液晶光栅之衍射性质研究
摘要
液晶光栅因为其光学性质在低电压驱动下能被电压控制,在光学存储和计算、衍射光学、三维图像显示以及光开关都有潜在的应用。
本论文应用半导体制程法,在基于ITO导电薄膜玻璃上制作条形导电玻璃,以E7这种通用的液晶材料与ITO导电玻璃组装形成纯液晶光栅盒。利用会聚偏振光干涉法对制作样品进行检测。之后我们讨论外加电压、温度对于液晶光栅盒衍射效应的影响。
进而对出射的各级衍射光偏振特性进行定性研究。通过对非零级衍射光偏振特性的研究,我们也发现,使用外加电压不仅可以控制各级衍射光的强度,同时也可以控制各级偏振光的偏振状态。
本实验的研究,对于影响液晶光栅衍射性质的外界变量进行了系统研究,为今后液晶光栅的应用打下了良好的基础。
Liquid crystal(LC) gratings have potential applications in optical processing and computing , diffractive optics, three-dimensional image displays and optical switching, since their optical properties can be electrically controlled with low driving voltage. Using semiconductor processing, the liquid crystal cells were fabricated in this experiment. And based on the Conoscope, the samples were detected. Then we investigated the diffraction properties of LC grating under different bias voltage and different temperature, and the polarization characteristics of light diffracted from this grating.
Fabrication of the LC samples
According to the structure of the electrode used in this study at first, we design the pattern of the mask(shown in Fig.1) using mapping software AUTOCAD, and the mask was produced by related manufacturers.
After finishing the production of mask, we begin fabrication of samples, fabrication flow of the samples is shown in Fig.2. According to spinning photoresist, exposure, photography and remove of photoresist, fabricate conductive strip ITO glass. this glass was formed the empty cell with a whole conductive ITO glass, then inject LC material to it and pack the cell in order to form the LC cell we need in this study.
Detection of LC samples
There are all sorts of defects due to various reasons in the process of fabricating samples. Therefore, the detection of the samples is a key element that the study carried out successfully. Qualified samples are shown as uniaxial crystal. We use conoscope to detect the state of uniaxial crystal. The system of conoscope is shown in Fig.3
However, after our analysis of the principle of its experiment, we found that it ensure the result of the experiment without lens when the two polarizer before and after the sample are mutually orthogonal, and the radius of probe light is large enough. The Fig.4 is the real simplified detection light-route graph.
The image on the screen is shown in Fig.5, it is a good sample when there is a clear dark cross strip on the screen.
The first part, the measurement of the diffraction under different bias voltage. Due to isotropic of dielectric constant of LC molecular, while the bias voltage is larger than the Vth, positive LC molecular begin to twist along the direction of electric field in the effect of the electric field, and the refraction in the LC cell creates a periodic change, it appears diffraction when the light enter the LC cell. Shown in Fig.6, we can observer the diffraction light intensities of 0,-1st and -2nd order with voltage up to the 100v.
When bias voltage is zero, the director of LC molecules is parallel to the x-axis. With the voltage to Vth=4v, LC molecules start tilting. Continue to increase the voltage slowly, there is a strong attenuation in 0th order, at the same time there are both strong enhance in -1st,-2nd order. But the process only appears between a small voltage range (4-6v), we refer to it as the beginning of tilting of LC molecules, the effect of diffraction appears, but it is instable. So the peak of diffraction only exists a small voltage range.
Shown in Fig.6, when the bias voltage is larger than 6v, the diffraction intensity of 2nd order increases as the voltage increases; when the voltage is larger than 10v, the diffraction intensity of 1st order increases as the voltage increases. After the voltage is up to about 25v, both diffraction intensities of 1st and 2nd order decrease as the voltage increases. In this process, we considered that LC molecules begin to tilt stably as the bias voltage become larger than 6v. From 6v to 10v, it is the start of the diffraction effect. From 10v to 25v, LC molecules continue tilting as the voltage increases, and birefringence of probe light continue increasing, it leads to the increase of diffraction effect.
