Planar liquid-crystal optical elements could help miniaturize mirror-based optical systems
Bragg-reflecting cholesteric liquid crystals can alter the reflected phase profile arbitrarily.
|Schematic illustration (left) and photos (right) show a standard cholesteric liquid crystal device. The green bars in the left figure are guides indicating positions with the same helix phase. Cholesteric liquid crystals reflect circularly polarized light with the same handedness as the helical structure and with wavelength fulfilling the Bragg condition. In the right figure, purple light with right circular polarization is reflected. (Image: Osaka University)|
Researchers at Osaka University (Osaka, Japan) developed a technology to control the light wavefront reflected from a cholesteric liquid crystal, which is a liquid crystal phase that has a helical structure. Although known for their ability to Bragg-reflect light, cholesteric liquid crystals could only be used as flat mirrors, reflecting light at the same angle as the incident angle. The new technology enables planar optical components to be made with functionality by design, contributing to the miniaturization of catoptric devices (mirror-based optical systems).1
The cholesteric liquid crystal is a liquid crystal phase in which the constituent rod-like molecules spontaneously form a helical structure (see figure). Owing to its structure, cholesteric liquid crystals exhibit Bragg reflection for circularly polarized light with the same polarization handedness as the helix, over a wavelength range determined by the refractive index and the helical pitch.
Their characteristic optical properties, as well as the fact that structure is formed by self-organization, have made cholesteric liquid crystals attractive for use as circular polarizers, light reflectors, and electronic paper. However, their ability to function only as a flat dielectric mirror in which light must follow the law of reflection posed a limit on the performance they could achieve, and hence usage of devices based on these materials.
Hiroyuki Yoshida, Assistant Professor, Junji Kobashi, a graduate student, and Masanori Ozaki, Professor at the Graduate School of Engineering, Osaka University discovered that the optical phase reflected from a cholesteric liquid crystal varied depending on the phase of the helical structure.
The distribution of optical phase (otherwise known as the wavefront) determines how the light propagates; for example, the wavefront of light propagating along a straight line has a planar profile, whereas the wavefront of light that converges has a curved (spherical) profile. On the other hand, the helix phase defines the relative orientation of the helical structure at a particular position in space, and can easily be controlled by defining the orientation of the liquid-crystal molecules on a substrate. Therefore, by patterning the orientational "easy axis" (the axis of equilibrium) in a standard, slab-like cholesteric liquid crystal device, the reflected wavefront can be designed arbitrarily.
1. Junji Kobashi et al.,Nature Photonics (2016); doi:10.1038/nphoton.2016.66