Zinc aluminogallate, Zn(AlxGa1−x)2O4 (ZAGO), a single-phase spinel structure, offers considerable potential for high-performance electronic devices due to its expansive compositional miscibility range between aluminum (Al) and gallium (Ga). Direct growth of high-quality ZAGO epilayers however remains problematic due to the high volatility of zinc (Zn). This work highlights a novel synthesis process for high-quality epitaxial quaternary ZAGO thin films on sapphire substrates, achieved through thermal annealing of a ZnGa2O4 (ZGO) epilayer on sapphire in an ambient air setting. In-situ annealing x-ray diffraction measurements show that the incorporation of Al in the ZGO epilayer commenced at 850 °C. The Al content (x) in ZAGO epilayer gradually increased up to around 0.45 as the annealing temperature was raised to 1100 °C, which was confirmed by transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy. X-ray rocking curve measurement revealed a small full width at half maximum value of 0.72 °, indicating the crystal quality preservation of the ZAGO epilayer with a high Al content. However, an epitaxial intermediate �–(AlxGa1−x)2O3 layer (� - AGO) was formed between the ZAGO and sapphire substrate. This is believed to be a consequence of the interdiffusion of Al and Ga between the ZGO thin film and sapphire substrate. Using density functional theory, the substitution cost of Ga in sapphire was determined to be about 0.5 eV lower than substitution cost of Al in ZGO. Motivated by this energetically favorable substitution, a formation mechanism of the ZAGO and AGO layers was proposed. Spectroscopic ellipsometry studies revealed an increase in total thickness of the film from 105.07 nm (ZGO) to 147.97 nm (ZAGO/AGO) after annealing to 1100 °C, which were corroborated using TEM. Furthermore, an observed increase in the direct (indirect) optical bandgap from 5.06 eV (4.7 eV) to 5.72 eV (5.45 eV) with an increasing Al content in the ZAGO layer further underpins the formation of a quaternary ZAGO alloy with a tunable composition.

NiO thin films with varied oxygen contents are grown on Si(100) and c-Al2O3 at a substrate temperature of 300 degrees C using pulsed dc reactive magnetron sputtering. We characterize the structure and optical properties of NiO changes as functions of the oxygen content. NiO with the cubic structure, single phase, and predominant orientation along (111) is found on both substrates. X-ray diffraction and pole figure analysis further show that NiO on the Si(100) substrate exhibits fiber-textured growth, while twin domain epitaxy was achieved on c-Al2O3, with NiO(111) k Al2O3(0001) and NiO[1 (1) over bar0]k Al2O3[10 (1) over bar0] or NiO[(1) over bar 10]k Al2O3[2 (1) over bar(1) over bar0] epitaxial relationship. The oxygen content in NiO films did not have a significant effect on the refractive index, extinction coefficient, and absorption coefficient. This suggests that the optical properties of NiO films remained unaffected by changes in the oxygen content.

InxGa1-x N is a strategically important material for electronic devices given its tunable bandgap, modulated by the In/Ga ratio. However, current applications are hindered by defects caused by strain relaxation and phase separation in the material. Here, we demonstrate growth of homogeneous InxGa1-x N films with 0.3 < x < 0.8 up to similar to 30 nm using atomic layer deposition (ALD) with a supercycle approach, switching between InN and GaN deposition. The composition is uniform along and across the films, without signs of In segregation. The InxGa1-x N films show higher In-content than the value predicted by the supercycle model. A more pronounced reduction of GPC(InN) than GPC(GaN) during the growth processes of InN and GaN bilayers is concluded based on our analysis. The intermixing between InN and GaN bilayers is suggested to explain the enhanced overall In-content. Our results show the advantage of ALD to prepare high-quality InxGa1-x N films, particularly with high In-content, which is difficult to achieve with other growth methods.

Electronic grade ZnGa2O4 epitaxial thin films were grown on c-plane sapphire substrates by metal-organic chemical vapor deposition and investigated using spectroscopic ellipsometry. Their thickness, roughness and optical properties were determined using a Multiple Sample Analysis based approach by the regression analysis of optical model and measured data. These samples were then compared to samples which had undergone ion etching, and it was observed that etching time up to four minutes had no discernible impact on its optical properties. Line shape analysis of resulting absorption coefficient dispersion revealed that ZnGa(2)O(4 )exhibited both direct and indirect interband transitions. The modified Cody formalism was employed to determine their optical bandgaps. These values were found to be in good agreement with values obtained using other popular bandgap extrapolation procedures. Published by Optica Publishing Group under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published articles title, journal citation, and DOI.

Featured Application The analysis of the transmission of Mueller matrices facilitates studies of optical activity in samples that also exhibit linear anisotropy and depolarization and may have a multilayered structure. Such studies are important for the development of applications in chiroptics. Optical chirality, in terms of circular birefringence and circular dichroism, is described by its electromagnetic and magnetoelectric material tensors, and the corresponding optical activity contributes to the Mueller matrix. Here, spectroscopic ellipsometry in the spectral range 210-1690 nm is used to address chiral phenomena by measuring Mueller matrices in transmission. Three approaches to determine chirality parameters are discussed. In the first approach, applicable in the absence of linear polarization effects, circular birefringence and circular dichroism are evaluated directly from elements of a Mueller matrix. In the second method, differential decomposition is employed, which allows for the unique separation of chirality parameters from linear anisotropic parameters as well as from depolarization provided that the sample is homogeneous along the optical path. Finally, electromagnetic modeling using the Tellegen constitutive relations is presented. The last method also allows structural effects to be included. The three methods to quantify optical chirality are demonstrated for selected materials, including sugar solutions, alpha-quartz, liquid crystals, beetle cuticle, and films of cellulose nanocrystals.

Thin films in the aluminosilicate (AlSiO) system containing up to 31 at. % Al and 23 at. % Si were prepared by reactive RF magnetron co-sputtering in order to investigate the dependence of film formation and optical properties on substrate temperature and Si and Al contents. The obtained films were amorphous with smooth microstructure. The growth rate at different substrate temperatures ranged from 1.2 to 3.3 nm/min and increase with increasing the Si target power. The roughness decreases and thickness increases with increasing Si content. The thickness of the films grown at a deposition temperature of 100 °C is found to be higher than the films deposited at 300 and 500 °C. The AlSiO-coated glasses have a higher transmission in the visible region than the uncoated glass. The spectroscopic ellipsometry analysis reveals that the refractive index value decreased with decreasing the Al content, having extinction coefficient values of zero in the measured spectral region and band gap values ≥ 3.4 eV. The obtained thin films have over 90% transmittance in the visible range and no systematic variation of transmittance was observed with substrate temperature. The results suggest that glass substrate coated with AlSiO thin films have improved optical properties.