After voltage is larger than 25v, the diffraction intensity of 0th order continues increasing, at the same time the diffraction intensities of 1st and 2nd order begin to decrease. In the process, as the voltage increases, electric field begin to have a pulling effect to LC molecules of the non-electrode slit regions. So the tilt of LC molecules of the non-electrode slit regions makes the refraction of light in non-electrode slit regions decrease,Δn between electrode slit regions and non-electrode slit regions decrease, and the effect of diffraction decreases.
The second part, the measurement of diffraction under different temperature. The nematic phase of LC material E7 we used in this study ranges from -10℃to +61℃, the temperature in our lab is +15℃, so we divide into two parts to change temperature: measurement of diffraction above room temperature and measurement of diffraction below room temperature.
We pay more attention to three points in the Fig.7. First, above +60℃diffraction intensities of 1st and 2nd have strong attenuation, at 62℃they approach 0, it is in line with the Tc of E7 at 61℃, and it shows the measurement of the experiment system effective. Second, from 20℃to 50℃,diffraction intensities of 1st and 2nd change as the voltage change, but change merely, it shows the optical property of E7 is stable in the temperature range. Third, above +50℃,diffraction intensities of non-0th order decrease and diffraction of 0th increases as the temperature. In the temperature range, temperature approaches to Tc, the physical properties of LC molecules approach to isotropic, the effect of birefringence decreases, and diffraction of non-0th order decreases, so it leads to the increase of diffraction of 0th.
From the Fig.8 we can see, above -13℃, as the temperature increase, diffraction intensities at all levels all decrease in varying degree. Then from -5℃to +5℃,diffraction intensities at all levels change merely as the temperature changes. At between +5℃and +7℃,diffraction intensities at all levels have strong enhance, above the temperature, diffraction intensities at all levels change merely as the temperature changes and keep good optical stability.
From the above two parts of variable-temperature experiment, we can see, from 7℃to 50℃,LC samples keep optical properties stable, under this temperature range we can use E7 to fabricate LC optical switching well. Below -5℃,diffraction efficiency of E7 is weak, and above +50℃diffraction intensities of non-0th order decrease as the temperature increase, so it is not suitable to fabricate LC grating of E7 as optical switching under the two temperature ranges.
The third part, the study of polarization characteristics of diffraction at all levels
In theory, the polarization states of non-0th order are the same as the state of 0th order, the direction of polarization of 0th order is the same as the probe light, and s-polarization.
See from right to left(direction of propagation of probe light), we define the angle of clockwise rotation of the polarizer as positive angle, we define the angle of counterclockwise rotation of the polarizer as negative angle. We rotate the polarizer from -90°to +90°and observer the changes of diffraction intensities. Fixed applied voltage in 14v. Shown in Fig.9, 10 and 11, they are curve diagrams of diffraction intensities of 0th ,1st and 2nd order under the rotation of polarizer respectively.
We can see from Fig.9, the change of diffraction intensities of 0th order under the rotation of polarizer illustrate that it is linearly polarized light, its direction of polarization is vertical direction, called s-polarization, lined in with thesis.
We can see from Fig.10, positive and negative 1st order light are not linearly polarized light, but they are elliptical polarized light.
We can see from Fig.11, positive and negative 2nd order light are also elliptical polarized light, but they approach to Circular polarized light closer than 1st order light.
We pay attention to the comparison between corresponding negative and positive diffraction orders. We choose that rotate the polarizer after the sample +45°and -45°respectively, observer the change of intensities of corresponding negative and positive diffraction orders under the change of voltage, and compare them. We can see from Fig.12,13,14 and 15, except low voltage, there are good antisymmetric arrangement of the diffraction intensities in corresponding negative and positive diffraction orders under different voltage.
For the antisymmetric arrangement of intensities of non-0th orders, we have to analyze the tilt of LC molecules in different regions under applied voltage.
