Two-beam Coupling Gain Coefficient Research Articles

Role of temperature in controlling performance of photorefractive organic glasses.

We present a detailed temperature-dependence study of dielectric, birefringent, conductive, and photorefractive (PR) properties of high-performance low-molecular weight organic glasses that contain 2-dicyanomethylene-3-cyano-2,5-dihydrofuran (DCDHF) derivatives. DCDHF organic glasses sensitized with C60 exhibit high two-beam coupling gain coefficients in the red-wavelength region. However, in the best performing DCDHF glasses at room temperature the PR dynamics are limited by slow molecular reorientation in the electric field. While orientational and, therefore, PR speed can be significantly improved by increasing the temperature above the glass-transition temperature of the material, the steady-state performance may worsen. Comprehensive study of the temperature dependence of various processes, which contribute to the PR effect in DCDHF glasses, clarifies the limiting factors and allows for optimization of the overall PR performance.

High-performance photorefractive organic glass with near-infrared sensitivity

A high-performance organic glass mixture comprised of two dicyanomethylenedihydrofuran derivatives is presented. A pronounced two-beam coupling effect was observed at a wavelength of 830 nm in an unsensitized composition. Sensitization with (2,4,7-trinitro-9fluorenylidene)malononitrile (TNFM) led to a significant increase in the two-beam coupling gain coefficient, reaching a net value of ∼370 cm−1 at an electric field of 45 V/μm at 1% TNFM, and resulted in an improvement in photorefractive speed.

Synthesis and characterization of photorefractive materials based on polymers containing photoconductors and nonlinear chromophores

We report the synthesis and characterization of a new, low- T g polymer exhibiting the photorefractive (PR) effect. The photorefractive polymer contains azo derivative as a second-order nonlinear optical chromophore and carbazole as a charge-transporting group. The photorefractive sample was fabricated by doping 2,4,7-trinitrofluorenone (TNF) of 2 wt.% as photosensitizer, which could be poled by applied electric field at room temperature to achieve large orientational birefringence without doping plasticizer. Photorefractive experiments showed a high two-beam coupling gain coefficient of 122 cm −1 at applied electric field of 85 V/μm. In comparison to host–guest polymer composite systems, long lifetime and high stability of the photorefractive sample was proved.

A PVK-Based High-Performance Photorefractive Polymer CompositeUnder a Low External Electric Field

We describe a PVK-based photorefractive polymer composite doped withdisperse red 1, poly-butylacrylate and TNF. Using the two-beamcoupling experiment, we investigate its photorefractive properties aswell as the dynamic process of the index gratings in the sample. Weobserve the photorefractive effect in the absence of an appliedexternal electric field. The two-beam coupling gain coefficient wasmeasured to reach a value as high as 140 cm-1 in the absence ofan external applied electric field and a value of 470 cm-1 at apoling dc electric field of 25V/µm.

Phase stability of the photoconductive polysiloxane-based photorefractive composites

This article discusses the phase stability of the photoconductive polysiloxane-based photorefractive composites as studied by the light-scattering method. The photorefractive properties of the composites were studied by the two-beam coupling technique. The effects of the electric field on the photorefractive property of the composites were investigated as well. The phase stability was found to be dependent on the chromophore concentration and storage temperature. The two-beam coupling gain coefficient of the composites increased with increasing electric field and chromophore content.

Fast response photorefractive guest-host polymer composite

Abstract A film of composite poly(N-vinylzarbazole): [[4-(dodecanecyl-methylamino) phenyl]methylene]-propanedinitrile: C60 was proven to possess attractive photorefractive properties with fast response time. The diffraction efficiency was fitted to the square of a biexponential formula and the fast response time was deduced to be 12 ms at an erasing beam intensity of 1 W/cm2. Meanwhile, the strong photorefractive effect was maintained. An amplitude of index grating as large as 3.1 × 10−3 at an electric field of 78 V/μ and fitted a two-beam coupling gain coefficient of 43 cm−1 at 57 V/μ were observed.

Fast response photorefractive guest-host polymer composite

A film of composite poly(N-vinylzarbazole): [[4-(dodecanecyl-methylamino) phenyl]methylene]-propanedinitrile: C60 was proven to possess attractive photorefractive properties with fast response time. The diffraction efficiency was fitted to the square of a biexponential formula and the fast response time was deduced to be 12 ms at an erasing beam intensity of 1 W/cm2. Meanwhile, the strong photorefractive effect was maintained. An amplitude of index grating as large as 3.1 × 10−3 at an electric field of 78 V/μ and fitted a two-beam coupling gain coefficient of 43 cm−1 at 57 V/μ were observed.