Several beetle species in the Scarabaeoidea superfamily reflect left-handed polarized light due to a circular Bragg structure in their cuticle. The right-handed polarized light is transmitted. The objective here is to evaluate cuticle chiral properties in an effective medium approach using transmission Mueller matrices assuming the cuticle to be a bianisotropic continuum. Both differential decomposition and nonlinear regression were used in the spectral range of 500-1690nm. The former method provides the sample cumulated birefringence and dichroic optical properties and is model-free but requires a homogeneous sample. The materials chirality is deduced from the circular birefringence and circular dichroic spectra obtained. The regression method requires dispersion models for the optical functions but can also be used in more complex structures including multilayered and graded media. It delivers the material properties in terms of model functions of materials permittivity and chirality. The two methods show excellent agreement for the complex-valued chirality spectrum of the cuticle.

Ramie is a plant whose fibers are used in fabrics. Ramie films are prepared by hot pressing and studied with transmission Mueller-matrix ellipsometry, which provides a complete description of polarizing and depolarizing sample properties. Symmetries of the Mueller matrices imply that the ramie films are linearly birefringent and act as waveplates. The linear birefringence is quantified by the differential decomposition of the Mueller matrices and the materials birefringence is found to be of the order of 0.05-0.08 with small dispersion in the visible spectral range. The films exhibit depolarization, which is quantified in terms of the depolarization index and varies from 0.9 in the infrared to 0.25 in the ultraviolet range. The deep understanding of ramie films polarization properties will pave the way for applications in optical and photonic devices.

In this work, amorphous thin films in Mg-Si-O-N system typically containing amp;gt; 15 at.% Mg and 35 at.% N were prepared in order to investigate especially the dependence of optical and mechanical properties on Mg composition. Reactive RF magnetron co-sputtering from magnesium and silicon targets were used for the deposition of Mg-Si-O-N thin films. Films were deposited on float glass, silica wafers and sapphire substrates in an Ar, N-2 and O-2 gas mixture. X-ray photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy, spectroscopic ellipsometry, and nanoindentation were employed to characterize the composition, surface morphology, and properties of the films. The films consist of N and Mg contents up to 40 at.% and 28 at.%, respectively and have good adhesion to substrates and are chemically inert. The thickness and roughness of the films increased with increasing content of Mg. Both hardness (16-21 GPa) and reduced elastic modulus (120-176 GPa) are strongly correlated with the amount of Mg content. The refractive index up to 2.01 and extinction coefficient up to 0.18 were found to increase with Mg content. The optical band gap (3.1-4.3) decreases with increasing the Mg content. Thin film deposited at substrate temperature of 100 degrees C shows a lower value of hardness (10 GPa), refractive index (1.75), and higher values of reduced elastic modulus (124 GPa) as compared to the thin film deposited at 310 degrees C and 510 degrees C respectively, under identical synthesis parameters.

The influence of structural configurations of indium aluminum nitride (InA1N) 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 (P-c) value of 0.75 is obtained from a sample consisting of 1.2 mu m InA1N nanospirals grown at 650 degrees C. A film-like structure is formed at temperatures lower than 450 degrees C. At growth temperatures higher than 750 degrees C, less than 0.1 In-content is incorporated into the InA1N nanospirals. Both cases reveal very low P-c-A red shift of wavelength at P-c peak is found with increasing nanospiral pitch in the range of 200-300 nm. The P-c decreases to 0.37 for two-turn nanospirals with total length of 0.7 mu m, attributed to insufficient constructive interference. A branch-like structure appears on the surface when the nanospirals are grown longer than 1.2 mu m, which yields a low P-c around 0.5, caused by the excessive scattering of incident light.

Chemical attribution signatures (CAS) associated with different synthetic routes used for the production of Russian VX (VR) were identified. The goal of the study was to retrospectively determine the production method employed for an unknown VR sample. Six different production methods were evaluated, carefully chosen to include established synthetic routes used in the past for large scale production of the agent, routes involving general phosphorus-sulfur chemistry pathways leading to the agent, and routes whose main characteristic is their innate simplicity in execution. Two laboratories worked in parallel and synthesized a total of 37 batches of VR via the six synthetic routes following predefined synthesis protocols. The chemical composition of impurities and byproducts in each route was analyzed by GC/MS-EI and 49 potential CAS were recognized as important markers in distinguishing these routes using Principal Component Analysis (PCA). The 49 potential CAS included expected species based on knowledge of reaction conditions and pathways but also several novel compounds that were fully identified and characterized by a combined analysis that included MS-CI, MS-EI and HR-MS. The CAS profiles of the calibration set were then analyzed using partial least squares discriminant analysis (PLS-DA) and a cross validated model was constructed. The model allowed the correct classification of an external test set without any misclassifications, demonstrating the utility of this methodology for attributing VR samples to a particular production method. This work is part one of a three-part series in this Forensic VSI issue of a Sweden-United States collaborative effort towards the understanding of the CAS of VR in diverse batches and matrices. This part focuses on the CAS in synthesized batches of crude VR and in the following two parts of the series the influence of food matrices on the CAS profiles are investigated.

A multivariate model was developed to attribute samples to a synthetic method used in the production of sulfur mustard (HD). Eleven synthetic methods were used to produce 66 samples for model construction. Three chemists working in both participating laboratories took part in the production, with the aim to introduce variability while reducing the influence of laboratory or chemist specific impurities in multivariate analysis. A gas chromatographic/mass spectrometric data set of peak areas for 103 compounds was subjected to orthogonal partial least squares - discriminant analysis to extract chemical attribution signature profiles and to construct multivariate models for classification of samples. For one- and two-step routes, model quality allowed the classification of an external test set (16/16 samples) according to synthesis conditions in the reaction yielding sulfur mustard. Classification of samples according to first-step methodology was considerably more difficult, given the high purity and uniform quality of the intermediate thiodiglycol produced in the study. Model performance in classification of aged samples was also investigated.

