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Credit: by Jianghao Xiong, Qian Yang, Yannanqi Li, and Shin-Tson Wu
Lately, planar liquid crystal (LC) optics is emerging as a novel holographic optical element (HOE). Besides the ability to record and reproduce arbitrary wavefront as in traditional HOEs, planar LC optics also exhibits unique properties like polarization selectivity, dynamic modulation, and large angular and spectral bandwidths. These advantages, combined with its ultrathin profile and high efficiency, make the planar LC optics extremely attractive for next-generation head-mounted displays, including augmented reality (AR) and virtual reality (VR), to pursue high image quality, lightweight, and compact form factor.
However, to enable a widespread application of this technology, the mass production issue must be seriously considered. So far, the most common fabrication method for planar LC optical elements still relies on lab-scale interferometers for the holographic exposure. This method is excellent for making centimeter-sized samples, but it becomes a bottleneck for large-scale and high throughput industrial manufacturing. On the contrary, another diffractive optical device—surface relief grating (SRG)—widely used in waveguide displays, benefits from nanoimprinting for mass production.
Nanoimprinting usually relies on a high-precision lithography method, like electron beam lithography, to write a master plate and then uses it to replicate copies. Although there are some issues associated with this technique, such as limited lifetime of the master plate and quality of replication, its major advantage of fast processing has led to preliminary success in SRG-waveguide displays.
In a new paper published in Light: Science & Applications, a team of scientists, led by Prof. Shin-Tson Wu from the College of Optics and Photonics, University of Central Florida, USA, proposed an intriguing concept called “holo-imprinting”, realizing the optical replication of planar LC optics. This method not only demonstrates the feasibility for mass production, but also eliminates the concern of master lifetime and imprinting quality due to its non-contact attribute.
Traditional HOEs that rely on the modulation of light intensity to form patterned fringes, which induces molecular diffusion. Planar LC optics, however, adopts a different mechanism of pattern recording, called photoalignment, which has been widely used in commercial LCD products, such as smartphones and TVs. Photoalignment molecules are extremely sensitive to the polarization state of light instead, among which the linear polarization produces the best alignment quality.
The formation of a high-quality linear polarization field is vital to the holographic exposure process. Previous interferometric methods mostly use two circularly polarized beams with opposite handedness (left and right) to form the linear polarization field pattern. Wu’s team, however, analyzed in the paper that two circularly polarized beams with same handedness could also produce high-quality linear polarization fields, but these two beams must be incident on the recording sample from opposite sides.
Coincidently, the lately developed reflective planar LC optics perfectly matches this requirement. Such a reflective LC optics is based on cholesteric liquid crystal, which has the self-assembly nature and forms stable helical structures. It only reflects the circularly polarized beam with same handedness as the helix. The reflected light has the same polarization state as the incident light.
Based on this principle, Wu’s team experimentally validated the concept and fabricated samples including gratings and lenses, which exhibit an excellent optical quality. For proof-of-concept, the sample size is around 5 cm. But the team pointed out that further scaling up the template size is readily achievable, by incorporating techniques like laser-scanning or multi-area exposure.
Journal
Light Science & Applications