Lately, liquid crystal (LC) planar optics are emerging as a new holographic optical element (HOE). Besides the ability to record and reproduce an arbitrary wavefront as in traditional HOEs, planar LC optics also exhibit unique properties such as polarization selectivity, dynamic modulation, and wide angular and spectral bandwidths. These advantages, combined with its ultra-thin profile and high efficiency, make 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 widespread application of this technology, the issue of mass production must be seriously considered. Until now, the most common fabrication method for planar LC optical elements still relies on lab-scale interferometers for holographic exposure. This method is excellent for making centimeter samples, but it becomes a bottleneck for large-scale, 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 typically relies on a high-precision lithography method, such as electron beam lithography, to write a master plate and then uses it to reproduce copies. Although there are some problems associated with this technique, such as limited life of the master plate and quality of replication, its major advantage of fast processing has led to preliminary success in waveguide displays. GIS.
In a new paper published in Light: Science & Applications, a team of scientists, led by Professor Shin-Tson Wu from the College of Optics and Photonics, University of Central Florida, USA, has proposed an intriguing concept called “holo-imprinting”. . , realizing the optical replication of planar LC optics. This method not only demonstrates the feasibility of mass production, but also eliminates concern for master life and print quality due to its non-contact attribute.
Traditional HOEs that rely on modulating light intensity to form patterned fringes, which induces molecular scattering. Planar LC optics, however, adopt a different pattern registration mechanism, called photoalignment, which has been widely used in commercial LCD products, such as smartphones and televisions. Photoalignment molecules are extremely sensitive to the polarization state of light, of which linear polarization produces the best alignment quality.
The formation of a high quality linear bias field is vital to the holographic exposure process. Previous interferometric methods mainly use two circularly polarized beams with opposite sensitivity (left and right) to form the linear polarization field pattern. Wu’s team, however, analyzed in the paper that two circularly polarized beams with the same laterality could also produce high-quality linear polarization fields, but these two beams must be incident on the recording sample of opposite sides.
Coincidentally, the newly developed reflective planar LC optic meets this requirement perfectly. Such reflective LC optics are based on cholesteric liquid crystal, which has the nature of self-assembly and forms stable helical structures. It only reflects the circularly polarized beam with the same sensitivity 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 excellent optical quality. For the proof of concept, the sample size is approximately 5 cm. But the team pointed out that a further increase in model size is easily achievable, incorporating techniques such as laser scanning or multi-zone exposure.
Light sciences and applications
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