  Despite the recent advances in augmented reality (AR), which has shown the
potential to significantly impact on our daily lives by offering a new way to
manipulate and interact with virtual information, minimizing visual discomfort
due to the vergence-accommodation conflict remains a challenge. Emerging AR
technologies often exploit focus-tunable optics to address this problem.
Although they demonstrated improved depth perception by enabling proper focus
cues, a bulky form factor of focus-tunable optics prevents their use in the
form of a pair of eyeglasses. Herein, we describe an ultrathin, focus-tunable
liquid crystal (LC) diffractive lens with a large aperture, a low weight, and a
low operating voltage. In addition, we show that the polarization dependence of
the lens, which is an inherent optical property of LC lenses, can be eliminated
using birefringent thin films as substrates and by aligning the optical axes of
the birefringent substrates and LC at a specific angle. The polarization
independence eliminates the need for a polarizer, thus further reducing the
form factor of the optical system. Next, we demonstrate a prototype of AR
glasses with addressable focal planes using the ultrathin lens. The prototype
AR glasses can adjust the accommodation distance of the virtual image,
mitigating the vergence-accommodation conflict without substantially
compromising the form factor or image quality. This research on ultrathin lens
technology shows promising potential for developing compact optical displays in
various applications.
