Iridescence Mimicking in Fabrics: A Ultraviolet/Visible Spectroscopy Study.

Poly(styrene-methyl methacrylate-acrylic acid) photonic crystals (PCs), with five different sizes (170, 190, 210, 230 and 250 nm), were applied onto three plain fabrics, namely polyamide, polyester and cotton. The PC-coated fabrics were analyzed using scanning electronic microscopy and two UV/Vis reflectance spectrophotometric techniques (integrating sphere and scatterometry) to evaluate the PCs' self-assembly along with the obtained spectral and colors characteristics. Results showed that surface roughness of the fabrics had a major influence on the color produced by PCs. Polyamide-coated fabrics were the only samples having an iridescent effect, producing more vivid and brilliant colors than polyester and cotton samples. It was observed that as the angle of incident light increases, a hypsochromic shift in the reflection peak occurs along with the formation of new reflection peaks. Furthermore, color behavior simulations were performed with an illuminant A light source on polyamide samples. The illuminant A simulation showed greener and yellower structural colors than those illuminated with D50. The polyester and cotton samples were analyzed using scatterometry to check for iridescence, which was unseen upon ocular inspection and then proven to be present in these samples. This work allowed a better comprehension of how structural colors and their iridescence are affected by the textile substrate morphology and fiber type.

Growth and Characterization of Sputtered InAlN Nanorods on Sapphire Substrates for Acetone Gas Sensing.

The demand for highly sensitive and selective gas sensors has been steadily increasing, driven by applications in various fields such as environmental monitoring, healthcare, and industrial safety. In this context, ternary alloy indium aluminum nitride (InAlN) semiconductors have emerged as a promising material for gas sensing due to their unique properties and tunable material characteristics. This work focuses on the fabrication and characterization of InAlN nanorods grown on sapphire substrates using an ultra-high vacuum magnetron sputter epitaxy with precise control over indium composition and explores their potential for acetone-gas-sensing applications. Various characterization techniques, including XRD, SEM, and TEM, demonstrate the structural and morphological insights of InAlN nanorods, making them suitable for gas-sensing applications. To evaluate the gas-sensing performance of the InAlN nanorods, acetone was chosen as a target analyte due to its relevance in medical diagnostics and industrial processes. The results reveal that the InAlN nanorods exhibit a remarkable sensor response of 2.33% at 600 ppm acetone gas concentration at an operating temperature of 350 °C, with a rapid response time of 18 s. Their high sensor response and rapid response make InAlN a viable candidate for use in medical diagnostics, industrial safety, and environmental monitoring.

Shear-Coated Linear Birefringent and Chiral Cellulose Nanocrystal Films Prepared from Non-Sonicated Suspensions with Different Storage Time.

Nanocelluloses are very attractive materials for creating structured films with unique optical properties using different preparation techniques. Evaporation-induced self-assembly of cellulose nanocrystals (CNC) aqueous suspensions produces iridescent films with selective circular Bragg reflection. Blade coating of sonicated CNC suspensions leads to birefringent CNC films. In this work, fabrication of both birefringent and chiral films from non-sonicated CNC suspensions using a shear-coating method is studied. Polarization optical microscopy and steady-state viscosity profiles show that non-sonicated CNC suspensions (concentration of 6.5 wt%) evolve with storage time from a gel-like shear-thinning fluid to a mixture of isotropic and chiral nematic liquid crystalline phases. Shear-coated films prepared from non-sonicated fresh CNC suspensions are birefringent, whereas films prepared from suspensions stored several weeks show reflection of left-handed polarized light. Quantification of linear and circular birefringence as well circular dichroism in the films is achieved by using a Mueller matrix formalism.

Experimental degradation of helicoidal photonic nanostructures in scarab beetles (Coleoptera: Scarabaeidae): implications for the identification of circularly polarizing cuticle in the fossil record.

Scarab beetles (Coleoptera: Scarabaeidae) can exhibit striking colours produced by pigments and/or nanostructures. The latter include helicoidal (Bouligand) structures that can generate circularly polarized light. These have a cryptic evolutionary history in part because fossil examples are unknown. This suggests either a real biological signal, i.e. that Bouligand structures did not evolve until recently, or a taphonomic signal, i.e. that conditions during the fossilization process were not conducive to their preservation. We address this issue by experimentally degrading circularly polarizing cuticle of modern scarab beetles to test the relative roles of decay, maturation and taxonomy in controlling preservation. The results reveal that Bouligand structures have the potential to survive fossilization, but preservation is controlled by taxonomy and the diagenetic history of specimens. Further, cuticle of specific genus (Chrysina) is particularly decay-prone in alkaline conditions; this may relate to the presence of certain compounds, e.g. uric acid, in the cuticle of these taxa.

Influence of InAlN Nanospiral Structures on the Behavior of Reflected Light Polarization.

