Browsing by Author "Reshetnyak, Victor"

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Liquid crystal lenses with tunable focal length
(2017-01-01)
Lin, Yi-Hsin
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Wang, Yu-Jen
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Reshetnyak, Victor
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光電工程學系
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Department of Photonics
Lenses with tunable focal length play important roles in nature by helping species avoid predators and capture prey. Many practical devices mimic lens concept for imaging, sensing, and detection. This review covers fundamental optics of lenses and its extension to lenses made of liquid crystals (LCs). Three main types of LC lenses are described, namely, lenses with curved surfaces, flat gradient-index lenses and composite lenses. The review discusses advantages of LC lenses over their isotropic counterparts, challenges in their fabrication and control, as well as a variety of potential applications. We also discuss the current challenges associated with nematic LC lenses and their solutions. LC lenses are already having significant impacts on optics and optometry, and these impacts will grow with discovering new LC materials and new lens designs.
Origins of Kerr Phase and Orientational Phase in Polymer-Dispersed Liquid Crystal
(2017-01-01)
Chang, Chia-Ming
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Lin, Yi-Hsin
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Reshetnyak, Victor
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Park, Chui Ho
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Manda, Ramesh
The anisotropic properties of nematic liquid crystals result in polarization dependency which leads to requirement of at least a polarizer in liquid crystal photonic devices. To develop polarizer free phase modulation, Kerr effect is one of the path. The phase modulation in polymer dispersed liquid crystals (PDLCs) is shown to have two parts: Kerr phase, which is the optical phase modulation linearly proportional to a square of electric field, and orientational phase. However, many puzzles are still under investigation: the origins of Kerr phase, the relation between Kerr phase and orientational phase, and how two-steps of electro-optical (EO) response relates to Kerr phase and orientational phase. We investigated the origins of Kerr phase and orientational phase in PDLC and their connection to two-step EO response. In our study, the Kerr phase is a result of LC orientation in the center of LC droplets. The orientational phase attribute to orientation of LC molecules near LC-polymer interfaces. The two phase in PDLC samples are adjustable depending on droplet size. We also found that two-steps EO response existing in small droplet (<333 nm) is related to Kerr phase and orientational phase. A modified PDLC model related to the Kerr phase and orientational phase is also proposed. Besides the conventional features of PDLC, such as polarization independent optical phase shift and response time independent of cell gap, we believe the Kerr phase and orientational phase with different response times (similar to msec) in PDLC pave a way for designing versatile photonic devices with pure optical phase modulation.
Origins of Kerr phase and orientational phase in polymer-dispersed liquid crystals
(2017-08-21)
Chang, Chia-Ming
;
Lin, Yi-Hsin
;
Reshetnyak, Victor
;
Park, Chui Ho
;
Manda, Ramesh
Polymer-dispersed liquid crystals (PDLCs) modulate the amplitude and optical phase of light. The optical phase modulation of PDLC can be dissected into two parts: Kerr phase and orientational phase according to the electro-optical (EO) response. We investigated the origins of the Kerr and orientational phases in PDLCs and their connection with the two-step EO response. The Kerr phase is attributed to LC orientation in the center of LC droplets. The orientational phase results from orientation of LC molecules near LC-polymer interfaces. Both phases can be adjusted by varying the droplet size. The two-step EO response in small droplets (<333 nm) is related to the Kerr and orientational phases, and possibly to rotation of point defects. A modified PDLC model considering the Kerr and orientational phases is proposed. Our findings suggest the possibility of versatile photonic devices using pure optical phase modulation. (C) 2017 Optical Society of America
Phase modulators with tunability in wavefronts and optical axes originating from anisotropic molecular tilts under symmetric electric field II: experiments
(2020-03-16)
Wang, Yu-Jen
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Lin, Yi-Hsin
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Cakmakci, Ozan
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Reshetnyak, Victor
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光電工程學系
We demonstrate, for the first time, an electrically-tunable and physically-planar freeform optical element made up of nematic liquid crystals (LCs). Continued on numerical study in previous paper (Part I), experimental results here show that it is possible to break the rotational symmetry of the wavefront through the use of uneven tilt angles of the LC molecules even though the electric potential is rotationally symmetric. Our optical element offers the ability to electrically tune the direction of the optical axis, the wavefront deviation, as well as the Zernike polynomials for general descriptions of wavefronts. Corresponding Zernike coefficients of a Zernike polynomial that are related to defocus and spherical aberration, which can be adjusted individually or together. The minimum wavefront deviation is >lambda/16. The Zernike coefficients related to coma aberration or the tilt of the optical axis are also electrically tunable. By incorporating our LC phase modulator with tunability of freeform wavefronts into a simple reflective optical system, we demonstrate convincing image performance for off-axis image aberration correction. This approach will inspire further development and design of LC optical elements for applications, such as hyperspectral imagers in aerospace optics, augmented reality, virtual reality, quantum information systems, innovative miniaturized reflective telescopic systems for astrophysics, planetary science, and earth science. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Polarization aberrations of electrically tunable liquid crystal mirrors
(2020-04-13)
Wang, Yu-Jen
;
Lin, Yi-Hsin
;
Cakmakci, Ozan
;
Reshetnyak, Victor
;
光電工程學系
Curved mirrors are able to fold optical paths and play important roles in compact optical systems in general. In this paper, we investigate the polarization aberrations of electrically tunable liquid crystal (LC) mirrors with two kinds of configurations (flat and curved ones). The LC mirrors exhibit spatially-continuous tunable wavefronts. The detailed wavefronts of two LC mirrors are related to angles of incidence, polarization of light, and the alignment direction of LC molecules. The key contribution of this paper is the development and characterization of a tunable liquid crystal mirror. The tunability of polarization aberration of LC mirrors should be able to provide extra parameters for optical engineers to design versatile optical systems. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Varifocal augmented reality adopting electrically tunable uniaxial plane-parallel plates
(2020-07-20)
Wang, Yu-Jen
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Lin, Yi-Hsin
;
Cakmakci, Ozan
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Reshetnyak, Victor
;
光電工程學系
Vergence-accommodation conflict (VAC) is a major challenge in optical-see through augmented reality (AR) system. To resolve this conflict, many approaches are proposed, for instance, by means of adjustment of the projected virtual image to coincide with the surroundings, called image registration, which is more often referred to as varifocal function. In this paper, a varifocal AR system is demonstrated by adopting electrically tunable liquid crystal (LC) plane-parallel plates to solve VAC problem. The LC plates provide electrically tunable optical paths when the directors of LC molecules are re-orientated with applied voltages, which leads to a corresponding change of light speed for an extraordinary wave. To provide a sufficient tunable optical path, three pieces of multiple-layered LC structures are used with the total thickness of the active LC layers (similar to 510 mu m). In experiments, the projected virtual image can be adjusted from 1.4 m to 2.1 m away from the AR system, while the thickness of LC plane-parallel plates are only less than 3 mm without any mechanical moving part. When light propagates in the uniaxial LC layers, the wave vector and the Poynting vector are different. The longitudinal displacement of the image plane is determined by Poynting vectors rather than wave vectors. As a result, the analysis of the AR system should be based on Poynting vectors during geometrical optical analysis. Surprisingly, the tunable range of the longitudinal displacement of Poynting vectors is 2-fold larger than the tunable range of the wave vectors. Moreover, the virtual image shifts in opposite directions with respect to the Poynting vectors and wave vectors. The proposed AR system is not only simple but also thin, and it exhibits a large clear aperture. The investigation here paves the way to a simple solution of the VAC problem for augmented reality systems. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement