No abstract

The surface of an azoaromatic polymer film is optically altered to produce local highly efficient diffraction gratings. The gratings obtained are stable but can be erased by heating the polymer above its glass transition temperature and no permanent damage of the film is observed. Multiple gratings can be simultaneously written and gratings can be overwritten. Atomic force microscopy was used to investigate the gratings produced on the surfaces. Possible mechanisms responsible for the surface alteration are discussed.

A series of amorphous azobenzene-containing polymers were cast as thin films and shown to produce both reversible volume diffraction gratings and high-efficiency surface gratings by laser irradiation at an absorbing wavelength. The latter process involves localized mass transport of the polymer chains to a high degree, as atomic force microscopy reveals surface profile depths near that of the original film thickness. A mechanism for this phenomenon is proposed which involves pressure gradients as a driving force, present due to different photochemical behaviors of the azo chromophores at different regions of the interference pattern. The phase addition of the two beams in the interference pattern leads to regions of high trans-cis-trans isomerization by the absorbing azo groups, bordered by regions of low isomerization. As the geometrical isomerization requires free volume in excess of that available in the cast films, the photochemical reaction in these areas produces a laser-induced internal pressure above the yield point of the material. It is proposed that the resulting viscoelastic flow from these high-pressure areas to lower-pressure areas leads to the formation of the regularly spaced sinusoidal surface relief gratings observed by a number of research groups, but previously unexplained. This mechanism of photoinduced viscoelastic flow agrees well with the results of experiments investigating the effect of the polarization state of the interfering writing beams and the photochemical behavior of the chromophore, the free volume requirements of the induced geometric changes, and the viscoelastic flow of the material.

The photochemical and thermal cis-to-trans isomerization of aromatic azo groups covalently bonded within polymers and its practical consequences as well as some other recent applications are reviewed. The kinetics and mechanism of the isomerization of azo polymers are briefly presented, and then several recent developments in azo polymers research are discussed. They include liquid crystallinity, nonlinear optical properties, monolayer assemblies, and the lightinduced birefringence and dichroism. Some of the phase transitions of liquid-crystalline azo polymers can be induced photochemically by the azo groups isomerization. Second-order optical nonlinearity can be obtained in polymers with donorand acceptor-substituted azo groups. Using the Langmuir-Blodgett technique, azo polymer monolayers can be built and used as "command surfaces" of liquid-crystal films. The light-induced birefringence is a reorientation phenomenon which is a consequence of the trans-cis-trans isomerization. Future possible applications for a variety of practical devices, such as display devices, optical modulators, optical waveguides, holography, and reversible optical storage are mentioned.

ethyl]ethylamino]-4-nitroazobenzene] (pDRIA) is used as an example to demonstrate the ability of amorphous high-Tg azoaromatic-containing polymers to reversibly store optical information. Writing, reading, erasing, and rewriting processes are performed using laser beams on a pDRIA film. This ability is based on the well-known trans-cis-trans isomerization process which was previously studied on liquid-crystalline polymers containing azoaromatic groups. Liquid crystallinity is not a necessary condition for optical storage. As well, erasing can be performed optically, eliminating the need to heat the polymer sample. The writing and erasing are statistical processes, and the level of optical anisotropy induced depends on the photon flux directed at the sample. The time evolution of the writing process is investigated. A computer simulation of these processes is performed taking into account a relaxation process in which some of the reoriented trans molecules are randomized by thermal motion. The computer model replicates the general features of the optical storage processes. Possible applications and future work on this class of polymers are also discussed.