For LC molecules in electrode regions, due to the effect of electric field, they just tilt along the inclination direction; For LC molecules in the center of non-electrode regions, they keep original state without effect of electric field. For the LC molecules in the edge of non-electrode regions, the LC molecules near the straight edges of the upper grating electrode are twisted toward the direction perpendicular to the initial alignment direction due to the lateral electric fields induced by these electrode edges, while being tilted to the direction normal to the substrates. And those near the lower electrode substrate are tilted out of the substrate surface only in the rubbing plane. Because the twist directions of LC molecules are different near the straight edges of the upper grating electrode, it leads to the antisymmetric arrangement of intensities of non-0th orders under different voltage.
本论文应用半导体制程法,在基于ITO导电薄膜玻璃上制作条形导电玻璃,以E7这种通用的液晶材料与ITO导电玻璃组装形成纯液晶光栅盒。利用会聚偏振光干涉法对制作样品进行检测。之后我们讨论外加电压、温度对于液晶光栅盒衍射效应的影响。
进而对出射的各级衍射光偏振特性进行定性研究。通过对非零级衍射光偏振特性的研究,我们也发现,使用外加电压不仅可以控制各级衍射光的强度,同时也可以控制各级偏振光的偏振状态。
本实验的研究,对于影响液晶光栅衍射性质的外界变量进行了系统研究,为今后液晶光栅的应用打下了良好的基础。
Liquid crystal(LC) gratings have potential applications in optical processing and computing , diffractive optics, three-dimensional image displays and optical switching, since their optical properties can be electrically controlled with low driving voltage. Using semiconductor processing, the liquid crystal cells were fabricated in this experiment. And based on the Conoscope, the samples were detected. Then we investigated the diffraction properties of LC grating under different bias voltage and different temperature, and the polarization characteristics of light diffracted from this grating.
Fabrication of the LC samples
According to the structure of the electrode used in this study at first, we design the pattern of the mask(shown in Fig.1) using mapping software AUTOCAD, and the mask was produced by related manufacturers.
After finishing the production of mask, we begin fabrication of samples, fabrication flow of the samples is shown in Fig.2. According to spinning photoresist, exposure, photography and remove of photoresist, fabricate conductive strip ITO glass. this glass was formed the empty cell with a whole conductive ITO glass, then inject LC material to it and pack the cell in order to form the LC cell we need in this study.
Detection of LC samples
There are all sorts of defects due to various reasons in the process of fabricating samples. Therefore, the detection of the samples is a key element that the study carried out successfully. Qualified samples are shown as uniaxial crystal. We use conoscope to detect the state of uniaxial crystal. The system of conoscope is shown in Fig.3
However, after our analysis of the principle of its experiment, we found that it ensure the result of the experiment without lens when the two polarizer before and after the sample are mutually orthogonal, and the radius of probe light is large enough. The Fig.4 is the real simplified detection light-route graph.
The image on the screen is shown in Fig.5, it is a good sample when there is a clear dark cross strip on the screen.
The first part, the measurement of the diffraction under different bias voltage. Due to isotropic of dielectric constant of LC molecular, while the bias voltage is larger than the Vth, positive LC molecular begin to twist along the direction of electric field in the effect of the electric field, and the refraction in the LC cell creates a periodic change, it appears diffraction when the light enter the LC cell. Shown in Fig.6, we can observer the diffraction light intensities of 0,-1st and -2nd order with voltage up to the 100v.
When bias voltage is zero, the director of LC molecules is parallel to the x-axis. With the voltage to Vth=4v, LC molecules start tilting. Continue to increase the voltage slowly, there is a strong attenuation in 0th order, at the same time there are both strong enhance in -1st,-2nd order. But the process only appears between a small voltage range (4-6v), we refer to it as the beginning of tilting of LC molecules, the effect of diffraction appears, but it is instable. So the peak of diffraction only exists a small voltage range.
