Fuzi Yang

Determination of the optical permittivity and thickness of absorbing films using long range modes

Analytic theory and numerical modelling are presented for the prism coupling of light to long range surface modes supported by a thin absorbing film. If both transverse magnetic and transverse electric modes can be excited, then by comparing theory to angle dependent reflectivity data the optical permittivity and thickness of the film may be unambiguously established. An experimental confirmation is presented for thin films of indium tin oxide (ITO) on glass substrates. The environment of the ITO is made optically symmetric, to support the long range mode, by use of a fluid which is index matched to the glass substrate. Attenuated total reflection, with prism coupling through the matching fluid, gives, with suitable fluid thickness, sharp resonant modes in the angle dependent reflectivity. Comparison of such data with modelling theory yields the ITO parameters.

Optical characterization of liquid crystals by means of half-leaky guided modes

Optical excitation of half-leaky guided modes has been used, for the first time to our knowledge, to characterize in detail the optical tensor profile in a liquid-crystal layer. The thin liquid-crystal layer is sandwiched between a high-index pyramid, with an index greater than the maximum index of the liquid crystal, and a glass substrate with an index lower than the minimum index of the liquid crystal. Analysis of this geometry shows that over a small angle range encompassed by a pseudocritical angle and a critical angle associated with the pyramid–liquid crystal and the pyramid–glass-substrate boundaries, respectively, there may exist sharp resonant features in the angle-dependent optical reflectivity. More particularly, if the director is tilted and twisted out of the plane of incidence there is strong TE-to-TM optical conversion over this small angular window, which is sensitive to details of the director profile. A 90° twisted, homogeneously aligned nematic liquid-crystal layer has been studied with this technique at two different wavelengths, 632.8 and 514.5 nm. Greater than 60% conversion from TM incident radiation to TE output has been recorded from sharp resonances in the half-leaky guided-wave angle window. If one fits these resonances with predictions from multilayer optics theory, one obtains extraordinary detail of the director profile in the cell. Since no metallic coatings are used, and all that is required are two glass plates of high and low index, respectively, the technique offers the potential of extremely useful applications in the examination of detailed director profiles in commercially fabricated cells.

Microwave liquid crystal wavelength selector

The combined use of a zero-order grating with liquid crystals to control microwaves is presented. A nematic liquid crystal is aligned in the 75 μm gaps of a grating comprised of 1-mm-thick aluminum slats. A set of resonant transmission frequencies is recorded for microwaves incident on this structure with their electric field perpendicular to the slats. These resonances are due to the excitation of coupled surface plasmons. A voltage applied between adjacent pairs of slats causes the liquid crystal to realign, allowing sensitive control of the transmitted microwave frequencies.

Optical fully leaky mode characterization of a standard liquid-crystal cell

Optical excitation of fully leaky guided modes is used to characterize in detail the optical tensor profile in a liquid-crystal layer confined between two standard glass plates as for a conventional cell. The angle-dependent reflectivity and transmissivity of such a structure are explored analytically, numerically, and experimentally. By suitable choice of incident and detected polarizations it is shown possible to obtain a detailed characterization of the director profile in the cell even though the leaky optical modes lead to rather broad features in the recorded data. Using a two-prism coupling technique, matching the prisms to the glass of the cell with identical index-matching fluid, allows access to sets of both reflectivity data and transmissivity data over a sufficient range of in-plane photon wave vectors to yield unambiguous director profiles when the data are compared with modeling theory. The specific cell explored in this study contains a homogeneously aligned ferroelectric smectic material in which there is a cusped, chevron, director profile. The results presented for such a complex structure show that this powerful new form of quantified conoscopy is likely to provide the primary route forward for optical characterization of conventional cells of this nature.

