Holographic behavior of a photopolymer at high thicknesses and high monomer concentrations: mechanism of photopolymerization

One of the main problems involved in the development of holography is that of the recording material used. In this study the behavior of a photopolymer is analyzed when thicknesses greater than 100 μm are used. The photopolymer is made up of acrylamide as monomer, triethanolamine as radical generator, yellowish eosin as sensitizer and polyvinyl alcohol as binder. The main problem is that an increase in thickness produces more scattering and consequently a decrease in the diffraction efficiency. In this study we analyzed the influence of the concentration of monomer on scattering and found that an increase in the concentration reduces scattering. The behavior of the material varies as the concentration of acrylamide is increased. Two polymerization mechanisms compete and so the behavior of the photopolymer is not linear, the reaction is slower and the expected maximum efficiency is not reached. It has been demonstrated experimentally that when the recording intensity is reduced the photopolymer behaves in a linear way. Received: 28 January 2000 / Revised version: 25 July 2000 / Published online: 13 December 2000     

Probing photo-induced alignment in poly(vinyl cinnamate) films by surface second-harmonic generation

Received: 20 September 1998 / Revised version: 18 December 1998     

Electric-field modulation measurements of complex electro-optic effects in poled polymer

Received: 10 July 1998/Revised version: 21 September 1998     

High photorefractive performance of low glass-transition-temperature polymeric composite doped with bifunctional chromophore

-1 and an index grating amplitude of about 4.3×10^-3 were obtained. Furthermore, high solubility of the dye in PVK and high stability of the photorefractive composite were also exhibited. Received: 7 January 1998/Revised version: 7 April 1998     

Three-dimensional reversible laser micromachining with subnanojoule femtosecond pulses based on two-photon photochromism

Three-dimensional reversible laser micromachining of polymer materials based on two-photon photochromism is demonstrated. Unamplified 60-fs, 0.5-nJ pulses of 790-nm Ti:sapphire laser radiation are used to induce a refractive-index change in a polymethyl methacrylate (PMMA) sample doped with spiropyran molecules through a two-photon absorption process. Waveguides are written in the bulk of spiropyran-doped PMMA samples by scanning these samples with respect to a tightly focused Ti:sapphire laser beam. Laser-induced fluorescence is used for on-line monitoring of the laser-micromachining process. The structures written in photochromic samples can be erased and reconfigured due to the reversibility of the photochromic effect. Received: 23 April 2003 / Published online: 6 June 2003 RID="*" ID="*"Corresponding author. Fax: +7-095/939-51-74, E-mail: zheltikov@top.phys.msu.su     

Optically controlled image storage in azobenzene liquid-crystalline polymer films

We report on optically controlled image storage based on photo-induced alignment in azobenzene liquid-crystalline polymer films. Measurements reveal that the polymer film possesses photo-induced birefringence of large magnitude and the characteristic of long-term optical storage. The photo-induced alignment process is used for storing images. The stored image is read from the film placed between two crossed polarizers. It is demonstrated clearly that the alignment of the azo chromophores as well as the stored image can be easily controlled by choosing the appropriate polarization of the irradiating light or by rotating the film. Received: 20 October 1998 / Revised version: 7 December 1998 / Published online: 31 March 1999     

Laser performance of pyrromethene 567 dye in solid matrices of methyl methacrylate with different comonomers

Laser performance of pyrromethene 567 (PM567) dye dissolved in pure poly(methyl methacrylate) homopolymer and its copolymers with 2-hydroxyethyl methacrylate, 2-hydroxyethyl acrylate, 1-vinyl-2-pyrrolidone, 2-phenoxyethyl acrylate, and 2,2,2-trifluoromethyl methacrylate has been investigated. The chosen monomers were selected to mimic solvents used in a previous study on the photophysical and lasing properties of PM567 in liquid solution in order to establish correlations between the lasing properties of PM567 in liquid and solid solutions. The vol/vol proportion of the different comonomers in each copolymer formulation was systematically varied, and the effect of each composition on the lasing properties of PM567 was evaluated. The laser samples were transversely pumped at 534 nm with 5.5-mJ pulses from a frequency-doubled Q-switched Nd:KGW laser. Lasing efficiencies of up to 30% and good photostabilities, with a drop of the initial laser output of 20% after 10000 pump pulses in some of the solid samples, were demonstrated. Received: 2 June 1999 / Revised version: 18 July 1999 / Published online: 3 November 1999     

Transmission behavior in a photorefractive polymer film

When a p-polarized beam propagates through a high-performance photorefractive polymer composite, poly dimethyl-4-(4^′-nitrophenylazo)benzene, its transmission behavior is influenced by three effects: the electroabsorption, the photorefractive coupling with the reflected beam from the rear surface, and the amplified scattering. From the measurements on the incidence angle dependence as well as the applied-electric-field dependence of the three effects, some conclusions are obtained. At a small incidence angle with a low applied electric field, both the amplified scattering and the electroabsorption are small whereas the coupling between the incident beam and the reflected beam plays a principal role. At a large incidence angle or with a high poling electric field, the transmission is influenced mainly by the amplified scattering and the electroabsorption. A poling electric field asymmetric loss to the amplification scattering is also observed. Received: 18 November 1998 / Revised version: 10 February 1999 / Published online: 12 April 1999     

Optical poling in a crosslinkable polymer system

Received: 30 September 1998 / Published online: 24 February 1999     

Holographic study of chain length in photopolymerizable compositions

We describe a study of the free-radical homopolymerization kinetics for a system based on acrylamide, triethanolamine and methylene blue by analyzing temporal variations of the diffraction efficiency. The modulation index is related to the component’s parameters, giving as a result a method that can be used to determine the chain length of the polymer and the kinetic rate constants. By using this method we have studied the effect of intensity on the kinetic parameters of the photopolymerization process. Received: 20 August 2001 / Published online: 7 February 2002     

