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     

Properties of CdF2:Ga as a medium for real-time holography

The metastable nature of the excited state of bistable Ga centers in semiconducting CdF₂ crystals allows for the use of CdF₂:Ga and CdF₂:Ga,Y crystals as materials for real-time holography over a wide range of response times (1–1000 ms). The characteristics of these materials and optimal conditions for their use are discussed in the framework of a model that describes the decay of photo-induced gratings written in them. Received: 20 November 2000 / Published online: 20 April 2001     

Determination of proton diffusion anisotropy by thermal decay of fixed holograms with K-vector perpendicular to the c-axis in LiNbO3:Fe

In this paper the thermal fixing of holographic gratings with K-vectors perpendicular to the crystallographic c-axis of LiNbO₃ is considered in order to obtain information about anisotropy of the proton thermal diffusion in this crystal. Specifically, thermal decays of fixed holograms in particular crystallographic directions are measured and related with proton diffusion. The values obtained are compared with previous data of decays of fixed holograms with K-vector parallel to the c-axis. The results show a high anisotropy of the thermal diffusion of protons in lithium niobate crystals.     

Photoinduced anisotropy in an azo-containing ionic liquid-crystalline polymer

Photoinduced anisotropy in an azobenzene ionic liquid-crystalline polymer was investigated through dichroism, birefringence and polarization holography. A dichroism degree of 1.58 and a birefringence value Δn ∼ 10⁻² were achieved in the polymer film at room temperature, and the polymer film was found to possess the characteristics of reversible and long-term optical storage. Particularly the stored birefringence could be enhanced to Δn ∼ 10⁻¹ by annealing the film, and it is attributed to the thermal self-organization of the molecules. Furthermore, linear- and circular-polarization holographic recordings were accomplished in the polymer film and pure polarization gratings were produced.     

Polarized-light-controlled holographic recording in an azobenzene-doped polymer film

A new polarized-light-controlled holographic recording based on the optical enhancement/restraint of self-diffraction has been demonstrated in an azobenzene-doped polymer film. It is found that a continuous variation of the polarization status as well as of the intensity of the pumping light results in a continuous variation of self-diffraction efficiency. The mechanism originates from the photo-induced anisotropy and polarization-dependent absorption. Both positive and negative replicas of an incident image were presented in real time by means of this incoherent–coherent optical conversion technique. Received: 30 March 2000 / Revised version: 28 September 2000 / Published online: 27 April 2001     

Kinetic characterization of photoinduced anisotropy in diarylethene film

Based on the theoretical model we have proposed, a complete study on the kinetics of photoinduced anisotropy in diarylethene films is performed. The kinetic curves of molecular concentration, photoinduced dichroism and birefringence are calculated, respectively. It is found that the colored molecular concentration decreases with the increase of the excitation exposure until saturation, and the photoinduced anisotropy increases to a maximum and then decreases gradually. The optimal exposure is 260 J/cm². In addition, the transmittance of probe beam reflecting the anisotropy is measured by experiment. The theoretical results are compared with experimental data, and basic concordance is found between both sets of data.     

Photofabrication of periodic microstructures in azodye-doped polymers by interference of laser beams

Volume holographic gratings and two-dimensional periodic microstructures in azodye-doped polymethylmethacrylate were fabricated, respectively, by interference of two coherent beams of a femtosecond laser and by interference of three coherent beams of a nanosecond laser. The dependence of the first-order Bragg diffraction efficiency and the photoinduced refractive-index modulation of the gratings on the intensity of the writing light was investigated. The measurements of the absorption spectra before and after irradiation with the writing light suggest that the photoinduced gratings were refractive-index-modulated gratings, which arose from a photoinduced decomposition reaction of the azodye molecules through multiphoton absorption. In the experiments involving the interference of three beams, the period of the two-dimensional periodic microstructures was changed by adjusting the angle between the three writing beams. Received: 10 July 2002 / Revised version: 5 September 2002 / Published online: 20 December 2002 RID="*" ID="*"Corresponding author. Fax: +81-774/955206, E-mail: jhsi@photon.jst.go.jp     

