JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS Vol. 8, No. 4, August 2006, p. 1529 - 1532 Channeled spectra simulation of an anisotropic poly-(phenylmethacrylic) ester of cetyloxybenzoic acid in tetrachloromethane J. BARAN a , M. POSTOLACHE b* , M. POSTOLACHE c a Institute of Low Temperature and Structure Research of the Polish Academy of Sciences, 2-Okolna Street 50-442, Wroclaw, Poland b “Gh. Asachi” Technical University, Faculty of Automatic Control and Computer Science, 53A Dimitrie Mangeron Blvd., Iaşi, RO–700050, Romania c “Al.I.Cuza” University, Faculty of Physics, 11 Carol I Blvd., 700506, Iaşi, Romania The values of the birefringence n ∆ in the visible range of a 10 7 molecular weight poly-(phenyl methacrylic) ester of cetyloxybenzoic acid (PPMAECOBA) in tetrachloromethane (TCM) liquid crystal was determined from the channelled spectra of thin layers, as previously described. The spectral composition of the emerging light from a device consisting by a thin liquid crystalline layer placed between the crossed polarizers is investigated using 3-D representations in Matlab. The proposed method permits to establish the spectral composition for a given thickness of the anisotropic layer. The thickness of liquid crystalline layer for which the channels or the maxima of a monochromatic radiation are obtained could also be estimated. (Received May 9, 2006; accepted July 20, 2006) Keywords: Liquid crystals, PPMAECOBA in TCM, Channelled spectra 1. Introduction The modern devices based on colour display uses liquid crystalline layers kept between two transparent protective plates [1-4] covered (on their internal faces) with transparent conducting films that facilitates the applying of an external electrostatic field between them [5]. The modern display is based on the two possible geometries of the uniaxial liquid crystalline layers: a homeotropic one, with mesogenic molecules having their long axes perpendicular on the protective plates, and a homotropic one, with mesogenic molecules oriented parallel to the plate surface. The two geometries can be realized by using some anchoring procedures [4] of the mesogenic molecules. The mesogenic molecules are oriented parallel to the field intensity, when the liquid crystal is characterized by a positive electric susceptibility, 0 > ∆ε , while when 0 < ∆ε they are oriented perpendicularly on the plate surface. Thus, the external electric field can improve the degree of order in the anisotropic layer, or it can disorganise the allignement of the mesogenic molecules [5]. When the initial orientation of the mesogenic molecules is perpendicular on the plates, the liquid crystals having 0 > ∆ε improves its orientation along the nematic director. On the other hand, when the initial orientation of the mesogenic molecules is parallel to the plates, the external electrostatic field changes the molecular orientation of the substance, rotating its mesogenic molecules with 90 degrees. When the liquid crystalline layers are used between two crossed polarizers, two situations could be obtained if the device is illuminated by a monochromatic radiation [6,7]: - total extinction is obtained when the mesogenic molecules are oriented perpendicularly on the plates; - monochromatic light is obtained after the second polarizer, when the mesogenic molecules are parallel to the plates. The radiant intensity is maxim when the nematic director makes 45 degrees with the transmission directions of the polarizers. The device consists of two crossed polarizers and the liquid crystalline layer between them having its director at 45 degrees compared with the transmission direction of the polarizers. When this device is illuminated with visible radiation, the emerging from the liquid crystalline layer visible components have a changed polarization state, thus the spectral composition of the emergent light from the device differs from that of the incident light [8-10]. The spectral changes in the light composition depend both on the thickness and on the birefringence of the liquid crystalline layer. For a given liquid crystal we can estimate, by using a program written in Matlab, the thickness of the anisotropic layer corresponding to a given spectral composition of the emergent light. 2. Theoretical An anisotropic uniaxial substance is characterised by two values of the refractive index when it is related to the