Electrically tunable cholesteric optical filters

一般描述。光学成像是我们与周围世界互动的基石。在某些情况下，这种互动不需要任何特殊的仪器，因为大自然赋予了人类视觉的力量。生物医学和化学分析、食品和农业、航空航天和国防等领域的技术应用越来越依赖于控制透光率和反射率的能力，以分离不同的光“颜色”。该项目建议开发简单紧凑的设备，可以选择性地反射或传输光谱中从紫外线到可见光和红外部分的不同颜色的光。这个想法是使用一种新发现的具有电控周期性的液晶结构。该装置在两个透明电极之间夹了一层液晶薄膜。通过改变施加的电压，可以改变结构的周期，从而改变薄膜透射或反射的光的颜色。该设备将有助于获得人眼无法看到的图像细节。目的是通过实验和理论表征这些液晶薄膜在各种器件几何形状和光传播方向控制下的电光性能，探索它们作为可调反射器、衍射光栅、光衰减器和光偏振旋转器的作用。该项目在现有技术（如改进的显示器）和新技术（如可调谐的“智能窗户”、百叶窗、衰减器、可调谐激光器、光学限制器、偏振旋转器）等领域具有巨大的社会效益。该研究为材料科学、电光现象、光学和高级显微镜的研究生和本科生提供了一个极好的教育平台。技术描述。当前光学成像特别是多光谱成像领域面临的挑战可以归结为两个方面的任务：(1)找到一种方法来方便地选择和调整光谱波段参数，如峰值波长和带宽；(2)使光谱成像变得紧凑和经济。本项目拟开发基于胆甾相液晶（lc）的新型电可调谐滤光片，具有斜螺旋结构（ChOH），以满足上述两个要求。最近发现的ChOH的特点是光轴（分子取向的方向）具有单模调制，其周期可通过外加电场调节。知识意义在于揭示了ChOH电光在从紫外到可见和红外的广谱范围内的潜力。具有定向器周期性调制但分子密度均匀的chh斜螺旋结构是LCs和一般软物质领域的新发现。它的前身，所谓的扭曲-弯曲向列相NTB，几年前才被发现，目前正享受着研究的爆炸式增长，因为斜螺旋结构的细节和它们对外部场的响应还不是很清楚。所提出的研究将产生新的知识，了解外源电场和约束几何如何塑造ChOH的结构，以及如何将ChOH用于设计和制造新一代电可调谐光学元件，如滤波器，光栅，偏振旋转器，衰减器。ChOH的电可调周期代表了lc电光响应的一种新模式。该项目的变革价值在于，在从紫外到可见和红外的广谱范围内，可调谐电光器件的新设计概念具有潜力。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

General description. Optical imaging is a cornerstone of our interaction with the surrounding world. In some cases, this interaction does not require any special instruments, as Nature provided humans with power of sight. Technological applications in areas such as biomedical and chemical analysis, food and agriculture, aerospace and defense increasingly rely on the ability to control light transmittance and reflectance, to separate different "colors" of light. The project proposes to develop simple and compact devices that could selectively reflect or transmit different colors of light in a broad range from ultraviolet to visible and infrared parts of the spectrum. The idea is to use a newly discovered liquid crystal structure with electrically controlled periodicity. The device represents a thin film of the liquid crystal sandwiched between two transparent electrodes. By changing the applied voltage, one changes the period of the structure and thus s the colors of light transmitted or reflected by the film. The device will help to image details that are not accessible by a human eye. The objectives are to characterize experimentally and theoretically electro-optical performance of these liquid crystalline films in various device geometries with controlled direction of light propagation, to explore them as tunable reflectors, diffraction gratings, light attenuators, and rotators of light polarization. The project has the potential for enormous societal benefits in areas ranging from existing technologies (such as improved displays) to new technologies, such as tunable "smart windows", shutters, attenuators, tunable lasers, optical limiters, polarization rotators. The proposed research provides a superb platform to educate students at graduate and undergraduate levels in materials science, electro-optic phenomena, optics, and advanced microscopy. Technical description. The current challenge in the field of optical imaging and especially multispectral imaging can be formulated as a two-fold task: (1) to find the means to easily select and adjust the spectral bands parameters such as peak wavelength and bandwidth and (2) to make the spectral imaging compact and affordable. This project proposes to develop new electrically tunable optical filters based on cholesteric liquid crystals (LCs) with oblique helicoidal structure (ChOH) that would fulfill both of the requirements above. ChOH, discovered very recently, features a single-mode modulation of the optic axis (the director of molecular orientations) with a period that is tunable by the applied electric field. The intellectual significance is in unveiling the potential of ChOH electro-optics in a broad spectral range from UV to Vis and IR. The oblique helicoidal structure of ChOH with periodic modulation of the director but homogeneous molecular density is new to the field of LCs and soft matter in general. Its predecessor, the so-called twist-bend nematic phase NTB, has been discovered only a few years ago and is currently enjoying an explosive growth of research as the fine details of the oblique helicoidal structures and their response to the external fields are not understood well. The proposed research will result in the new knowledge of how the structure of ChOH is shaped by the externally applied electric field and confinement geometry and how ChOH can be used in the design and fabrication of the new generation of electrically tunable optical elements such as filters, gratings, polarization rotators, attenuators. The electrically adjustable period of ChOH represents a new mode of the electro-optical response of LCs. The transformative value of the project is in the potential for new design concepts for tunable electro-optical devices performing in a broad spectral range from UV to Vis and IR.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.