Study Of Photorefractive Effect In Asymmetrical Liquid Crystal Cells With Photoconductive Interlayers

As information technology continues to progress, the requirements of display technology are becoming higher and higher. Holographic display with excellent qualities has already aroused widespread interest. As one of holographic recording and displaying materials with great potential, liquid crystal(LC) shares many merits, such as strong birefringence, low applied voltage etc, and therefore becomes a research hotspot. To better understand the leading role played by surface effect in photorefractive effect, we designed a new type of asymmetrical liquid crystal cells with different photoconductive layers, and explored the photorefractive properties of this kind of liquid crystal cells in two aspects.In Chapter 3, we study the photorefractive properties of asymmetric liquid crystal cell under different experimental conditions by fixing LC thickness at 12.7 μm. The influence of photorefractive layer on photorefractive effect was studied by changing the direction of applied field and incident plane, and then got some insight of the impact of photoconductive layer on photorefractive effect. In the two-beam coupling experiments,two exponential gain coefficient curves with opposite trend were observed. These intriguing trend may be associated with the dominant grating position. According to the first-order diffraction curve, we have tentatively proposed the physical model of charge transportation, which can be used to elucidate the experimental results. In the four cases studied, the raising of response rate with increasing applied voltage was almost the same. After the voltage reaches a certain value, the response time can be shortened to about 10.0 ms, and even down to 5.0 ms in the fastest case. In addition, we also tested a symmetrical liquid crystal cell with the Zn Se photoconductive layers on both side for comparison. The experimental results thus obtained indicates that the asymmetrical one has an obvious advantage in fast response over its symmetrical counterpart, and the threshold voltage is lower. In other words, this structure of liquid crystal cell has great potential in potential in video rate holographic display.After being convinced about the dominant role played by the surface gratings, in Chapter 4, the influence of thickness on sample’s photorefractive properties was studied by measuring the exponential gain coefficient and response time of different thickness samples, in order to reveal the contribution of surface effect and bulk effect. The experimental results show that the gain coefficient is inversely proportional to the thickness, and can be as large as 4607 cm-1 in 3.5 μm sample, this is the largest gain coefficient obtained up to date, to the best our knowledge. This implies that the surface effect plays a dominant role in energy transfer, while response time is determined by acombination of surface effect and bulk effect, which increases with increasing thickness.Theoretical analyses show that with increasing of sample’s thickness, the bulk effect will be enhanced, and therefore the overall response rate will be reduced since the slow charge carrier migration rate in liquid crystal. What is worth mentioning that the response time will be slowed down in samples which is too thin, since the molecular freedom of liquid crystal molecules are restricted and the molecular reorientation are largely hindered in such samples. In order to improve the transmittivity of samples, we introduces a Zn Se layer with the thickness of 500 nm to original asymmetric structure,which effectively enhanced the transmission rate of the sample.