Generation of optical vortices is described in cholesteric liquid crystals with a singular point in the spatial distribution of a helix phase. The phenomenon uses the fact that a Bragg reflected light phase varies in proportion to the spatial phase of the helix, both at normal and oblique incidences. Our proposal enables high-efficiency, polychromatic generation of optical vortices without the need of a cumbersome fabrication process and fine-tuning.
Patterned cholesteric liquid crystals enable the fabrication of circular-polarization sensitive, re ective di ractive optical elements based on Bragg re ection. Here we report the fabrication of large-angle re ective deector, which show de ection angles as large as 48.9°. The devices are fabricated by polarization holography, whereby two orthogonal circular polarized beams are superimposed on a photoalignment layer to create a linear modulation of the orientational easy axis. The devices show high circular-polarization extinction ratios, making them potentially useful as functional re ectors and polarization splitters.
Recent advances in nanofabrication techniques are opening new frontiers in holographic devices, with the capability to integrate various optical functions in a single device. However, while most efficient holograms are achieved in reflection-mode configurations, they are in general opaque because of the reflective substrate that must be used, and therefore, have limited applicability. Here, we present a semi-transparent, reflective computer-generated hologram that is circularly-polarization dependent, and reconstructs different wavefronts when viewed from different sides. The integrated functionality is realized using a single thin-film of liquid crystal with a self-organized helical structure that Bragg reflects circularly-polarized light over a certain band of wavelengths. Asymmetry depending on the viewing side is achieved by exploiting the limited penetration depth of light in the helical structure as well as the nature of liquid crystals to conform to different orientational patterns imprinted on the two substrates sandwiching the material. Also, because the operation wavelength is determined by the reflection band position, pseudo-color holograms can be made by simply stacking layers with different designs. The unique characteristics of this hologram may find applications in polarization-encoded security holograms and see-through holographic signage where different information need to be displayed depending on the viewing direction.
Cholesteric liquid crystals are known to exhibit a circular-polarization selective Bragg reflection band over a wavelength region determined by their reflective index and pitch.In a planar device where the molecules are aligned unidirectionally at the substrate This article is protected by copyright. All rights reserved. 2 surface, cholesteric liquid crystals act as flat dielectric mirrors, reflecting light at an angle fulfilling the law of reflection. Here, we show that by introducing a random structure inspired by the Morpho didius butterfly into the distribution of the helix phase, the reflected light phase becomes randomized, leading to a diffuse reflection that spans over an angle greater than ±30°.The reflection maintains its circular polarization selectivity, making these materials unique compared to the butterfly itself and other man-made structures mimicking the butterfly's structure.
Singular optics is an emerging field, in which an optical wave with a helical wavefront is handled for optical manipulation and nanofabrication. We present a method to generate optical vortices via reflection from cholesteric liquid crystals with planar alignment. Numerical and experimental results show that the device is operable at multiple wavelengths with theoretically high conversion efficiency (lossless) without requiring fine-control of retardation. Although the operational wavelength range is limited by the birefringence and helix pitch of the material, broadband operation is possible by expanding the reflection band width by either introducing a pitch distribution or stacking multiple devices.C0NTACT Hiroyuki Yoshida
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