Abstract
The instantaneous response of liquid crystal (LC) molecules near the dye-adsorbed layer of azo dye-doped LCs (ADDLCs) in early-stage photoexcitation was observed through the pumped-attenuated total reflection (PATR) method. As the sensing depth of the attenuated total reflection (ATR) method is in the order of hundreds of nm, in situ observation using the PATR method directly inspected the restructuring of the adsorbed layer in the early-stage photoexcitation, which demonstrated the competition between adsorption and desorption on the isotropic surface without the interference of the elastic continuum in the bulk of a cell. Analyzing the frequency of the oscillation-like PATR signal, which reflects the evolution of the orientation of the LC molecules, revealed that, depending on the dose of the pump light, the structural evolution of the adsorbed layer is fast at the beginning of photoalignment in ADDLCs and slows down with the accumulation of the adsorbed layer. A suitable intensity of the pump light in the early stage of the photoalignment process can improve the reform of the adsorbed layer to strengthen its anchoring.
Highlights
IntroductionA well-known method of alignment uses a mechanical rubbed polymer surface to induce a preferable liquid crystal (LC) director orientation due to its simplicity [1,2]
The liquid crystal (LC) alignment technique is a crucial subject in LC science
Based on the merit of pumped-attenuated total reflection (PATR) technology and further improvements from previous work [21,25], our findings show that the competitive adsorption-desorption phenomenon at the early stage of the photoexcitation is strongly dependent on the intensity of the pumped laser, which influences the final morphology and effective anchoring of the dye-adsorbed layer at the early stage of the photoexcitation
Summary
A well-known method of alignment uses a mechanical rubbed polymer surface to induce a preferable LC director orientation due to its simplicity [1,2]. This rubbing technique has several disadvantages, such as generating dust, electrostatic charge, and physical damage on the surface during the rubbing treatment [1,2]. Azo dyes undergo diffusion, adsorption, or desorption on the irradiated surface [13,14]. The final orientation of LC molecules on the surface is determined by the competition between light-induced and spontaneous adsorption and desorption [2,13,15,16]. The restructuring of the dye-adsorbed layer at the early stage of photoexcitation (the first several hundred seconds of irradiation) will affect the afterward evolution of the surface anchoring [17]
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