Dynamics and steady-state properties of photorefractive poly(N-vinylcarbazole)-based composites sensitized with (2,4,7-trinitro-9-fluorenylidene)malononitrile in a 0–3 wt % range

This paper reports on the characterization of six low-Tg poly(N-vinylcarbazole)-based photorefractive (PR) composites sensitized with (2,4,7-trinitro-9-fluorenylidene)-malononitrile (TNFM) in different concentrations, ranging from 0 to 3 wt %. At 780 nm, two-beam coupling gain coefficients, four-wave mixing diffraction efficiencies, and photoconductivities were measured versus electric field, writing beam intensity, and temperature. Dynamic measurements pointed out that chromophore reorientation is not rate-limiting in any of the six samples. In samples with sensitizer concentrations up to 1.24 wt %, increasing the sensitizer concentration leads to a faster grating buildup through a faster charge generation. The grating buildup in these samples is rate- limited by the photogeneration speed. We provide evidence that the TNFM− anions, formed by photoreduction of TNFM, can act as a trap, similar to what has been observed in C60-sensitized samples. As a result, above 1.49 wt % of TNFM, the larger amount of traps produced by photoreduction of the sensitizer reduces the mobility of the charges. Then, the grating buildup speed becomes mobility limited, and smaller buildup rates are observed. Except for the sample with 3 wt % TNFM, increased writing beam intensities or sensitizer concentrations give rise to a larger dynamic range. The different behavior of the sample with the largest sensitizer concentration is explained theoretically in terms of the trap density. The PR phase shifts were found to decrease with increasing writing beam intensity or sensitizer concentration. This provides evidence that the PR phase shifts are controlled by the charge mobility rather than by the photogeneration efficiency. PR measurements as a function of temperature and electric field evolve as predicted by theory.

Photovoltaic contribution to counter-propagating two-beam coupling in photorefractive lithium niobate

A direct observation of the role of the photovoltaic effect in counter-propagating two-beam coupling in photorefractive iron-doped lithium niobate has been made. Through the application of a strong external field we directly observe that the photovoltaic effect is the dominant mechanism for a counter-propagating two-beam optical limiting geometry. We find that the contribution to counter-propagating two-beam coupling gain coefficient from the photovoltaic effect is approximately five times greater than that arising from diffusion mechanisms alone.

Synthesis and characterization of bi-functional photorefractive polymers

A novel bi-functional photorefractive acrylate polymer with pendant carbazolyl groups and azo derivatives as side chains was synthesized, in which azo derivatives as electro-optic chromophores and carbazolyl as photoconductive moiety were covalently linked on the acrylate backbone. Photorefractive experiments showed that a high two-beam coupling gain coefficient of 93cm−1, diffraction efficiency of 12% and electro-optic coefficient of 26pm/V were obtained. With increasing writing beam's intensity, the two-beam coupling enhanced gradually, and then reached saturation. Using the method of four-wave mixing, the photoisomerization grating was observed.

Photorefractive effect in a new composite based on bifunctional host polymer

A new photorefractive composite based on a bifunctional methacrylate copolymer with N-methacryloxypropyl-3-(p-nitrophenyl)azo carbazole and N-methacryloxypropyl carbazole as pendant side chains, which has high stability and potential applications, was synthesized. A two-beam coupling gain coefficient of 9.4 cm−1 and an electrooptic coefficient of 9.3 pm/V were measured at the applied electric field of 92.4 V/μm by a typical two-beam coupling experiment and a compensation electrooptic measurement technique. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 189–194, 2000

Influence of Ce and Co doping ions on photorefractive effect of SBN:61 crystals

Ce-, Co-, and (Ce,Co)-doped Sr 0.61Ba 0.39Nb 2O 6 (SBN:61) crystals with different concentrations were grown by Czochralski technique in the congruent composition. It is the first report of double-doped SBN crystals with Ce and Co. The doping ions Ce and Co occupy 15- (or 12-) fold coordinated and 6-fold coordinated lattice sites in crystal, respectively. A little change of the lattice constants induced by doping ions were detected by XRD experiments. Absorption spectra show that Ce 3+ and Co 3+ are dominant valence status in SBN:61 crystal. Ce and Co doping enhances two-beam coupling gain coefficient and photorefractive response speed, due to these doping ions donating or capturing electrons during the photorefractive process.

Photorefractive effect in a new bi-functional host polymer based composite

We report observations of the photorefractive effect in a low glass transition temperature composite based on a bi-functional side chain copolymer of N-methacryloxypropy1-3-(p-nitrophenyl)azo carbazole with N-methacryl-oxypropyl carbazole. A two-beam coupling gain coefficient of 9.4cm−1 and an electro-optic coefficient of 9.3pmV−1 were measured at the applied electric field of 92.4Vμ−1 by the typical two-beam coupling experiment and the compensation electro-optic measurement technique.