The use of Mueller matrices for studies of polarizing properties and cuticle structure of scarab beetles are partly reviewed. Specifically we show how the polarization of the reflected light can be quantified in terms of degree of polarization and ellipticity. It is also shown that sum decomposition of Mueller matrices reveals cuticle reflection characteristics in different spectral regions, e.g. in terms of mirrors and circular polarizers. With a differential decomposition of cuticle transmission Mueller matrices, we determine the spectral variation in the fundamental optical properties circular birefringence and dichroism. (C) 2017 Elsevier Ltd. All rights reserved.

Ca-Si-O-N thin films were deposited on commercial soda-lime silicate float glass, silica wafers and sapphire substrates by RF magnetron co-sputtering from Ca and Si targets in an Ar/N-2/O-2 gas mixture. Chemical composition, surface morphology, hardness, reduced elastic modulus and optical properties of the films were investigated using X-ray photoelectron spectroscopy, scanning electron microscopy, nanoindentation, and spectroscopic ellipsometry. It was found that the composition of the films can be controlled by the Ca target power, predominantly, and by the reactive gas flow. Thin films in the Ca-Si-O-N system are composed of N and Ca contents up to 31 eq. % and 60 eq. %, respectively. The films thickness ranges from 600 to 3000 nm and increases with increasing Ca target power. The films surface roughness varied between 2 and 12 nm, and approximately decreases with increasing power of Ca target. The hardness (4-12 GPa) and reduced elastic modulus (65-145 GPa) of the films increase and decrease with the N and Ca contents respectively. The refractive index (1.56-1.82) is primarily dictated by the N content. The properties are compared with findings for bulk glasses in the Ca-Si-(Al)-O-N systems, and it is concluded that Ca-Si-O-N thin films have higher values of hardness, elastic modulus and refractive index than bulk glasses of similar composition. (C) 2017 Elsevier B.V. All rights reserved.

Supported lipid membranes represent an elegant way to design a fluid interface able to mimic the physicochemical properties of biological membranes, with potential biotechnological applications. In this work, a diacyl phospholipid, the 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol (DPPTE), functionalized with a thiol group, was immobilized on a gold surface. In this molecule, the thiol group, responsible for the Au S bond (45 kJ/mol) is located on the phospholipid polar head, letting the hydrophobic chain protrude from the film. This system is widely used in the literature but is no less challenging, since its characterization is not complete, as several discordant data have been obtained. In this work, the film was characterized by cyclic voltammetry blocking experiments, to verify the SAM formation, and by reductive desorption measurements, to estimate the molecular density of DPPTE on the gold surface. This value has been compared to that obtained by quartz crystal microbalance measurements. Ellipsometry and impedance spectroscopy measurements have been performed to obtain information about the monolayer thickness and capacitance. The film morphology was investigated by atomic force microscopy. Finally, Monte Carlo simulations were carried out, in order to gain molecular information about the morphologies of the DPPTE SAM and compare them to the experimental results. We demonstrate that DPPTE molecules, incubated 18 h below the phase transition temperature (T = 41.1 +/- 0.4 degrees C) in ethanol solution, are able to form a self-assembled monolayer on the gold surface, with domain structures of different order, which have never been reported before. Our results make possible rationalization of the scattered results so far obtained on this system, giving a new insight into the formation of phospholipids SAMs on a gold surface.

There is an increasing demand for glass materials with better mechanical and optical properties for display and electronic applications. This paper describes the deposition of novel thin films of Mg-circle divide-Si-O-N onto float glass substrates. Amorphous thin films in the Mg-Si-O-N system with high nitrogen and magnesium contents were deposited by reactive RF magnetron co-sputtering from Mg and Si targets in Ar/N-2/O-2 gas mixtures. The thin films studied span an unprecedented range of compositions up to 45 at% Mg and 80 at% N out of cations and anions respectively. Thin films in the Mg-Si-O-N system were found to be homogeneous and transparent in the visible region. Mechanical properties like hardness (H) and reduced elastic modulus (Er) show high values, up to 21 GPa and 166 GPa respectively. The refractive index (1.87-2.00) increases with increasing magnesium and nitrogen contents. (C) 2016 Elsevier Ltd. All rights reserved.

Transmission Mueller-matrix spectroscopic ellipsometry is applied to the cuticle of the beetle Cetonia aurata in the spectral range 300-1000 nm. The cuticle is optically reciprocal and exhibits circular Bragg filter features for green light. By using differential decomposition of the Mueller matrix, the circular and linear birefringence as well as dichroism of the beetle cuticle are quantified. A maximum value of structural optical activity of 560 degrees/mm is found. (C) 2016 Optical Society of America

In this report we present studies on beetles of the Scarabaeidae family. The selected beetles show brilliant colors and in addition interesting polarization features. Mueller matrices of such beetles are of large interest to explore for biomimetics and for the understanding of the biological relevance of the observed polarization phenomena. Several species of the Scarabaeidae family have been studied by Hodgkinson, Goldstein  and our group to mention some. Ellipticity, degree of polarization and other derived parameters have been reported and Arwin et al. also did optical modeling to determine structural parameters of the scutellum part of the exoskeleton of Cetonia aurata. Mueller matrices are very rich in information about the sample properties and can also be analyzed by addressing depolarization. Cloude showed that a depolarizing Mueller matrix can be represented by a sum of up to four non-depolarizing Mueller matrices weighted by the eigenvalues of the covariance matrix of the Mueller matrix. These eigenvalues are all positive for a physically realizable Mueller matrix and this, so called sum decomposition can be used to filter matrices and obtain a measure of experimental fidelity. The result of the decomposition can also be used to describe a Mueller matrix as a set of basic optical elements having direct physical meaning, such as polarizers and retarders. Pioneering work on decomposition of Mueller-matrix images, including studies of beetles, was performed by Ossikovski et al. We have also previously demonstrated this with Cloude as well as regression decomposition of Mueller matrix spectra and images measured at near-normal incidence on C. aurata. Using Cloude decomposition we found that the experimentally determined Mueller matrix of C. aurata decomposes into a set of a mirror and a circular polarizer. Those results were then the basis for a more stable regression decomposition where the result was confirmed.

