Transversely graded polarization volume gratings fabricated by freeform holographic photoalignment.

Polarization volume gratings (PVGs) based on chiral nematic liquid crystals offer a great potential as polarization-dependent holographic optical elements, but it is not easy to fabricate PVGs with varying pattern periods in the transverse plane. Here, we fabricate a PVG with an in-plane gradient of the pattern period by performing two-beam interference photoalignment on a flexible polyimide substrate. The pattern period varies depending on the local interference angle, which is controlled by the bent shape of the flexible substrate. We demonstrate fabrication of a PVG with a linearly graded sub-micrometer period, showing the potential of the proposed method to fabricate designer PVGs.

Circularly-polarized, semitransparent and double-sided holograms based on helical photonic structures.

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.

Polychromatic Optical Vortex Generation from Patterned Cholesteric Liquid Crystals.

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.

Polarization-independent submillisecond phase modulation utilizing polymer/short-pitch cholesteric liquid crystal composite.

A broadband, polarization-independent phase modulation spanning the visible range is demonstrated using a polymer/cholesteric liquid crystal composite with optical pitch in the ultraviolet. Polarization insensitivity is achieved as a result of two effects: (1) optical anisotropy of the rod-like molecules is canceled out by the short helical pitch, and (2) stabilization of the Grandjean texture by the polymer network suppresses depolarization. Polarization-independent modulation of the refractive index by approximately 0.045, corresponding to a phase modulation of π at 500 nm, is achieved with submillisecond response times. Our material system opens new avenues for polarization-independent, tunable optical devices, such as narrow bandpass filters, gratings, and adaptive lenses.

Tunable enhanced 0th-order transmission in a metal-dielectric hole array covered with a subwavelength liquid crystal layer.

We demonstrate tunable, enhanced 0th-order transmission through a metal-dielectric nanohole array device with a subwavelength-thick liquid crystal (LC) layer. The LC filled the nanoholes and formed a subwavelength covering layer, which is then capped by a top cover layer. The wavelength where the transmittance dip associated with the LC occurs is determined by the anisotropic refractive-index component of the LC, which is normal to the surface of the hole array. A low-refractive-index cover layer suppresses unwanted higher-order diffraction, which results in an enhancement of the 0th-order transmission, which is closely related to laterally propagating surface plasmon polaritons. The proposed design is expected to help realize tunable plasmonic devices with high optical transmittance.