Photorefractive Materials Research Articles

Special Issue: “Photorefractive Materials, Effects, and Devices: Control of Light and Matter”

This forthcoming special issue of Applied Physics B - Lasers and Optics, a leading international journal, focuses on photorefractive materials, effects, and devices where light modifies material properties and/or vice versa. The feature is intended to highlight the latest achievements in this field and to stimulate research in this important area of optical physics. It will be published in June 2009. Accepted articles qualify automatically for presentation at the Topical Meeting „PhotorefractiveMaterials, Effects, andDevices: Control of Light andMatter“ that will take place from June 11-14, 2009 in BadHonnef, Germany (http://www.pr09.de).

Application of interference microscopy to the study of hologram build-up in LiNbO3 crystals

Interference microscopy was applied to direct microscopic observation of temporal evolution of phase holograms in LiNbO3:Fe photorefractive crystals. First a hologram was recorded in the sample, and diffraction efficiency was monitored during hologram build-up using inactinic laser light. Thus kinetics of hologram build-up could be determined. The initial hologram was erased using white light. Then a series of write-erase cycles were performed with increasing exposure times. Holograms were observed by interference microscope after each exposure. The time elapsed between the exposure and the microscopic observation was negligible compared to the relaxation time of the hologram. The obtained temporal evolution of the grating profile gives a deeper insight into the physical mechanism of hologram formation in photorefractive materials than simple diffraction efficiency measurements. A congruently grown sample of LiNbO3 doped with 10−3mol/mol Fe in melting was studied by this method. Sample thickness was set to 300μm to allow correct microscopic observation. Plane-wave holograms were recorded in the samples using an Ar–ion laser at λ=488.0nm of grating constants of 3, 6.5 and 8.8μm.

Intrinsic optical bistability in Kerr ferroelectric materials

In this paper we investigate the intrinsic optical bistability (IOB) in a ferroelectric (FE) single layer using an alternative analysis. The FE material is considered to have an intensity-dependent refractive index where the third order nonlinear susceptibility χ(3) acts like Kerr coefficient. The nonlinear response of the FE medium is modeled using the Landau-Khalatnikov (LK) dynamical equation with the nonlinear anharmonic potential obtained from the Landau-Devonshire free energy expressed in terms of polarization. Within a single frequency approximation, the electromagnetic wave equation is written in terms of the polarization P rather than the electric field E as the dependent variable. With the application of the nonlinear boundary conditions we have derived expressions for both reflectance and transmittance as a function of the electric field incident amplitude, polarization and other material parameters. The formalism proves to be more suitable for FE materials since most of these materials have highly linear and nonlinear coefficients especially when the operating frequency is in the resonance region. The effects of thickness, operating frequency and temperature on BaTiO3 single film are investigated theoretically. The results presented here agree in principle with the recent experimental observations of intrinsic OB in BaTiO3 monocrystal and other FE photorefractive materials.

Simulation of micro/nanometer-scale self-organized lightwave network (SOLNET) using the finite difference time domain method

A simulator for self-organized lightwave network (SOLNET) is developed. The simulator is based on the finite difference time domain method. SOLNET enables us to construct self-aligned coupling waveguides between misaligned micro/nanometer-scale optical devices by self-focusing, which arises from an increase in refractive index induced by write beams in photo-refractive materials. The SOLNET simulator reveals that an L-shaped nanometer-scale optical waveguide of SOLNET is grown from a 0.5-μm-wide optical waveguide when write beams are introduced from the optical waveguide into photo-refractive materials, where a wavelength filter is embedded. The SOLNET simulator also reproduces coupling path construction between a 2-μm-wide optical waveguide and a 0.5-μm-wide optical waveguide having a wavelength filter on the core edge. The optical waveguides are placed with misalignment of 0.5-μm offset. By introducing write beams from the 2-μm-wide optical waveguide, the incident write beams and reflected write beams from the wavelength filter merge into one optical waveguide in the photo-refractive materials, constructing a self-aligned coupling waveguide.

