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Liquid Crystal-Based Tunable Diffraction Gratings and Polarization-Dependent Lens Arrays : 액정 기반의 조정 가능한 회절격자 및 편광 의존 렌즈 어레이에 관한 연구

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Authors

김지윤

Advisor
이신두
Major
공과대학 전기·컴퓨터공학부
Issue Date
2015-08
Publisher
서울대학교 대학원
Keywords
Diffraction gratingLens arrayNematic liquid crystalDye-doped ferroelectric liquid crystalReactive mesogenImprinting
Description
학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2015. 8. 이신두.
Abstract
In recent years, as the employment of optical systems such as display, communication, and data storage has expanded explosively, more sophisticated functions are required to these optical systems. Therefore, fundamental optical components playing significant role in the operation of these optical systems including diffraction gratings, lenses, beam splitters, prisms, and optical filters are widely studied in order to assign advanced and particular functions to optical devices. Among various fundamental optical components described above, the diffraction gratings and the lens arrays composed of regularly aligned lenses have been considered as key elements which are studied most widely due to their wide applications and relatively simple operations and realizations. Since the tuning capability is essential for most recent optical systems, it is clearly required for the diffraction grating and the lens array to change the optical properties according to the external stimuli. For several decades, a number of types of tunable diffraction gratings and lens arrays have been suggested, and the optical properties of most of these devices were modulated by the applied electric voltage, the incident electromagnetic wave, or applied pressure. Especially, based on the large electro-optic modulation of liquid crystals (LCs) resulting from the high optical anisotropy and the electrically or optically tuning capability, a variety of tunable diffraction gratings and lens arrays using LC have been suggested. However, LC diffraction gratings suffer from the very limited diffraction pattern due to the difficulty in achieving unconstrained alignment, or the slow and small change in the diffraction characteristics when optically controlled. Furthermore, the applications of LC lens arrays are also limited owing to the sensitivity of LC alignment on the geometrically uneven substrate or the restricted focusing effect compared to other types of lens.
This thesis primarily aims to demonstrate new types of LC diffraction gratings and LC lens arrays. It consists of three major categories, first one and the second one of which concern the electrically tunable LC grating, and the optically tunable ferroelectric LC (FLC) grating. The last one concerns the electrically tunable LC square lens array and the LC lenticular lens array.
Firstly, a fully continuous LC grating device with the alternating semi-circular alignment which exhibits the switching effect between the diffraction orders independent of the thickness of the LC cell is demonstrated. The continuous phase modulation in the LC grating with the rotational symmetry was achieved on a micro-imprinted surface where the semi-circular alignment of the LC was spontaneously produced. This LC grating device in the hybrid geometry exhibited the perfect continuity of the phase retardation and the switchable diffraction as a function of an applied voltage. It was also found that the symmetry of the input polarization direction with respect to the grating patterns results in the interchange between two symmetric grating configurations.
Secondly, an all-optically switchable FLC grating constructed in an alternating binary configuration with different optical properties from domain to domain is demonstrated. A dye-doped FLC is uniformly aligned in one type of domains whereas it is infiltrated into the photo-polymerized networks of reactive mesogens in the other. Compared to conventional nematic LC cases, this FLC-based grating allows more efficient all-optical modulation and faster diffraction switching in subsecond since the optical response associated with the dye molecules in the layered state is less hindered than in the orientationally ordered state.
Finally, an LC-based square lens array with two focusing modes according to the polarization state of the input light is demonstrated. The homogeneously aligned LC layer is placed on an array of static square lenses fabricated using a photo-curable polymer whose refractive index is matched with the refractive index of the LC. For the input beam polarized parallel to the easy axis of the LC, the focal length is varied with the applied voltage. For the perpendicularly polarized input beam, the focal length is independent of the applied voltage and remains constant. An LC-based lenticular lens array, having a polarization-dependent focusing effect, fabricated through a simple imprinting process is also demonstrated. The input polarization-dependence of our LC lenticular lens array arises mainly from the index matching scheme between the polymer lenticular lens and the LC on it.
In conclusion, throughout this thesis, new types of LC diffraction gratings including electrically tunable LC grating having continuous alignment geometry and optically switchable FLC binary grating with superb switching capability are demonstrated. Moreover, electrically tunable LC square lens array and LC lenticular lens array with perfect polarization-dependence are also described. These advanced fundamental optical devices will be useful for devising a variety of sophisticated optical systems.
Language
English
URI
https://hdl.handle.net/10371/119106
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