Multi-wavelength Filters Of Templated Blue Phase Liquid Crystal

The blue phase of liquid crystal,which is composed of a unique twisted double helix structure and defects,usually exists between the chiral nematic phase and isotropic phase,.Blue Phase Liquid Crystal(BPLC)exhibits many advantages such as the fast response time,sub-millisecond order;self-assembled double twisted cylinder structure;macro-optical iostropic state.However,BPLCs also show obvious shortcomings,such as poor thermal stability,the temperature range of blue phase liquid crystals is usually only 1-2 degrees.To solve the problem of poor thermal stability of blue phase liquid crystals,researchers have proposed a method of polymer stabilization of BPLCs,which can expand the temperature range of BPLCs to near hundreds of degrees.On this basis,the concept of templated blue phase liquid crystal(T-BPLC)is proposed.The BPLC can be reconstructed by polymer template and conventional nematic liquid crystal.The temperature range of BPLC can reach hundreds of degrees.The Cholesteric Liquid Crystals(CLCs)have been widely applied in the optical filters due to the Bragg reflection with helical structure.The central wavelength and bandwidth of CLC filters can be easily tuned by thermal or electrical modulation.However,the bandwidth of CLC filters is too broad to apply in a multi-wavelength filters and fabrication process of multi-films CLC filters s is too complex.In this paper,we demonstrate a multi-wavelength filter with narrow band using blue phase liquid crystal(BPLC)templates in visible band.The multi-layer T-BPLC without intermediate dielectric layer may reflect multi wavelength in one filter.First,the fabrication process of templated mono-domain BPLC was studied.By adjusting the proportion of chiral materials of polymer-stabilized blue-phase liquid crystals(PS-BPLCs),mono-domain BPLCs with reflective wavelengths of red,green and blue were obtained.The mono-domain blue phase was reconstructed by template process.Meanwhile,the properties of polymer mono-domain BPLC and templated mono-domain blue phase liquid crystal were analyzed.The reflective wavelengths of polymer-stabilized mono-domain BPLC and templated mono-domain BPLC with chiral agent content of 3.6wt%,3.42wt%and3.13wt%were 455 nm/457 nm,521 nm/525 nm and 651 nm/655 nm,respectively.The bandwidths of waveforms are 12 nm/12 nm,14 nm/14nm and 16 nm/18 nm,respectively.The central reflection wavelength and bandwidth of the two waveforms is basically the same.Next,the variations of the central wavelength of mono-domain BPLCs at different incident angles are investigated.While the incident light angle changes from 0 ~o to60 ~o,the blueshift of the central wavelength is 59 nm.The blue shift of the cholesteric liquid crystal filter is 120 nm,and the angle characteristic of the center wavelength of mono-domain BPLC is more stable.The characteristics of multilayer template filters are also studied in the dissertation.Blue-phase liquid crystals with multilayer templates can reflect multiple wavelengths in the same filter.Compared with the CLC filter,the BPLC filter may get much narrower reflection bandwidth and more stable wavelength and bandwidth with temperature shift.Furthermore,the central wavelength and bandwidth can be easily tunable by combining templates of different wavelengths and the optimization of the material proportion.The existing manufacturing process of templated BPLCs is complex.The BPLCs template is formed by washing out the residues of liquid crystals and other unreacted element after the blue-phase liquid crystals are stabilized,which wastes a lot of materials and time.In this paper,a digital laser direct writing method is proposed to realize the direct writing of BPLC templates.A double twisted helical structure is constructed directly in the polymer film by exposing the vortex light formed in the spatial light modulator to form a BPLCs template.The optical direct writing method is simple and easy to control,but need higher requirements for the use of laser.