The influence of structural configurations of indium aluminum nitride (InAlN) nanospirals, grown by reactive magnetron sputter epitaxy, on the transformation of light polarization are investigated in terms of varying structural chirality, growth temperatures, titanium nitride (TiN) seed (buffer) layer thickness, nanospiral thickness, and pitch. The handedness of reflected circularly polarized light in the ultraviolet-visible region corresponding to the chirality of nanospirals is demonstrated. A high degree of circular polarization (Pc) value of 0.75 is obtained from a sample consisting of 1.2 µm InAlN nanospirals grown at 650 °C. A film-like structure is formed at temperatures lower than 450 °C. At growth temperatures higher than 750 °C, less than 0.1 In-content is incorporated into the InAlN nanospirals. Both cases reveal very low Pc. A red shift of wavelength at Pc peak is found with increasing nanospiral pitch in the range of 200-300 nm. The Pc decreases to 0.37 for two-turn nanospirals with total length of 0.7 µm, attributed to insufficient constructive interference. A branch-like structure appears on the surface when the nanospirals are grown longer than 1.2 µm, which yields a low Pc around 0.5, caused by the excessive scattering of incident light.

Linear Birefringent Films of Cellulose Nanocrystals Produced by Dip-Coating.

Transparent films of cellulose nanocrystals (CNC) are prepared by dip-coating on glass substrates from aqueous suspensions of hydrolyzed filter paper. Dragging forces acting during films' deposition promote a preferential alignment of the rod-shaped CNC. Films that are 2.8 and 6.0 µm in thickness show retardance effects, as evidenced by placing them between a linearly polarized light source and a linear polarizer sheet in the extinction configuration. Transmission Mueller matrix spectroscopic ellipsometry measurements at normal incidence as a function of sample rotation were used to characterize polarization properties. A differential decomposition of the Mueller matrix reveals linear birefringence as the unique polarization parameter. These results show a promising way for obtaining CNC birefringent films by a simple and controllable method.

Pitch profile across the cuticle of the scarab beetle Cotinis mutabilis determined by analysis of Mueller matrix measurements.

Helicoidal structures of lamellae of nanofibrils constitute the cuticle of some scarab beetles with iridescent metallic-like shine reflecting left-handed polarized light. The spectral and polarization properties of the reflected light depend on the pitch of the helicoidal structures, dispersion of effective refractive indices and thicknesses of layers in the cuticle. By modelling the outer exocuticle of the scarab beetle Cotinis mutabilis as a stack of continuously twisted biaxial slices of transparent materials, we extract optical and structural parameters by nonlinear regression analysis of variable-angle Mueller-matrix spectroscopic data. Inhomogeneities in the beetle cuticle produce depolarization with non-uniformity in cuticle thickness as the dominant effect. The pitch across the cuticle of C. mutabilis decreased with depth in a two-level profile from 380 to 335 nm and from 390 to 361 nm in greenish and reddish specimens, respectively, whereas in a yellowish specimen, the pitch decreased with depth in a three-level profile from 388 to 326 nm.

Modeling of light interaction with exoskeletons of scarab beetles.

Some beetles of the family Scarabaeidae produce brilliant metallic-looking colors by their pure dielectric exoskeletons and reflect light with a high degree of circular polarization. In the present work, we discuss three models for simultaneously describing scattering, spectral, and polarization characteristics of scarab beetles. Each model consists of three slabs: an outer thin epicuticle, an exocuticle having a helicoidal structure, and a thick uniform slightly absorbing endocuticle. Scattering features are defined by rough interfaces of the epicuticle and/or nonuniformities of the exocuticle. As an example, a slightly modified model of an earlier study of Chrysina aurata is considered. The modification is aimed at including surface and volume nonuniformities that affect not only spectral and polarization properties but also scattering. Another example of using the proposed models is based on the analysis of image formations of a specimen of the species Mimela chinensis, which was studied in a polarizing microscope at different magnifications. The results show that the proposed models can be applied for explanation of light interaction with the exoskeletons of scarab beetles.

Neutral shielding and cloaking of magnetic fields using isotropic media.

A method for designing magnetic shields that do not perturb applied multipole fields in the static regime is developed. Cylindrical core-shell structures with two layers characterized by homogeneous isotropic permeabilities are found to support neutral shielding of multipole fields and unique cloaking solutions of arbitrary multipole order. An extra degree of freedom is provided by every layer added to the structure which may be exploited with an effective design formula for cloaking of additional field terms. The theory is illustrated with numerical simulations.

Polarizing properties and structure of the cuticle of scarab beetles from the Chrysina genus.

The optical properties of several scarab beetles have been previously studied but few attempts have been made to compare beetles in the same genus. To determine whether there is any relation between specimens of the same genus, we have studied and classified seven species from the Chrysina genus. The polarization properties were analyzed with Mueller-matrix spectroscopic ellipsometry and the structural characteristics with optical microscopy and scanning electron microscopy. Most of the Chrysina beetles are green colored or have a metallic look (gold or silver). The results show that the green-colored beetles polarize reflected light mainly at off-specular angles. The gold-colored beetles polarize light left-handed near circular at specular reflection. The structure of the exoskeleton is a stack of layers that form a cusplike structure in the green beetles whereas the layers are parallel to the surface in the case of the gold-colored beetles. The beetle C. gloriosa is green with gold-colored stripes along the elytras and exhibits both types of effects. The results indicate that Chrysina beetles can be classified according to these two major polarization properties.

