Photorefractive Grating Formation Research Articles

Formation of Dynamic Photorefractive Gratings in a LiNbO3:Cu Surface-Doped Crystal

We present results of experimental studies, theoretical analysis, and numerical modeling of specific features of photorefractive gratings that are formed by a high-contrast interference pattern of writing laser beams in an X-cut plate of a LiNbO3:Cu crystal created from a metal film by diffusion doping.

Формирование динамических фоторефрактивных решеток в кристалле LiNbO-=SUB=-3-=/SUB=- : Cu с поверхностным легированием

The results of experimental studies, theoretical analysis and numerical simulation of the features of the formation of photo-refractive gratings by the interference pattern of recording laser beams with high contrast in a LiNbO3: Cu X-cut plate created by diffusion doping from a metal film are presented.

In situ observation and monitoring of the dynamic behavior of field-induced photorefractive grating formation process with digital holographic microscopy

Both the spatial and temporal evolutions of photorefractive (PR) grating formation by field-induced enhancement in Mn:Fe:KTN crystal are visualized in situ and estimated quantitatively by means of digital holographic microscopy. A series of sequential phase maps are retrieved numerically from the recorded digital holograms to explore the quantitative characteristics of the observed procedure. Further investigations reveal that the properties of PR grating, i.e. amplitude of index modulation and writing time of the grating, can be improved drastically by an external field. The improvement PR grating is attributed to enhanced space-charge fields induced by an external field. This effect can be of interest for many relevant applications, such as electroholographic switching, and in situ and real-time monitoring of both the grating structure and its refractive index profile.

Dark current and light illumination effects on grating formation during periodic long-term operation in photorefractive polymers

Photorefractive grating formation dynamics in long-timescale writing and the effects of periodic writing through the control of writing beam irradiation or electric field application were investigated using typical photorefractive polymers. Both dark current and writing beam irradiation affected grating formation dynamics. Dark current in polymers changed the effective trap density over time through deep trap filling and/or detrapping and thus affected grating formation considerably. The writing beam irradiation also affected grating development in the presence of an electric field owing to the accumulation of filled deep traps. However, grating development recovered after the elimination of the electric field freed up the filled deep traps.

Photorefractive effects in polymer dissolved liquid crystal composites dopes with fullerene derivatives

We investigated the photorefractive performance of the polymer dissolved liquid crystalline composite (PDLCC), in which liquid crystalline polymer and low-molar-mass liquid crystal are miscible without phase separation, doped with three kinds of fullerene derivatives with different length of alkyl groups. The photorefractive performance was improved for the photorefractive PDLCC doped with fullerene derivatives with long length of alkyl groups. The photorefractive grating formation originates in the cooperative reorientation of the liquid crystalline director and the space charge field was estimated using the elastic continuum theory and the field for the PDLCC doped with the functionalized fullerene with longer alkyl side groups was larger than that for the PDLCC doped with conventional fullerene C60.

Talbot effect by a photorefractive volume phase grating

In our proposal a light intensity distribution generated by an incoherently illuminated planar amplitude grating is projected into a photorefractive crystal. This 3D distribution is mapped as an index refractive perturbation via the photorefractive effect thereby generating a volume phase grating. The self-imaging phenomenon in the Fresnel field of this volume phase grating coherently illuminated is theoretically and experimentally analyzed. A model to simulate this volume grating that considers the 3D light intensity distribution formed in the crystal combined with the photorefractive grating formation theory is proposed. A path-integral approach to calculate the self-image patterns which account for the inhomogeneous propagation through the photorefractive grating is employed. The experimental and theoretical results show that the self-images location coincides with that of the self-images generated by planar phase grating of the same period. Moreover, the self-images visibility depends on three parameters: the exit pupil diameter of the incoherent recording optical system, the external electric field applied on the crystal, and the crystal thickness. To study the visibility behavior, a phase parameter which includes the three mentioned parameters is proposed. The self-images visibility shows the typical sinusoidal dependence found in planar phase grating. A good agreement between theoretical and experimental results is observed.

Organic Photorefractive Materials and Applications

AbstractThis review describes recent advances and applications in the field of organic photorefractive materials, an interesting area in the field of organic electronics and promising candidate for various aspects of photonic applications. We describe the current state of knowledge about the processes involved in the formation of photorefractive gratings in organic materials and focus on the chemical and photo‐physical aspects of the material structures employed in low glass‐transition temperature amorphous composites and organic photorefractive glasses. State‐of‐the art materials are highlighted and recent demonstrations of photonic applications relying on the reversible holographic nature of the photorefractive materials are discussed.