Chirality, tailored by external morphology and internal composition, has been realized by controlled curved-lattice epitaxial growth (CLEG) of uniform coatings of single-crystalline In_xAl_1-xN nanospirals. The nanospirals are formed by sequentially stacking segments of curved nanorods on top of each other, where each segment is incrementally rotated around the spiral axis. By controlling the growth rate, segment length, rotation direction, and incremental rotation angle, spirals are tailored to predetermined handedness, pitch, and height.  The curved morphology of the segments is a result of a lateral compositional gradient across the segments while maintaining a preferred crystallographic growth direction, implying a lateral gradient in optical properties as well. Left- and right-handed nanospirals, tailored with 5 periods of 200 nm pitch, as confirmed by scanning electron microscopy, exhibit uniform spiral diameters of ~80 nm (local segment diameters of ~60 nm) with tapered hexagonal tips.  High resolution electron microscopy, in combination with nanoprobe energy dispersive X-ray spectroscopy and valence electron energy loss spectroscopy, show that individual nanospirals consist of an In-rich core with ~15 nm-diameter hexagonal cross-section, comprised of curved basal planes. The core is surrounded by an Al-rich shell with a thickness asymmetry spiraling along the core. The ensemble nanospirals, across the 1 cm² wafers, show high in-plane ordering with respect to shape, crystalline orientation, and direction of compositional gradient. Mueller matrix spectroscopic ellipsometry shows that the tailored chirality is manifested in the polarization state of light reflected off the CLEG nanospiral-coated wafers. In that, the polarization state is shown to be dependent on the handedness of the nanospirals and the wavelength of the incident light in the ultraviolet-visible region.

We use angle-dependent Mueller-matrix spectroscopic ellipsometry (MMSE) to determine Mueller matrices of Scarabaeidae beetles which show fascinating reflection properties due to structural phenomena in the exocuticle which are often depolarizing. It has been shown by Cloude [1] that a depolarizing matrix can be decomposed into a sum of up to four non-depolarizing matrices according to M= aM­­₁+bM₂+cM₃+dM₄, where a, b, c and d are eigenvalues of the covariance matrix of M. Using the same eigenvalues the matrices M_i can be calculated. This method provides the full solution to the decomposition with both the non-depolarizing matrices and the weight of each of them in the sum.

An alternative to Cloude decomposition is regression decomposition. Here any Mueller matrix can be decomposed into a set of matrices M_i which are specified beforehand. Whereas in Cloude decomposition the only constraint on the matrices is that they are physically realizable non-depolarizing Mueller matrices, we can now limit the constraint and only use Mueller matrices representing pure optical devices having direct physical meaning, such as polarizers, retarders, etc. This leaves a, b, c, d as fit parameters to minimize the Frobenius norm M^exp -M^reg where M^exp is the experimentally determined Mueller matrix to be decomposed and M^reg is the sum of all M_i. Depending on M^exp an appropriate choice of M^reg matrices has to be made and different values of a, b, c and d are obtained through regression analysis.

We have previously shown that regression decomposition can be used to show that the Mueller matrix of Cetonia aurata can be decomposed into a sum of a circular polarizer and a mirror [2]. Here we expand the analysis to include angle-resolved spectral Mueller matrices, and also include more species of Scarabaeidae beetles.

One effect of the decomposition is that when depolarization is caused by an inhomogeneous sample with regions of different optical properties the Mueller matrices of the different regions can be retrieved under certain conditions. Regression decomposition also has potential to be a classification tool for biological samples where a set of standard matrices are used in the decomposition and the parameters a, b, c, d are used to quantify the polarizing properties of the sample.

[1] Cloude S.R. 1989. Conditions for the physical realisability of matrix operators in polarimetry. Proc. SPIE 1166, Polarization Considerations for Optical Systems II, pp. 177-185

[2] Arwin H, Magnusson R, Garcia-Caurel E, Fallet C, Järrendahl K, De Martino A, Ossikovski R, 2015. Sum decomposition of Mueller-matrix images and spectra of beetle cuticles. Opt. Express, vol. 23, no. 3, pp. 1951–1966

A Mueller matrix of a sample can be used to determine the polarization of  reflected light  for  incident light with arbitrary polarization. The polarization can be quantified  in terms of ellipticity, polarization azimuth and degree of polarization. We apply spectroscopic Mueller-matrix ellipsometry at multiple angles of incidence  to study the cuticle of beetles and derive  polarization features for incident unpolarized light.  In particular we address chiral phenomena in scarab beetles,  the origin of their structural colors and the observed high degree of circular polarization is discussed. Results from beetles in the Scarabaeidae subfamilies Cetoniinae and Rutelinae are presented including specimens with broad-band silver- or gold-like colors with metallic shine as well as specimens with narrow-band green or red reflectors. The variation of polarization with angle of incidence and occurrence of both left-handed and right-handed polarization from a single species are presented. We also use Mueller-matrix spectra in electromagnetic modeling and show how to determine structural parameters including cuticle layer thicknesses and optical properties. Interference oscillations in the observed spectra are due to allowed optical modes and we show how to develop a structural model of a cuticle based on this effect. Sum decomposition of  Mueller matrices measured on a depolarizing cuticle of a beetle is briefly discussed.

The scarab beetle Cetonia aurata is known to reflect light with brilliant colors and a high degree of circular polarization. Both color and polarization effects originate from the beetles exoskeleton and have been attributed to a Bragg reflection of the incident light due to a twisted laminar structure. Our strategy for mimicking the optical properties of the Cetonia aurata was therefore to design and fabricate transparent, chiral films. A series of films with tailored transparent structures of helicoidal In_xAl_1-xN nanorods were grown on sapphire substrates using UHV magnetron sputtering. The value of x is tailored to gradually decrease from one side to the other in each nanorod normal to its growth direction. This introduces an in-plane anisotropy with different refractive indices in the direction of the gradient and perpendicular to it. By rotating the sample during film growth the in-plane optical axis will be rotated from bottom to top and thereby creating a chiral film. Based on Muellermatrix ellipsometry, optical modeling has been done suggesting that both the exoskeleton of Cetonia aurata and our artificial material can be modeled by an anisotropic film made up of a stack of thin layers, each one with its in-plane optical axis slightly rotated with respect to the previous layer. Simulations based on the optical modeling were used to investigate how pitch and thickness of the film together with the optical properties of the constitutive materials affects the width and spectral position of the Bragg reflection band.