Sum regression decomposition of spectral and angle-resolved Mueller matrices from biological reflectors.

We show spectroscopic Mueller-matrix data measured at multiple incidence angles of the scarab beetle C. aurata. A method of regression decomposition can decompose the Mueller matrix into a set of two matrices representing one polarizer and one dielectric reflector. We also report on a tentative decomposition of the beetle C. argenteola using the same method.

Simulation of light scattering from exoskeletons of scarab beetles.

An approach for simulation of light scattering from beetles exhibiting structural colors originating from periodic helicoidal structures is presented. Slight irregularities of the periodic structure in the exoskeleton of the beetles are considered as a major cause of light scattering. Two sources of scattering are taken into account: surface roughness and volume non-uniformity. The Kirchhoff approximation is applied to simulate the effect of surface roughness. To describe volume non-uniformity, the whole structure is modeled as a set of domains distributed in space in different orientations. Each domain is modeled as an ideal uniformly twisted uniaxial medium and differs from each other by the pitch. Distributions of the domain parameters are assumed to be Gaussian. The analysis is performed using the Mueller matrix formalism which, in addition to spectral and spatial characteristics, also provides polarization properties of the scattered light.

Scattering and polarization properties of the scarab beetle Cyphochilus insulanus cuticle.

Optical properties of natural photonic structures can inspire material developments in diversified areas, such as the spectral design of surfaces for camouflage. Here, reflectance, scattering, and polarization properties of the cuticle of the scarab beetle Cyphochilus insulanus are studied with spectral directional hemispherical reflectance, bidirectional reflection distribution function (BRDF) measurements, and Mueller-matrix spectroscopic ellipsometry (MMSE). At normal incidence, a reflectance (0.6-0.75) is found in the spectral range of 400-1600 nm and a weaker reflectance <0.2 in the UV range as well as for wavelengths >1600 nm. A whiteness of W=42 is observed for mainly the elytra of the beetle. Chitin is a major constituent of the insect cuticle which is verified by the close similarity of the measured IR spectrum to that of α-chitin. The BRDF signal shows close-to-Lambertian properties of the beetle for visible light at small angles of incidence. From the MMSE measurement it is found that the beetles appear as dielectric reflectors reflecting linearly polarized light at oblique incidence with low gloss and a low degree of polarization. The measured beetle properties are properties that can be beneficial in a camouflage material.

Curved-lattice epitaxial growth of In(x)Al(1-x)N nanospirals with tailored chirality.

Chirality, tailored by external morphology and internal composition, has been realized by controlled curved-lattice epitaxial growth of In(x)Al(1-x)N nanospirals. The curved morphology of the spiral segments is a result of a lateral compositional gradient while maintaining a preferred crystallographic growth direction, implying a lateral gradient in optical properties. Individual nanospirals show an asymmetric core-shell structure with curved basal planes. Mueller matrix spectroscopic ellipsometry shows that the tailored chirality is manifested in the polarization state of light reflected off the nanospirals.

Cuticle structure of the scarab beetle Cetonia aurata analyzed by regression analysis of Mueller-matrix ellipsometric data.

Since one hundred years it is known that some scarab beetles reflect elliptically and near-circular polarized light as demonstrated by Michelson for the beetle Chrysina resplendens. The handedness of the polarization is in a majority of cases left-handed but also right-handed polarization has been found. In addition, brilliant colors with metallic shine are observed. The polarization and color effects are generated in the beetle exoskeleton, the so-called cuticle. The objective of this work is to demonstrate that structural parameters and materials optical functions of these photonic structures can be extracted by advanced modeling of spectral multi-angle Mueller-matrix data recorded from beetle cuticles. A dual-rotating compensator ellipsometer is used to record normalized Mueller-matrix data in the spectral range 400 - 800 nm at angles of incidence in the range 25-75°. Analysis of data measured on the scarab beetle Cetonia aurata are presented in detail. The model used in the analysis mimics a chiral nanostructure and is based on a twisted layered structure. Given the complexity of the nanostructure, an excellent fit between experimental and model data is achieved. The obtained model parameters are the spectral variation of the refractive indices of the cuticle layers and structural parameters of the chiral structure.

Spectroscopic ellipsometry analysis of silicon nanotips obtained by electron cyclotron resonance plasma etching.

Silicon nanotips fabricated by electron cyclotron resonance plasma etching of silicon wafers are studied by spectroscopic ellipsometry. The structure of the nanotips is composed of columns 100-140 nm wide and spaced by about 200 nm. Ellipsometry data covering a wide spectral range from the midinfrared to the visible are described by modeling the nanotip layer as a graded uniaxial film using the Bruggeman effective medium approximation. The ellipsometry data in the infrared range reveal two absorption bands at 754 and 955 cm(-1), which cannot be resolved with transmittance measurements. These bands indicate that the etching process is accompanied with formation of carbonaceous SiC and CH(n) species that largely modify the composition of the original crystalline silicon material affecting the optical response of the nanotips.