Inorganic Photorefractive Crystals Research Articles

Review of Organic Photorefractive Materials and Their Use for Updateable 3D Display.

Photorefractive materials are capable of reversibly changing their index of refraction upon illumination. That property allows them to dynamically record holograms, which is a key function for developing an updateable holographic 3D display. The transition from inorganic photorefractive crystals to organic polymers meant that large display screens could be made. However, one essential figure of merit that needed to be worked out first was the sensitivity of the material that enables to record bright images in a short amount of time. In this review article, we describe how polymer engineering was able to overcome the problem of the material sensitivity. We highlight the importance of understanding the energy levels of the different species in order to optimize the efficiency and recording speed. We then discuss different photorefractive compounds and the reason for their particular figures of merit. Finally, we consider the technical choices taken to obtain an updateable 3D display using photorefractive polymer. By leveraging the unique properties of this holographic recording material, full color holograms were demonstrated, as well as refreshing rate of 100 hogels/second.

Polymeric photorefractive surface waves

We theoretically studied photorefractive surface waves supported by polymers with orientationally enhanced photorefractive effect. We find that the orientational enhancement and the dependency of the quantum efficiency of generating mobile holes on the electric field make the diffusion nonlinearity in photorefractive polymers dependent not on only the variation of light intensity but also on external electric field, which is very different from the external electric field independent diffusion nonlinearity in inorganic photorefractive crystals. Consequently the profile, frequency and especially the penetration depth of polymeric photorefractive surface waves strongly depend on external electric field, which makes it more controllable than that in photorefractive inorganic crystals. The stability of polymeric photorefractive surface waves, the effects of birefringence component and electro-optic component of orientationally enhanced photorefractive nonlinearity on the formation of x or y-polarized polymeric photorefractive surface waves are also discussed.

Recent advances in photorefractive two-beam coupling

In this article, we review two approaches that we have taken to improve photorefractive two-beam coupling: (1) the enhancement of the effective trap density in a well-known inorganic photorefractive crystals (KNbO3:Fe) and (2) the improvement of a relatively new concept of organic–inorganic hybridized liquid crystal cells.

Influence of intensity on the steady and transient state space-charge fields in photorefractive polymers

We have proven theoretically that there are sublinear, linear and superlinear relations between the response rates and total incident intensity for different cases of traps in photorefractive polymer materials. These relations were observed in inorganic photorefractive crystals many years ago. Also, the steady-state space-charge field is a function of the total incident intensity, which has also been found in inorganic photorefractive crystals. We have measured the relations of the steady-state diffraction efficiency and the response rate with respect to the total incident intensity in the photorefractive composite consisting of the polymer (N-vinylcarbazole) (PVK) doped with 4,4'-n-pentylcyanobiphenyl (5CB) and C60. The results obtained show that the composite belongs to the case of low trap density.

Effect of the applied electric field on the steady state and temporary state space-charge field in photorefractive polymers

Abstract In this paper, after considering the dependence of quantum efficiency of charge generation, carrier mobility, and trapping rate on the applied electric field, the relations of steady state space-charge field and response rates of space-charge field are discussed with respect to the applied electric field in photorefractive polymers. It is found that the steady state properties of the space-charge field with respect to the applied electric field are in agreement with those of inorganic photorefractive crystals. However, for the transient processes, there are different function forms between the response rates and the applied electric field, such as superlinear functions and sublinear functions. Also, the response rates are determined by the properties of traps.

Effect of the applied electric field on the steady state and temporary state space-charge field in photorefractive polymers

In this paper, after considering the dependence of quantum efficiency of charge generation, carrier mobility, and trapping rate on the applied electric field, the relations of steady state space-charge field and response rates of space-charge field are discussed with respect to the applied electric field in photorefractive polymers. It is found that the steady state properties of the space-charge field with respect to the applied electric field are in agreement with those of inorganic photorefractive crystals. However, for the transient processes, there are different function forms between the response rates and the applied electric field, such as superlinear functions and sublinear functions. Also, the response rates are determined by the properties of traps.

Theory of space-charge field with a moving fringe in photorefractive polymers

Resonance of space-charge field can occur in photorefractive polymers through a moving fringe and can be described by a forced oscillation equation with damping. The resonance frequency and the magnitudes of both the amplitude and the imaginary part of the space-charge field at the resonance frequency are functions of three experimental parameters: total incident intensity, applied electric field, and grating wave vector. Thus, the diffraction efficiency of grating and the gain coefficient of two-wave mixing are dependent on the three experimental parameters. Our theory can also be applied to inorganic photorefractive crystals.

Correlation between photoconductivity and holographic response time in a photorefractive guest host polymer

The standard model for photorefractivity, which was originally developed for inorganic photorefractive crystals, yields a relation between the pertinent parameters of photoconductivity and the holographic response time. We investigated a photorefractive guest host polymer with respect to these properties by photoconduction and holographic experiments. Our system consists of a photoconducting siloxane polymer, which is sensitized by trinitrofluorenone, and an azo chromophore as nonlinear optical moiety. We compare measurements of photoconduction and response time as a function of the external electric field and the incident light intensity in order to test the applicability of the standard model to photorefractive polymers.

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.

Photorefractive Polymers and Composites

AbstractMaterials demonstrating a photorefractive effect are principal candidates for numerous applications, including high‐density optical data storage, optical image processing, phase conjugated mirrors and lasers, dynamic holography, optical computing, pattern recognition, etc. Considerable progress has been made in the research on photorefractive polymers and composite materials in the last few years. These materials have many advantages over inorganic photorefractive crystals, including large optical nonlinearities, low dielectric constants, low cost and ease of fabrication. A large number of materials, including those exhibiting an extremely large photorefractive effect, have been developed. In addition, a number of interesting phenomena particular to polymeric photorefractive materials have been reported and corresponding mechanisms have been proposed to account for these phenomena. Possible applications of these materials have been explored with the demonstration of a volume holographic storage device based on photorefractive polymers. This paper reviews the latest developments of the young and exciting field of polymeric photorefractive materials.