Amir Tork

Polymer-stabilized liquid crystal for tunable microlens applications

We investigate the electro-optical properties of polymer stabilized nematic liquid crystals produced by in situ photopolymerization technique using Gaussian laser beam. The distribution of refractive index in such structure under the action of a homogeneous electric field reveals a non-homogeneous lens-like character, approximately reproducing the intensity transverse distribution in the photopolymerizing beam.

Photochromic behavior of spiropyran in polymer matrices

The photoexcitation, relaxation, and optical erasure regimes of spiropyran- (SP-) doped polymer films were studied. Cellulose acetate, poly(vinyl acetate), and poly(methyl methacrylate) (PMMA) were used as host polymer matrices. We studied the character of the photoreaction for both coloring and bleaching processes. Reversible holographic recording in SP-PMMA films and the origin of the photochemical fatigue was studied upon repeated UV-visible irradiation cycles.

Self-written gradient double claddlike optical guiding channels of high stability

Composite photopolymerizable material and self-writing method are developed, allowing the creation of very stable (in time) gradient double-clad optical waveguides with W form of refractive index profile. The guide is formed by the mutual counter diffusion of reactive and nonreactive molecules followed by photopolymerization. After the guide formation by a Gaussian intensity shaped beam, the mixture is simply fixed by uniform irradiation without additional processing. More than 2cm length permanent self-guiding channels are created. The obtained results could be useful for “optical bonding” of dissimilar optical devices.

Photochromic materials for holographic data storage

Photochromism and real-time holographic recording were characterized for two principal classes of photochromic compounds: spiropyrans and fulgides. For spiropyran molecules, important thickness and writing intensity effects were observed. Concerning fulgide, in PMMA matrix, the closed form presents a maximum of absorption centered at 525 nm upon irradiation at 365 nm. We have determined the photoreaction rate constants kUV and kvis respectively for the coloring and bleaching process: kUV equals 1.2 X 10-3 s-1 and kVIS equals 11.1 X 10-3 s-1. Photochemical fatigue resistance in different polymer matrices was investigated. We found a loss of 9, 11, 13 and 35 percent respectively in PS, CA, PMMA and PVK. Concerning holographic recording, we obtained diffraction efficiency (eta) equals 0.65 percent in PMMA films 30 micrometers thick.

Characterization of dye-doped PMMA, CA, and PS films as recording materials

The photoinduced reversible color change and in-situ recording of fulgide Aberchrome 670 doped polymethyl methacrylate (PMMA), cellulose acetate (CA) and polystyrene (PS) were investigated. Upon UV and visible exposure, closed-form absorbency followed first-order kinetic. The rate constants KUV and KVIS for respectively the coloring and bleaching process were determined. In PMMA matrix KUV equals 1.2 * 10-3 s-1 and KVIS equals 11.1 8 10-3 s-1, in CA matrix kUV equals 2.7 * 10-3 s-1 and kVIS equals 6.4 8 10-3 s-1 and in the case of PS film kUV equals 2.1 * 10-3 s-1 and kVIS equals 11.9 * 10-3 s-1 were obtained. These results show that, KVIS is much larger than KUV for all matrices. Photochemical fatigue resistance in different polymer matrices was investigated. We found a loss of 9, 11 and 13 percent in PS, CA and PMMA respectively, after 10 repeated UV and visible cycles. The real time holographic recording in fulgide doped PMMA films were studied. We have analyzed the effect of the photochromic concentration, the thickness of the film and the recording intensity on the diffraction efficiency. The highest diffraction efficiency is obtained for the concentration of 5 percent of the fulgide dye in PMMA film with an exposure energy of 10 mw/cm2. For the same sample we have not observed any diffraction beam when the sample was illuminated by an intensity of 3 mw/cm2.

