Study On Liquid Crystal Polarization Grating With Large Deflection Angle

Liquid crystal polarizing grating is a periodic nanostructure in which liquid crystal molecule azimuth angle continuously changes from 0 degrees to 180 degrees.The liquid crystal polarizing grating can diffract the first order beam by applying continuous phase changes to the incident beam.When the light wavelength is determined,the smaller the grating period,the larger the beam deflection angle.The+1 order or-1 order beam theoretical diffraction efficiency can be 100%and the diffraction angle of beam can reach±40°.Liquid crystal polarization gratings have the characteristics of small size,light weight and polarization selectivity compared with other non-mechanical beam steering technology,and have great application prospects in laser communication,remote sensing monitoring,VR and AR wearable devices.The current problem is that the liquid crystal polarizing grating period is difficult to achieve less than 2 microns,so the beam deflection angle for the visible light band is generally difficult to exceed 15°,and many applications require the beam deflection range to exceed 20°.The diffraction efficiency of deflection beam of small period liquid crystal polarization grating is too low for 1550nm which is common optical communication wavelength.Application of liquid crystal polarization grating is limited.The specific preparation process of the liquid crystal polarization grating has not been reported internationally,and the optical field diffraction characteristics of the grating are also unclear.The thesis has conducted research on the above problems.The performance comparison of the liquid crystal photo-alignment film is based on the completion of the light reaction in the light control alignment film to obtain the optimal exposure time,and then the anchoring ability of the alignment film is judged based on the total light reaction.It was found that the exposure time of OPA51 domestic materials can be reduced from 18 minutes to 7 minutes compared with the ROP imported materials that have been used all the time,and it was confirmed that OPA51photo-alignment film has sufficient anchoring energy,which is determined as our laboratory photo-alignment film to be used in the future.The main light rays of the two coherent circularly polarized beams in the optical path are separated,and the beam cross section has a gaussian distribution of light intensity,which makes the two coherent light at the edge of the superimposed light field have light intensity differences,resulting in the ellipticity of the linear polarization state of the superimposed light field and the decrease of the grating fringe contrast at the edge.The of ellipticity of the superposed light field of two beams of coherent light under non-equal light intensity was calculated,and the experiment was designed to analyze the decrease of orientation of liquid crystal molecules under the exposure of elliptically polarized light,and it was concluded that the orientation of liquid crystal decreased significantly when the light intensity ratio of the two coherent light was greater than 1.2,resulting in a significant decrease in the stripe contrast of liquid crystal polarization grating.Therefore,the light intensity ratio of the two coherent light should be strictly limited between 1.2 and 1.0,so as to avoid the quality decline of the liquid crystal polarization grating caused by the ellipsometization of the superposition light field.in addition,the preparation period is less than 5 microns grating two coherent light the light separate distance,make preparation of grating effective area is significantly reduced,propose solving the countermeasure of:the preparation of light path in the holes after the aperture of lens focal length longer,the coherent beam intensity distribution of flattened gaussian curve,thereby increasing the superposition of effective area of the light field.In the preparation of small-period liquid crystal polarization gratings,it was also found that the photo-alignment film can only induce a thin layer of liquid crystal alignment,and the thickness(?) of the liquid crystal layer is also constrained by the grating period.The larger the fan-out bending stress,the smaller the thickness(?) of the liquid crystal layer that can be induced to orient.This study quantitatively measured the relationship between the grating period and the thickness dc of the liquid crystal layer that can be induced to align:(?)≈0.1λ.However,the thickness d of the liquid crystal layer in the polarization grating must also satisfy the condition of(?) to achieve the best diffraction efficiency.The thickness d of the liquid crystal layer constrained by the diffraction efficiency is much larger than the critical thickness(?).For this purpose,a process of spin-coating multi-layer liquid crystal and layer-by-layer photo-alignment orientation is proposed.The spin coating thickness of the liquid crystal layer can be reduced to 100 nm using propylene glycol methyl ether acetate(PGMEA)as a solvent to dilute the liquid crystal polymer,the multi-layer liquid crystal spin coating can maintain a 15 mm diameter defect-free grating area,and a 2μm,14mm×14mm aperture liquid crystal polarization grating beam folding device is prepared,so that the deflection angle of 532nm laser can reach±15°and the diffraction efficiency can reach 98%,reaching the best reported level of similar devices.Simulation calculation shows that:when the wavelength of incident light is close to the geometric length of the grating period,the diffraction efficiency will drop sharply.For example,when the 2μm grating beam deflection device with a 1550nm laser incidence,the diffraction efficiency drops to 30%;for the 3μm grating beam deflection device,the diffraction efficiency rises to 66%.Although the deflection angle is large to31°,such energy efficiency is not acceptable in laser communication applications,therefore,if we want to continue to increase the exit deflection angle,it is not feasible to simply reduce the grating period,so we can only use the method of grating cascade.In general,a half-wave plate is inserted into each grating cascade.In this study,the rotation direction of the fan column of two adjacent gratings is reversed to eliminate the half-wave plate inserted in front of each grating,reduce the interface loss,and reduce the thickness of the beam splitter.When the beam passes through the first-order grating and then enters the second-order grating,oblique incidence occurs,which not only leads to a significant decrease in diffraction efficiency,but also a large gap between the efficiency of+1order and-1order,which can be alleviated by increasing the period of the second-order grating.After optimization,the cascade beam deflection device should be designed with small periodic grating in the first level and large periodic grating in the second level.For example,4.5μm grating is used in the first level and 8μm grating in the second level.The deflection angle of the 1550nm laser cascade beam folding device can reach 31°,and the diffraction efficiency of the+1 order and-1order levels is 83%and 75%respectively,which are acceptable energy efficiency and energy gap..Through the research of thesis,the application potential of liquid crystal polarization grating in laser radar,laser communication and other engineering fields is effectively tapped,which accelerates the industrialization process..