Photorefractive Properties of Co-Doped Potassium Sodium Strontium Barium Niobate Crystals

The transmissivity and the results of two-beam coupling and self-pumped phase conjugation (SPPC) experiments on Co-doped potassium sodium strontium barium niobate (KNSBN) single crystals are reported. The Co ions doped into KNSBN can form a deep energy level near 2.25 eV. The two-beam coupling gain coefficients of the Co-doped KNSBN are nearly 20 cm-1 at 2 teta = 370 for the transmission photorefractive index grating and larger than 29.2 cm-1 for the reflecting photorefractive index grating. A striking single-beam asymmetric transmission was observed. The maximum SPPC reflectivity is larger than 70% at teta = 49.70 for an intensity W = 77 mW, which is the largest value measured in the family of KNSBN crystals.

Response Characteristics of High-Performance Photorefractive Liquid Crystals

The time response of high-performance photorefractivity in a thick-grating regime was characterized in high- (H-LC) and low-molar-mass liquid crystal (L-LC) mixtures (HL-LCMs) doped with fullerene. A high two-beam coupling gain coefficient of over 250 cm-1 was achieved at a low applied dc field of 0.16 V/µm. The photorefractive grating formation time was varied between 0.12 and 0.5 s, which strongly depended on the two-beam power ratio, the two beam intensities and the applied dc field.

Method for determining the two-beam coupling gain coefficients of photorefractive crystals

A new method is proposed for determining the two-beam coupling gain coefficients of photorefractive crystals with both o- and e-polarized lights. This method enables one to determine simultaneously and precisely the gain coefficients of a crystal for o- and e-polarized lights while the fanning effect is diminished. Experimental demonstrations are presented.

Photorefractive Bragg diffraction in high- and low-molar-mass liquid crystal mixtures

Orientational photorefractive Bragg diffraction is observed in high- and low-molar-mass liquid crystal mixtures doped with fullerene (C60). These novel materials are in a nematic phase without phase separating. In the functionalized materials, we observe a high two-beam coupling gain coefficient (Γ=75 cm−1) with a low applied field of 4 V/μm, low total losses (32 cm−1 including scattering, reflection, and absorption), a high diffraction efficiency (9%), and response time of 200 ms.

Electrooptic and photorefractive properties of ferroelectric barium-calcium titanate crystals

Barium-calcium titanate of congruently melting composition Ba0·77Ca0·23TiO3 is a novel photorefractive crystal. The refractive, electrooptic and photorefractive properties of an undoped as grown sample are investigated: The refractive indices are slightly smaller than those of BaTiO3, but the electrooptic coefficients r 113 = 36 ± 3 pm V−1 and r 333 = 140 ± 5 pm V−1 (light wavelength 514·5 nm) are larger. Light-induced absorption and a sublinear photoconductivity indicate the participation of a shallow level in the charge transport. There are no space charge limiting effects; bulk photovoltaic fields of the order of 15 kV m−1 are present. The obtained two-beam coupling gain coefficients for ordinarily and extraordinarily polarized light are 160 m−1 and 425 m−1, respectively. In summary, barium-calcium titanate is a very promising crystal which will compete with barium titanate.

Anomalous power dependence of reflectivity and response time of KNSBN:Cu self-pumped phase conjugators

The reflectivity and time response of self-pumped phase conjugators using a Cu-doped potassium sodium barium strontium niobiate (KNSBN:Cu) crystal in both internal and external geometries are investigated. The light source used is an Ar + laser operating at 514.5 nm. It was observed that the reflectivity remains constant at low power and begins to drop above a certain threshold power in both phase conjugators, and the response time of the internal phase conjugator increases with power. It was shown with two-beam coupling measurements that some of the observed phenomena result from a drop in the codirectional and contradirectional two-beam coupling gain coefficients of the crystal above a threshold intensity of 43 W cm −2.

PHOTOREFRACTIVE POLYMERS

▪ Abstract Rapid advances in the field of photorefractive polymers and composites have in the brief time since their inception in 1991 led to the development of high-performance materials with refractive index modulations approaching 0.01, dif-fraction efficiencies close to 100%, and net two-beam coupling gain coefficients exceeding 200 cm−1in samples typically 100 μm thick. This paper reviews the current state of research, from the most successful synthetic strategies to produce polymeric photorefractive materials, to their emerging uses in applications. Two-beam coupling and four-wave mixing measurement techniques are presented and their importance in the characterization of the photorefractive properties of new materials is explained. The physics of the photorefractive effect in polymers is discussed with emphasis placed on the differences compared with the traditional inorganic photorefractive crystals. In particular, the orientational enhancement mechanism, which is believed to be responsible for the high performance of most of the low-glass-transition-temperature systems, is discussed in detail.