S. M. Silence

Net two-beam-coupling gain in a polymeric photorefractive material

Two-beam-coupling gain coefficients exceeding the absorption coefficient are demonstrated for the first time to our knowledge in an organic photorefractive system. The material is based on the photoconducting polymer poly(N-vinylcarbazole), doped with the optically nonlinear chromophore 3-fluoro-4-N,N-diethylamino-β-nitrostyrene and sensitized for charge generation with 2,4,7-trinitro-9-fluorenone. The photorefractive performance is significantly better than that of any previously described organic. Diffraction efficiencies as large as 1% in a 125-μm sample, grating growth times of the order of 100 ms, and beam-coupling gain coefficients >10 cm−1 were observed at 647 nm (writing intensity of 1 W cm−2, applied field of 40 V μm−1).

C60 sensitization of a photorefractive polymer

The fullerene molecule C60 is shown to act as a useful sensitizer of a recently discovered photorefractive polymer. Measurements of the steady‐state diffraction efficiency, grating growth rate, and other photorefractive properties are presented as a function of C60 concentration, writing intensity, and applied electric field. The dc photoconductivity, grating growth rate, and steady‐state diffraction efficiency all increase by as much as a factor of 20 upon doping with up to 0.2 wt % C60. The sensitization appears to result from a small increase in the carrier generation efficiency and a larger increase in the useful optical absorption at the operating wavelength, 647 nm.

Subsecond grating growth in a photorefractive polymer

We survey the dynamics of the photorefractive effect in a methyl methacrylate copolymer with the nonlinear chromophore p-nitroaniline in a pendant side group doped with a charge-transport agent, diethylaminobenzaldehyde diphenylhydrazone, a material that represents a new class of photorefractive polymer. The grating growth times are several orders of magnitude smaller than that for the previous epoxy-based photorefractive polymers and fall below 1 s at the highest intensities used. Grating competition and revelation effects suggest that charge carriers other than photogenerated holes are mobile. A sublinear dependence of growth rate on writing intensity implies that shallow traps may also be present.

Optical trap activation in a photorefractive polymer.

Optical trap activation has been observed to increase the steady-state diffraction efficiency by a factor of 3 in a photorefractive (PR) polymer, poly(methylmethacrylate):(l,3-dimethyl-2,2-tetramethylene-5-nitrobenzimidazoline):C60 (PMMA:DTNBI:C60). The increased diffraction efficiency is accompanied by an increase in the PR storage lifetime but also by an increase in the characteristic grating growth time of the PR grating. The optical activation of the deep trapping sites is observed to be a sublinear function of the optical irradiation power. Both spatially uniform trap activation and spatial patterning of the photorefractive response of the material are demonstrated.

ELECTRIC-FIELD-SWITCHABLE STRATIFIED VOLUME HOLOGRAMS IN PHOTOREFRACTIVE POLYMERS

Two- and four-layer stratified volume holograms have been fabricated from the photorefractive polymeric material poly(methyl methacrylate): (1,3-dimethyl-2,2-tetramethylene-5-nitrobenzimidazoline):C(60) and characterized by holographic four-wave mixing experiments. Coherent addition of diffracted fields from the individual layers is observed, leading to a diffraction efficiency that increases with the square of the active layer thickness. Electricfield switching of the diffraction efficiencies of individual layers is demonstrated. The angular selectivity of the diffraction efficiency is also characterized for one, two, and four active layers. The angular width of the peaks narrows with increasing total structure thickness in agreement with theory.

Optical properties of poly(N-vinylcarbazole)-based guest-host photorefractive polymer systems.

The photorefractive properties of poly(N-vinylcarbazole) doped with a variety of nonlinear optical chromophores and sensitizing agents are surveyed. Steady-state diffraction efficiencies of greater than 10(-3) and two-beam coupling gain exceeding the absorption loss are found in six materials combinations. The effect of the structure of the nonlinear optical chromophore on the photorefractive properties is discussed.

