| Holographic polymer-dispersed liquid crystal?H-PDLC?grating consists of alternate polymer-rich region and liquid crystal-rich region with periodic distribution of refractive index.The addition of a small concentration of Ag Nanoparticles?NPs?has a considerable effect on the diffraction and electro-optical characteristics of an H-PDLC grating.This kind of holographic grating has advantages of easy fabrication,high efficiency,and widely application in optical switch,zoom lens,organic laser,photonic crystal,etc.What's more,it can also be applied into display device,for example,imgae splitter for3D autostereoscopic display.The images on the even pixels and odd pixels of LCD can be separated from each other and transmitted to the left eye and right eye of the observer respectively.It possesses high brightness and contrast ratio,and ability of 2D/3D switch.However,there are two disadvantages of holographic grating-based image splitting:the dispersion of stereoscopic color images and the fixed viewing angle of the eyes of the observer.Regarding the first problem,the reported method has very complicated fabrication and high requirement of optical path so that it is easy to cause errors.Regarding the second problem,one has to observe the separated images at a designated location and cannot move within a certain range.The two problems restrict the practical application of this holographic grating-based imgae splitter.To solve the above problems,the main researches of this dissertation are listed as follows:Firstly,the physical mechanism behind the observed improvement in the diffraction and electro-optic properties of H-PDLC grating doped with Ag NPs was studied in this dissertation.A small concentrations of spherical metal NPs,specifically,Ag and Au NPs with the same quality and size,were added into PDLC material.The diffraction efficiency and electro-optic properties were experimentally measured for contrast.The results show that the diffraction efficiency of holographic gratings are improved through the addition of Ag and Au NPs,for example,the phase separation became more smooth and the diffraction efficiency rose nealy 30%.It is notable that Ag-NPs-doped gratings have far superior performance than those containing Au NPs.According to Mie theory,the dissertation theoretically simulated the localized surface plasmon resonance?LSPR?wavelengths of the Ag and Au NPs within the PDLC,which were 530 nm and 581 nm respectively,and find that the laser wavelength used to fabricate the holographic gratings?532 nm?is much closer to the SPR wavelength of Ag NPs than that of Au NPs.It can be proved that the LSPR occurred during the recording of holographic grating,which enhanced the interaction between the light and the PDLC and further improve the phase separation between LC and pre-polymer.Besides,the Ag NPs-doped PDLC material can also be used to fabricate reflection grating.The experimental results proved that it has better diffraction and electric-optic characteritics.The refractive index modulation of doped reflection gratings reached 4.8×10-3 experimentally so that it can be used to design cascades for four grating-based dynamic gain equalizer.Simulated results showed that the fluctuation range of a typical gain spectrum for an erbium-doped fiber amplifier can be reduced from 3.3 dB to 0.23 dB.Aiming at the sensitivity of PDLC material to the single particular wavelength,the dissertation solved the problem through appropriate combined action of two initiator systems:photoinitiator,Methylene Blue and coinitiator,p-toluenesulfonic acid as well as photoinitiator,Rose Bengal and coinitiator,N-phenylglycine.The H-PDLC gratings that can be fabricated using 632.8nm,532nm and 441.6nm laser beams simultaneously were fabricated.The highest first-order diffraction efficiency of red,green and blue gratings were 57%,75%and 33%when the exposure time is 20min,2min and 20min.The decrease of the exposure time and increase of diffraction efficiency can be further obtained by increasing the laser intensity of 632.8 nm and 441.6 nm laser.A cylindrical lens was adopted to form a cylindrical wave,which interferes with a plane wave,generating interference pattern with a continuously varying period.The PDLC material was put into the interference field to fabricate an H-PDLC grating with varied period.The experimental results showed that the period of the grating with width of 20 mm has variation of 500 nm,which accorded with the theoretical values.The central period and varied scope can be modified by changing the angle between two interference beams and the distance between the cylindrical lens and sample,respectively.Besides,two-dimensional H-PDLC grating with varied period has also been fabricated by twice horizontal and vertical exposure,and the exposure time should be experimentally optimized.The results showed that the phase separation between the prepolymer and LC was most evident with the first exposure at 2 s and the second exposure at 10 s.The period exhibits a symmetrical square lattice in a diagonal direction,with an asymmetrical rectangular lattice in off-diagonal locations.Imgae splitter for 3D autostereoscopic display based on tri-color H-PDLC grating was designed and fabricated.Due to the advantage of sensitive to all red,green,and blue wavelengths,image splitting of left and right piexls for RGB full color were obtained by changing laser wavelengths insteade of changing the exposure angles.The left and right contrast ratio of red gratings are 75%and 86.4%,those of green gratings are 65.9%and83.3%,those of blue gratings are 65.2%and 82.9%,respectively.The contrast ratio for the whole image splitter is more than 65%.Based on one-dimensional H-PDLC grating with varied period,the wide-viewing-angle switchable image splitter were designed.Because of the varied period within the structure,the incident light bringing even and odd pixels information passed through the gratings and separated them into right and left viewing fields with a wider range.Therefore the observer can move his/her eyes within a range instead of keeping them at a fixed location and feel stereoscopic vision.This dissertation analyzed the physical reason for the improvement of the properties of H-PDLC gratings by the doping Ag NPs,and fabricated Ag NPs-doped tri-color H-PDLC grating and period-chirped one-and two-dimensional H-PDLC grating.Using the unique characteristics of the two gratings,image splitters for 3D autostereoscopic display were designed.The experimental results preliminarily confirmed the feasibility of the holographic grating as an image splitter,which lays a solid foundation for the practical application of the device. |
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