Mueller-matrix characterization of beetle cuticle: polarized and unpolarized reflections from representative architectures.

We discuss the use of Mueller matrices for characterizing the reflecting properties of beetles, including matching patterns of significant elements to specific cuticular architectures. In the case of illumination by natural light, the parameters of the reflected light are related to the elements of the first column of the matrix. The green and red beetle Stephanorrhina guttata is shown to be a narrowband polarization-preserving reflector apart from depolarizing white patches, the green Calloodes grayanus a narrowband left-circular reflector, and the gold Anoplognathus parvulus behaves as a broadband left-circular reflector. Comparison of experimental and simulated matrices confirms that the beetle reflectors are natural analogs of all-dielectric thin-film reflectors. However, the gold Chrysina resplendens, which was formerly known as Plusiotis resplendens and reflects both right-handed and left-handed light, is represented by an ensemble of laterally incoherent chiral thin-film reflectors.

Chiral photonic film and flake.

We show that chiral photonic flake has potential as a physical effect colorant that contributes both bright interference colors and a characteristic polarization spectrum. An analysis of the polarizing properties of chiral film and flake indicates that the Stokes spectrum s(3) v. lambda is suitable for characterization. s(3) Is shown to be invariant both to azimuthal rotation of a flake and to incoherent summation of the light from an array of flakes with random orientations. We form chiral photonic flake experimentally by scraping film material from nanoengineered chiral films on glass. Three basic architectures are used, a standard chiral medium that supports a single Bragg resonance, a threaded chiral medium that supports right-handed and left-handed resonances at different wavelengths and a threaded chiral medium that supports two right-handed resonances at different wavelengths. In a separate set of experiments a twist defect is added to each basic structure. Experimental measurements of s(3) spectra from film and flake show the expected signatures of the circular Bragg resonances and of the spectral holes caused by the defects.

Observation and modeling of polarized light from scarab beetles.

The light reflected from scarab beetles illuminated with unpolarized white light is analyzed ellipsometrically and displayed as the sum of an elliptically polarized spectrum I(p) and an unpolarized spectrum I(u). A chirped stack of chiral resonators, each with a characteristic Bragg wavelength and partial realignment of birefringent material to a fixed axis, is proposed as a model for simulation of both reflection and polarization spectra. Possible mechanisms that effectively eliminate impedance mismatch at the air-elytron interface and allow some beetles to exhibit nearly perfect circularly polarized reflections are discussed. Results are presented for three representative beetles, Ischiosopha bifasciata, which is shown to be a narrowband left-circular polarizer; Chrysophora chrysochlora, a broadband left-circular polarizer; and Chrysina woodi, an elliptical polarizer. The methods that are developed are applicable to the more general problem of synthesis of reflectors with prescribed reflection and polarization spectra.

Biaxial thin-film coated-plate polarizing beam splitters.

We present a design for a biaxial thin-film coated-plate polarizing beam splitter that transmits the p-polarized component of a beam of light without change of direction and reflects the s-polarized component. The beam splitter has a periodic structure and is planned for fabrication by serial bideposition in mutually orthogonal planes. Recent experimental data for form-birefringent silicon is used to establish the feasibility of the design for a beam splitter to be used at 1310 nm and at an angle of 45 degrees in air.

Structurally perturbed chiral Bragg reflectors for elliptically polarized light.

The structure of an inorganic chiral medium represented as a stack of identical form-birefringent layers that twist steadily with increasing thickness is perturbed by realigning a fraction of each layer to a fixed direction. Experimental results show that the resulting chiral-birefringent composite medium exhibits Bragg resonance with elliptically polarized light, and simulations indicate that Bragg reflectors can be designed for any polarization including linear.

Inorganic positive uniaxial films fabricated by serial bideposition.

The physical vapor deposition process of serial bideposition is adapted to the fabrication of uniaxial optical coatings. During the coating process the vapor impinges at an angle of incidence of about 70 on to the substrate, and a stepwise axial rotation with 90 increments causes a columnar structure to grow normal to the substrate. Symmetry considerations that follow from the choice of 90 for the stepwise increment ensure that the film is achiral and has negligible in-plane linear birefringence. Optical characterization techniques confirm that films of tantalum oxide, titanium oxide and zirconium oxide are positive uniaxial with ne -no in the range 0.10 to 0.14.

Supermodes of chiral photonic filters with combined twist and layer defects.

We consider the circularly polarized localized modes of chiral photonic structures with combined central twist and isotropic layer defects. The general filter is shown to suffer from anomalous remittance and saturation of linewidth as the thickness of the structure is increased. However, by choosing parameters that phase match the elements of the round-trip matrix of the isotropic layer defect, we demonstrate the existence of supermodes that maintain exceptional purity of polarization state and exponential decrease in linewidth as the thickness is increased.