Two-beam Coupling Experiments Research Articles

Photorefractive Effects and Structure/Property Correlation of Oligothiophenes Functionalized with Nonlinear Optical Chromophores

A homologous series of fully functionalized regioregular oligo(3-alkylthiophenes) 10−12 bearing a nonlinear optical (NLO) chromophore was synthesized. An alternating sequence of bromination and Stille cross-coupling reactions was developed for the synthesis of these oligomers and regiochemically pure, trimethylsilyl-substituted bithiophene organostannane 2c was utilized as the building block. The resulting materials were shown to form stable amorphous films exhibiting a large photorefractive (PR) effect. Two-beam coupling and degenerate four-wave mixing experiments demonstrate large optical net gain and high diffraction efficiency. It was also found that the PR performance of these oligomers depends on the π-conjugation chain length with an optimized conjugation length.

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

%Synthesis and Properties of Photorefractive Polymers Containing Indole-Based Multifunctional Chromophore as a Pendant Group

Novel photorefractive chromophores with indole and nitrobenzene as the push and pull groups, respectively, were synthesized and subsequently reacted with methacryloyl and acryloyl chloride to give multifunctional monomers. Polymers were obtained by copolymerization of these monomers with methyl methacrylate (P1, P2) or butyl acrylate (P3). These polymers were very soluble in organic solvents to be fabricated into optically clear polymer films and showed glass transition (Tg) at ca. 150 °C for P1, P2 and at 8 °C for P3, respectively. Large nonlinear optical coefficients (d33 as high as 49 pm/V) and photoconductive sensitivity (on the order of 10-9 S·cm-1/W·cm-2) were identified with high Tg polymers P1 and P2. Preliminary investigation of photorefractivity was carried out with low Tg polymer (P3) by two-beam coupling and four-wave mixing experiments. It was observed that P3 doped with 1% TNF showed two-beam coupling gain of 2.2 cm-1 at 75 V/μm and diffraction efficiency of 0.44% at 60 V/μm. When the photoc...

Charge carrier photogeneration, trapping, and space-charge field formation in PVK-based photorefractive materials

We studied the dark conductivity ${(j}_{\mathrm{dark}}),$ the photoconductivity ${(j}_{\mathrm{photo}}),$ and the charge carrier photogeneration efficiency \ensuremath{\eta} of poly(N-vinylcarbazole)--based photorefractive (PR) materials with different glass-transition temperatures ${(T}_{g})$ and chromophore content $({\ensuremath{\rho}}_{\mathrm{CHR}}).$ Measurements were carried out at wavelengths similar to those used in degenerate four-wave mixing (DFWM) and two-beam coupling (2BC) experiments. Both thick (37 \ensuremath{\mu}m) and thin samples (\ensuremath{\approx}1 \ensuremath{\mu}m) were analyzed. Photoconductivity experiments at different temperatures show that both ${j}_{\mathrm{dark}}$ and ${j}_{\mathrm{photo}}$ are thermally activated. For ${j}_{\mathrm{dark}}$ the activation is not related to the glass-transition temperature of the blends, whereas photocurrents exhibit a universal behavior with respect to ${T}_{r}{=T}_{g}\ensuremath{-}T.$ The charge carrier photogeneration efficiency \ensuremath{\eta} was measured by xerographic discharge experiments. \ensuremath{\eta} was found to be independent of both ${T}_{g}$ and of ${\ensuremath{\rho}}_{\mathrm{CHR}}.$ The photoconductivity gain factor G defined as the number of charge carriers measured in photoconductivity in relation to the number of carriers initially photogenerated as determined by the xerographic experiments is used to compare the results of photoconductivity and xerographic discharge experiments. G is found to be much smaller than unity even for thin samples, which indicates that the mean free path of the photogenerated charge carriers is less than 1 \ensuremath{\mu}m at photoelectrical equilibrium. Using Schildkraut's model for the space-charge field formation in organic PR materials, trap densities ${T}_{i}$ of approximately ${10}^{17}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ could be derived from G. The field and temperature dependence of ${T}_{i}$ is independent of ${\ensuremath{\rho}}_{\mathrm{CHR}}$ and might account for the universal ${T}_{r}$ dependence of ${j}_{\mathrm{photo}}.$ The estimated trap densities are used to calculate the first-order Fourier component of the space-charge field in the PR materials illuminated with a sinusoidal intensity pattern. Modifying Schildkraut's model so that the tilt between the applied electric field and the index of refraction grating is taken into account yields saturation fields of approximately 100 V/\ensuremath{\mu}m in agreement with findings from PR experiments. The dramatic decrease of the space-charge field when the temperature exceeds the glass-transition temperature of the blend is fully explained by a decrease in trap density. The fact that the trap density depends on the temperature with respect to ${T}_{g}$ and not on the absolute temperature suggests that the relevant traps are most likely of conformational nature.