Fabrication of three-dimensional periodic microstructures by means of two-photon polymerization

Received: 7 October 1998     

Incoherent-to-coherent optical conversion by means of azobenzene liquid-crystalline films

Received: 20 July 1998/Revised version: 21 September 1998     

Optical study of dye-containing fluorinated polyimide thin films

Thin films of dye-containing fluorinated polyimide have been obtained by adding the dye powder to the polyamic acid resin and by spin coating the resulting solution on silica and silicon substrates. 6FDA (4,4’-hexafluoroisopropylidene diphthalic anhydride) and DAB (diaminobenzophenone) have been used as precursor monomers and rhodamine B as dye. The influence of the rhodamine-B molecule on the completeness of the imidization process has been studied by coupled thermogravimetric and mass-spectrometric analyses (TG-MS) of pure and doped polyamic acid resin and by FT-IR analysis of samples before and after curing. Optical emission, excitation and absorption spectra have been collected in order to study spectroscopic and aggregation characteristics of rhodamine as a function of the deposition parameters. Received: 2 March 2000 / Accepted: 24 July 2000 / Published online: 9 November 2000     

Photophysical and lasing properties of pyrromethene 567 dye in solid poly(trifluoromethyl methacrylate) matrices with different degrees of crosslinking

The effect of increasing the rigidity of the host matrix by controlled crosslinking of the polymeric chains on the lasing and photophysical properties of pyrromethene 567 (PM567) was evaluated. To this end, the dye was dissolved in 2,2,2-trifluoromethyl methacrylate (TFMA) together with different amounts of the crosslinking monomer ethylene glycol dimethacrylate (EGDMA). Increasing the concentration of this crosslinker monomer resulted in significant increases in both lasing efficiency and photostability. Good correlation between fluorescence and lasing properties has been observed, indicating the existence of an optimum degree of crosslinking for which lasing efficiency and photostabilty reach their highest values. These new solid materials show lasing efficiencies up to 18% when transversely pumped at 534 nm and good photostabilities, with a decrease in the initial laser output of 45% after 10000 pump pulses. Received: 16 February 2001 / Revised version: 9 April 2001 / Published online: 8 June 2001     

Orientational photorefractive effects observed in poly(vinyl alcohol)/liquid crystal composites

60 ) as a photoconductive sensitizer under an applied dc field. Orientational photorefractivity was demonstrated by observing Raman–Nath diffraction beams with an external dc field. The photorefractive gratings were partially memorized even in the absence of the applied dc field. Received: 12 January 1998/Revised version: 2 February 1998     

New photopolymerizable holographic recording material based on polyvinylalcohol and 2-hydroxiethylmethacrylate (HEMA)

We have developed a new polyvinylalcohol-based photopolymeric holographic recording material. The recording is obtained by the copolymerization of acrylamide and 2-hydroxiethylmethacrylate (HEMA). Diffraction efficiencies near 70% are obtained with exposures of ≈65 mJ/cm² in materials of 110-μm thickness. The addition of HEMA increases the thickness of the film, which results in improved storage capacity for holographic optical storage. Received: 7 March 2002 / Published online: 2 May 2002     

Development and characterization of Yb-Er laser glass for high average power laser diode pumping

A new strong erbium laser glass (SELG) based on a boro-alumo-phosphate composition is reported. We discuss the synthesis and chemical properties together with spectroscopic and thermo-mechanical data. The new glass composition shows excellent laser performance and withstands high-average power pump radiation. We present laser results at 1.54 μm from flashlamp and laser pumping. In tests with laser-diode pumped Q-switched Er-Yb microchip lasers, we have achieved up to 150 mW of average output power and generated 1.2 kW in peak power. Co²⁺:MgAl₂O₄ was here used as the saturable absorber. Received: 21 December 2001 / Revised version: 14 April 2002 / Published online: 8 August 2002     

Saturation effect in azobenzene polymers used for polarization holography

The saturation behavior of diffraction efficiency has been experimentally studied and theoretically modeled in azobenzene polyesters by recording polarization gratings with two beams of orthogonal circular polarization. The model is an extension of the phenomenological expression of Kakichashvili, derived from the rate equations for photoanisotropic materials, taking into account the effect of saturation with time and intensity. The saturation model of the polyester material can be used for the computer simulation of a polarization holographic data storage system under development. The simulation is based on fast Fourier transforms, taking the parameters of the recording material and the optical processes into account. Agreement between measured and simulated results proves the applicability of the saturation model as an efficient tool in the optimization of the system. Received: 16 September 2002 / Published online: 3 April 2003 RID="*" ID="*"Corresponding author. Fax: +36-1/463-4194, E-mail: pvarhe@math.bme.hu     

Photoinduced macroscopic chiral structures in a series of azobenzene copolyesters

A study of the propagation of elliptically polarized light and the resulting formation of macroscopic chiral structures in a series of azobenzene side-chain copolyesters, in which the morphology is varied from liquid crystalline to amorphous, is reported. Real-time measurements are presented, showing the dynamic behavior of the photoinduced rotation of the polarization ellipse in the different samples. The relationship between the ellipticity of the recording light and the linear birefringence induced is studied. A numerical solution that takes into account the influence of the photoinduced linear dichroism on the light propagation through the samples is presented. Received: 10 June 2002 / Revised version: 26 July 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +45-4677/4588, E-mail: lian.nedelchev@risoe.dk RID="**" ID="**"Permanent address: Department of Chemistry and Physics, The Nottingham Trent Unversity, Clifton Lane, Nottingham NG11 8NS, England