Controllable Chiral Liquid Crystal Self-Organized Micro/Nanostructure And Photonic Applications

Liquid crystals(LCs)are mesophases between crystalline solid and isotropic liquid,which perfectly integrate the anisotropy of crystalline solid and the fluidity of liquid.Soft matter features of LCs render them responsive to external stimuli such as light,electric fields,magnetic fields and mechanical forces,resulting in rearrangement of LC molecules as well as the change of their optical,electrical and other properties.In this paper,the properties and applications of cholesteric liquid crystals(CLCs)with one-dimensional(1D)self-organized helical structures and liquid crystal blue phases(BPs)with three-dimensional(3D)self-organized cubic structures are investigated.Laser emission based on an electrically reconfigured fingerprint texture of a CLC helical superstructure was achieved by judiciously designing the composition of material and the structure of device.Such lasing demonstrated herein was caused by a combination of the high-order distributed feedback resulted from a periodic refractive index modulation in fingerprint stripes and the waveguide effect of the sandwiched cell structure.Furthermore,laser with a narrow band-width of less than 0.5 nm can be switched between two orthogonal emission directions;the emission wavelength as well as the lasing types(i.e.,single-wavelength emission,muti-wavelength emission and random lasing)can be manipulated readily with a comparative low external electric-stimulation owing to the variety of configurations and reconfigurations of such soft helix.This work not only corroborates the lasing possibility of a CLC helix showing fingerprint texture with a helical pitch significantly exceeding the short pitch required in common photonic band edge lasing,but also provides multi-functional controllable lasing properties such as emission wavelength,emission intensity,emission types and emission directions of the CLC laser via low external electric fields.BPs,which are commonly observed in strong chiral systems,possessing cubic structures with lattice constant dimensions of hundreds of nanometers built by double twisted cylinders formed by LC molecules have been regarded as a special soft crystalline material.Achieving uniform and patterned orientation of crystallographic direction is generally complicated and challenging regarding inorganic and organic crystalline materials.Taming and tailoring the ordering of such soft,cubic lattices along predetermined or desired directions,and even imparting a prescribed pattern on lattice orientation,are challenging,due to the entropy-domination attribute of soft cubic BPs.Herein,we combine the "top-down" photopatterning process with the "bottom-up" self-organized property of LC molecules to realize designed micropatterning of a crystallographic direction of soft cubic BPs superstructure,exhibiting an alternate uniform and random orientation of the lattice crystallographic direction with high resolution.Besides,such an oriented soft lattice can sensitively respond to various external stimuli such as temperature,electric field resulting in tunable reflection waveband.Because of the rewritable trait of the photoalignment film,such patterned superstructures can be erased and rewritten on-demand by light.Based on the controllable optical characteristics of crystallographic direction of soft cubic BPs,two types of reflective diffractive optical elements(DOEs)have been demonstrated enabled by surface photopatterning process:1)Regulable waveband-selective amplitude modulation DOEs,which derives from the periodic reflection intensity between the uniform oriented BP lattice and random oriented BP lattice.2)Phase-modulation DOEs,which owing to the periodic reflected phase from the patterned BPs.Based on the above methods,1D/3D diffraction gratings,Fresnel lens,and fork-grating which is capable of generating vortex beams are designed and prepared.These two types of DOEs exhibit peculiarly waveband-selectivity,which means that light diffraction occurs selectively to certain wavelength.Based on the soft attribute of BPs,these devices can respond to various external stimuli such as temperature and electric fields leading to a tunable wavelength-selectivity.Furthermore,a preferable feature of the devices is photo-induced dynamical erasability and rewritability due to rewritable characteristic of the photoalignment film,which results in realizing not only the conversion among different DOEs,but also the switching between amplitude modulation and phase modulation.The versatile DOEs achieved by patterned BPs lattice would open extensive possible applications such as photo-communication,holographic display,quantum computing,spectral scanning and multi-channel optics.