Novel Helielectric Nematic Liquid Crystals With High Fluidity And Nonlinear Optical Properties

Ferroelectricity is a spontaneous polarization state possessed by dielectrics and is commonly found in crystalline（solid）material systems with low symmetry.Fluid or high-fluidity soft-matter materials usually exhibit high symmetry and thus are contrary to the requirement of ferroelectricity.The introduction of strong polarity or ferroelectricity is a strategy that has received much attention in the field of new materials for liquid crystals.However,it has been challenging in liquid crystals and even soft matter fluidic materials.The emerging ferroelectric nematic liquid crystals have opened an entirely new era for soft matter,not only opening the way to explore exotic polar states of matter in condensed matter physics but also providing the opportunity to develop entirely new liquid crystal technologies.Ferroelectric nematic liquid crystals possesses a variety of transformative properties compared to conventional liquid crystals and soft matter materials,including ultra-high dielectric constants,strong nonlinear optical response,low voltage driving,and high fluidity.Chirality or helicity is one of the most fundamental properties for assembling soft matter super-structured materials,spanning from atomic to macroscopic biological level scales.It is very much anticipated and exciting to see what kind of novel helical structures will be induced and what unique properties will be exhibited when chirality is coupled with such fluidic ferroelectric states.Considering the two unique properties of chirality and fluid ferroelectric state,this thesis introduces chirality into the ferroelectric nematic liquid crystal.It reveals a new fluid polarization state,helielectric nematic（HN*）.Furthermore,the unique periodic polarization structure and chiral structure of the helielectric nematic phase are exploited to further investigate its properties in nonlinear optical phase matching and circular dichroism.The main contents of this thesis are summarized as follows.（1）The presence of a helielectric nematic phase was revealed by introducing chirality into N_F liquid crystals using chiral molecules S1 and S2,structurally similar to RM734,and the commercial chiral molecule R811.The coupling between strong local polarity and chirality achieves the polarization state of the near-helical arrangement of the polarity vectors.This unique helielectric structure in fluids is highly tunable in a wide range from hundreds of nanometers to tens of micrometers.By adjusting the concentration of doped chiral molecules,HN*phase can be maintained up to room temperature while maintaining excellent nonlinear optical and dielectric properties.The helielectric structures in fluids are similar to helimagnets in magnetic systems,offering the possibility of exploring new polarization structures in electrical systems similar to the exotic topological spin structures in magnetic systems,as well as designing room-temperature flexible devices using their high mobility and excellent electro-optical properties.（2）Based on the fact that HN*liquid crystals are ideal polarized helical structures with strong localized electric dipoles parallel to the long axis of the molecule.Associated with traditional nonlinear optics,a periodic polarization structure with quasi-phase matching is achieved.By tuning the pitch of HN*liquid crystal（i.e.,tuning the polarization period）by varying the concentration of chiral molecules,it is found that the concentration of chiral molecule R811 has a significantly enhanced second harmonic generation（SHG）signals at0.8%-1.2%when the pitch of HN*liquid crystal is 5-8μm,with R811/RM734=1.1/98.9 being the optimal concentration.The maximum SHG conversion efficiency of 50μm HN*liquid crystal is about 0.0071%,and the SHG conversion efficiency of 2 cm HN*liquid crystal material can be estimated at 3%.For the first time,the nontrivial phase-matching technique is realized in a high-fluidity liquid material,which can be used as an ideal liquid platform for exploring light-matter nonlinear interactions.（3）Based on the realization of the nontrivial phase-matching technique for HN*liquid crystals,a very strong second harmonic generation circular dichroism（SHG-CD）is achieved by changing the chirality of chiral molecules to obtain left-or right-handed HN*structure.The anisotropy factor2)_（-）of HN*liquid crystals is up to 1.8,which is close to the theoretical limit value of 2.0.At the same time,this strong second harmonic generation circular dichroism is characterized by visualization.In addition,the SHG-CD of HN*liquid crystals is electrically tunable and reconfigurable.With its high fluidity and easy preparation,HN*liquid crystals provide new ideas for the design of novel flexible nonlinear optical devices and chiral probes.