Effect of co-sensitization in new hybrid photo-refractive materials based on PVK polymer matrix and inorganic LiNbO3 nano-crystals

A new class of photo-refractive (PR) composite materials based on poly(N-vinylcarbazole) (PVK) (co-)sensitized with nano-crystalline lithium niobate (LiNbO3) is presented. The steady-state performance, as well as the kinetics of the photo-refractive grating formation, was investigated by degenerate four-wave mixing (DFWM) and two-beam-coupling (TBC) experiments. We found an optimum content of LiNbO3 nano-particles of only 10−3 wt.%. Even at concentrations as low as 10−7 wt.% a notable effect of the nano-crystals was detected. This yields materials with improved PR performance compared to that of the standard PR polymer material sensitized by TNF only. The role of LiNbO3 nano-particles is discussed in detail: The particles support generation and transport of the free-charge carriers. Furthermore, they increase the electron trap density.

Effect of Incoherent Illumination on Two-Beam Interaction of Light Waves in a Bismuth Titanate Crystal

Experimental investigations and theoretical analysis of the effect of external illumination on the dynamics of formation of photorefractive reflection gratings in a (100)-cut bismuth titanate crystal showed that, in the case of counter interaction of laser beams with a wavelength of 633 nm, incoherent green light may cause a change in the sign of the two-beam gain.

Fast photorefractive polymer composites based on nanocrystalline J-aggregates of the cyanine dyes

The hole and electron drift mobility, photoelectric properties as well as influence of temperature, dark current and also relative position of oxidation potentials of the interacting components on photorefractive characteristics of the composites from aromatic polyimide doped with nanocrystalline J-aggregates of a cyanine dye are presented. The nanocrystalline J-aggregates were used as the photosensitizers and also as nonlinear optical chromophores. The experimental electric field dependence of the charge carrier photogeneration efficiency is well fitted by Onsager equation when the quantum yield of the bound thermalized electron–hole pairs ϕ 0=1 and the initial separation of the charges in the pair r 0=14.1 Å. The charge photogeneration time constant determines the response time of the photorefractive grating formation. The polymer composites exhibit gain coefficient Γ=218 cm −1 and response time shorter than 20 ms at applied electric field 50 V/μm and a intensity of each writing beam 30 mW/cm 2. The net gain coefficient Γ− α=143 cm −1.

Photoelectric, nonlinear optical and photorefractive properties of polyimide doped with J-aggregates of cyanine dye

Abstract Novel solid-state photorefractive compositions consisting of the aromatic polyimide ( T g is about 230 °C) and the thiacarbocyanine dyes were prepared. These systems have the high net gain coefficient, Γ − α , about 360 cm −1 . The dyes form the nanocrystalline J -aggregates that are responsible for photoelectric sensibility in the region of action of Ar–Kr laser and are known to have also very high third-order macroscopic polarizability. The Kerr electrooptical effect is mainly responsible for the light induced refractive index change. It is established that the charge photogeneration time constant determines the time of the photorefractive grating formation and depending on the dye used may be about 20 ms. So, it became possible to develop the solid-state high-performance photorefractive layers based on the polymer with high T g .

Nonlinear charge transport in photorefractive semiconductor quantum wells

Nonlinear electron transport and hot carriers in GaAs/AlGaAs quantum wells produce interesting and beneficial effects on the formation of photorefractive gratings, including nonlocal space-charge screening and the prevention of grating slow-down at high fields.

Bulk photovoltaic effect and photorefractive grating formation in lithium niobate with picosecond light pulses

I investigated photorefractive grating formation in materials exhibiting the bulk photovoltaic effect on illumination with weak, very short light pulses. For a light pulse shorter than the time constants of the carrier, the existing theory of grating formation, as well as the expression for the photovoltaic current, breaks down. Nevertheless, my experiment, which is the first quantitative comparison of this kind to my knowledge, showed no meaningful difference between the pulsed and cw excitations of Fe:LiNbO3 for the same average intensity. I developed a simple model in which the bulk photovoltaic effect was implemented as a spatial shift in the initial carrier distribution, and I derived an analytical expression for the space-charge field. The resulting expression agreed with that of the quasi-cw approximation in the limit of a small spatial shift, thus supporting my experimental results.