Spectral Mueller matrices are very rich in information about physical properties of a sample. We have recently shown that polarizing properties like ellipticity and degree of polarization can be extracted from a Mueller matrix measured on a beetle cuticle (exoskeleton). Mueller matrices can also be used in regression analysis to model nanostructures in cuticles. Here we present the use of sum decomposition of Mueller matrices from these depolarizing biological reflectors to explore the fundamental character of these reflectors. The objective is to decompose a Mueller matrix into well- defined ideal non-depolarizing matrices corresponding to mirrors, circular polarizers, halfwave retarders etc.Generally it is possible to decompose a measured depolarizing Mueller matrix M into four (or fewer) non-depolarizing matrices according to M=λ1M1+λ2M2+λ3M3+λ4M4, where λ1, λ2, λ3 and λ4 are eigenvalues of the covariance matrix of M. Two strategies for decomposition will be discussed. A Cloude decomposition will provide the eigenvalues and also the Mi’s although the latter will contain severe noise in some spectral regions. However, a major advantage with the Cloude decomposition is that the number of nonzero eigenvalues is directly obtained, i.e. the number of contributing Mi matrices. In an alternative decomposition, the Mi’s are assumed and the eigenvalues are found by regression analysis based on M. In the case with two non-zero eigenvalues we define a model Mueller matrix MD=αRM1+βRM2 with αR+βR=1. With αR as adjustable parameter, the Frobenius norm ||M-MD|| is minimized for each wavelength in the spectral range of M. For more complex structures, the regression can be extended by adding more matrices up to a total of four. Advantages with a regression approach are its simplicity and stability compared to a Cloude decomposition.Mueller-matrix spectra of beetle cuticles are recorded with a dual rotating compensator ellipsometer in the spectral range 400 – 900 nm at angles of incidence in the range 20 - 75°. The application of decomposition on biological reflectors is demonstrated on M measured on the beetle Cetonia aurata, which represents a narrow-band chiral Bragg reflector with two non-zero eigenvalues. A decomposition in an ideal mirror and a circular polarizer is feasible. In another example, the broad-band and gold-colored beetle Chrysina argenteola, we show that more than two eigenvalues can be nonzero, especially at oblique incidence, and additional matrices are involved.

Spectral Mueller matrices measured at multiple angles of incidence as well as Mueller matrix images are recorded on the exoskeletons (cuticles) of the scarab beetles Cetonia aurata and Chrysina argenteola. Cetonia aurata is green whereas Chrysina argenteola is gold-colored. When illuminated with natural (unpolarized) light, both species reflect left-handed and near-circularly polarized light originating from helicoidal structures in their cuticles. These structures are referred to as circular Bragg reflectors. For both species the Mueller matrices are found to be nondiagonal depolarizers. The matrices are Cloude decomposed to a sum of non-depolarizing matrices and it is found that the cuticle optical response, in a first approximation can be described as a sum of Mueller matrices from an ideal mirror and an ideal circular polarizer with relative weights determined by the eigenvalues of the covariance matrices of the measured Mueller matrices. The spectral and image decompositions are consistent with each other. A regression-based decomposition of the spectral and image Mueller matrices is also presented whereby the basic optical components are assumed to be a mirror and a circular polarizer as suggested by the Cloude decomposition. The advantage with a regression decomposition compared to a Cloude decomposition is its better stability as the matrices in the decomposition are determined a priori. The origin of the depolarizing features are discussed but from present data it is not possible to conclude whether the two major components, the mirror and the circular polarizer are laterally separated in domains in the cuticle or if the depolarization originates from the intrinsic properties of the helicoidal structure.

Optical properties of chiral nanostructured films made of Al1-xInxN using a new growth mechanism - curved-lattice epitaxial growth - are reported. Using this technique, chiral films with right- and left-handed nanospirals were produced. The chiral properties of the films, originating mainly from an internal anisotropy and to a lesser extent from the external helical shape of the nanospirals, give rise to selective reflection of circular polarization which makes them useful as narrow-band near-circular polarization reflectors. The chiral nanostructured films reflect light with high degree of circular polarization in the ultraviolet part of the spectrum with left- and right-handedness depending on the handedness of the nanostructures in the films.

Spectroscopic Mueller-matrix ellipsometry at variable angles of incidence is applied to beetle cuticles using a small (50 -100 μm) spot size. It is demonstrated how ellipticity and degree of polarization of the reflected light can be derived from a Mueller matrix providing a detailed insight into reflection properties. Results from Cetonia aurata, Chrysina argenteola and Cotinis mutabilis are presented. The use of Mueller matrices in regression analysis to extract structural and optical parameters of cuticles is briefly described and applied to cuticle data from Cetonia aurata whereby the pitch of the twisted layered structure in the cuticle is determined as well as the refractive indices of the epicuticle and the exocuticle.

Materials can be tailored on the nano-scale to show properties that cannot be found in bulk materials. Often these properties reveal themselves when electromagnetic radiation, e.g. light, interacts with the material. Numerous examples of such types of materials are found in nature. There are for example many insects and birds with exoskeletons or feathers that reflect light in special ways. Of special interest in this work is the scarab beetle Cetonia aurata which has served as inspiration to develop advanced nanostructures due to its ability to turn unpolarized light into almost completely circularly polarized light. The objectives of this thesis are to design and characterize bioinspired nanostructures and to develop optical methodology for their analysis.

Mueller-matrix ellipsometry has been used to extract optical and structural properties of nanostructured materials. Mueller-matrix ellipsometry is an excellent tool for studying the interaction between nanostructures and light. It is a non-destructive method and provides a complete description of the polarizing properties of a sample and allows for determination of structural parameters.

Three types of nanostructures have been studied. The rst is an array of carbon nanobers grown on a conducting substrate. Detailed information on physical symmetries and band structure of the material were determined. Furthermore, changes in its optical properties when the individual nanobers were electromechanically bent to alter the periodicity of the photonic crystal were studied. The second type of nanostructure studied is bioinspired lms with nanospirals of In_xAl_1–xN which reflect light with a high degree of circular polarization in a narrow spectral band. These nanostructures were grown under controlled conditions to form columnar structures with an internally graded refractive index responsible for the ability to reflect circularly polarized light. Finally, angle-dependent Mueller matrices were recorded of natural nanostructures in C. aurata with the objective to refine the methodology for structural analysis. A Cloude sum decomposition was applied and a more stable regression-based decomposition was developed for deepened analysis of these depolarizing Mueller matrices. It was found that reflection at near-normal incidence from C. aurata can be described as a sum reflection o a mirror and a left-handed circular polarizer. At oblique incidence the description becomes more complex and involves additional optical components.

