Stephen T. Kowel

Focusing by electrical modulation of refraction in a liquid crystal cell

The creation of a field-controlled variation of the index of refraction in a liquid crystal cell has been analyzed and experimentally verified. To obtain a spherical lens utilizing a simple electrode structure and capable of focusing arbitrary incoming polarizations requires four flat nematic liquid crystal cells. With electrodes fabricated well within the current capability of photolithography, near diffraction-limited performance in terms of the optical transfer function is predicted. The focusing capability of a liquid crystal lens was demonstrated using a single cell with linear transparent electrodes. A plano-convex cylindrical lens for a single incoming polarization was thus created. While the cell had a crude electrode structure, it affirmed all the major qualitative predictions. The fringing along the edge of the electrodes required for eventually obtaining near diffraction-limited performance was observed.

High-resolution liquid-crystal phase grating formed by fringing fields from interdigitated electrodes.

We report the formation of thin anisotropic phase gratings in a nematic liquid-crystalline film by use of lateral (fringing) electric fields induced by transparent interdigitated electrodes. These gratings yield high diffraction efficiency (>30%) with a strong dependence on the readout beam incidence angle. In addition, the formation of a defect wall is observed that has a significant effect on the diffraction properties of the phase grating.

LANGMUIR-BLODGETT MULTILAYERS OF POLYMER-MEROCYANINE-DYE MIXTURES

Deposition studies of mixed monolayers of poly(methylmethacrylate) (PMMA) and merocyanine chromophore (M22) were carried out by the Langmuir-Blodgett technique. The equilibrium surface pressure vs. specific area of the monolayers at the air-water interface was measured for several mixtures. The specific area vs. film composition at a fixed surface pressure deviated from linear behavior. The deposition of the monolayer onto a glass substrate was Z type for mixtures with high PMMA content, while Y type was preferred for mixtures with high dye content. The former structure exhibited second harmonic generation and should be of considerable interest in exploring the non-linear optical effects in Langmuir-Blodgett films. When prepared under a class 100 modular laminar flow hood and observed with reflection microscopy, the films showed no evidence of aggregate formation, suggesting that the PMMA-M22 mixtures were miscible.

Enhanced second harmonic generation from multilayered langmuir/blodgett films of dye

Abstract The first measurements of second harmonic generation (SHG) with multilayer enhancement in non-alternating layer Langmuir/Blodgett films of a hemicyanine dye is reported. The first observation of SHG in layers of a homogeneous mixture of hemicyanine and poly (methyl) methacrylate is also reported.

Second-harmonic generation in Langmuir-Blodgett films of hemicyanine-poly(octadecyl methacrylate) and hemicyanine-behenic acid

Abstract Second-harmonic generation (SHG) is used to probe the structure and order in monolayer and multilayer Langmuir-Blodgett (LB) films of highly polarizable dye molecules deposited on glass. Optical measurements on LB monolayers of hemicyanine-poly(octadecyl methacrylate) (PODMA) and hemicyanine-behenic acid mixtures in conjuction with data from compression isotherms show that the hemicyanine-PODMA system is immiscible and that the hemicyanine-behenic acid system is miscible. Incomplete Y-type LB multilayers of 54 mol.% hemicyanine in PODMA exhibited an erratic increase, which was less than linear, in the SHG with the number of layers deposited on the upstroke. Multilayer films of hemicyanine interleaved with behenic acid showed a slightly greater than quadratic increase in SHG with the number of hemocyanine layers deposited.

Adaptive spherical lens

This paper reports the creation of a tandem liquid crystal cell arrangement capable of simulating the performance of a spherical lens. Electrical modulation of the index of refraction creates focusing behavior. The modulation is induced by a set of electrodes whose individual voltages are arranged to provide a cylindrical exit wave front for a uniform plane-wave input. Two such cells with orthogonal electrodes arranged in cascade create spherical lens performance. Experimental evidence using a lens with a relatively small number of electrodes is presented. Applications for such structures, once improved, would include real-time focus adjustment for optical disk readers and hand-held photographic equipment as well as aberration compensation for long path-length optical systems.

Use of Fabry-Pérot devices for the characterization of polymeric electro-optic films

Electro‐optic properties of thin polymeric films can be studied by incorporating the films into the spacer layers of transmission‐mode Fabry–Perot devices formed using metal mirrors. The metal mirrors, which also serve as electrodes, are used for the application of an electric field for poling the polymeric material and for subsequently modulating the index of refraction through the electro‐optic effect. A model for transmission modulation is developed, and experimental characterization of the devices by angles tuning at a fixed wavelength is presented. Suitable approximations can be made to allow determination of the net trapped charge and the electro‐optic coefficients of the spacer layer. This paper demonstrates the use of polymeric thin‐film etalon devices for the study of new materials and indicates their potential use as electro‐optic devices.

Future applications of ordered polymeric thin films

Abstract Polymeric ultrathin film systems need to be developed in the context of applications where their unique combinations of properties promise revolutionary improvements in performance or cost effectiveness. The applications examined include electron beam resists for microlithography and nanolithography, insulating films in semiconductor devices, non-linear optical elements and coatings for communications and computing, as well as highly permselective membranes in biotechnology. In this paper, we will review some of the most appealing suggestions and evaluate their current status. Improvements in film characterization and deposition based on Langmuir-Blodgett techniques are also suggested.

Quadratically enhanced second harmonic generation in polymer-dye Langmuir-Blodgett films: A new bilayer architecture

Abstract A new double bilayer ABCC dippind sequence for the Langmuir-Blodgett deposition process has resulted in a one hundred-fold increase in second harmonic signal generation for ten ABCC sequences compared to one ABCC sequence. The AB bilayers are comprised of hemicyanine side chains on a polyether backbone. Two different polymer-dye compositions are used in each bilayer in order to have dipoles pointing in the same direction upon “Y” type deposition. Hydrophilic-hydrophilic interactions between polymer-dye bilayers are prevented by interleaving with a CC behenic acid bilayer, which also provides interlayer local field insulation.

Second-order nonlinear optical properties of poled coumaromethacrylate copolymers

Second-order nonlinear optical properties of newly designed and synthesized coumaromethacrylate side-chain polymers are reported. The optimum poling conditions were determined experimentally. The optimum poling temperature for these side-chain polymers is well above the glass transition temperature. The second harmonic coefficient of films poled by coronaonset at elevated temperature and the linear electro-optic coefficient of films poled by contact electrodes were measured. The stabilized value of the second harmonic coefficient, d33, at 1064nm fundamental wavelength was found to be 13 pm/V. The linear electro-optic coefficient, r33, exhibits strong dispersion ranging from 2 to 12pm/V in the wavelength range 477 to 1115 nm.