With the development of virtual reality (VR) and augmented reality (AR) technology, the existing display technology is facing challenges and opportunities for providing vivid experience. Among many display technologies, holography shows extraordinary advantages in multi-dimensional optical recording and reconstruction, and is envisioned as the way to the ultimate visual feast.
Nowadays, holography is facing the dual challenges of massive data processing and limited optoelectronic system. Traditional optical holography is limited by static optical response. And computer-generated holography (CGH) suffers from large pixel size, which brings poor display performance. As an ultra-thin intelligent planar device, metasurfaces can modulate multi-dimensional optical information with subwavelength pixels, which is helpful to settle the above issues.
Therefore, multiplexed holographic metasurfaces emerge. Among many multiplexing parameters, polarization is insensitive to human eyes, but can provide innumerable possibilities. Compared with orbital angular momentum (OAM) or frequency, polarization multiplexed strategy is easier to implement and integrate with system. To efficiently reconstruct polarization-multiplexed holographic videos, it needs to develop suitable holographic encoding rules and metasurface design strategies.
To satisfy the above requirements, the research group led by Prof. Yongtian Wang and Prof. Lingling Huang from the School of Optics and Photonics, Beijing Institute of Technology, proposed a phase-only metasurface design strategy, which can realize polarization-multiplexed holographic videos in the optical band with high performance including high conversion efficiency and large field of view. The research results are published in Photonics Research, Volume 4, No. 10, 2022 (Shifei Zhang, Lingling Huang, Guangzhou Geng, Junjie Li, Xiaowei Li, Yongtian Wang. Full-Stokes polarization transformations and time sequence metasurface holographic display[J]. Photonics Research, 2022, 10(4): 04001031).
Fig. 1 Schematic of polarization multiplexed holography for time sequence holographic video.
The metasurfaces are composed of amorphous silicon (α-Si) nanofins. By designing the rectangular cross section and rotation of nanofins, the phase of the off-diagonal components of Jones matrix can be modulated independently. Meanwhile, through the improved vectorial genetic algorithm, two phase-only holograms of circular polarization basis can efficiently reconstruct multiple polarized images.
Fig. 2 Design principle of polarization multiplexed holography for time sequence holographic video
By changing the input or output polarization states, the holographic video with 106°full field of view and 16 frames is smoothly displayed. Due to the unique design method, the reconstructed image has no twin image and obvious zero order. Input and output polarization modulations are independent. At the same time, the conversion efficiency reaches 40.2% in experiment.
Dr. Zhang, who participated in this work, said, "Compared with previous schemes, we expand the modulation possibility to two full-Stokes spaces of input and output. Here, we encode holograms at subwavelength resolution, which ensures the absolute advantages of metasurfaces in field of view and information capacity. And the phase-only reconstruction greatly improves efficiency. We hope that this research will contribute to applications of holographic display and wavefront engineering, especially for those requiring compact forms."
The research group uses phase-only metasurfaces to efficiently reconstruct the polarization multiplexed holographic videos, which shows large field of view, high conversion efficiency and huge information capacity. Such scheme may provide solutions for wearable dynamic and near eye display applications. At the same time, optical encryption with high security can be designed by taking input and output polarization states as double secret keys. And the applications of all solid-state LiDAR, holographic optical tweezers based on such smart compact elements are possible.