by suitably selecting the layer thicknesses.

Light coupling and enhanced backscattering in layered plasmonic nanocomposites.

Peculiar enhanced backscattering of light as well as selective vapor sensing were recently observed in a layered plasmonic nanocomposite which consisted of gold nanospheres randomly distributed in a sol-gel glass thin film on top of a soda-lime glass substrate, including a buried leaky waveguide. In order to understand the underlying physical mechanisms, we performed three-dimensional transfer-matrix numerical simulations and calculated the reflectance in both backward and specular directions as functions of the incidence angle. First, assuming a layered periodic particle arrangement, we confirmed that backscattering took place at grazing incidence if the spatial period in the layers was chosen within an optimal range, in agreement with theoretical predictions. Then, using a pseudo-random particle arrangement to describe the actual nanocomposite, we revealed that strong backscattering could nevertheless persist for specific particle distributions, in spite of their randomness. This behavior was tentatively explained by putting backscattering in relation with the particle interdistance statistics. Finally, we showed that backscattered reflectance was much more sensitive than specular reflectance to the adsorption of water vapor either on the surface or inside the likely porous structure of the glass host.