Joseph A. Sherwin

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.

On calibration of a nominal structure–property relationship model for chiral sculptured thin films by axial transmittance measurements

Abstract A chiral sculptured thin film is fabricated from patinal titanium oxide using the serial bideposition technique. Axial transmittance spectrums are measured over a spectral region encompassing the Bragg regime for axial excitation. The same spectrums are calculated using a nominal structure–property relationship model and the parameter space of the model is explored for best fits of the calculated and measured transmittances. Ambiguity arising on calibrating the model against axial transmittance measurements is shown to be resolvable using non-axial transmittance measurements.

Orthorhombic Materials and Perfect Lenses

The potential of orthorhombic materials for perfect lenses is theoretically examined.

Nominal model for structure-property relations of chiral dielectric sculptured thin films

The Bruggeman formalism is implemented to estimate the permittivity dyadics of chiral dielectric sculptured thin films (STFs), called thin-film helicoidal bianisotropic mediums (TFHBMs), modelled as helicoidally arranged stacks of ellipsoidal inclusions in vacuum. Spectral maximums of several observable optical properties of axially excited TFHBM slabs are examined as functions of inclusion shape, volume fraction and orientation. Changes in the transverse aspect ratio of the ellipsoidal inclusions significantly affect the predicted optical properties of the TFHBM. A critical value for the transverse aspect ratio of the ellipsoids can be found such that optical activity disappears when all other parameters are fixed. The orientation of the inclusions significantly affects the value of optical property maximums, and can be made to suppress optical activity completely. A distribution of orientations decreases various measures of optical activity.

Nominal model for the optical response of a chiral sculptured thin film infiltrated with an isotropic chiral fluid

Abstract An ellipsoid-based structure–property relationship model for a chiral sculptured thin film (STF) infiltrated with a chiral fluid is presented. The effect of the constitutive and microstuctural parameters on the optical response of the chiral STF is explored. Specific structure–property relations are developed. The model predicts that infiltration by a chiral fluid shifts the spectral location of the Bragg regime, and this shift is linearly dependent on the degree of chirality of the fluid. The bandwidth of the Bragg regime also changes. Finally, the model also predicts that the chiral fluid can either enhance or diminish the optical activity exhibited by a chiral STF. Which of these effects occurs depends on the microstructural and constitutive parameters of the chiral STF and the constitutive parameters of the chiral fluid.

Homogenization of similarly oriented, metallic, ellipsoidal inclusions using the Bruggeman formalism

Abstract The effective relative permittivity dyadic of a composite material made by randomly embedding parallel, ellipsoidal, isotropic, metallic inclusions in an homogeneous, isotropic, dielectric host material is computed using the Bruggeman formalism with exact depolarization dyadics. Numerical calculations carried out for iron inclusions at 670 nm free-space wavelength indicate that the inclusion volume fraction at the percolation threshold is direction-dependent, being lower in those directions that the ellipsoids have longer extents. When the longest principal semi-axis of an ellipsoidal inclusion exceeds a certain relative size – about three to five times as large as the other two principal semi-axes – the Bruggeman estimate of the effective permittivity along this direction does not indicate a percolation threshold, becoming merely a simple weighting of the permittivities of the two materials in relation to their volume fractions.

Nominal model for the optical response of a chiral sculptured thin film infiltrated by an isotropic chiral fluid-oblique incidence

Numerical calculations on the response of a nonaxially excited chiral sculptured thin film (STF) infiltrated with a chiral fluid are presented. The effects of the constitutive and microstructural parameters on the optical response of the chiral STF are explored. Two Bragg regimes manifest themselves when the chiral STF is excited nonaxially. Infiltration of a chiral STF with a chiral fluid can either blue- or red-shift the spectral location of one Bragg regime, but the shift of the spectral location of the other Bragg regime is largely indifferent to the chirality of the fluid. The model also predicts that the chiral fluid can either enhance or diminish the optical activity exhibited by the chiral STF within either manifested Bragg regime. Which of these effects occurs depends on the microstructural and constitutive properties of the chiral STF as well as the constitutive properties of the chiral fluid. The angle of incidence of the plane wave also effects the response of the chiral STF.

Ellipsoid-based model of structure-response relationships for chiral sculptured thin films

Dielectric versions of helicoidal bianisotropic mediums (HBMs) have been realized recently as chiral sculptured thin films (STFs). With a view to understanding their structure-response relationships, these STFs are modeled as periodically arranged stacks of dielectric ellipsoidal inclusions in air. The inclusions are assumed to be randomly dispersed and similarly oriented in each stack, and the Bruggeman formalism is adopted for local homogenization. The constitutive properties are examined as functions of inclusion shape, volume fraction, and orientation angles. Optical signatures of the modeled thin-film HBM (TFHBM) layers, assumed axially excited, are calculated after solving a boundary value problem. Several conclusions drawn from the calculated spectrums of co- and cross-polarized reflectances and transmittances, true and apparent circular dichroisms, true and apparent linear dichroisms, ellipticity transformation, and optical rotation are presented.

Displacement in a continuously twisted structurally chiral medium due to axial loading

Loading of a linear, continuously twisted, structurally chiral medium (CTSCM) parallel to its helicoidal axis is analyzed, after assuming that the medium is locally triclinic. The roles of certain composite compliances are established, with a view to eventual use of CTSCMs for ultrasonic, infrared and optical applications.

Errata to: “nominal model for structure-property relations of chiral dielectric sculptured thin films”

In the subject paper, calculations reported for the stated angular values of @g were actually made for @p/2 - @g. Consequently, Figures 1-7 are incorrect, and a few quantitative, but notqualitative, conclusions drawn therefrom are also incorrect. The error in the computer program came to light very recently, after the printing of the journal issue. We deeply regret the error. Corrected versions of Sections 5 and 6 of the paper are presented here, and will also be available in the forthcoming Ph.D. Thesis of the first author.