Shown in Fig.6, when the bias voltage is larger than 6v, the diffraction intensity of 2nd order increases as the voltage increases; when the voltage is larger than 10v, the diffraction intensity of 1st order increases as the voltage increases. After the voltage is up to about 25v, both diffraction intensities of 1st and 2nd order decrease as the voltage increases. In this process, we considered that LC molecules begin to tilt stably as the bias voltage become larger than 6v. From 6v to 10v, it is the start of the diffraction effect. From 10v to 25v, LC molecules continue tilting as the voltage increases, and birefringence of probe light continue increasing, it leads to the increase of diffraction effect.
After voltage is larger than 25v, the diffraction intensity of 0th order continues increasing, at the same time the diffraction intensities of 1st and 2nd order begin to decrease. In the process, as the voltage increases, electric field begin to have a pulling effect to LC molecules of the non-electrode slit regions. So the tilt of LC molecules of the non-electrode slit regions makes the refraction of light in non-electrode slit regions decrease,Δn between electrode slit regions and non-electrode slit regions decrease, and the effect of diffraction decreases.
The second part, the measurement of diffraction under different temperature. The nematic phase of LC material E7 we used in this study ranges from -10℃to +61℃, the temperature in our lab is +15℃, so we divide into two parts to change temperature: measurement of diffraction above room temperature and measurement of diffraction below room temperature.
We pay more attention to three points in the Fig.7. First, above +60℃diffraction intensities of 1st and 2nd have strong attenuation, at 62℃they approach 0, it is in line with the Tc of E7 at 61℃, and it shows the measurement of the experiment system effective. Second, from 20℃to 50℃,diffraction intensities of 1st and 2nd change as the voltage change, but change merely, it shows the optical property of E7 is stable in the temperature range. Third, above +50℃,diffraction intensities of non-0th order decrease and diffraction of 0th increases as the temperature. In the temperature range, temperature approaches to Tc, the physical properties of LC molecules approach to isotropic, the effect of birefringence decreases, and diffraction of non-0th order decreases, so it leads to the increase of diffraction of 0th.
From the Fig.8 we can see, above -13℃, as the temperature increase, diffraction intensities at all levels all decrease in varying degree. Then from -5℃to +5℃,diffraction intensities at all levels change merely as the temperature changes. At between +5℃and +7℃,diffraction intensities at all levels have strong enhance, above the temperature, diffraction intensities at all levels change merely as the temperature changes and keep good optical stability.
From the above two parts of variable-temperature experiment, we can see, from 7℃to 50℃,LC samples keep optical properties stable, under this temperature range we can use E7 to fabricate LC optical switching well. Below -5℃,diffraction efficiency of E7 is weak, and above +50℃diffraction intensities of non-0th order decrease as the temperature increase, so it is not suitable to fabricate LC grating of E7 as optical switching under the two temperature ranges.
The third part, the study of polarization characteristics of diffraction at all levels
In theory, the polarization states of non-0th order are the same as the state of 0th order, the direction of polarization of 0th order is the same as the probe light, and s-polarization.
See from right to left(direction of propagation of probe light), we define the angle of clockwise rotation of the polarizer as positive angle, we define the angle of counterclockwise rotation of the polarizer as negative angle. We rotate the polarizer from -90°to +90°and observer the changes of diffraction intensities. Fixed applied voltage in 14v. Shown in Fig.9, 10 and 11, they are curve diagrams of diffraction intensities of 0th ,1st and 2nd order under the rotation of polarizer respectively.
We can see from Fig.9, the change of diffraction intensities of 0th order under the rotation of polarizer illustrate that it is linearly polarized light, its direction of polarization is vertical direction, called s-polarization, lined in with thesis.
We can see from Fig.10, positive and negative 1st order light are not linearly polarized light, but they are elliptical polarized light.
We can see from Fig.11, positive and negative 2nd order light are also elliptical polarized light, but they approach to Circular polarized light closer than 1st order light.
