Bifocal Lenses, with Liquid Crystal Bilayers, Allow Flexible Light Modulation

A bilayer, bifocal lens with adjustable focal points could allow more flexible modulation of light in applications for optical computing, bioimaging, AR and VR, and other areas.

The lens, which was developed at Hunan University, is composed of bilayer structures made with a liquid crystal cell and liquid crystal polymer. Each liquid crystal layer has a different function in controlling light modulation.

“Most liquid crystal-based devices are made from single-layer structures, but this limits light field modulation to a confined area,” research team leader Fan Fan said. “We used bilayer structures composed of a liquid crystal cell and a liquid crystal polymer to realize more complex and functional modulation of incident light.”

Researchers developed a bifocal lens based on two layers of liquid crystal structures. The intensities for the two focal lengths can be easily adjusted by applying external voltage. Courtesy of Fan Fan, Hunan University.

While some bifocal lenses can create different focal points depending on the polarization of the incident light, the bilayer, bifocal lens takes this a step further by allowing active manipulation of the polarization states of the output beams. The bilayer lens splits left-handed circularly polarized light into two focused light beams — one with left-handed circular polarization and one with right-handed circular polarization. The left-handed and the right-handed circularly polarized light can be controlled independently.

The bilayer, bifocal lens allows the user to establish two focal points with different intensities. The liquid crystal cell layer of the lens enables the lens to rapidly change foci intensity in response to an external voltage. The relative intensity of each focal point can be adjusted arbitrarily. In previously developed bifocal lenses, the focal point could only be adjusted through mechanical rotation of the wave plate.

As proof of concept, the researchers created longitudinal and transverse bifocal lenses that were designed to split a portion of left-handed circularly polarized incident light into two convergent beams with orthogonal helicity. The researchers showed that the relative intensity of the two foci could be adjusted arbitrarily by using the electrically controlled polarization conversion capabilities of the liquid crystal cell. They confirmed that the point spread function of the bilayer, bifocal lens corresponded to theoretical calculations.

The researchers also demonstrated broadband polarization and edge imaging with the bifocal lens. They showed that the new lens can be used for polarization imaging, which is used to enhance image contrast, and for edge imaging, which is used to highlight the outlines of objects to make it easier to see fine details and certain shapes.

In polarization imaging, the separation distance between the two foci is large (2 mm). In edge imaging, a small separation distance (0.03 mm) is used with equal intensity for the two focal points. The team incorporated the lens into imaging systems for polarization and edge imaging, and the bifocal lens performed well for both types of imaging.

“In virtual or augmented reality devices, bifocal lenses are commonly used to adjust the distance of the image display to overcome vergence-accommodation conflict, which can cause visual discomfort and eye strain,” Fan said.

The bifocal lenses are made from bilayer structures of liquid crystal cells and liquid crystal polymer. These lenses split light with a left-handed circular polarization (LPC) into two focused light beams with left and right-handed circular polarization (RPC). Courtesy of Fan Fan, Hunan University.

The design for the bilayer, bifocal lens was inspired by the development of multifunctional holographic devices. “Researchers have devised many methods to improve the information capacity of holographic devices, including holographic devices based on multilayer structures,” Fan said. “We thought this type of structure could be useful beyond the field of holographic displays, so we tried to expand its application scenarios.

“We believe that the light control mechanism we created using the multilayer structure could also be used to design other optical devices, including holographic devices and beam generators, or for optical image processing,” Fan said.

The researchers are currently working on the design and manufacture of additional multifunctional devices based on the bilayer structures for use in other research applications.

To make the lenses practical for commercial use, the researchers said that it will be necessary to lower the cost of mass production of the lenses, incorporate the ability to adapt to different environments into the lenses, and develop fast, accurate layer-to-layer alignment technology.

“With this research, we aimed to illustrate the huge potential of bilayer structures for optical devices and the advantages of liquid crystal devices in electrical tunability,” Fan said. “We hope these unique features inspire scientists to develop even more advanced applications.”

The research was published in Optics Letters (www.doi.org/10.1364/OL.537415).