The Photorefractive Effect in Monolithic Structural Polyimides

We report here the photorefractive effect in monolithic structural polyimides containing the triphenylamino-( p-nitrophenyl)-ethylene group as a nonlinear optical (NLO) chromophore and hole transfer. Three types of polyimides were synthesized to control the loading ratio of the NLO chromophore; 50, 30 and 17 mol% for polyimides 1, 2 and 3 respectively. In the two-beam coupling experiment, asymmetric energy transfer was observed in the case of polyimides 1 and 2, providing evidence for a phaseshifted grating after corona-poling treatment, whereas the energy transfer of polyimide 3 was not observed. This result denotes that the NLO chromophore plays an important role in monolithic photorefractive materials, and a greater than 30 mol% concentration of NLO chromophore is required for creating sufficient space charge distribution.

Preparation and optical properties of a poly(N-vinylcarbazole) material

We report on the photorefractive properties of a stable polymeric material based on the host polymer, poly(N-vinylcarbazole). Two-beam coupling and four-wave mixing experiments were used to measure the performance of devices constructed by sandwiching the material, composed of host, plasticizer, photo-sensitizer, and non-linear optical chromophore, between transparent electrodes. A steady-state diffraction efficiency of 60% and a two-beam coupling gain of 22 cm−1 in excess of the absorption of 4 cm−1 were observed at the laser's operating wavelength of 670 nm. The devices are easily prepared and have lifetimes in excess of one year with no indication of degradation when stored at ambient temperatures.

Photorefractivity and transmission-type holograms of a poly(N-vinylcarbazole)-based composite with N-4-nitrophenyl-(l)-prolinol as chromophore

The photorefractivity of a poly(N-vinylcarbazole)/2,4,7-trinitro-9-fluorenone/N-4-nitrophenyl-(l)-prolinol/N-ethylcarbazole composite was investigated by two-beam coupling and degenerate four-wave mixing experiments. The composite in a weight ratio 35 : 1 : 50 : 15 exhibited a 22% steady-state diffraction efficiency and 70 cm -1 two-beam coupling coefficient at the applied external field of 55 V/μm. Zero applied field transmission-type holograms were observed in the photorefractive sample as well as the spin-coated thin film. The dynamic decay of diffraction light indicated that there existed two contributors with different phase-shift, and the slow decay time can be as long as several h under continuous illumination by the readout beam.

Synthesis and properties of polysiloxane-based electrooptic materials

In this study, new functionalized polysiloxane material was synthesized. The polysiloxane-based electrooptic material with side-chain charge-transporting functionalities and nonlinear optical chromophores was investigated through a three-step synthetic route. Elementary analysis and NMR spectra indicate that the polymer has a high degree of substitution. Detailed physical properties showed that the polymer has a glass transition of around 91°C, and its photoconductivity and electrooptic coefficient increase with the increasing applied electric field. Photorefractivity of the system has been demonstrated through the asymmetric energy transfer in a two-beam coupling experiment. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3854–3860, 1999