The coupling and superposition behavior of higher-order diffraction image based on photorefractive nematic liquid crystal materials

We report some photorefractive higher-order diffraction phenomena of two kinds of photorefractive nematic liquid crystal materials. In the experiment, we observed the superposition of higher-order diffraction images, the coupling of higher-order diffraction lights, and the coupling between higher-order diffraction lights and incident beams. The observed behavior suggests the complexity of the coupling process. The properties of higher-order diffraction images are discussed theoretically, which are in good agreement with experimental results.

Pit Distribution Design for Computer-Generated Waveguide Holography

Multilayered waveguide holography (MWH) is one of a number of page-oriented data multiplexing holographies that will be applied to optical data storage and three-dimensional (3D) moving images. While conventional volumetric holography using photopolymer or photorefractive materials requires page-by-page light exposure for recording, MWH media can be made by employing stamping and laminating technologies that are suitable for mass production. This makes devising an economical mastering technique for replicating holograms a key issue. In this paper, we discuss an approach to pit distribution design that enables us to replace expensive electron beam mastering with economical laser beam mastering. We propose an algorithm that avoids the overlapping of even comparatively large adjacent pits when we employ laser beam mastering. We also compensate for the angular dependence of the diffraction power, which strongly depends on pit shape, by introducing an enhancement profile so that a diffracted image has uniform intensity.

Grey photovoltaic solitons in two-photon photorefractive materials under the open-circuit case

In steady-state conditions, we predicted that grey photovoltaic solitons of the optical wave evolution equation could exist in two-photon photorefractive materials under the open-circuit case. The characteristics of the grey photovoltaic soliton, such as the intensity profile, the intensity full width at half-maximum, the transverse velocity and the phase profiles of these grey photovoltaic solitons are discussed in detail.

MULTI-WAVELENGTHS OPTICAL SWITCHING AND TUNABLE FILTERS USING DYNAMIC SUPERIMPOSED PHOTOREFRACTIVE BRAGG GRATING

We present a new scheme for all optical multi-wavelengths switching and filtering using photorefractive materials to route optical signals without converting to electronic state. For this purpose the photorefractive effect which is a nonlinear optical effect seen in certain crystals and other materials that respond to light by altering their refractive index is used. When a photorefractive material is illuminated by patterned command light of intensity I(x), a dynamic superimposed Bragg grating can be obtained which is used for optical multi-wavelength switching and filtering purposes.

Nanoparticle-Based Photorefractive Polymers

Photorefractivity has attracted intense attention owing to its ability to spatially modulate the refractive index under non-uniform light illumination. In particular, photorefractive polymers are appealing materials as they enable the high non-linear performance that underpins many areas of photonics. The incorporation of nanoparticles into photorefractive polymers shows an enormous potential owing to the broad spectroscopic tuning range and the high photogeneration efficiency, which are inaccessible to traditional photorefractive materials. This article reviews the recent developments in the field of nanoparticle-doped photorefractive polymers. The merit and functionality of these hybrid materials are summarized and future challenges are discussed. The application of nanoparticle-doped photorefractive polymers under two-photon excitation is also described, which facilitates a promising new area of high-density optical data storage, the third-generation of optical data storage.

