Liquid Crystal Hierarchical Superstructure Manipulation and Their Applications

Author:Ma Ling Ling

Supervisor:lu yan qing hu wei


Degree Year:2019





Nature is always an inexhaustible source of inspirations for scientists and engineers to learn and develop sophisticated materials and advanced technology.It utilizes simplest building blocks to achieve remarkable assemblies of complex materials and hierarchical systems.Such fascinated hierarchical structures ranging from micrometers to nanometers endow these materials with elegant characteristics and functions on the macro level,such as the anti-reflective nature of insect wings,the beautiful structural color of peacock feathers,the self-cleaning ability of lotus leaves and the "Shoes" of water striders.Liquid crystals(LCs)have attracted particular attention due to their excellent controllability of self-assembly behavior and resultant physical properties.Recently,based on the helical superstructures in cholesteric LCs(CLCs),applications such as spectrum scanning,photolithography masks,vortex generation,and optical anti-counterfeiting have been proposed.On the other hand.much effort has been devoted to the hierarchical superstructures in lamellar smectic LCs(SLCs),which can be used in soft lithographic templates,superhydrophobic surfaces,particle trapping and assembly,photolithography masks and so on.Therefore,it is significantly important to realize the free manipulation of LC-based hierarchical superstructures.However,due to the entropy-driven nature of molecular assembly,fully controlling the hierarchical architecture of LCs is still challenging.Here,we achieve multi-dimensional manipulations of both the cholesteric helical superstructures and the smectic lamellar focal conic domains by combining the“top-down”photopatteming process with the“bottom-up,self-assembly behavior,and also study their related applications.The major results are summarized as followings:1.We demonstrate a free manipulation of CLC helical superstructures.Through this method,defect-free CLC gratings can be easily prepared over a scale of several square centimeters.A dynamic photo-patterning technique is further adopted to rationally design and arbitrarily control the in-plane helical axes of CLCs.Subsequently,the growth of unique CLC stripe textures,including Archimedean spiral and wave-like continuous grating,are demonstrated conveniently.Additionally,the dynamic growing and disappearing processes of the Archimedean spirals are investigated.To find out the general rules behind the CLC helical axis manipulation enabled by the photo-alignment,we systematically study the relationship between the designed alignments and the directions of obtained stripes in two contiguous domains.Such hierarchical superstructures are quite promising for applications such as non-mechanical beam-steering devices and lithography masks.This study will bring brand new opportunities for the design of novel advanced photonic devices based on the proposed helical CLC superstructures.2.We investigate the light-driven rotating and pitch tuning behaviors of CLC gratings in semi-free films,which are formed by spin-coating the CLC mixtures onto planarly photoaligned substrates.The doped azobenzene chiral molecular switch supplies great flexibility to realize the continuous grating rotation.The maximum continuous rotational angle reaches 987.80.Moreover,dependencies of light-driven rotating and pitch tuning on the dopant concentration and exposure are studied.The model of director configuration in the semi-free film is constructed.Also,precise beam steering and synchronous micromanipulation are demonstrated.Our work may provide new opportunities for applications in beam steering,micromanipulation and sensing.3.The 3D smectic layer origami via a 2D preprogrammed photoalignment film is accomplished.Full control of hierarchical superstructures is demonstrated,including the domain size,shape,and orientation,and the lattice symmetry of fragmented toric focal conic domains(FCDs).The unique symmetry breaking of resultant superstructures combined with the optical anisotropy of the LCs induces an intriguing polarization-dependent diffraction.This work broadens the scientific understanding of self-assembled soft materials and may inspire new opportunities for advanced functional materials and devices.4.We present an efficient solution for four-dimensional visual imaging based on self-assembled asymmetric microlens arrays in SLCs.These microlenses can exhibit multifoeal functionality emd polarization selectivity due to their rationally designed phase profiles and asymmetries.With a specific sample featured by radially increased unit sizes and azimuthally varied FCD orientations,the discriminability of four-dimensional infornation,including the three-dimensional space and polarization information,is achieved in a single snapshot via referring to the coordinates of clearest images.Such designed microlenses are also very potential for the demultiplexing of depth/polarization information respectively.This work will unlock a variety of revolutionary apparatuses and lighten extensive applications.