Response Of Langmuir Research Articles

A theoretical model for the pyroelectric response in Langmuir–Blodgett films

Understanding the structure of pyroelectric materials is important to elucidate the nature of the temperature-dependent microscopic and macroscopic electric polarisation. Pyroelectric materials must have a non-centrosymmetric structure. Previously, researchers [C.A. Jones, PhD thesis, University of Durham, (1987); J. Mater. Chem. 1 (1991) 819; Langmuir 11 (1995) 4623] have reported the pyroelectric effect of non-centrosymmetric ultrathin LB films prepared using the alternate layer Langmuir–Blodgett (LB) deposition technique. They have proposed three main mechanisms to explain the origin of the pyroelectric activity, namely, tilting, proton transfer, and ionic processes. Linear and cyclic polysiloxane materials with aliphatic and aromatic side groups have been studied in this work. These materials have been alternately deposited with eicosylamine to form the active pyroelectric material in metal–LB film–metal (MIM) devices, whose pyroelectric coefficients have been measured using a quasi-static measurement technique [W.H. Majid, Abd., PhD thesis, Univ. of Sheffield (1994); Mater. Sci. Eng., C, Biomim. Mater., Sens. Syst. 3 (1995) 197; Thin Solid Films 327–329 (1998) 369]. The relation between pyroelectric mechanism(s) and microscopic and macroscopic pyroelectric response for polysiloxane/eicosylamine alternate layer LB films will be explained in this work. Results indicate that the physical mechanism by which the pyroelectric activity arises in the LB films is critically dependent upon their structural quality, the ions within the multilayer arrangement and the molecular dipole moments of the molecules.

Microscopic calculations of linear and quadratic optical response in model Langmuir–Blodgett multilayers

Microscopic calculations are reported of linear and quadratic nonlinear optical response in Langmuir–Blodgett films. The films are modeled as ordered layers of molecules that can tilt from the vertical and have a higher polarizability and first hyperpolarizability in the head than in the tail. The local electric field varies little after the first layer and negligibly after the second, which simplifies the analysis. Tilted two-layer Y-type films can have quadratic susceptibility components as large as X- or Z-type ones. Overall the results provide a systematic account of the linear and nonlinear optical properties as a function of molecular and film structure that should assist interpretation of experimental results.

Modification of nonlinear optical response in Langmuir–Blodgett films by ultraviolet laser-induced bleaching

Permanent modifications of the second-order nonlinear susceptibility χ(2)(−2ω;ω,ω) of certain Langmuir–Blodgett (LB) films can be induced photochemically by local irradiation with UV laser light, with the spatial resolution limited only by the laser spot size. In a LB film consisting of alternating monolayers of poly[N-(p-perfluorodecylsulfonylphenyl)-L-prolinol acrylate] and a ‘‘buffer’’ polymer, the kinetics of the laser bleaching process, as studied by optical second-harmonic generation, is found to be first order. The generation of a ‘‘χ(2)-grating’’ in the LB film is also demonstrated.