Theory of Photorefractive Grating Formation in the Presence of the Bulk Photovoltaic Effect

A simple analytical expression for the evolution of photorefractive grating was derived for photovoltaic materials. The theory is based on the band transport model under the quasi-cw approximation, and a small light modulation was assumed. The resultant expression is consistent with the existing theory and is applicable to all the time regions of the evolution of the grating. It accounts for the major characteristics of the photorefractive effect observed in Fe:LiNbO3 including the apparently anomalous large phase shift of the grating, the shift of the grating during erasure, and the quasi-equivalence of the bulk photovoltaic effect to an external field. We also conducted numerical simulations based on the coupled wave theory. The result is consistent with the experiment reported elsewhere, except for the large beam coupling that was experimentally observed. This can, however, be ascribed to an origin external to the grating formation.

Direct observation of orientation limit in a fast photorefractive polymer composite

We report on a photorefractive polymer with a 4-ms-response time in transient four-wave mixing experiments at 0.5 W/cm2 writing irradiance, 95 V/μm applied electric field, and a grating period of 3.1 μm. Complementary transient ellipsometry, however, reveals orientational birefringence response which leads the four-wave mixing response all the way to its saturation, despite complex dynamics in these processes. Orientation does not limit the dynamic formation of photorefractive gratings in this polymer, which suggests that even faster photorefractive responses are possible for polymer composites with improved charge generation and transport properties.

High-performance photorefractivity in high- and low-molar-mass liquid crystal mixtures

High-performance photorefractivity in a thick-grating regime was observed in high- (H-LC) and low-molar-mass liquid crystal (L-LC) mixtures (HL-LCMs) doped with fullerene. The photorefractive performance strongly depended on the concentration ratio of H-LC and L-LC. A high net gain coefficient over 600 cm−1 was achieved with low applied dc fields (0.7 V/μm) and a fringe spacing of 2.8 μm. The gain coefficients and the resolution increased with increasing the concentration of the H-LCs. The photorefractive grating formation time was varied between 0.12 and 3.5 s, which depended on the concentration of the H-LCs.

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.

Kinetics of photorefractive grating formation with an external applied field at larger modulation depths

In this paper, we use the `hopping' model to analyze the dynamic behaviors of the first three-order harmonics of the space-charge field at large modulation with an external dc and ac field, respectively. We present the respective exact and approximate dynamic analytical solutions in the presence of a dc field and a sinusoidal ac field. For a supplied dc field, the real parts of the first three-order harmonics of the space-charge field will decrease with increasing spatial frequency, but the imaginary part of each higher-order harmonic displays maximal values at the respective spatial frequencies. There are peak values for the real parts of the higher-order harmonics with increasing external electric fields, but no peak values for the real part of the fundamental harmonics and every imaginary part. For the applied ac field, we present for the first time the approximate analytical solution of the fundamental harmonic, and found that the real part of the fundamental space-charge field varies at the same frequency as that of the applied field, the imaginary part oscillates at twice that frequency. These results are in agreement with those derived from the band-transport model.

Photorefractive grating formation in the region of large intensity modulation

A new method of measuring Bragg diffraction more accurately is introduced. When three formulas of photorefractive grating formation are given as functions of intensity modulation as m, 2m (1+ 1−|m| 2) , and 2m (1+ 1−|M| 2) , where M = m [1 + ( K K o ) 2] , and K is the grating wavenumber and K o is the Debye inverse screening length, their predictions for the Bragg diffraction are derived analytically and numerically by using the interrelation between two-beam energy coupling and Bragg diffraction in the photorefractive crystal. By fitting the predictions to the experimental data the behavior of the formulas is checked in the region of large m. It is shown that the general formula, 2m (1+ 1−|m| 2) , predicts the Bragg diffraction well within the experimental error even for m equal to unity, while the other two hold only for m smaller than 0.2 in the energy-gain orientation and smaller than 0.5 in the energy-loss orientation. In the limit of small K where the formula reduces to 2m (1+ 1−|m| 2) , it is shown to predict Bragg diffraction always larger than the experimental data. In the limit of small m where the formula reduces to m, it is shown to predict Bragg diffraction always smaller than the experimental data.

Resonant two-photon processes for nonvolatile holography in photorefractive crystals under continuous-wave illumination

We investigate the resonant two-photon (two-step) processes of photorefractive grating formation and identify the regimes in which nonvolatile holography is possible. We develop a charge-transport model to describe this behavior for a photorefractive crystal with a single active impurity species under continuous-wave illumination. For the cases that allow for nondestructive reconstruction of gratings, we evaluate the maximum refractive-index perturbation and the response rate with respect to illumination intensities and impurity characteristics. We evaluate the importance of the impurity intermediate-state occupancy. Holographic data storage system issues are also discussed. The present results are consistent with previously reported photorefractive behavior and predict additional properties that characterize these resonant processes. The analysis may be used further to study other related two-photon phenomena of interest in holographic storage systems.