Liquid crystals open route to planar optical elements

The cholesteric liquid crystal is a liquid crystal phase in which the constituent rod-like molecules spontaneously form a helical structure. Owing to its structure, cholesteric liquid crystals exhibit Bragg-reflection for circularly polarised light with the same polarisation 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-organisation, have made cholesteric liquid crystals attractive for use as circular polarisers, light reflectors, and electronic papers.

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 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 (also known as the wavefront) determines how the light propagates; for example, light propagating along a straight line has a flat profile, whereas light that converges has a curved (parabolic) 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 in a standard, slab-like cholesteric liquid crystal device, the reflected wavefront can be designed arbitrarily.

The device shows high circular polarisation selectivity even when the helix phase is patterned; the technology thus provides a platform to develop unique optical devices that can be tuned from being fully reflective to transmissive, depending on the incident polarisation.