Abstract
We report the observation and measurement of second-harmonic generation in self-assembled ultra thin film nonlinear optical materials using a femtosecond high repetition rate laser system. Second-harmonic intensity, as a function of the incident angle in these films, has been measured using incident p-polarized and s-polarized optical beam components. The second-order nonlinear optical susceptibilities of the thin films have also been determined. Using a curve fitting method and a crystal reference material, we have obtained second-order susceptibilities c333 = 6.17 ± 0.18 pm/V and c311 = 0.68 ± 0.02 pm/V at a fundamental wavelength of 1,200 nm. Based on linear molecular model approximation, we have also used the fitted data to investigate the average orientation distribution of the chromophore dipoles in the self-assembled film. The result indicates that the average tilt angle of the chromophore dipoles away from the substrate normal line is 25.2° ± 0.8°.
Highlights
Recent interest in conjugated organic molecular and polymeric materials has been stimulated in part due to their intrinsic large optical nonlinearities
Since the first external electric field poled second order nonlinear optical (NLO) polymers were reported in 1982 [1,2], intensive studies have led to the development of commercial polymer guided-wave nonlinear optical devices [3]
We report here detailed studies of the electrostatic self-assembly technique for the creation of noncentrosymmetric thin films for NLO
Summary
Recent interest in conjugated organic molecular and polymeric materials has been stimulated in part due to their intrinsic large optical nonlinearities. Due to the thermal and temporal-dependent relaxation of chromophore alignment in conventional electric field-poled guest-host nonlinear optical polymer materials, the development of alternative techniques to form such materials without the need for electric field poling has been investigated. These techniques include the electrostatically self-assembly method by means of alternate adsorption of polycation and polyanion to build multilayer thin films on solid substrate [8,9], fabricating NLO superlattices by self-assembling via covalent linking [10], and the Langmuir method to prepare monolayers at the air-water interface [11], which provide new possibilities to fabricate NLO thin films without poling by an external electric field
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