Electrically tunable cholesteric optical filters

Kent State University, Kent OH

Investigators

Abstract

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.