Study On Photoalignment Of Methyl-Red-Doped Cholesteric Liquid Crystal Fingerprint Textures

Cholesteric liquid crystals(CLCs)with self-assembled helical superstructures has been widely utilized in the fields including liquid crystal(LC)lasers,reflective films,dynamic lithography masks,phase gratings,and etc.due to their unique one-dimensional chiral photonic crystal superstructures as well as stimuli responsive property.On the other hand,alignment technique has always been the key for improving the performance of liquid crystalline devices.By providing regular alignment for the local LC molecules,the ones adjacent will also be aligned orderly due to their long-range orientational ordering,leading to particular physical anisotropies such as refractive index,dielectric constants,elastic coefficients,phases,etc.Recently,photoalignment has been a dominant alignment technique in LC field.The noncontact photoalignment process not only overcome the prejudicial contamination problems due to dust particles and electrostatic charges occurred in the conventional rubbing technique,but also could achieve multi-domain and high-resolution alignment in various substrates including curved-surfaces and flexible substrates.Here,we present an alternative asymmetric approach to photoalign the fingerprint textures of Methyl Red(MR)doped CLCs(MDCLCs)in a predictable and rewritable manner.Apart from that,we also elaborately analyze and characterize the evolution of the fingerprint textures and their electro-optic characteristics.The main investigation could be divided into two parts as follows,1.The regular alignment of LC molecules was achieved by photoaligning the MDCLC molecules through the irradiation of a linearly polarized beam.The initial helical structures of CLCs were dependent on the MR molecules which were stimulated by the incident light,leading to the reactions of the trans-cis isomerization,adsorption and desorption.The corresponding fingerprint texture grating was obtained by the modulation of the AC electric field.The vector of the grating could be manipulated precisely by control the polarization direction of the incident beam and the ratio between the cell gap and the helical pitch.As a result,the electrically-manipulated dynamic CLC superstructures were realized by irradiating the sample with two orthogonal linearly and circularly polarized beams,respectively.In addition,the principle of the alignment provided by MRs was analyzed carefully and compared with the experiment results,which were completely agreed with each other.2.Based on the experiment mentioned above,the 2D geometric patterning of MDCLC fingerprint texture grating was achieved by the two-step photoalignment technique due to the excellent alignment property and rewritable ability of MRs.The grating vector could be manipulated precisely by controlling the polarization direction of the incident beam and the geometric pattern of the grating could be designed arbitrarily by utilizing photomasks with various shapes.Apart from that,the planar alignment of LC molecules was instead with the hybrid alignment by introducing the nanoparticles POSS.By applying an external electric field to the sample,the fingerprint texture grating would rotate along the center axis of the grating.The grating could be switched from the Rotation-Mode to the On-off-Mode by the multi-domain photoalignment through photomasks due to the competitive relationship between the POSS and MRs,leading to the CLC superstructure which possessed both R-Mode and O-Mode.