Metamaterials in the form of chiral nanostructures have shown great potential for applications such as chemical and biochemical sensors and broadband or wavelength tunable circular polarizers. Here we demonstrate a method to produce tailored transparent chiral nanostructures with the wide-bandgap semiconductor Al_xIn_1 − xN. A series of anisotropic and transparent films of Al_xIn_1 − xN were produced using curved-lattice epitaxial growth on metallic buffer layers. By controlling the sample orientation during dual magnetron sputter deposition, nanospirals with right-handed or left-handed chirality were produced. Using a dual rotating compensator ellipsometer in reflection mode, the full Mueller matrix was measured in the spectral range 245–1700 nm at multiple angles of incidence. The samples were rotated one full turn around their normal during measurements to provide a complete description of the polarization properties in all directions. For certain wavelengths, unpolarized light reflected off these films becomes highly polarized with a polarization state close to circular. Nanostructured films with right- and left-handed chirality produce reflections with right- and left-handed near-circularly polarized light, respectively. A model with a biaxial layer in which the optical axes are rotated from bottom to top was fitted to the Mueller-matrix data. Hence we can perform non-destructive structural analysis of the complex thin layers and confirm the tailored structure. In addition, the refractive index, modeled with a biaxial Cauchy dispersion model, is obtained for the Al_xIn_1 − xN films.

We demonstrate an electrically tunable 2D photonic crystal array constructed from vertically alignedcarbon nanofibers. The nanofibers are actuated by applying a voltage between adjacent carbon nanofiberpairs grown directly on metal electrodes, thus dynamically changing the form factor of the photoniccrystal lattice. The change in optical properties is characterised using optical diffraction andellipsometry. The experimental results are shown to be in agreement with theoretical predictions andprovide a proof-of-principle for rapidly switchable photonic crystals operating in the visible that can befabricated using standard nanolithography techniques combined with plasma CVD growth of thenanofibers.

The attractive shiny metallic colour of jewel scarabs is originating from the structure of the exoskeleton.For some directions and wavelengths of the incident light this structure will also cause the reflectedlight to have a large ellipticity (near-circular polarization). This is due to that the exoskeleton is ahelicoidal structure, formed by layers of chitin molecules. The reflected light is most commonly lefthandedpolarized but right-handed polarization is also observed. In this work six species of Scarabbeetles from the Chrysina genus are investigated. The complete Mueller-matrix is measured with adual rotating compensator ellipsometer (RC2, J.A.Woollam Co., Inc.). The results are presented ascontour plots where we represent different parameters as a function of incidence angle 2[25; 75]and wavelength 2[240; 1000]nm of the incident beam. Parameters of particular interest are the m41element of the Mueller-matrix, which is related to the circular polarization behaviour, the degree ofpolarization, the ellipticity and the absolute value of the azimuth angle. From ocular observationsthrough left- and right-circularly polarizing filters all specimens showed clear polarization effects interms of colour changes. However, the Mueller matrix ellipsometry measurements showed two generaltypes of polarization behaviour depending on the studied species. Chrysina macropus and Chrysinaperuviana had a smaller range of m41 values around zero. Much larger m41 variations were observedfor Chrysina argenteola, Chrysina chrysargyrea and Chrysina resplendens. Chrysina gloriosa hadboth types of polarization behaviour depending on if the measurements where made on the green orgolden parts of this striped beetle. Comparisons among samples of beetles from the same species wereconducted. For instance, different specimens of Chrysina resplendens show rather large differences inthe polarization response whereas specimens of Chrysina chrysargyrea showed very similar polarizationbehaviour. All studied specimens did in some sense reflect both right- and left-handed polarizedlight. In many cases very high ellipticities (near-circular polarization states) were observed. Modelsof structures generating the observed polarization effects as well as biological aspects will also bediscussed.Figure 257: Three pictures of C. chrysargyrea from left to right taken with aleft-circular polarizer, no filters and with a right-circular polarizer in front of thecamera. Two contour plots of m41 for C. chrysargyrea showing a large region withleft-handed near-circular polarization and C. resplendens showing a large regionwith right-handed near-circular polarization.

One hundred years ago Michelson discovered circular polarization in reflection from beetles. Today a novel Mueller-matrix ellipsometry setup allows unprecedented detailed characterization of the beetles polarization properties. A formalism based on elliptical polarization for description of reflection from scarab beetles is here proposed and examples are given on four beetles of different character: Coptomia laevis - a simple dielectric mirror; Cetonia aurata - a left-hand narrow- band elliptical polarizer; Anoplognathus aureus - a broad-band elliptical polarizer; and Chrysina argenteola - a left-hand polarizer for visible light at small angles, whereas for larger angles, red reflected light is right-handed polarized. We confirm the conclusion of previous studies which showed that a detailed quantification of ellipticity and degree of polarization of cuticle reflection can be performed instead of only determining whether reflections are circularly polarized or not. We additionally investigate reflection as a function of incidence angle. This provides much richer information for understanding the behaviour of beetles and for structural analysis.

Despite of using chiral metamaterials to manipulate light polarization states has been demonstrated their great potential for applications such as invisible cloaks, broadband or wavelength-tunable circular polarizers, microreflectors, etc. in the past decade [1-6], operating wavelength in ultraviolet-visible range is still a challenge issue. Since these chiral structures often consist of metallic materials, their operation is designed for the infrared and microwave regions [2-4]. Here, we show how a controlled curved-lattice epitaxial growth (CLEG) of wide-bandgap AlInN semiconductor curved nanocrystals [7] can be exploited as a novel route for tailoring chiral nanostructures in the form of twisted nanorods (TNRs). The fabricated TNRs are shown to reflect light with a high degree of polarization as well as a high degree of circular polarization (that is, nearly circularly polarized light) in the ultravioletvisible region. The obtained polarization is shown to be dependent on the handedness of the TNRs.

