Traditional refractive lenses have numerous uses, but also several problems. Most notably, the phase of a wave has to be continuous at both surfaces, with phase accumulating continuously as the wave propagates through the lens. This means that, to create a macroscopic deflection of light, a macroscopic thickness of lens is required, which often makes the lenses undesirably heavy or bulky. They are also insensitive to polarization – which is sometimes an advantage but does limit the possibilities that can be realized.

Resonating problems

In 2011 researchers at Harvard University unveiled an alternative type of lens called a metasurface, which uses dielectric or metallic resonators to interfere directly with the electromagnetic field of the wave. This changes the phase discontinuously at a single point in space, allowing for flat lenses. Unfortunately, the resonators have to be smaller than the wavelength of the radiation. This is relatively easy in the case of infrared radiation, but becomes increasingly difficult for shorter wavelengths such as visible light, for which nanometre-scale resonators are required, thus limiting the prospects for mass production.

Hiroyuki Yoshida, Masanori Ozaki and Junji Kobashi at Osaka University in Japan have taken a different approach, using two surfaces with a layer of cholesteric liquid crystals – liquid crystals with a helical structure that are chiral – between them. The rod-shaped molecules can form themselves into helical structures that interact with light waves, reflecting light with the same circular polarization as the helix, while transmitting light with the opposite polarization unperturbed. The researchers realized that they could control the phase imprinted on the reflected light (and thereby shape the reflected wavefront) by controlling the phase of the helices at every point around the surface of the lens.

Spiral self-assembly

This required controlling the orientation of every liquid crystal on one surface, and the rest of each helical structure would self-assemble from that starting orientation, almost like a spiral staircase building itself from the bottom step. The researchers achieved this using a process called photo-alignment, which required just a commercial LCD projector and a rotatable waveplate. "You don't need to make tiny structures," says Yoshida, "so we didn't need any nanofabrication techniques."

The researchers fabricated two optical devices from liquid crystals: a deflector to focus reflected light and a special type of lens called a Fresnel lens, which is used for theatrical lights and some camera lenses. Both devices achieved almost total circular-polarization selectivity. Yoshida believes this could be useful in smart glasses, because it could enable all of the light from a projector to be reflected into the wearer's eye, whereas current designs transmit half of this light out through the lens.

One advantage of the lenses over metasurfaces is that the patterns are not fixed, but can be made responsive to temperature or electric field, potentially allowing the optical effects to be switched on and off at the touch of a button. The researchers are currently only able to deflect the reflected beam by about 0.5° but are working to optimize this – something that involves miniaturizing the patterns of the cholesteric liquid crystals. Yoshida told that they "have confirmed that we can go up to 7° or so, but the technology to miniaturize the patterns is still under development".

Path to real-world device

Surface scientist Hiroshi Yokoyama of Kent State University in Ohio describes the work as "pretty significant". "Not everything is new," he says, "but it certainly has novel aspects: the applications seem to be very up to date and there's a strong interest in developing thin-film lenses and optical components." Shin-Tson Wu of the University of Central Florida agrees, saying that the work solves "a long-standing challenge to realize light deflection with cholesteric liquid crystals".

However, both Yokoyama and Wu note that the results were measured using a monochromatic laser, and say that the lens would probably encounter problems with broadband light. "A single glass lens has a chromatic aberration [it focuses different wavelengths at different distances]; this is also true, or even more significantly so, in the case of a pattern-alignment liquid-crystal lens," explains Yokoyama. "That has to be solved in order to make this a real, working device."

The research is published in Nature Photonics.