Electric field‐dependent nonphotorefractive gratings in a nonlinear photoconducting polymer

The molecularly doped polymer system polyvinylcarbazole:Lophine 1:2,4,7‐trinitro‐9‐fluorenone, which is both photoconductive and optically nonlinear, forms a grating whose diffraction efficiency is strongly dependent on the externally applied electric field. Two‐beam coupling measurements show no energy transfer between the beams and grating translation measurements and reveal an index of refraction grating that is not phase shifted relative to the light intensity pattern at any value of the externally applied field. These results lead to the conclusion that, even though diffraction is field dependent, the grating is not photorefractive in origin. This illustrates the importance of two‐beam coupling measurements for characterization of potentially photorefractive polymers. A possible origin of the nonphotorefractive grating based on quadratic electroabsorption is discussed.

Photorefractive polymers - A status report

A photorefractive system is one which is simultaneously photoconductive and electrooptic. Diffraction gratings or holograms can be produced in a photorefractive material by the photogeneration, drift or diffusion and subsequent trapping of mobile charges. The grating is produced by the internal space charge field set-up by these charges which, via the electrooptic effect, produces an index of refraction grating. Until 1990 all photorefractive systems were inorganic crystals such as LiNbO,, BaTiO,, B,,SiO,, InP:Fe, GaAs, or multiple quantum well materials. In 1990, the first observation of the photorefractive effect in an organic material, a carefully grown, doped molecular crystal was described. This was followed in short time by the discovery of photorefractive polymers. The early photorefractive systems, polymeric and crystalline, were inefficient compared to inorganic systems but in recent years the efficiency and sensitivity of photorefractive polymers have become equal to inorganic crystals. This paper describes the origins of the photorefractive effect and the design, synthesis and characterization of photorefractive polymers. In addition an orientational enhancement of the photorefractive diffraction efficiency is discussed. This orientational enhancement does not occur in crystalline systems, since it relies on the ability of the optically nonlinear chromophores to be aligned not only by an externally applied field, but also by the sinusoidally varying space charge field produced during photorefractive grating formation.

Nonlinear optical properties of photorefractive polymers

This paper describes the recent progress of our group in the field of photorefractive polymers. Three separate systems are discussed. The first two consist of polymers with optically nonlinear molecules attached in a pendant side-chain configuration, which are made photoconductive through the addition of a molecular charge-transport agent. The photorefractive properties are investigated using degenerate four-wave mixing (FWM) as well as two-beam coupling (2BC) techniques. The latter technique is essential for confirming the photorefractive origin of the FWM signals through the observation of asymmetric 2BC. The performance of various sensitizing agents used to generate mobile charges at long wavelengths is also compared for such systems. The third system consists of a charge-transporting polymer, which is made nonlinear through the addition of a optically nonlinear guest molecule. The photorefractive properties of this system are found to be superior of those of other polymeric systems, with diffraction efficiencies of 1% in a 125 micrometers thick sample for gratings written with red light (647 and 676 nm), and a 2BC gain coefficient (Gamma) which exceeds the absorption coefficient by 7.2 cm-1 at 753 nm. These properties are comparable to those of inorganic crystals, and should generate interest in organic photorefractive materials for device applications.

Recent progress in photorefractive polymers: materials and structures

A new class of photorefractive polymers is described in which an inert polymer binder is doped with a great molecule called a dual-function dopant, which has the dual functions of providing optical nonlinearity as well as charge transport as required for the photorefractive effect. These materials are the first photorefractive polymers to show both nondestructive readout as sufficiently low reading power and optical trap activation, in which pre-irradiation by a uniform light beam increases the concentration of deep trapping sites in the material. In the area of potential applications, a new sample configuration is described in which photorefractive polymers are fabricated into an electric field switchable stratified volume holographic structure. Individual layers may be activated by applied electric field leading to improved diffraction efficiency and angular selectivity.