Abstract
In a pure nematic liquid crystal (NLC) cell, optically induced charge carriers followed by transports in double border interfaces of orientant/LC and indium-tin-oxide (ITO)/orientant (or LC) can cause removal of screening of the static electric field inside the LC film. This is called surface photorefractive effect (SPR), which induces director field reorientation at a low DC electric field beyond the threshold at a reduced Fréedericksz transition and, as a result, a modulation of the LC effective refractive index. The studies conducted on the photoinduced opto-electrical responses in pure nematic LC cells biased with uniform static DC electric fields support the SPR effect (attributed to the photoelectric activation of the double interfaces). The SPR effect was further studied in LC cells with photoresponsive substrates, which act as a source of a bell-shaped electric field distribution in the LC film if no ITO electrode was employed. In an equipped cell with ITO, the photovoltaic electric field induces charge carrier redistribution in the ITO film, hence the SPR effect. This paper is aimed at highlighting all the evidences supporting ITO film as one of the fundamental sources of the SPR effect in pure NLC cells under the condition of applying low optical power and low DC voltage. An optically induced fringe electric field stemming from inhomogeneous photo-charge profiles near the electrode surfaces is expected in the LC film due to the semiconducting behavior of the ITO layer.
Highlights
The surface photorefractive (SPR) effect describes a change in refractive index near the surface of a material owing to an optically induced redistribution of electrons and holes
In pure nematic liquid crystal (NLC) cells, the SPR effect is realized as a director field reorientation driven by a nonuniform electric field of photogenerated-charge carriers under the action of a visible light beam, which was, for example, manifested in aberrational patterns formation and characterizations [4,5]
Opto-Electronic Behavior of the ITO Thin Films In NLC cells, LC films are commonly confined between ITO-equipped glass covers (ITO film had a thickness in the range of a few hundred nm)
Summary
The surface photorefractive (SPR) effect describes a change in refractive index near the surface of a material owing to an optically induced redistribution of electrons and holes. In pure (undoped) nematic liquid crystal (NLC) cells, the SPR effect is realized as a director field reorientation driven by a nonuniform electric field of photogenerated-charge carriers under the action of a visible light beam, which was, for example, manifested in aberrational patterns formation and characterizations [4,5]. Further investigations confirmed the dominancy of the surface charge modulation in the photorefractive effect observed in pure NLCs, while rejecting the hypothesis of anchoring tuning at the LC cell surfaces [11] It was understood from a diminished threshold voltage verified selectively at the anode in the laser on state, and decreasing the effective operating DC voltage [11,12,13]. Since this effect exists independent of the LC mixtures and orientation layers, one has to draw the attention to the relevant optical and semiconducting properties of the ITO layer participating in the SPR effect, as will be discussed in the following
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