The thermal stability of the electric field induced poled order in a new class of second-order optically nonlinear polymers, "donor-imbedded" side-chain polyimides containing no flexible connectors or tethers to the nonlinear optical (NLO) chromophore, is investigated. In these polymers, the electron-donor part of the chromophore is a diaryl-substituted amine that is incorporated as a part of the polymer backbone. The donorimbedded systems used in this study have exceptional chemical stabilities at elevated temperatures (350 degrees C) and impressive poled order stability at extremely high temperatures (300 degrees C). In both respects, they were significantly more stable than a true side-chain polyimide with a similar NLO-active chromophore covalently linked to the polymer backbone by a flexible tether group.
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.
The applicability of the technique of two-photon four-level photochemistry for the production of near-IR holograms is critically evaluated. Although the current best system (biacetyl in a polycyanoacrylate matrix) has low recording sensitivity, several applications of the material have been demonstrated. Hologram efficiencies of up to 70% recorded at 752 nm are shown. Recording of extended holographic images and production of holographic optical elements are demonstrated. Finally, we have recorded holograms directly with GaAlAs diode lasers.
Two-photon holographic recording with cw laser sources in the 750-1100-nm spectral range is accomplished using recording materials consisting of an alpha-diketone dissolved in a poly-alpha-cyanoacrylate host. Gating of the holographic sensitivity by irradiation with incoherent UV radiation is demonstrated. Hologram diffraction efficiencies as large as 10% are achieved.
Two-photon holography with continuous-wave laser sources is accomplished by using carbazole dissolved in a polymethyl methacrylate host matrix as a recording medium. Gating of the holographic sensitivity for 4880-A radiation by irradiation with 3336-A radiation is demonstrated. It is shown that two-photon holographic recording can be accomplished with a wide variety of molecules characterized by four-level spectroscopic systems with cascade excited metastable intermediate levels.
Vibronic radiationless transitions in large polyatomic molecules can be thought of as a process whereby the molecule, initially prepared in a discrete quasistationary state, makes a transition to an adjoining vibronic continuum belonging to a lower electronic state of the same multiplicity. In many instances the transition is analogous to penetration through a barrier between two "nested" potential sheets far away from an actual intersection of the sheets. Simultaneous distortions of vibrational and electronic parts of the wavefunction are required for such a tunneling process. The vibrational distortion manifests itself in the familiar Franck-Condon effect. The electronic distortion can be caused by nontotally symmetric vibrations of the molecule either because of the displaced nuclear configuration, to which the electrons instantly respond (Herzberg-Teller effect), or because of the protracted response of the electrons to the kinetic energy of nuclear motion (nonadiabatic effect or the breakdown of the Born-Oppenheimer approximation). It is found here that Herzberg-Teller coupling is more effective in causing radiationless transitions than the breakdown of the Born-Oppenheimer approximation when the potential surfaces involved are not close to an intersection.