Photorefractive Bragg gratings in nematic liquid crystals aligned by a magnetic field

Photorefractive Bragg gratings are observed in low-molar-mass nematic liquid crystals doped with electron donor and acceptor molecules. This is accomplished by alignment of the nematic liquid crystals in a 0.3 T magnetic field, which produces thicker homeotropic aligned samples than traditional surfactant techniques. Grating fringe spacings as low as 3.7 μm are achieved with 176-μm-thick samples, producing grating Q values of 33. Up to this point, low molar mass nematic liquid crystals have exhibited photorefractive gratings with Q⩽1. Asymmetric two-beam coupling and photoconductivity experiments are performed to verify the photorefractive origin of the gratings.

Improved Properties of Stoichiometric LiNbO3 for Electro-Optic Applications

High quality stoichiometric LiNbO3 single crystals, 40 mm in diameter and 80-100 mm in length, have been successfully grown by the double crucible Czochralski method equipped with automatic powder supply system using radio-frequency heating and automatic diameter control system. Although some difficulties still exist in growing 3 inch size single crystals from offcongruent composition melts, our results demonstrate that stoichiometric LiNbO3 crystals containing much less nonstoichiometric defects than congruent LiNbO3 exhibit some advantageous properties for holographic data storage applications. This was confirmed by the results of two-beam coupling experiment and digital hologram test. In digital holographic data storage, we have demonstrated that stoichiometric LiNbO3 shows more than 10 times larger photorefractive sensitivity than congruent LiNbO3 In order to interpret the high performance of stoichiometric LiNbO3, we compared several relevant parameters such as electro-optic constants, photoconductivity, and photogalvanic constants between stoichiometric and congruent LiNbO3 crystals. We report that material parameters are strongly dependent on the density of nonstoichiometric defects and control of these defects is of key importance in improving and tailoring those properties.

Optical amplification with high gain in hybrid-polymer–liquid-crystal structures

Efficient coherent light amplification with a very high net exponential gain Γ≈2600 cm−1 has been observed in sandwich-type hybrid-polymer–liquid-crystal structures. The amplification is a result of an energy exchange between two beams in a two-beam coupling experiment. The reported gain coefficient has been measured in a tilted sample configuration within a Raman–Nath diffraction regime at electric fields of the order of 1 V/μm.

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.

Enhancement of Photorefractivity in the Isotropic Phase of Polymer Liquid Crystals

The photorefractivity of a series of polymer liquid crystals that possess a 4-nitrobenzylideneaniline moiety or a cyanobiphenyl moiety as a mesogen and simultaneously as a nonlinear optical chromophore was investigated using two-beam coupling experiments. A large diffraction efficiency (η) of 65% under application of a 37 V/μm external electric field was obtained in the isotropic phase of a polymer liquid crystal mixed with charge transporting compounds. The photorefractivity was compared between the liquid crystalline polymers and amorphous polymers. The photorefractivity was found to be significantly enhanced in the isotropic phase of the liquid crystalline polymers compared to that of the amorphous polymers with almost the same molecular structure.

Permanently poled fully functionalized photorefractive polyesters

Crystallization and phase separation are severe degradation mechanisms of photorefractive (PR) guest–host systems. Additionally, the response time of PR guest–host systems is in some cases limited by a large contribution of slow chromophore reorientation to the PR effect. The synthesis of fully functionalized systems with glass transition temperatures well above room temperature is a way to restrict these degradation mechanisms and to reduce the PR effect to a pure charge generation and transport phenomenon. We present novel, fully functionalized PR polymers based on polyester. Permanent poling of the systems is demonstrated and quantitatively investigated. The PR properties are measured by degenerate four-wave mixing and two-beam coupling experiments as a function of the external electric field and the degree of permanent poling. One of the systems shows response times of approximately 200 ms with Δn≈1×10-3. To our knowledge, this work presents the first characterization of PR efficiencies for fully functionalized polymers as a function of the permanent Pockels coefficient χ(2)(-ω;ω, 0).

