Ian J. Hodgkinson

Thin-films field-transfer matrix theory of planar multilayer waveguides and reflection from prism-loaded waveguides

A standard 2 × 2 matrix method-used in thin-film optics is applied to planar multilayer optical waveguides. All modes are required to satisfy substrate-to-cover field-transfer equations that reduce to the equation γcm11 + γcγsm12 + m21 + γsm22 = 0 for bound modes and leaky waves. Expressions are derived for the field profiles and the power in each medium. A first-order perturbation theory is developed and applied to absorbing multilayer guides and to the reflection of plane waves from the prism-loaded lossy multilayer guide. The latter leads to experimental arrangements for measuring losses in which the gap thickness and propagation constant are accessible parameters.

Empirical equations for the principal refractive indices and column angle of obliquely deposited films of tantalum oxide, titanium oxide, and zirconium oxide

Values of the transmittance T(s) and the phaseretardation D were recorded in situ at two angles duringthe growth of thin films of tantalum oxide, titanium oxide, andzirconium oxide for deposition angles theta(nu) in the range40 degrees -70 degrees . Column angles for the same films were determinedex situ from scanning electron microscopy photographs ofdeposition-plane fractures. We show that the experimental columnangles are smaller than the corresponding values predicted by thetangent-rule equation psi = tan(-1)(0.5 tan theta(nu)) and that the experimental values fit a modifiedform of the equation psi = tan(-1)(E(1) tan theta(nu)) where E(1) is less than 0.5. We also show that theprincipal refractive indices are represented well by quadraticfunctions of the deposition angle, for example, n(1)(theta(nu)) = A(0) + A(2) theta(nu)(2).

Vacuum deposition of chiral sculptured thin films with high optical activity.

We present the technique of bideposition to realize thin-film helicoidal bianisotropic mediums (TFHBMs) that exhibit high optical activity. We show, by experiment as well as by simulation, that the optical rotation produced by these chiral sculptured thin films is roughly proportional to the square of the local linear birefringence. Experimental measurements on bideposited TFHBMs of titanium oxide yield a typical value of 5 degrees /mum for the effective specific rotation in the short-wavelength regime; the corresponding value determined for the standard unideposited TFHBMs is 1 degrees /mum. Both types of TFHBMs are highly optically active in comparison with quartz, fluorite films, and cholesteric liquid crystals. Bideposited TFHBMs will lend themselves to many different types of optical devices.

Serial bideposition of anisotropic thin films with enhanced linear birefringence

We describe a serial bideposition technique in which a tilted substrate is rotated stepwise by half a turn about a normal axis during the evaporation of a metal oxide from a single electron-beam source. Coatings formed by the new method develop a columnar nanostructure that is perpendicular to the substrate and has greatest width or bunching perpendicular to the common deposition plane. With appropriate choice of deposition parameters, the method produces biaxial films with large birefringence, principal axes aligned parallel and perpendicular to the substrate, and improved uniformity. Measured phase retardances for light incident normally on the films are double the corresponding values for tilted-columnar films.

Spacerless circular-polarization spectral-hole filters using chiral sculptured thin films: theory and experiment

Abstract In the Bragg wavelength regime, a sculptured thin film with chiral micromorphology permits light of a particular circular polarization state to pass virtually unhindered, but light of the other polarization state is reflected almost completely. On cascading two such films, but with one film twisted with respect to the other by an angle of 90°, a hole appears in the reflection spectrum. This is proved theoretically as well as experimentally in this paper. Also, it is shown that the design wavelength of the filter can be tuned by a modest amount by tilting the filter with respect to the incident beam.

Inorganic Chiral Optical Materials

Recent advances in birefringent thin-film technology have led to vacuum-deposited inorganic chiral materials of significance for optics. The new materials have a double-start-screw nanostructure and exhibit large optical activity. At the circular Bragg resonance, which can be engineered for visible or near infrared wavelengths, the materials tend to transmit circularly polarized light of one handedness and reflect the other. A large range of standard thin-film materials can be used, but best results are achieved with evaporants that yield large form birefringence, such as tantalum oxide, titanium oxide, and zirconium oxide for visible wavelengths and silicon for the near infrared. Multilayered fabrication has been demonstrated, and potential applications include solid-state sources, reflectors, filters, and detectors for circularly polarized light.

Sculptured-thin-film spectral holes for optical sensing of fluids

A novel class of fluid sensors is proposed based on monitoring the optical properties of multi-section chiral sculptured thin films (STFs) that function as spectral reflection holes. Using a nominal model that treats a chiral STF as a two-phase composite material with locally biaxial dielectric properties, we predict that the presence of a fluid in the porous film results in a red-shift of the spectral holes. Several device operation modes are proposed, and their relative merits are compared. Proof-of-concept experiments with both circularly polarized and unpolarized incident light confirm the red-shift of the spectral holes, and demonstrate operation in a practical situation.

Anisotropy in thin films: modeling and measurement of guided and nonguided optical properties: application to TiO 2 films

The main purpose of this research is to study the anisotropic behavior of dielectric material in thin-film form. First we present a theory based on a 4 × 4 transfer matrix linking tangential components of the electromagnetic field on one interface to the tangential components of the electromagnetic field on the other interface of an anisotropic thin film. A biaxial model is associated with the columnar structure of the layer. The comparison between measurements of the transmission in normal incidence in cross-polarized light and of guided-mode propagation constants with the calculations allows us to study the biaxial behavior of TiO(2) films. The excellent consistency between measurements and computations demonstrates the validity of the model based on the columnar structure.

Chiral mirror and optical resonator designs for circularly polarized light: suppression of cross-polarized reflectances and transmittances

Abstract A left-handed chiral sculptured thin film (STF) that reflects strongly at the wavelength of the circular Bragg resonance tends to partially convert the handedness of incident LCP (left-circularly-polarized) light to RCP (right-circularly-polarized). We show that the cross-polarized component of the reflected RCP beam can be eliminated by interference with an additional RCP beam that is reflected at the interface of an isotropic cover and an AR (antireflecting) layer. For best results the refractive index and thickness of the AR layer need to accommodate a phase change on reflection that occurs at the chiral film. Effective suppression of the reflectances RRR, RRL, RLR and the transmittances TRL, TLR can be achieved by sandwiching the chiral reflector between such amplitude and phase-matched AR coatings. Co-polarized chiral reflectors of this type may form efficient handed optical resonators. For LCP light the optical properties of such a handed resonator are formally the same as the properties of the isotropic passive or active Fabry–Perot resonators, but the handed resonator is transparent to RCP light.

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.