Holographic recording mechanisms of gratings in indium oxide films using 325 nm helium–cadmium laser irradiation

UV (325 nm) holographic recording of gratings in indium oxide films fabricated by reactive pulsed laser deposition has been investigated as a function of growth temperature, oxygen pressure and angle of incidence of the plasma plume on the substrate. The influence of the ambient environment (air or vacuum) and the film temperature during recording has also been studied. Large steady state refractive index changes up to 6×10^-3 were observed in layers grown at an oblique angle of 75°. About 77% of the magnitude of these changes residues after thermal annealing and is attributed to UV-induced permanent structural rearrangements. In contrast, refractive index changes in films grown at normal incidence were smaller in magnitude and completely reversible. Received: 30 July 2001 / Accepted: 20 November 2001 / Published online: 23 January 2002     

Scattering Noise Properties of Holographic Photopolymers by Means of Real-Time Transmittance Analysis

We propose a simplified model to analyse the temporary behaviour of transmittance for holographic photopolyreefs, based on the existing first-harmonic diffusion theory, with assumptions of constant polymerization coefficient and negligible diffusion effect for noise gratings. By applying this model to measure real-fime transmittance of our novel blue-sensitized photopolymer, the bleaching time constant of dye and polymerization rate are extracted, and the effect of the material composite on the noise property is revealed. This provides a prompt method to assess the performances of holographic photopolymers.     

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     

Estimation of trap concentration in photorefractive crystals by a technique of adaptive holographic interferometry

12 GeO₂₀, Bi₁₂SiO₂₀, Bi₁₂TiO₂₀). Received: 17 June 1998/Revised version: 3 August 1998     

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

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

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     

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     

Holographic recording in amorphous side-chain polymers: a comparison of two different design philosophies

Recently, Bieringer et al. [1] reported on the optical properties of liquid-crystalline side-chain polymers. These photoaddressable systems showed light-induced modulations Δn of their refractive index of up to 0.06. We present similar materials and compare them with a novel ansatz reaching birefringence levels of up to 0.2 [2]. This was achieved by donor–acceptor substitutions of the chromophores and by introducing a new type of second side chain. We discuss the different photophysical properties of the two materials under the point of view of potential applications. Received: 16 November 1998 / Revised version: 18 December 1998 / Published online: 7 April 1999     

Optical holographic memory using angular-rotationally phase-coded multiplexing in a LiNbO3:Fe crystal

We propose a new holographic memory scheme based on random-phase-coded multiplexing in a photorefractive LiNbO₃:Fe crystal. Experimental results show that rotating a diffuser placed as a random-phase modulator in the path of the reference beam provides a simple yet effective method of increasing the holographic storage capabilities of the crystal. Combining this rotational multiplexing with angular multiplexing offers further data-storage possibilities. The advantage of using post-image versus pre-image phase modulation of the object beam is demonstrated. Received: 15 August 2000 / Published online: 10 January 2001     

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     

Electrooptic properties of lithium niobate crystals for extremely high external electric fields

The electrooptic effect in lithium niobate crystals (LiNbO₃) for extremely high externally applied electric fields of up to 65 kV/mm is investigated. Homogeneous electrooptic refractive-index changes of up to 4.8×10^-3 are found for ordinarily polarized light. No quadratic electrooptic effect is observed. An upper bound for the quadratic electrooptic coefficient of |s₁₃|≤2.3×10^-21 m²/V² is determined. Electrooptic, angular, and wavelength tuning of the Bragg condition of a thermally fixed hologram are demonstrated. Received: 29 October 2002 / Revised version: 14 January 2003 / Published online: 26 March 2003 RID="*" ID="*"Corresponding author. E-mail: ml@uni-bonn.de