We pay attention to the comparison between corresponding negative and positive diffraction orders. We choose that rotate the polarizer after the sample +45°and -45°respectively, observer the change of intensities of corresponding negative and positive diffraction orders under the change of voltage, and compare them. We can see from Fig.12,13,14 and 15, except low voltage, there are good antisymmetric arrangement of the diffraction intensities in corresponding negative and positive diffraction orders under different voltage.
For the antisymmetric arrangement of intensities of non-0th orders, we have to analyze the tilt of LC molecules in different regions under applied voltage.
For LC molecules in electrode regions, due to the effect of electric field, they just tilt along the inclination direction; For LC molecules in the center of non-electrode regions, they keep original state without effect of electric field. For the LC molecules in the edge of non-electrode regions, the LC molecules near the straight edges of the upper grating electrode are twisted toward the direction perpendicular to the initial alignment direction due to the lateral electric fields induced by these electrode edges, while being tilted to the direction normal to the substrates. And those near the lower electrode substrate are tilted out of the substrate surface only in the rubbing plane. Because the twist directions of LC molecules are different near the straight edges of the upper grating electrode, it leads to the antisymmetric arrangement of intensities of non-0th orders under different voltage.
引文
[1]田芊、廖延彪、孙利群,工程光学.北京:清华大学出版社,2006:248-249
[2] Susumu Sato, Liquid-Crystal Lens-Cells with Variable Focal Length. Japanese Of Applied Physics VOL. 18, No. 9, SEPTEMBER, 1979 pp. 1679-1684
[3] Hongwen Ren and Shin-Tson Wu, Variable-focus liquid lens by changing aperture. Applied Physics Letters 86, 211107 (2005).
[4] Marco Peccianti, Claudio Conti, and Gaetano Assanto, Antonio De Luca and Cesare Umeton, All-optical switching and logic gating with spatial solitons in liquid crystals. Appl. Phys. Lett. 81, 3335 (2002).
[5] Chie-Tong Kuo and Shuan-Yu Huang, Enhancement of diffraction of dye-doped polymer film assisted with nematic liquid crystals. Appl. Phys. Lett. 89, 111109 (2006).
[6]二元光学,百度百科. http://baike.baidu.com/view/1837590.html?fromTaglist
[7]二元光学技术与应用,徐平. http://www.szns.gov.cn/main/qy/tsqy/bslt/2008090255472.shtml
[8]赵凯华,新概念物理教程:光学.北京:高等教育出版社,2004:195,317-318,205,270-318.
[9]潘笃武、贾玉润、陈善华,光学(上).上海:复旦大学出版社,1997:234-242.
[10]周殿清、于国萍,用偏光显微镜研究液晶的相变及光学特性(PPT). http://wlsyzx.whu.edu.cn:8080/info/shifanzhongxin/f/10486_1_f_6.ppt
[11] B.Bahadur, Liquid Crystals - Applications and Uses, World Scientific Press, Singarpore (1990).
[12]松本正一、角田市良,液晶之基础与应用.台北:国立编译馆,1996:37-38,73,82-85.
[13] Andrew J. Lovinger, Karl R. Amundson and Don D. Davis, Chem. Mater. 6, 1726 (1994).
[14] Grant R. Fowles, Introduction to Modern Optics, 2nd ed., University of Utah,New York (1975).
[15] A. Yariv, Optical Electronics in Modern Communications. New York :Oxford University Press , 1997:Fifth edition, Chap. 1.
[16] A. Yariv, Quantum Electronics. New York :Wiley, 1988:Third edition, Chap. 5.
[17] E. B. Priestley, P. J. Wojtowicz and P. Sheng, Introduction to Liquid Crystals , Princeton, New Jersey(1975), Chap. 1, Chap. 8.
[18] Peter J. Collings and Michael Hird,“Introduction to Liquid Crystals Chemistry and Physics”, Taylor and Francis(1997), Chap. 10.
[19]高鸿锦、董友梅,液晶与平板显示技术.北京:北京邮电大学出版社,2007:157-158,180.
[20]许千树,液晶之种类及物理化学特性. http://www.tcfsh.tc.edu.tw/adm/special/far-sighted/creature/download/961024_lee.pdf
[21]游璞,光学.北京:高等教育出版社,2003:179.