Polarization effects in reflection from the cuticle of scarab beetles studied by spectroscopic Mueller-matrix ellipsometry

H. Arwin^*, T. Berlind, J. Birch, L. Fernandez Del Rio, J. Gustafson, J. Landin,

R. Magnusson, C. Åkerlind, and K. Järrendahl

Department of Physics, Chemistry and Biology, Linköping University, Sweden

^*corresponding author: han@ifm.liu.se

Abstract- Many scarab beetles exhibit structural colors and complex polarization phenomena in reflection. These effects are characterized with spectroscopic Mueller-matrix ellipsometry in our work. The polarization ellipse of reflected light as well as the degree of polarization is presented including variations with angle of incidence and wavelength. Emphasis is on beetles showing chiral effects and structural modeling of cuticle nanostructure is discussed.

Background Since one hundred years it is known that some scarab beetles reflect elliptically polarized light as demonstrated by Michelson for the beetle Chrysina resplendens [1]. The handedness of the polarization is in a majority of the cases left-handed but also right-handed polarization has been found [2,3]. The ellipticity varies with wavelength and viewing angle but can be close to +1 or -1 (right or left circular polarization, respectively) and in addition these beetles may exhibit beautiful structural colors. The polarization and color effects are generated in the outer part of the exoskeleton, the cuticle. These natural photonic structures are often multifunctional and play important roles for survival of beetles, e.g. for hiding from or scaring predators, for intraspecies communication, etc. [4]. However, such structures may find use in many commercial applications and a major motivation for detailed studies of natural photonic structures is that they inspire to biomimetic applications [5,6].

Approach Our objective is to use spectral Mueller-matrix data on scarab beetles to parameterize reflection properties in terms of polarization parameters and degree of polarization. The studied beetles all are phytophagous and include species from the Cetoniinae subfamily (e.g. Cetonia aurata and Coptomia laevis,), the Rutelinae subfamily (e.g. Chrysina argenteola and Chrysina resplendens) and the Melolonthinae subfamily (Cyphochilus insulanus). Furthermore, structural modeling is presented on Cetonia aurata and a few more beetles to demonstrate that structural parameters can be extracted by advanced modeling of Mueller-matrix data.

Experimental A dual rotating compensator ellipsometer (RC2, J. A. Woollam Co., Inc.) is used to record all 16 Mueller-matrix elements m_ij (i,j=1..4) in the spectral range 300 – 900 nm at angles of incidence in the range 20-70º. The elements are normalized to m₁₁ and thus have values between -1 and +1. All measurements are performed on the scutellum (a small triangular part on the dorsal side of the beetles) with focusing optics resulting in a spot size of the order of 50-100 mm. The software CompleteEASE (J. A. Woollam Co., Inc.) is used for analysis.

Results and discussion As an example, Fig. 1 shows contour plots of Mueller-matrix data measured on Cetonia aurata. This beetle has a metallic shine and if illuminated with unpolarized white light it reflects left-handed polarized green light as revealed by the non-zero Mueller-matrix elements m₁₄ and m₄₁ in the green spectral region for angles of incidence below about 45º. This is clearly seen in the graph to the right in Fig. 1 which shows a spectrum for Mueller-matrix element m₄₁at 20º as well as fitted model data. A model based on a twisted lamella structure, also called Bouligand structure, is used to model the chiral nanostructure [4]. Given the complexity of the nanostructure, an excellent model fit is achieved. The obtained model parameters are the spectral variation of the refractive index of the birefringent lamellas and the pitch. The model also includes a dielectric surface layer.

Fig.1. Left: Mueller-matrix data on Cetonia aurata. Each contour plot shows m_ij, where i and j correspond to the row and column, respectively. m₁₁ =1 and is not shown but is replaced with a photo of the beetle. Right: Experimental and model-generated Mueller-matrix element m₄₁at an angle of incidence of 20º.

From the Mueller-matrix data one can also determine so called derived parameters including azimuth and ellipticity of the polarization ellipse and the degree of polarization. The variations of these parameters with angle of incidence are presented for a selection of scarab beetles. Examples of both left-handed and right-handed polarization effects are shown and the importance of degree of polarization will be discussed.

Concluding remarks Mueller-matrix spectra at oblique incidence are very rich in information about reflection properties and allows parameterization of polarization parameters of the reflected light. Both left-handed and right-handed reflected light is found in scarab beetles. Mueller-matrix data can also be used for a detailed modeling of the nanostructure of the cuticle of beetles.

AcknowledgementsFinancial support was obtained from the Knut and Alice Wallenberg foundation and the Swedish Research Council. The Museum of Natural History in Stockholm, the National Museum of Natural Science in Madrid, the Berlin Museum of Natural History and the Natural History Museum in London are acknowledged for loan of beetles.

REFERENCES

1. Michelson, A. A. “On Metallic Colouring in Birds and Insects,” Phil. Mag., 21, 554-567, 1911.
2. Goldstein, D. H. “Polarization properties of Scarabaeidae,” Appl. Opt., 45, 7944-7950, 2006.
3. Hodgkinson, I., Lowrey, S., Bourke, L., Parker, A. and McCall, M. W. “Mueller-matrix characterization of beetle cuticle polarized and unpolarized reflections from representative architectures,” Appl. Opt., 49, 4558-4567, 2010.
4. Vukusic, P. and Sambles, J. R. “Photonic structures in biology,” Nature, 424, 852-855, 2003.
5. Lenau, T. and Barfoed, M. “Colours and Metallic Sheen in Beetle Shells - A Biomimetic Search for Material Structuring Principles Causing Light Interference,” Adv. Eng. Mat., 10, 299-314. 2008.
6. Parker, A. R. and Townley, H. E “Biomimetics of photonic nanostructures,” Nature Nanotech., 2, 347-351, 2007.