Laser Written Permanent Gratings in a New Liquid Crystalline Organometallic Polymer

Interest in materials for optical storage effect has been intense in the last years. In this paper we report the observation of permanent gratings in a new liquid crystalline side-chain polymer containing a mononuclear cyclopalladated azobenzene complex. Two-beam coupling experiments indicate photorefractive properties. A gain of 900 cm−1, without applied electric field, was determined by two beam coupling measurements. A significant feature of this high gain material is that this effect occurs in an easily processable pure compound.

Orientational gratings in dye-doped polymer-dispersed liquid crystals induced by the photorefractive effect

We report the observation of orientational gratings induced by the photorefractive effect in dye-doped polymer-dispersed liquid crystals. The photorefractive origin of the grating-induction effect is supported by the results of two-beam coupling experiments and by the possibility of erasing the grating by uniform illumination. For all the samples a stable memory effect was observed.

Synthesis and Characterization of New Photorefractive Polymers with High Glass Transition Temperatures

Polyamides based on 2',5'-diamino-4-(dimethylamino)-4-nitro-stilben (DDANS) and aliphatic diacids represent a well-established class of polymers for nonlinear optical (NLO) applications, in which the NLO units are fixed in the polymer backbone with their dipole moments oriented transversly to the polymer main chain. The introduction of a hole transport (HT) agent, [4((diphenylhydrazono)-methyl)phenyl]diethylamin (DEH) or 4-(bis(2-hydroxyethyl)amino)benzaldehyde 1,1-diphenylhydrazone (BBDH), was found to generate or enhance photorefractivity in these materials. Using appropriate chemical pathways, the NLO and HT moieties were connected in various ways, and polymer systems with dramatically different properties were obtained. Thus, (i) polyamides based on DDANS and aliphatic diacids, (ii) polyamide esters based on DDANS, BBDH, and adipic acid, (iii) polymer blends of the DDANS polyamides and a polyester based on BBDH and adipic acid, and (iv) DDANS polyamides doped with DEH have been prepared. All types of polymer systems were processed into transparent thin films by spin coating and could be oriented at elevated temperatures, applying a corona discharge poling technique. Second-order NLO coefficients (d 33 ) of up to 45 pm/V at a fundamental wavelength of 1542 nm have been measured. The poling efficiency was found to depend strongly on the content of the hydrazone moiety and the polymer system, decreasing with increasing content of hydrazone. Preliminary temperature-dependent two-beam coupling experiments demonstrated photorefractivity for DDANS-based systems, even at temperatures below the glass transition.

Competing photorefractive gratings in organic thin-film devices

Recently, amorphous organic photorefractive materials have generated great excitement because of their excellent performance, which permits applications in high-density holographic storage, real-time image processing, and phase conjugation. However, the heterostructure of the devices (consisting of glass cover slides, transparent electrodes, and the photorefractive material) and the tilted recording and readout geometry commonly used result in multiple reflected beams in addition to the normal object and reference beams. This result leads to several photorefractive gratings competing inside the photorefractive polymer device. We prove the coexistence of these gratings by two-beam coupling and four-wave mixing experiments and demonstrate how to distinguish between them.

Electrostrictive behavior observed in a low glass-transition temperature photorefractive polymeric composite during a two-beam coupling experiment

A movement of refractive index grating was observed in two-beam coupling behavior with a low glass-transition temperature photorefractive polymeric film as the poling electric field was discharged. The photorefractive polymeric composite is poly(N-vinylcarbazole) doped with 2,5-dimethyl-4-(p-nitrophenylazo)anisole, N-ethylcarbazole and 2,4,7-trinitro-9-fluorenone in a weight ratio of 25:40:34:1. A theoretical explanation based on electrostrictive effect was given, and some evidential experiments, including degenerate four-wave mixing and Michelson interference measurement were also presented.