C. A. Walsh

Orientational decay in poled second‐order nonlinear optical guest‐host polymers: Temperature dependence and effects of poling geometry

The orientational decay of chemically and thermally stable high‐temperature chromophores doped into thin films made from polyimides and a variety of other polymeric hosts has been investigated. The chromophores are aligned using electric field poling and second‐harmonic generation (SHG) is used to probe the decay of the electric field poling induced alignment. The decay rate of the SHG signal from films poled using both a corona discharge and side‐by‐side in‐plane electrodes was measured. When electrodes are chosen so that the effects of charge injection are minimized, little difference has been observed between the orientational decays from films poled using the two methods for either an amorphous preimidized polyimide host or a highly anisotropic film poled during imidization. The films imidized during poling showed significant orientational stability at 250 °C for over 15 h after a fast initial partial decay. In addition, the decay of the SHG signal was measured as a function of temperature below the g...

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.

Re‐evaluation of the thermal stability of optically nonlinear polymeric guest‐host systems

Guest‐host polymer systems with potential use in electro‐optic devices are discussed. The polymer host is a polyimide and the guest chromophores are 2,4,5‐triarylimidazoles (lophines). Poling stabilities have been obtained by extrapolating the second harmonic generation decay using a stretched exponential function and extrapolated lifetimes greater than a year at 80 °C have been obtained. In addition, an apparent relationship between the stability of poled order and the glass transition temperature is discussed.

The design, synthesis, and evaluation of chromophores for second‐harmonic generation in a polymer waveguide

In order to develop nonlinear optical chromophores for frequency doubling applications, it is important to understand the magnitudes of the relevant chromophore properties that will be required. In this paper the use of organic polymers with attached optically nonlinear chromophores to form waveguide second‐harmonic‐generation devices will be discussed. In particular, the use of such a frequency doubling device in optical storage applications will be considered. To this end limits on the quantity μβ are obtained where μ is the dipole moment and β the hyperpolarizability. Experimental values for electron donor/acceptor‐substituted benzenes, stilbenes, and tolanes are compared to these requirements. None of the chromophores treated here have both sufficiently high optical nonlinearity and sufficiently low optical absorption at a doubled optical frequency of 400 nm to be practical in the specific application described.

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 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.

Picosecond photoionization and geminate recombination in an organic donor—acceptor complex

Abstract The initial steps involved in the charge separation process in thin films of the organic charge-transfer complex formed from poly-N-vinylcarbazole (PVK) and 2,4,7-trinitro-9-fluorenone (TNF) are observed directly using polarization selective pico-second transient grating techniques. When forming a grating with parallel polarized writing beams, a nonexponential decay of the transient grating signal is observed. When forming a grating with perpendicularly polarized writing beams, the decay of the transient grating signal is observed to increase and to be described adequately as a single exponential. These transient grating signals have been investigated as a function both of an applied electric field and as a function of the TNF:PVK ratio. All of these results are consistent with a four-level kinetic model that includes charge hopping, geminate recombination, and field-assisted charge separation.

Photorefractivity in doped nonlinear organic polymers

This paper describes the properties of a new class of materials exhibiting the photorefractive effect, doped nonlinear organic polymers. Photorefraction (at 647.1 nm) was established by a combination of hologram erasability, correlation with photoconductivity and electro-optic response, and enhancement by external fields in numerous samples (178 to 533 micrometers thick) of two nonlinear epoxy materials doped with hole transport agents based on p- diethylaminobenzaldehyde-diphenyl hydrazone (DEH). Diffraction efficiencies up to 0.1% were observed at bias fields near 100 kV/cm. A useful property of these materials is that poling of the nonlinear chromophores is partially reversible, permitting partial control of the grating readout independent of the space-charge field formed. The polarization anisotropy of grating readout is consistent with the photorefractive mechanism. Two-beam coupling measurements of both absorption and index gratings show (1) the absorption gratings are approximately 10 times smaller than the index gratings, and (2) the phase shift of the index grating is near 90 degree(s), which cannot occur via photochromism, heating, or any other process except photorefractivity.

From electric field-induced second harmonic generation (EFISH) to electro-optic measurements of nonlinear chromophores

Experimental measurements of molecular hyperpolarizabilities can be useful in two ways. First, they allow comparison between different chromophores, so that structure-function relationships can be understood and increasingly nonlinear compounds synthesized. This application requires only good relative measurements. The second and ultimately more important application is prediction of the macroscopic nonlinear optical properties of custom materials. The latter application requires accurate absolute values for the molecular parameters. Examples of both types of comparisons are discussed below. We describe experimental measurements on several types of nonlinear optical chromophores, and the choice of conventions and reference standards that leads to accurate predictions of electro- optic coefficients in poled polymers. The degree to which nonlinearity and thermal stability are correlated is also discussed.