Anders Grunnet-Jepsen

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.

Spectroscopic determination of trap density in C60-sensitized photorefractive polymers

Abstract A new and simple method for the determination of the trap density in C 60 -sensitized photorefractive polymers is presented. We show that the radical anion of C 60 acts as the primary photorefractive hole trap, using the strong near-infrared absorption of C 60 − to quantify the anion concentration in situ. The spectroscopically determined concentration correlates well with the photorefractive trap density obtained from analysis of the photorefractive performance. In our model, optical irradiation is needed to activate the trapping sites, and the nonlinear optical chromophore acts as a temporary reservoir for the photogenerated holes (compensator). This model is further clarified using cyclic voltammetry.

Homodyne detection of ultrasonic surface displacements using two-wave mixing in photorefractive polymers

We demonstrate an improved laser-based receiver for the detection of small ultrasonic surface displacements. The receiver is based on a homodyne interferometer using a photorefractive polymer as an adaptive beam combiner. The interferometer requires no path-length stabilization and can process speckled beams from rough surfaces. In experiments using a 10 mW diode laser at 676 nm, we have measured a shot noise-limited surface displacement sensitivity that is within a factor of three of the ideal limit for a lossless, plane wave interferometer.

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.

Space charge field enhancement in photorefractive materials by applied sinusoidal fields: An approximate analytical solution

Abstract The second order temporal differential equation, known to describe the space charge field in the high frequency region, is solved approximately in a closed analytical form. The condition for multiple resonances is found. The maxima and minima in the space charge field are shown to correspond to the roots of the zero and first order Bessel functions respectively. The analytical approximations for the space charge field are compared with numerical results for the complete range of frequencies, and it is shown that there is excellent agreement between the two for the high frequency region which includes all the resonances.

DYNAMICS OF THREE COUPLED LONG JOSEPHSON JUNCTIONS

Abstract The dynamics of a system of three long Josephson transmission lines coupled at a common end point is investigated. We report several periodic fluxon states and trace out the corresponding zero field steps. The boundary conditions at the common point lead to a very different stability of steps for odd and even numbers of fluxons. In addition we find two “normal state” branches for the IV curve, where either two or three of the branches are in their normal state.

Recent advances in high-gain photorefractive polymers

This talk focuses on the development and characterization of new high-gain photorefractive polymers, with emphasis on a particular stable composite composed of the photoconductor poly(N-vinylcarbazole) (PVK), the charge generator C/sub 60/, the nonlinear optical chromophore 4-piperidinobenzylidene malononitrile (PDCST), and the liquid plasticizer butylbenzyl phthalate (BBP). The optical properties of this material, denoted PVK:PDCST:BBP:C/sub 60/, have been reported recently, and the material has been shown to exhibit a high two-beam coupling gain coefficient, fast response time, over-modulation of diffraction efficiency, high sensitivity, and high stability. Using such high-gain coefficients, we believe we have recently observed amplified scattering (beam fanning) in a photorefractive polymer for the first time.

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.