Matrix effects on the photochemical fatigue resistance of fulgide-doped polymers

Fulgide Aberchrome 670-doped polymer films are studied. The closed form of this chromophore presents a maximum of absorption centered at about 525 nm and is formed upon irradiation of its colorless open form at 365 nm. We have determined the photoreaction rate constants, kUV and kVIS, respectively, for the coloring and bleaching processes. It was found that photochemical fatigue of fulgide doped in the mixture of a cyanoacrylate adhesive with 10% of dioctoepoxy monomer is essentially absent after 40 UV/visible irradiation cycles. We thus report, for the first time, a simple and rapid preparation of dye-doped polymer films in a solvent-free process that shows significant improvements in fatigue behavior. Our previous investigation of conventional polymer matrices prepared by gravity deposition of dilute dye-polymer solutions found losses of 9, 11, 13, and 35% after only 8 UV/visible irradiation cycles, respectively, in polystyrene (PS), cellulose acetate (CA), poly(methyl methacrylate) (PMMA), and polyvinyl carbazole (PVK).

xPs and RBS Analysis of the Composition and Structure of Barium Titanate Thin Films to be Used in DRAMs

We report the synthesis of polycrystalline barium titanate BaTiO3 thin films on silicon substrate prepared by sol- gel technique. The structure and composition of ferroelectric film ms have been analyzed by using x-ray photoelectron spectroscopy and Rutherford backscattering spectroscopy. These methods provide information on the core level electronic states and the film composition. The results indicate the films have no distinctive deviation from the stoichiometry within the accuracy of both spectroscopic methods. Thus, this study of surface structure and composition of ferroelectric films could lead to a better understanding of the observed change of electrical and optical properties and to the design of improved devices such as dynamic random access memories.

Characterization of the fulgide-doped PMMA films and investigation of photochromic reaction of Langmuir-Blodgett films as recording materials

Photochemical characterization and holographic recording of fulgide Aberchrome 670 and 540-doped polymethyl methacrylate (PMMA) were investigated. Upon UV and visible exposure, closed-form absorbency followed first-order kinetic. The real time holographic recording in fulgides doped PMMA films were studied. The effect of dye concentration, thickness of the film and the recording intensity on diffraction efficiency was reported. We used the Langmuir-Blodgett (LB) technique in order to transferring a compact multilayer of fulgide spread on water surface between two thin films of cellulose acetate (CA). The preliminary results of the surface pressure-area isotherms obtained by LB show the transfer of the fulgide between two CA thin films. Finally, the photochromic reaction of fulgide in LB films was investigated.

Fulgide-doped photopolymers as reversible storage media

The photoinduced reversible color change and holographic recording in Fulgide (Aberchrome 670) doped photopolymers was investigated. The rate constants KUV and KVIS for respectively the coloring and bleaching process, photochemical fatigue resistance and real time holographic recording in fulgide doped photopolymer film was investigated.

In-fiber variable optical attenuation with ultra-low electrical power consumption

The variable optical attenuator (VOA) is an important part of agile optical telecommunication systems. VOAs built on so called free space technologies carry important drawbacks in terms of mechanical reliability, size and optical loss. Evanescent field approach have been used to design VOAs with very low insertion loss. Thermo-optic modulation mechanism was mainly used to control the attenuation level, which unfortunately requires from 10 to 100 times more electrical power compared to above mentioned free space architectures. This power consumption issue may be very challenging in high count arrays of VOAs. At the same time, liquid crystals (LC) have been proved to require very low electrical power for operation. In the present work, we report the creation of evanescent field modulation based VOA with extremely low insertion loss (below 0.1dB) and low electrical power consumption by removing a portion of the original fibers cladding and replacing it by a specifically synthesized composite LC material, which have an ordinary refractive index lower than the glass one. The initial orientation of LC molecules provides an effective refractive index of the electro-optic cladding that is equal to the refractive index of the original silica cladding. We then create a LC molecule reorientation by the so-called Fredericksz effect by applying to the LC material an electrical voltage. This reorientation changes the refractive index around the depressed cladding area and brings to the partial leakage of the guided radiation into the cladding area, achieving thus attenuation levels above 50dB. Measured maximum electrical power consumption of the VOA is in the microwatt range.