Electrically controlled surface diffraction gratings in nematic liquid crystals

Abstract

Photorefractive diffraction gratings were studied in cells of homeotropically aligned pentyl-cyanobiphenyl liquid crystal. These holographic gratings were induced by the simultaneous and nonsimultaneous application of dc and coherent optical electric fields. The observed behavior was consistent with a predominantly surface-mediated photorefractive effect. Beam coupling was observed in all cases and led to a model involving screened and unscreened interfacial trapped charges driving a modulation of the easy axis. Holographic gratings could be switched on and off by the application of a small voltage.

Surface-Mediated Beam Coupling in Nominally Pure Nematic Liquid Crystal

“…Photorefraction in liquid crystals has recently attracted significant attention since it is characterized with strong optical nonlinearities at low light intensities and voltage [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. Currently, liquid crystalline materials are considered a leading candidate for those applications that do not require long storage times.…”

“…Tabiryan et al proposed to use a photorefractive aligning surface with this aim [10]. Singer, Reshetnyak, Reznikov et al found that surface-mediated effect could also arise if photo-induced charges in the aligning layers changed the distribution of ions in the LC bulk [11] or due to the photochemical reaction on the aligning surface [12]. It was shown recently that surface mediated photorefractivity can be also initiated due to a light-induced adsorption of fullerene-based dopant [13].…”

On liquid crystal diffractive optical elements utilizing inhomogeneous alignment

“…Several types of LC optical elements have been proposed [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. The needed spatial distribution of the LC director is achieved in the majority cases by means of an inhomogeneous electric field [1][2][3][4][5][6][7][8][9][10][11][12] or with a thickness variations [1,2,13,14].…”

“…The needed spatial distribution of the LC director is achieved in the majority cases by means of an inhomogeneous electric field [1][2][3][4][5][6][7][8][9][10][11][12] or with a thickness variations [1,2,13,14]. Adaptation of such approaches for micron-scaled systems poses technological difficulties due to that electrodes are required or due to geometrical non-uniformities.…”

Electrically tunable, optically induced dynamic and permanent gratings in dye-doped liquid crystals

“…Moreover, by doping a polymer layer with dye, such as azo-dye, gratings can be also written directly into that layer [20]. It was also shown that the diffraction efficiency of such doped-polymer-mediated gratings can be varied (but not enhanced) by application of an ac/dc field [12], [24]. Patterns [3], [23] written in cells without special coating of the cell surfaces persisted in time, but typically decayed over a period of a few hours.…”