[22]陈玉娉,以液晶制作光波导与电开关元件之特性研究,第三章.高雄:国立中山大学,2006.
[23]刘秉侠,离子交换条形光波导的制备与特性测试及Y分支光波导的设计,第四章.长春:吉林大学,2007
[24] Zhan He, Toshiaki Nose, Susumu Sato, Polarization properties of an amplitude nematic liquid crystal grating. Opt. Eng. 37(11) 2885–2898 (November 1998)
[2] Susumu Sato, Liquid-Crystal Lens-Cells with Variable Focal Length. Japanese Of Applied Physics VOL. 18, No. 9, SEPTEMBER, 1979 pp. 1679-1684
[3] Hongwen Ren and Shin-Tson Wu, Variable-focus liquid lens by changing aperture. Applied Physics Letters 86, 211107 (2005).
[4] Marco Peccianti, Claudio Conti, and Gaetano Assanto, Antonio De Luca and Cesare Umeton, All-optical switching and logic gating with spatial solitons in liquid crystals. Appl. Phys. Lett. 81, 3335 (2002).
[5] Chie-Tong Kuo and Shuan-Yu Huang, Enhancement of diffraction of dye-doped polymer film assisted with nematic liquid crystals. Appl. Phys. Lett. 89, 111109 (2006).
[6]二元光学,百度百科. http://baike.baidu.com/view/1837590.html?fromTaglist
[7]二元光学技术与应用,徐平. http://www.szns.gov.cn/main/qy/tsqy/bslt/2008090255472.shtml
[8]赵凯华,新概念物理教程:光学.北京:高等教育出版社,2004:195,317-318,205,270-318.
[9]潘笃武、贾玉润、陈善华,光学(上).上海:复旦大学出版社,1997:234-242.
[10]周殿清、于国萍,用偏光显微镜研究液晶的相变及光学特性(PPT). http://wlsyzx.whu.edu.cn:8080/info/shifanzhongxin/f/10486_1_f_6.ppt
[11] B.Bahadur, Liquid Crystals - Applications and Uses, World Scientific Press, Singarpore (1990).
[12]松本正一、角田市良,液晶之基础与应用.台北:国立编译馆,1996:37-38,73,82-85.
[13] Andrew J. Lovinger, Karl R. Amundson and Don D. Davis, Chem. Mater. 6, 1726 (1994).
[14] Grant R. Fowles, Introduction to Modern Optics, 2nd ed., University of Utah,New York (1975).
[15] A. Yariv, Optical Electronics in Modern Communications. New York :Oxford University Press , 1997:Fifth edition, Chap. 1.
[16] A. Yariv, Quantum Electronics. New York :Wiley, 1988:Third edition, Chap. 5.
[17] E. B. Priestley, P. J. Wojtowicz and P. Sheng, Introduction to Liquid Crystals , Princeton, New Jersey(1975), Chap. 1, Chap. 8.
[18] Peter J. Collings and Michael Hird,“Introduction to Liquid Crystals Chemistry and Physics”, Taylor and Francis(1997), Chap. 10.
[19]高鸿锦、董友梅,液晶与平板显示技术.北京:北京邮电大学出版社,2007:157-158,180.
[20]许千树,液晶之种类及物理化学特性. http://www.tcfsh.tc.edu.tw/adm/special/far-sighted/creature/download/961024_lee.pdf
[21]游璞,光学.北京:高等教育出版社,2003:179.
[22]陈玉娉,以液晶制作光波导与电开关元件之特性研究,第三章.高雄:国立中山大学,2006.
[23]刘秉侠,离子交换条形光波导的制备与特性测试及Y分支光波导的设计,第四章.长春:吉林大学,2007
[24] Zhan He, Toshiaki Nose, Susumu Sato, Polarization properties of an amplitude nematic liquid crystal grating. Opt. Eng. 37(11) 2885–2898 (November 1998)