Striving for clarity: Dynamic liquid crystal lenses

Liquid crystal (LC) lenses with electrically tunable focal length have attracted much attention for their potential in ophthalmic applications, three-dimensional displays, cell phones, virtual reality (VR), and augmented reality (AR). The optical mechanism of gradient refractive-index (GRIN) LC lenses mainly results from the electrically adjustable distribution of refractive indices for incident polarized wave by means of the molecular orientations of LC.

Although LC lens was first proposed in 1979, many challenges hinder the applications of LC lenses because of the chemical as well as physical limitations of LCs. For example, it has been a challenge to have large aperture size as well as short focal length (or large lens power) of GRIN LC lenses at the same time. To obtain short focal length at large aperture size (>10 mm), the LC layers must be thickened by means of a multilayered LC structure that exhibits polarization independence, large aperture size, and large tunable range in the lens power without sacrificing response time. However, in this way high driving voltage (∼90 V_rms) and the unavoidable haze that result affect the image quality of LC lenses and hinder practical applications.

Such a haze in GRIN LC lenses using rod-like LC molecules results from light scattering because of poor alignment and orientational fluctuations of LC molecules. The orientation fluctuation originates from a thermotropic characteristic of LCs. Many research studies present methods to reduce light scattering induced by the orientation fluctuations in LC phase retarders, such as enlargement of operating voltages or magnetic fields, thinner LC layer, and augmentation of anchoring energy to help LC molecules align better along certain directions as well as against the perturbation.

“The haze has been a long-lasting problem in GRIN LC lens, especially since we want to go for a large aperture size—30 mm or even larger for ophthalmic applications. We discuss the root cause of haze of the LC lens resulting from the orientational fluctuations of LC molecules,” said Yi-Hsin Lin, a professor at National Yang Ming Chiao Tung University, where a recent study published in the Journal of Optical Microsystems was conducted.    

Theoretical and experimental results confirm that the elastic constant and the electric field help to minimize or suppress the haze. Additionally, the haze is linearly proportional to birefringence squared divided by elastic constant. As a result, the imaging quality of the LC lens is improved after considering those factors. The driving voltage is also reduced 4 times from 80V_rms to 18 V_rms by removing the buffering layer in the LC lens structure.

Based on the elastic continuum theory of nematic LCs, an LC with larger elastic constant needs to receive more energy in order to perturb the LC molecules, just like more force or energy must be applied to pull a spring with large spring elastic constant. Thus, the LC lens with LC materials of higher elastic constant could reduce haze induced by orientation fluctuations. This research can provide a better understanding for engineers and researchers to develop better GRIN LC lenses. The applications of such dynamic lenses are versatile: eyeglasses, AR/VR, portable imaging systems, and machine vision.

Read the Gold Open Access paper by Lin et al., “Orientation fluctuation in liquid crystal lenses,” J. Opt. Microsys. 3(4) 041204 (2023) doi: 10.1117/1.JOM.3.4.041204