Qi Hong Wu

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.

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.

Spectral-hole filter fabricated using sculptured thin-film technology

Abstract A Fabry–Perot description of the three-layer spectral-hole filter (SHF) fabricated using sculptured thin-film technology is presented. The identical outer layers are made of a thin-film helicoidal bianisotropic medium, and individually act as Bragg reflectors of one circular polarization. The middle layer, made of a homogeneous, isotropic, dielectric medium, functions as a quarter-wave spacer layer at the center-wavelength of the Bragg regime. The measured transmittance spectrums of the fabricated SHF are evaluated relative to exact theoretical results.

Effective principal refractive indices and column angles for periodic stacks of thin birefringent films

An effective medium theory is developed for periodic stacks of thin films A and B, one or both of which may be birefringent because of an effect such as columnar microstructure. The equivalent medium is itself described as columnlike, and the theory permits calculation of the angle of the effective columns together with the magnitudes of the effective principal refractive indices. Several examples that relate to possible applications of the method are discussed, and it is shown that optical media with predetermined refractive indices and birefringence may be engineered. For example, the layered combination of a low-index material A with zero or small normal-incidence refractive anisotropy and a high-index material B with large anisotropy may be used to make a medium-index material with considerable normal-incidence anisotropy.

Dispersion equations for vacuum-deposited tilted-columnar biaxial media.

We consider the application of the Bragg-Pippard (BP) equations for form birefringence to a tilted-columnar biaxial thin film with columns of index n(c) and voids of known index n(v). In such a situation the three forward BP equations that express the principal refractive indices n1, n2, and n3 as functions of n(c), n(v), the packing fraction p(c), and the depolarization factors L1, L2, and L3 can be inverted. The procedure described for adding dispersion to the principal indices involves entry to the BP model via the inverted equations, modification of n(c) to allow for dispersion, and then exit from the model via the forward BP equations. We discuss the introduction of composite columns to the model to allow for angular dependence of n(c) and the selection of suitable dispersion functions for bulk tantalum oxide, titanium oxide, and zirconium oxide. Theory and experiment both show that the dispersion of the normal-incidence birefringence Deltan of the thin films is several times larger than the dispersion of the individual principal refractive indices.