Hanna Schwartz, P. Krief, J. Becker, Lev Shapiro, Vladimir Khodorkovsky, Jacob Klug, E. Kovalev, Guilia Meshulam, Garry Berkovic, Zvi Kotler, Schlomo Efrima
Langmuir-Blodgett films have been prepared from amphiphilc molecules containing an indandione-based nonlinear chromophore. Study of the pressure-area (π-A) isotherm enabled us to find optimal conditions for monolayer transfer to a glass substrate. The multilayer films thus formed exhibited strong optical second harmonic generation with a bulk nonlinear co-efficient equal to the ideal value predicted by the product of the chromophore density and its known molecular hyperpolarizability.
One limitation of using electric field induced second harmonic (EFISH) to determine the molecular first hyperpolarizability (beta) of nonlinear optical molecules lies in the fact that part of the second harmonic signal comes from the second hyperpolarizability (gamma) produced by mixing two optical fields with the DC field. In analyzing EFISH results, the second hyperpolarizability contribution of the studied molecules is generally neglected. We present a modified time resolved EFISH technique that allows us, in a single experiment, to determine separately the beta and the gamma contributions. We study para-nitro aniline dissolved in Glycerol, a highly viscous solvent, and apply the DC field via a high voltage pulse with a fast rise time of approximately 40 nsec. As a result, the orientation of the molecules under the applied electric field is slow relative to the build-up of the field, enabling us to directly measure only the DC induced second harmonic (gamma contribution), at the beginning of the HV pulse. The pure beta contribution is determined from the difference between this signal and the conventional EFISH signal at the plateau of the HV pulse. Our result confirm that the gamma contribution is indeed less than 10% of the total.
The second order nonlinearity of conjugated organic molecules involving, 1,3 indandione derivatives as an acceptor moiety has been studied. Varying the donor from dialkylamino to the chemically similar substituent, N- carbazolyl resulted in a drastic reduction of electric field induced second harmonic (beta) values. For some molecules, even a small negative value of (beta) was received. Quantum chemical calculations indicate that the decrease occurs as a result of two overlapping transitions, which contribute to (beta) with opposite signs. The charge transfer band gives a positive (beta) zzz along the molecular long axis, while a transition essentially within the carbazole moiety provides a negative (betazzz contribution to (betaEFISH. Thus, these molecules must be described with a 2D model as opposed to the 'classical' model of 1D nonlinear optical chromophores. The prediction of the 2D model was verified experimentally by using a combination of two methods, EFISH and Hyper-Rayleigh Scattering, which probe different combination of the (beta) tensor elements.
We report on a new unique photochromic material which is based on a reversible formation - cleavage of a C-C bond. The bicyclic bindon derivative, 2 undergoes a photochemical and/or thermally induced ring opening to form the isomer 3. The form 3 presents a conjugated donor-acceptor system and exhibits a considerable second-order optical nonlinearity as found by the field induced SHG measurements. The photochromic conversion is also observed in the crystalline form indicated visually by a crystal red-to-green color change. We have studied the reversible ring opening - closure process in liquid and polymeric solutions. Optical and thermal switching and the NLO efficiency of these guest- host polymer are reported.
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