D. P. West

Determining the cubic susceptibility χ (3) of films or glasses by the Maker fringe method: a representative study of spin-coated films of copper phthalocyanine derivation

An experimental analysis technique is described for accurate extraction of the coherence length and the complex third-order nonlinear susceptibility χ(3) in films or bulk optical glasses from the Maker fringes obtained by third-harmonic generation measurements. This method permits calculation of the third-order susceptibility of a sample without the need for assumptions as to the magnitude or the phase of the nonlinearity relative to a reference material such as the substrate. To illustrate the utility of the method we study spin-coated films of copper 2,3,9,10,16,17,23,24-octa(1,4,7,10-tetraoxaundecyl)phthalocyanine.

Holographic index-contrast prediction in a photorefractive polymer composite based on electric-field-induced birefringence

The physical properties of a hologram written in a photorefractive polymer composite are predicted from a macroscopic model based on ellipsometry. An electric poling field increases the birefringence of the composite. The way that the bulk birefringence changes with the electric field is used to predict the holographic index contrast, and, by comparison with experiment, accurate deductions of the holographic space-charge field are made. A photorefractive polymer composite was used that contained 47.5 wt. % 1-(2′-Ethylhexyloxy)2,5-dimethyl-4-(4″ nitrophenylazo)benzene electro-optic dye. When sinusoidal modulation in optical intensity is used with high contrast, the higher spatial harmonics of the modulation of the holographic space-charge field become important. The amplitude of the first-order modulation in the space-charge field is accordingly reduced by 13% relative to the predictions of the standard model of photorefractivity in the case of a high-saturation field.

Photorefractive polymer composite trapping properties and a link with chromophore structure

The photorefractive properties and the phase stability of polymer composites are dependent on the detail of the alkyl chain substituent attached to the electro-optic dye within the composite. Photorefractive composites based on poly (N-vinylcarbazole) (PVK), sensitized with trinitrofluorenone (TNF) and mixed with a concentration of 47.5 wt. % of electro-optic dye have been tested for photorefractive performance. Two alternative azo dyes of identical molecular weight have been used to produce alternative composites; both dyes were modified to suppress spatial isomerism and incorporated an eight carbon alkyl chain at the electropositive end of the chromophore: either a straight octyl chain or a branched ethylhexyl chain was substituted. The reorientational enhancement of photorefractive performance is similar in the composites resulting from these dyes. The dye with a straight octyl chain led to a composite with improved holographic performance. The dye with a branched ethylhexyl chain led to a composite ex...

Effect of field-dependent photogeneration on holographic contrast in photorefractive polymers

The field-dependence of the charge photogeneration efficiency reduces the holographic contrast in an organic photorefractive composite. In this article, the Braun model for field-dependent photogeneration efficiency is incorporated into the standard model of photorefractivity. Using this formalism, the effect on the space-charge field, and thereby holographic index contrast, is calculated directly without the need for empirically found parameters. The dependence of the space-charge field on experimental geometry is also analyzed. A powerful description of the effective space-charge field is thus derived and is experimentally verified. This analysis is then used to compare the observed holographic contrast to that predicted from transmission ellipsometry measurements.

NLO properties of polymeric Langmuir-Blodgett films of sulfonamide-substituted azobenzenes

The behaviour at the air/water interface of polymeric materials containing sulfonamide-substituted azobenzene chromophores has been investigated using surface pressure versus area measurements. Langmuir-Blodgett (LB) films of the polymers have been built up and then studied using X-ray diffraction. The polymer which gave the best organised LB film has been used to prepare an alternating Y-type multilayer structure with a polymer inactive in non-linear optics. The obtained film has a well-ordered polymeric LB structure giving three Bragg peaks. A series of films with various numbers of bilayers within the range from 2 to 80 has been prepared and the second harmonic generating properties of the films investigated. The measured intensity of the SH signal produced from incident light of wavelength 1064 nm was proportional to the square of the number of bilayers. The alternating LB films exhibited a non-linear susceptibility of up to 24 pm V –1 corresponding to a chromophore non-linear vector susceptibility of magnitude χ zzz (2) =170 pm V –1 .

Processes limiting the rate of response in photorefractive composites

Abstract The processes limiting the rate of response of highly diffractive, reorientationally enhanced photorefractive polymer composite materials are identified from a series of degenerate four wave mixing and Mach–Zehnder interferometric measurements. In the regime of low intensity writing beams charge generation limits the rate of holographic grating formation, but at higher intensities charge transport or reorientation of dye molecules can restrict the rate of grating formation more strongly. A grating risetime of 540 ms is observed in a composite of high dye content with high reorientational mobility of the dye molecules. In this case it is proposed that the charge carrier mobility of the doped poly(N-vinylcarbazole):2,4,7-trinitro-9-fluorenone (PVK:TNF) matrix is the principal limiting factor in grating response rates.

Poling and relaxation of dipoles in dispersive media

The poling of dipoles in dispersive media under the influence of an applied electric field and the subsequent relaxation after the removal of the applied field have been analyzed. Both the poling and relaxation processes are described by simple and exact analytical solutions to the rotational diffusion equation. The resulting expressions for the order parameters are power laws with respect to time and hence have no characteristic lifetimes. The poling and relaxation rates are, in fact, determined by a single material parameter: the diffusion constant, D0. These theoretical results have been experimentally verified for dipolar molecules doped into an amorphous polymer host. An excellent agreement between theory and measurement was found.

Photorefractive trapping and the correlation between recording and erasure dynamics in a polymer composite

Abstract The fast time constants of holographic contrast growth and erasure for various photorefractive polymer composites were measured under a wide range of experimental conditions. Either of two sensitizers, 2,4,7-trinitro-9-fluorenone (TNF) or (2,4,7-trinitro-9-fluorenylidene)malonodinitrile (TNFDM), were used in a degenerate four-wave mixing experiment under varying conditions of field, wavelength and intensity. Collected together, the fast growth and erasure time constants were found to be correlated with a coefficient of 0.96. From this correlation we conclude that the charge generation process during holographic recording and the optical de-trapping process during hologram erasure are indistinguishable. Hence in accordance with the photorefractive crystal model, hole recombination with the sensitizer anions forms the primary charge trapping mechanism in the majority of photorefractive polymer composites.