Design of optimized photorefractive polymers: A novel class of chromophores

Abstract

It is demonstrated that the microscopic mechanism of the photorefractive (PR) effect in organic composites with low glass transition temperatures involves the formation of refractive index gratings through a space-charge field-modulated Kerr effect. A tensorial formulation of the macroscopic aspects of the PR Kerr effect and its microscopic interpretation is presented. The second-order dipole orientation term containing the anisotropy of the first-order optical polarizability α(−ω;ω) is shown to yield the dominant contribution to the Kerr susceptibility χ(3)(−ω;ω,0,0). A class of special chromophores having negligible second-order polarizabilities β(−ω;ω,0) and large dipole moments μ has been identified in order to optimize this term. These chromophores are not subject to the efficiency-transparency tradeoff typically encountered with second-order nonlinear optical (NLO) chromophores, providing highly transparent materials with large PR Kerr response. Contrary to previous approaches in this field, the best-performing PR polymers are then expected to employ chromophores that would be useless for second- order applications (negligible β). We report PR of the material 30% 2,6-di-n-propyl-4H-pyran-4-ylidenemalononitrile (DPDCP): 15% N,N′-bis(3-methylphenyl)- N,N′-bis(phenyl)benzidine (TPD):55% poly(methyl methacrylate) (PMMA):0.3% C60 as an illustration of this principle. A 100 μm thick film of this material exhibits a steady-state diffraction efficiency of η=25% and net two-beam coupling of Γ=50 cm−1 at a bias field of 100 V/μm and a wavelength of 676 nm. The macroscopic Kerr susceptibility of the material is related to molecular electronic properties of the chromophore DPDCP which were independently determined by experiments in solution and by quantum chemical calculations.

Stereoelectronic, Vibrational, and Environmental Contributions to Polarizabilities of Large Molecular Systems: A Feasible Anharmonic Protocol

“…In this case we only limit ourselves to the dynamic polarizability computed at 633 nm, for which experimental measurements exist. 56 Results are summarized in table 5. The discussion is very similar as in the two previous cases: the electric anharmonicity gives again the greatest contribution to the zero-point polarizability, whose inclusion shifts the computed value in the right direction with respect to the experimental measurement.…”

Konjugierte Oligomere mit terminaler Donor‐Acceptor‐Substitution

“…Wortmann und Mitarbeiter [7][8][9] haben den nach Gleichung (32) zoukas et al [6] verwenden in Gleichung (30) sin q/2 anstelle von c und definieren den "Mischparameter" MIX als Àcos q, was (2c 2 À1) in Gleichung (33) entspricht. Marder und Mitarbeiter [2][3][4][5] führten den Parameter BLA für die Bindungslängen-Alternanz in linearen D-p-A-Ketten ein.…”

“…Eine Reihe von Parametern wie BLA, [2][3][4][5] MIX [6] oder c 2 [7][8][9] wurde zur Quantifizierung zwitterionischer Grenzstrukturen vorgeschlagen, die in Abschnitt 5 diskutiert werden sollen. Es sei allerdings hier schon vermerkt, dass das "Gewicht" der jeweiligen Resonanzstrukturen von externen Faktoren wie dem Lösungsmittel oder einem elektrischen Feld abhängt.…”

Dark decay of holograms in photorefractive polymers

“…This gives rise to a space-charge field E SC , which modulates the refractive index of the material through the linear electrooptic effect and orientational effects. 2,3 Photorefractivity in amorphous polymers has been intensively investigated, 4,5 and these systems have been widely recognized as potential active media in rewritable holographic optical memories for security applications, 6 in associative memories, 7 or in adaptive ultrasound sensors. 8 Due to the rather low dielectric constants of polymers ( Ͻ10), oppositely charged carriers show a rather strong tendency to recombine.…”

“…11,3 The experimental data were normalized to the index modulation achieved at the end of recording and fitted by bi-or triexponential decay functions. In order to obtain a unified measure for the general decay dynamics, we calculated the combined logarithmic averages of the relaxation times ͗ ͘ according to 12…”