Suchergebnisse

The cuticle of the scarab beetle Cotinis mutabilis reflects left-handed polarized light indicating the presence of a helicoidal structure. Different in-depth pitches in the cuticle are corroborated by optical microscopy images of the cuticle which originally is yellowish or reddish but becomes greenish after gently scratching its top side. Using the Mueller-matrix formalism the degree of polarization (total and circular) of reflected light is determined for unpolarized incident light. The effects of the finite thickness of the cuticle on the broadening and strength of the selective Bragg reflection are discussed on the basis of dispersion relations for optical modes in helicoidal structures and simulated spectra. (C) 2017 The Author(s). Published by Elsevier Ltd. ; Funding Agencies|Conacyt-Mexico; Knut and Alice Wallenberg foundation; Swedish Research Council; Carl Tryggers foundation; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]

Birefringent chitin films were prepared by a dipping technique from aqueous suspensions of chitin nanocrystals in a nematic liquid crystal phase. In the films, chitin nanocrystals are preferentially oriented along the withdrawal direction. Normal incidence transmission Mueller-matrix (M) spectroscopic ellipsometry measurements as a function of sample rotation were used to investigate the optical birefringence in the spectral range 0.73 to 5 eV. Analysis of eigenvalues and depolarization data reveal that the Mueller matrix corresponds to a pure retarder for photon energies below 4 eV and is depolarizing in the range 4 to 5 eV. By modeling the chitin film as a slab with in-plane anisotropy the birefringence was determined. The determination of birefringence was extended to include the range of 4 to 5 eV by a differential decomposition of M. (C) 2016 Optical Society of America ; Funding Agencies|Concayt-Mexico; Knut and Alice Wallenberg foundation; Swedish Research Council; Carl Tryggers foundation; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]

The cuticle of the beetle Chrysina chrysargyrea reflects left-handed polarized light in the broad spectral range from 340 to 1000 nm. Interference oscillations in the experimental Mueller-matrix spectroscopic ellipsometry data reveal that transparent materials comprise the cuticle. A spectral analysis of the interference oscillations makes evident that the pitch profile across the cuticle is graded. The graded pitch and effective refractive indices are determined through non-linear regression analysis of the experimental Mueller matrix by using a cuticle model based on twisted biaxial dielectric slices. Non-uniformity in cuticle thickness as well as in pitch profile near the cuticle surface account for depolarizance of the Mueller matrix. Transmission electron microscopy supports the reliability of the results. ; Funding Agencies|Conacyt-Mexico; Knut and Alice Wallenberg foundation; Centre in Nano Science and Nano Technology (CeNano) at Linkping University; Carl Tryggers foundation; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkping University (Faculty Grant SFO-Mat-LiU) [2009-00971]

Chiral nanocrystalline cellulose (NCC) free-standing films were prepared through slow evaporation of aqueous suspensions of cellulose nanocrystals in a nematic chiral liquid crystal phase. Mueller matrix (MM) spectroscopic ellipsometry is used to study the polarization and depolarization properties of the chiral films. In the reflection mode, the MM is similar to the matrices reported for the cuticle of some beetles reflecting near circular left-handed polarized light in the visible range. The polarization properties of light transmitted at normal incidence for different polarization states of incident light are discussed. By using a differential decomposition of the MM, the structural circular birefringence and dichroism of a NCC chiral film are evaluated. ; Funding Agencies|Knut and Alice Wallenberg foundation; Swedish Research Council; Carl Tryggers foundation; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009-00971]; Brazilian agency FAPESP; Brazilian agency CNPq; FAPESP [2014/12424-2]

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 ; Funding Agencies|Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University; Carl Tryggers Foundation [CTS:31]; Knut and Alice Wallenberg Foundation [2004.0233]; Consejo Nacional de Ciencia y Tecnologia (CONACYT); Vetenskapsradet (VR) [621-2011-4283]

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. ; Funding Agencies|Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [SFO-Mat-LiU 2009-00971]