Measurement of the refractive indices of a ferroelectric liquid crystal

Optical excitation of half‐leaky guided modes has been used to determine the refractive indices of a ferroelectric liquid crystal (Merck‐SCE13) in a homeotropically aligned state. The uniform homeotropic alignment is realized, with no surface treatment, by the application of an in‐plane dc electric field. This applied field, of the order of 5×105 V m−1 is sufficient to fully unwind the S*c helix giving a uniformly tilted homeotropic monodomain for optical characterization. Analytic and numerical modeling results indicate that, for a slab with its optic axis tilted in a plane orthogonal to the plane of incidence, two distinct critical angles appear in the half‐leaky guided mode response. These independently relate simply to the ordinary and extraordinary refractive indices of the S*c material. By fitting theoretical angle dependent reflectivities to those recorded experimentally the two refractive indices have been obtained for a range of temperatures in both the S*c and SA phases.

Determination of the permittivity of nematic liquid crystals in the microwave region

Two 3 mm thick microscope glass plates, having one face plus their two long edges coated by a thick metallic film, are spaced 75 μm apart by mylar spacers. Because of the metallic coatings on the inner faces the structure acts as a single metallic slit. The space between the two coated plates is filled with aligned nematic liquid crystal (E7, Merck/BDH) and the cell is inserted in an absorber aperture. This single metallic slit geometry supports resonant modes when microwaves are incident with their polarization (E-field) perpendicular to the slit. The structure gives a set of Fabry-Perot-like resonant transmission frequencies. These frequencies move when a voltage is applied between the two plates, the liquid crystal being first aligned homogeneously, then realigning homeotropically with the applied field. By minotoring these changes a fast and easy to use procedure for determining the permittivity and its anisotropy for nematic liquid crystals in the microwave region has been developed. The parameters determined for E7 are εe = 3.17 (ne = 1.78 ± 0.01) and εo = 2.72 (no = 1.65 ± 0.01), (Δn ≈ 0.13) in the 40.0–60.0 GHz region.

Determination of the microwave permittivities of nematic liquid crystals using a single-metallic slit technique

A technique is presented for quantifying the microwave permittivities of small quantities of material. Here a 75 μm gap between two 3 mm thick aluminum plates is filled with a nematic liquid crystal. This filled slit is inserted in an absorber aperture. A set of transmission peaks are recorded as a function of incident wavelength for microwaves polarized with their electric field perpendicular to the slit direction. When a voltage is applied between the two plates, the liquid crystal realigns and the shift of the resonant peaks gives the anisotropic permittivities of the nematic material in the microwave region.

Microwave liquid-crystal variable phase grating

A voltage-controlled variable phase grating, at microwave frequencies, is described and its response characterized. It comprises a stack of 71 aluminium strips of 1mm thickness separated by 75μm spaces, filled with aligned nematic liquid crystal. For microwaves polarized normal to the grating strips there are a set of resonant transmitted frequencies. By varying the voltages applied across the liquid crystal layers and their distribution, a variable phase microwave grating is realized. This allows low-voltage control of output beam profile and intensity.

The study of the optical properties of obliquely evaporated nickel films using IR surface plasmons

Abstract The anisotropic optical properties of obliquely evaporated nickel films were deduced from observations of the surface plasmon resonance at 3.391 microm wavelength. Using the attenuated total reflection technique the radiation was coupled through an air-gap to nickel films deposited at different angles of condensation. For each nickel film, experimental data were recorded in the form of reflectivity against angle of incidence for two orthogonal geometries. For one, the direction of condensation lies in the plane of incidence, and for the other, it lies in the plane perpendicular to the plane of incidence. Fitting the recorded data to the predictions of Fresnel theory with an anisotropic permittivity tensor allowed the evaluation of this tensor. Then, using the determined tensor with Bruggemans effective medium theory, the surface profile, depolarization factor and volume composition were deduced. The results obtained are discussed in the light of our knowledge of such a system.

Determination of the Optical Constants and Thickness of a Highly Absorbing Film Using the Attenuated Total Reflection Technique

Abstract Theoretical analysis of the Kretschmann configuration for attenuated total reflection experiments shows that near the critical angle the reflectivity is strongly dependent upon the dielectric constants and thickness of a thin absorbing layer on the prism surface. Numerical calculations based on Fresnels equations illustrate this clearly for thin absorbing films. Using this property of highly absorbing films it has then been possible to determine the optical constants of a thin film of phthalocyanine over the visible region of the spectrum.