Synthesis and photorefractivity of poly[methyl-3-(7-dibenzo[a,g]carbazolyl)-propylsiloxane

7H-Dibenzo[a,g]carbazole-substituted polysiloxane (PSX-[a,g]BCz) has been synthesized by hexachloroplatinate (IV) hydrate polymerization from poly(methylhydrosiloxane) and 7-ally-7H-dibenzo[a,g]carbazole. PSX-[a,g]BCz composite showed large orientational birefringences because of both large dipole moments and high-polarizability anisotropies of P-IP-DC chromophore associated with the effective conjugation along the polyene. The 50-μm thick photorefractive material containing 30 wt % 2-[3-[(E)-2(piperidino)-1-ethenyl]-5,5-dimethyl]-2-cyclohexenyliden]malononitrile showed a diffraction efficiency of 51% at 55 V/μm, which corresponded to a Δn of 3.45 × 10−3. PSX-[a,g]BCz composite shows a fast time constant of 0.42 s at 34 °C and 55 V/μm, which corresponded to the space-charge field of 12 V/μm under 70 V/μm. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1783–1791, 2008

Coupled Bright-Dark Soliton Pairs in Polymeric Photorefractive Materials

We have shown that under appropriate steady state conditions, two-component coupled bright-dark soliton pairs can be established in biased polymeric photorefractive materials. These bright-dark soliton pairs are possible if both carrier beams possess same wavelength and polarization. They are mutually incoherent and stable against small perturbation.

Synthesis and optical properties of benzocarbazole-substituted polysiloxanes for polymeric photorefractive materials

A novel polysiloxane having benzo[ a]carbazole and benzo[ a, i]carbazole pendant groups has been synthesized and characterized. We studied the electric-field-induced birefringence, the photoconductivity and the photorefractive (PR) properties of the polymeric PR composites. The PR composite showed a diffraction efficiency of 53% at 45 V/μm. A PR response time of 1.4 s at wavelength 633 nm was obtained, which was found to be higher than that of the carbazole-substituted polysiloxane composite.

The Effect of the Electro Optic Chromophore on the Space Charge Field Formation in Polymeric Photorefractive Composites

We investigated the effect of the electro-optic chromophore concentration on the space charge field formation in polymeric photorefractive composites. For polymeric photorefractive materials, the electro optic chromophore is one of the most substantial components, because it is to provide a refractive index modulation in response to the electric field and probably can act as trap which is essential part to promote space charge field inside the material. The photoconductivity was decreased as the chromophore ratio increased. However, the space charge field measured by our characterized method remains unchanged. This result can be explained that presence of chromophore may reduce the photoconductivity, but also facilitate the charge trapping process.

Synthesis, photoconductivity, and photogeneration efficiency of polysiloxanes bearing hole-conductors for photorefractive materials

Two series of polysiloxanes, namely, PSX-DCZ and PSX-TPD, with hole-transporting moieties have been synthesized and their electronic absorption, electrochemical behavior, quantum efficiency of charge generation, and photoconductivity is also studied. Our results show that the photocurrent were found to be ca. 3280 nA for PSX-DCZ/C 60 and ca. 3400 nA for the PSX-TPD/C 60 at wavelength of 633 nm and an electric field of 60 V/μm. We also obtained the quantum efficiency of charge generation as high as 25.2 × 10 −5 for PSX-DCZ/C 60 and 31.1 × 10 −5 for PSX-TPD/C 60 (at λ = 633 nm, E = 60 V/μm). The photoconductivity of these polymers is found to be greater than that of PVK/C 60.

Theory-inspired nano-engineering of photonic and electronic materials: Noncentrosymmetric charge-transfer electro-optic materials

Electronic and photonic properties of organic materials are well-known to depend on the intra- and intermolecular interaction of π-electron orbitals. We describe the systematic use of coupled quantum and statistical mechanical calculations to design materials with precisely controlled charge perturbation and charge transport (under the influence of applied electric fields) critical to the realization of desired photonic and electronic properties. Control of electro-optic activity is taken as an example because this phenomenon requires that acentric (dipolar or charge-transfer) molecules be organized in a lattice arrangement exhibiting finite noncentrosymmetric symmetry. Noncentrosymmetric organization of dipolar π-electron chromophores is also an arrangement of potential relevance to optimization of the performance of electronic, photovoltaic, and photorefractive materials. In our theoretical calculations, the roles of inter- and intramolecular electrostatic interactions and the effects of applied electrical and optical fields are explicitly taken into account. A variety of experimental tests of theoretical predictions are executed and a new class of materials is introduced and evaluated. Theoretically-inspired molecular and nanoscopic material design is shown to yield electro-optic coefficients ( r 33) at telecommunication wavelengths on the order of 500 pm/V (15 times greater than lithium niobate). The optimization of auxiliary properties including optical loss, thermal stability and conductivity is also discussed.

