Lakshman De Silva

Natural and Nanoengineered Chiral Reflectors: Structural Color of Manuka Beetles and Titania Coatings

A parallel study of natural and nanoengineered structurally chiral reflecting coatings is described. It is shown that the nanostructures are different in a minor way but are optically equivalent. Refractive index matching of nanoengineered chiral coatings on a plane substrate is shown to improve the saturation of structural color. Optical and electron microscopies reveal complexity in the multilayered chiral coatings that produce green metallic-like reflections from manuka (scarab) beetles. In particular, the reflectors are shown to have the form of small concave pits and troughs that are filled with contouring chiral material. Each chiral microreflector presents a range of pitch and tilt to an incident beam of light. Physical properties of the textured coatings are related to optical properties such as spectral reflectance, angle of spread, and perceived color, which has a high degree of saturation due to the filling of the pits. Observations of overlapping chiral mediums in beetle reflectors have inspired nanoengineering of related handed media such as Bragg reflectors for elliptically polarized light.

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.

All-silicon polarizing filters for near-infrared wavelengths

We discuss the design and fabrication of optical filters that are transparent to a preferred linear polarization, but are partially or substantially reflective to the orthogonal linear polarization. The filters are fabricated by depositing thin films of form-birefringent silicon onto silicon wafers using the serial bideposition method. Characteristic curves that are presented for the in-plane principal refractive indices np and ns of the porous films as functions of vapor impingement angle facilitate critical aspects of the filter design. These include matching of a p principal index and an s principal index for films deposited at different angles, and realization of p and s principal indices that satisfy the antireflection condition for the silicon substrate in air.

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.

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.

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.

Layered and continuous handed materials for chiral optics

Thin film optical materials deposited with a helical nanostructure can be used to fabricate devices that respond differently to right-handed and left-handed circularly polarized light. Together these inorganic materials and devices promise the framework of chiral optics, in which the basic polarization states of propagating light are circular. In the presentation we show that one period of a continuous chiral material can be replaced with three or more sub- layers of oriented biaxial material, and compare the performance that is expected of elements and devices such mirrors, spacerless spectral-hole filters and circularly polarized lasers that use continuous and layered chiral materials respectively. The investigation shows that, for the same number of turns and local linear birefringence, the performance of the layered material is slightly poorer than that of the continuos material. However, in practice the small loss in performance of the layered chiral material may be offset by inherently larger local linear birefringence and improved structural fidelity due to superiority of optical monitoring over quartz-crystal monitoring.

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.

Inorganic polarizing materials grown by physical vapor deposition

Currently there is a need for retarders that do not degrade under elevated temperatures and intense illumination for use in displays and devices such as rear projection televisions. A stack of inorganic planar layers with alternating high and low refractive indices behaves as a form birefringent uniaxial material, but the sign of the birefringence is always negative. In the presentation we outline the use of serial bideposition with 90 deg incremental substrate rotations to generate positive uniaxial materials with a typical difference of 0.12 between the ordinary and extraordinary refractive indices. Optical methods for displaying the axial symmetry of the coatings and for measuring the birefringence are described and contrasted with previous and current work on inorganic biaxial materials.

Modeling optical reflectance from chiral micromirrors embedded in manuka beetles

Optical and electron microscopies reveal complexity in the multilayered chiral coatings that produce green metallic-like reflections from manuka (scarab) beetles. In particular the reflectors are shown to have the form of small concave pits and troughs that are filled with contouring chiral material. Each chiral micro-reflector presents a range of pitch and tilt to an incident beam of light. The presentation attempts to relate these physical properties to optical properties such as spectral reflectance, angle of spread and perceived color of the beetles.