Langmuir-Blodgett films for non-linear optics
In recent years there has been considerable interest in media which display significant non-linear optical properties; the telecommunications industry may exploit chin films of such materials for signal processing applications. The Langmuir-Blodgett (LB) technique provides a means of depositing organic layers of a precisely defined thickness. Moreover, by alternating layers of different materials, supermolecular arrays may be fabricated in which there is no centrosymmetry, and therefore the second-order non-linearity of the constituent molecules may be exploited. An investigation of the properties of water-surface monolayers of a number of novel materials with potentially large non-linearities is described. Several of these compounds are shown to form high quality homogeneous or heterogeneous LB films. The optical and electrical properties of the layers are characterized by optical absorption spectroscopy, surface plasmon resonance, and measurements of capacitance, whilst their structure is examined by electron diffraction. Monolayers of a nitrostilbene dye are shown to exhibit: on exceptionally high degree of crystalline order. Data are also given for theoretical calculations of non-linear coefficients and for the relative efficiency of second harmonic generation from bulk samples of various materials. Studies of second harmonic generation from monolayer and alternate multilayer films are reported. Optical non-linearity in an alternating donor-acceptor: inverted donor-acceptor dye system is demonstrated for the first time; the results are analysed in terms of second harmonic surface susceptibilities, and the value of the second-order hyperpolarizability determined for the first bilayer is found to be much superior to that expected by the simple addition of the hyperpolarizabilities of the separate layers. Monolayers containing a mixture of hemicyanine and cadmium arachldate are found to give rise to second harmonic generation which is enhanced relative to that obtained from a pure monolayer of the dye. Corresponding changes in the absorption spectra of the layers can be observed. These findings may have important implications for improving the efficiencies of any non-linear optical device which utilises IB films.