Photovoltaic spatial soliton pairs in two-photon photorefractive materials

We report the existence of incoherently coupled bright–bright steady state photovoltaic soliton pairs in two-photon photorefractive material under open circuit conditions. Based on WKBJ method and paraxial ray approximation, we have obtained coupled equations describing dynamical evolution of spatial soliton pairs. In the steady state regime, the present analysis leads to the identification of existence equation of bright–bright solitons, which captures a plethora of soliton pairs. We have undertaken linear stability analysis which shows that these solitons are stable.

Selective Erasure of Holograms in Photorefractive and Photopolymer Materials for Rewritable and Secure Data Storage

A selective erasure/update method of multiplexed photorefractive and photopolymer holograms by use of the double Mach–Zehnder (DMZ) interferometric arrangement is proposed. The DMZ arrangement is used as the optical phase-control device for holographic recording/erasure/update operations, which can produce a pair of π-phase-shifted interference patterns with a simple optical setup. We intend to erase the holograms recorded in not only the inherently rewritable photorefractive crystal but also the photopolymer material generally used for the write-once (i.e., unerasable) holographic recording. It is experimentally confirmed that our method can erase the write-once type photopolymer holograms as well as the photorefractive holograms, although there will be a certain degree of restriction on the rewrite cycles limited by the monomer density, the degree of multiplexing and so on.

Photorefractive Binary System Based on Ferroelectric Liquid Crystals

In this paper, results of studies regarding the photorefractive effect in bistable surface stabilized ferroelectric liquid crystals are reported. In particular, an innovative approach toward the optical control of polarization switching in C70-doped bistable devices by means of a photogenerated space-charge field is described. The photorefractive nature of the resulting refractive index gratings is confirmed by two-beam coupling and direct phase shift measurements. The influence of an applied field and of light intensity on grating formation is also studied. The photorefractive gratings obtained by using this approach are stable, easily erasable, and rewritable. The described devices are “binary” systems, in which the refractive index switches only between two possible values. Bistable systems constitute a completely new class of photorefractive materials, with peculiar properties which distinguish them from previously developed media.

Photorefractive star-shaped molecular glassy materials containing tricyanopyrroline-based chromophore

The authors report the near-IR sensitive star-shaped molecule [D-(CBz-TCP)3] containing bifunctional chromophores. A nonlinear optical active tricyanopyrroline-based plasticizer was also synthesized and introduced into the prepared molecular glass so as to control the glass transi-tion temperature of the photorefractive matrix. In a doped glass, they observed significantly enhanced photorefractive performances with regard to the asymmetric energy transfer and the diffraction efficiency at 830nm. In the presence of a small amount of (2,4,7-trinitro-9- fluorenylidene)malononitrile, they could demonstrate a very large net two beam coupling gain higher than 300cm−1 and a diffraction efficiency of 58%.

Plasma ion induced Au nanocluster formation on silica

Inclusion of metal ions in photorefractive materials followed by thermal annealing leads to an increase in optical absorption. The glow discharge as well as thermal annealing have been used to change the optical properties of suprasil. We used the effects of the low-energy ions to induce the nucleation of nanocrystals on and near the surface of a silica nano-layer containing low concentrations of Au, produced by using the co-deposition of Au and silica thin film on a suprasil substrate. Au-nanoclusters were formed both by thermal annealing and glow discharge, then observed and studied by Rutherford backscattering spectrometry (RBS) and by optical absorption photospectrometry.