Dynamics of azo-dye-doped liquid crystals

Abstract

Two novel approaches enable better insights into liquid-crystal photoalignment.Because of their potential importance for photoalignment, azodye-doped liquid crystals (ADDLCs) have been studied extensively over the past decade. Photoalignment of liquid-crystal (LC) molecules can avoid unnecessary contamination due to mechanical rubbing. When ADDLC cells are exposed to certain wavelengths, azo-dye molecules on a surface diffuse, adsorb, and desorb sequentially because of photoisomerization. [1][2][3][4][5] This causes LC-structure reorientation by excited dye molecules. In related studies, the optical characteristics of ADDLC photoexcitation have been obtained on the basis of pump-probe twistnematic experiments. 1, 2 To better understand the transient as well as the permanent behavior of ADDLCs, here we present two novel approaches to observe the reorientation of LC molecules on short 5 and long timescales, 6 respectively.The dye-adsorbed layer is formed during photoalignment. The dye molecules can interact with the surrounding LC molecules through intermolecular forces. As a result, the LC's director (i.e., the LC structure's preferred direction) is reoriented perpendicular to the pump field. 7 After irradiation of the sample for a sufficiently long period, a ripple structure is generated parallel to the pump field. 7 To explore the structural variation of the dye-adsorbed layer, we channeled unpolarized light into a traditional pump-probe twist-nematic experiment, (which is generally used for observing light scattering during formation of the dye-adsorbed layer). The transmitted probe light was scattered during photoexcitation because of the dye's adsorption. Because the ripple structure is formed gradually, it is regular, thus enhancing the dye's surface anchoring (see Figure 1). At the same time, Rayleigh scattering from isolated (or separated) dye-adsorbed grains changes to Mie scattering since the regular ripple structure has a similar spatial scale as the wavelength of the incident light. The increase in transmission intensity is more centralized for Mie than for Rayleigh scattering in the presence of the regular ripple structure. 8 After the ripple structure has formed, the dye molecules' adsorption increases the depth of the grooves, which in turn enhances the surface-anchoring coefficient. The Mie scattering does not vary significantly because the spacing of the ripple structure remains the same in the optical view. Our experimental results verify that the formation of the ripple structure correlates with light scattering induced by adsorption of dye molecules on the surface.Attenuated total-reflection (ATR) measurements are highly sensitive to variations in the dielectric constants close to the surface (approximately 100nm from the surface in LC systems) within the depth of the evanescent wave, which is not affected by the elastic continuum. 9 Our pumped-ATR experiments result in improved measurements compared to the traditional ATR approach. We channeled green pump light-accompanied by a red probe beam...