The Mueller matrix elements m_ij representing the polarization response from a nanostructured materialis determined by the constituent materials optical properties and the superstructure. Here, we investigate how chiral structures in form of helicoidally stacked uniaxial layers determine m_ij as a functionof polarization state, wavelength, incidence angle and azimuthal angle of the incoming light. The studied parameters include the layer materials ordinary/extraordinary optical properties, Euler angle values, and layer thickness as well as the thickness and pitch of the helicoidal superstructure. Sub- and superstructure inhomogeneity is also introduced. From the Fresnel-based calculations, m_ij aswell as the degree of polarization, ellipticity and azimuth of the polarization ellipse are obtained and presented as contour and trace plots to give a complete view of the polarization behavior. The results from the calculations are compared with Mueller matrix spectroscopic ellipsometry measurements of both natural and synthesized helicoidal structures. The measurements were performed with a dualrotating compensator system (RC2, J.A. Woollam Co., Inc.) for wavelengths in the range from 245 to 1000 nm and incident angles from 20 to 75°. For some measurements the azimuthal angle of the incident light was varied. The investigated natural chiral structures were exoskeletons from several beetles in the scarab subfamilies Cetoniinae and Rutelinae. As predicted from the calculations it isobserved that the reflection from these beetles can have a high degree of polarization and high ellipticity (near-circular polarization). Both left- and right-polarization was observed. The synthesized structures are helicoidal nanorods of Al_1−xIn_xN grown on sapphire substrates with metal-nitride seedlayers using UHV magnetron sputtering. Due to an internal composition gradient (a variation of x) in the crystalline structure, the nanorods will tilt away from the substrate normal. Helicoidal structures can thus be obtained by rotating the substrate around its normal during deposition. Samples with different pitch and layer thickness with right-handed as well as left-handed chirality were grown. Also for these structures both left and right near-circular polarized light is observed. By combining calculations, ellipsometry measurements and scanning electron microscopy characterization we get agood input to build layered models of the natural and synthetic samples. After regression fitting agood agreement between calculated and measured optical data were obtained.

Carbon nanofibers were used as building blocks for two-dimensional photonic crystal slabs. Electron beam lithography and chemical vapor deposition were used to fabricate regular arrays and random patterns of nanofibers. The optical properties of the samples were investigated using a diffraction measurement setup, as well as reflection ellipsometry. We find that carbon nanofiber regularity has a strong effect on both diffractive and specular optical properties. This shows that ellipsometry can be a valuable tool to study properties of carbon nanofiber arrays. It also shows that carbon nanofibers provide an interesting candidate as building blocks for nanostructured optical components.

The effect of peak power in a high power impulse magnetron sputtering (HiPIMS) reactive deposition of TiO(2) films has been studied with respect to the deposition rate and coating properties. With increasing peak power not only the ionization of the sputtered material increases but also their energy. In order to correlate the variation in the ion energy distributions with the film properties, the phase composition, density and optical properties of the films grown with different HiPIMS-parameters have been investigated and compared to a film grown using direct current magnetron sputtering (DCMS). All experiments were performed for constant average power and pulse on time (100W and 35 mu s, respectively), different peak powers were achieved by varying the frequency of pulsing. Ion energy distributions for Ti and O and its dependence on the process conditions have been studied. It was found that films with the highest density and highest refractive index were grown under moderate HiPIMS conditions (moderate peak powers) resulting in only a small loss in mass-deposition rate compared to DCMS. It was further found that TiO2 films with anatase and rutile phases can be grown at room temperature without substrate heating and without post-deposition annealing.

We present a study of the optical properties of three-armed square nanospirals made of silver and realized as nanostructured thin films with Glancing Angle Deposition. Optical property variations with polarization were investigated using numerical simulations. For each polarisation case, two principal resonances were determined corresponding to excitation of plasmonic modes of nanospirals which resonances frequency depended on the dimensions of the nanospirals. Calculation of current flows in the nanospirals showed patterns resembling those observed in U-shaped resonators. In particular, a mode with anti-parallel current flow in opposite arms indicates the existence of a magnetic-like resonance in the square nanospirals. We present also generalized spectroscopic ellipsometry measurements obtained on one sample at an incidence angle of 25, and evidence conversion between polarization states even for light polarized in the plane containing one of the arms. The measurements showed good agreement with the results of calculations for an ideal structure. The differences in the conversion of polarization between measurements and calculations were mainly attributed to the existence of structural non-idealities in the actual sample.

The thermal behaviour of an organic photovoltaic (OPV) binary system comprised of a liquid-crystalline fluorene-based polymer and a fullerene derivative is investigated. We employ variable-temperature ellipsometry complemented by photo-and electroluminescence spectroscopy as well as optical microscopy and scanning force nanoscopy to explore phase transitions of blend thin films. The high glass transition temperature correlates with the good thermal stability of solar cells based on these materials. Furthermore, we observe partial miscibility of the donor and acceptor together with the tendency of excess fullerene derivative to segregate into exceedingly large domains. Thus, for charge generation less adequate bulk-heterojunction nanostructures are poised to develop if this mixture is exposed to more elevated temperatures. Gratifyingly, the solubility of the fullerene derivative in the polymer phase is found to decrease if a higher molecular-weight polymer fraction is employed, which offers routes towards improving the photovoltaic performance of non-crystalline OPV blends.

Carbon nanofibers (CNFs) are used as components of planar photonic crystals. Square and rectangular lattices and random patterns of vertically aligned CNFs were fabricated and their properties studied using ellipsometry. We show that detailed information such as symmetry directions and the band structure of these novel materials can be extracted from considerations of the polarization state in the specular beam. The refractive index of the individual nanofibers was found to be n(CNF) = 4.1.

Photonic crystals with lattice constant in the visible range of light can be fabricated using freestanding carbon nanofibres. These types of structures are optically complicated, with geometric effects both from the lattice and from individual scatterers. In this report, results from pilot studies of the optical properties of such samples are presented.

Ellipsometric measurements on samples with vertically aligned arrays of free-standing carbon nanofibres with lengths up to 1600 nm are presented. The carbon nanofibres were grown on silicon wafers with a 70 nm layer of titanium and a 15 nm layer of titanium nitride using plasma-CVD with nickel as catalyst. Electron beam lithography was used to create the nanofibre lattice. Samples with varying periodicity and length of the carbon fibers, i.e. film thickness, were investigated.

In this pilot study measurements were performed with a dual rotating compensator ellipsometer in the spectral range 245-1700 nm. The instrument provides the full Mueller matrix of the sample and measurements were performed at multiple angles of incidence and different sample orientations.

The samples show no difference in the optical response when rotated 90°, but at other orientations changes in the spectrum are observed below a wavelength of approximately 1000 nm. The analysis of the ellipsometric data is presented and different modeling approaches are discussed.