C. L. Thompson

High performance photorefractive polymer with improved stability

A novel photorefractive polymer composite is reported which exhibits a high two-beam coupling gain coefficient (Γ=200 cm−1 at 120 V/μm, fast response time (τg=50 ms at 1 W/cm2), overmodulation of diffraction efficiency, and high sensitivity. The composition and processing have been altered to yield greatly improved material stability.

Measurement of the spatial phase shift in high-gain photorefractive materials.

The correct determination of the spatial phase shift ø(p) in photorefractive materials is crucial to the proper characterization of novel materials. It is shown that the grating translation techniques commonly used for the measurement of ø(p) need to be reevaluated for high-gain materials. Strong energy and phase coupling leads to nonuniform slanted gratings, which result in an apparent dependence of the phase shift of the beam ratio and the optical polarization. A revised theory is presented, and analytical solutions are obtained for the special case of ø(p)?pi/2 . Numerical solutions for arbitrary ø(p) are in good agreement with measurements in a photorefractive polymer.

Gain enhancement by moving gratings in a photorefractive polymer

Abstract The use of moving gratings allows dramatic enhancement of the two-beam coupling gain in a photorefractive polymer. The advantage of using multi-layer structures to increase the effective interaction length is also demonstrated and a small signal optical amplification as large as 500 is achieved using a three-layer sample, the highest reported to date in a photorefractive polymer. From an investigation of the time dependence and detuning frequency dependence, we determine that hole conduction is dominant and that the mobility life-time product reaches μ τ =3×10 −10 cm 2 /V at an applied electric field E 0 =71 V/μm.

Recent advances in photorefractive polymer materials

This paper summarizes current research to develop photorefractive polymeric materials with improved speed, material stability, and high beam coupling gain. The recent demonstration of significantly improved two-beam coupling marks the entry into a gain regime which enables the observation of new effects for the first time, such as beam fanning and self-pumped phase-conjugation. These effects have previously been observed only in a few thick high gain inorganic photorefractive crystals. The large beam coupling forces the reinterpretation of such traditional characterization techniques such as the grating translation method for the determination of the spatial phase of the index grating. Our subsequent material study focuses on several compositional variations to investigate the effect on the photorefractive performance of varying the chromophore and charge transporting polymer.

Mechanisms of photorefractivity in polymer composites

We present a summary of the underlying mechanisms which govern the appearance and dynamics of the photorefractive effect in polymeric materials. Charge transport properties in polymers and their influence on the build-up of space- charge fields are discussed, and the beneficial effects from orientational enhancement of in situ room-temperature poling of nonlinear optical chromophores are reviewed. We introduce a new high performance low-glass-transition-temperature polymer composite and discuss its photorefractive properties as characterized by a thorough investigation of photoconductivity, two-wave beam coupling gain, grating phase shift, and diffraction efficiency as a function of intensity, applied electric field, and grating spacing.

Large gain photorefractive polymers

We describe on-going research to develop photorefractive polymeric materials with improved speed, material stability, and high beam coupling gain. The demonstration of significantly improved two-beam coupling is shown to mark the entry into a gain regime which enables the observation of new effects for the first time, such as beam fanning and self-pumped phase-conjugation. These effects have previously been reserved to a few thick high gain inorganic photorefractive crystals. We discuss how the large beam coupling has forced the reinterpretation of such traditional characterization techniques such as the grating translation method for the determination of the spatial phase of the index grating. Our subsequent material study focuses on several compositional variations to investigate the effect that varying the chromophore and charge transporting polymer has on the photorefractive effect.