Research Progress of Photopolymers for the Preparation of Holographic Optical Waveguide

Abstract

Holographic optical waveguide is the most practical waveguide coupling technology for head-mounted augmented reality displays (HMD-AR) due to its advantages of small size, lightweight, clear imaging, flexible design, and a large eye-movement range for users. To enhance the imaging capability of the HMD-AR system and expand its field of view, the volume holographic gratings (VHG) used as the coupling-in and coupling-out elements must have a high refractive index modulation (Δn). Photopolymer is one of the most promising recording media for manufacturing high-performance transparent VHG. The working principle of holographic optical waveguide in HMD-AR and the preparation principle of photopolymer VHG are introduced, the recent research progress of photopolymers in this field is reviewed, the influence of different components in photopolymer formulations and the post-treatment methods of the recording media on their holographic optical properties are summarized, mainly including: (1) the composition of the photoinitiator systems; (2) the crosslink density, molecular weight and refractive index of the film-forming resins; (3) the refractive index, addition and size of the writing monomers; (4) the additive of nanoparticles, chain transfer agents, free radical scavengers and plasticisers; (5) post-treatment methods such as heating and/or light. It is expected to provide guidance for designing new components, developing recording media with higher Δn, and fabricating HMD-AR with better performance.

Acrylate dendritic monomer for ultrafast holographic recording with pulse exposure

“…[4][5][6][7][8] Its recording principle is based on the formation of a holographic grating with refractive index modulation due to the photopolymerization of monomer in the coherent bright region and the migration of monomer from dark region to bright region under two coherent beams interference. 9,10 High diffraction efficiency and low selection angle of recorded gratings are favored to achieve holograms multiplexing in ultrafast holography. 11 Zhao et al investigated the holographic performances of an acrylamide/polyvinyl alcohol (AA/PVA) photopolymer with different Eosin Y concentration under 8 ns single-pulse holographic exposure.…”

High-Performance Holographic Photopolymers Based on Low Refractive Index Host Matrices Containing Fluoroelastomers

“…In addition, the compatibility between the monomers and the host matrix determines the transparency of the photopolymer. Most important, the difference in refractive index and the degree of segregation between the monomers and host matrix and the concentration of the monomers jointly determine the Δ n of the VHG. ,− Therefore, the matching of monomer and host matrix is very important for preparing a high-performance holographic photopolymer. ,, …”

“…Holographic optical elements (HOEs) have been widely used in near-eye display systems such as virtual reality, augmented reality (AR), mixed reality, and head-up display fields due to their advantages of lightweight, small size, convenient production, easy miniaturization, and functional integration. , Photopolymers possess great application potential in fabricating transparent HOEs, so they have gained widespread attention in recent years. − The refractive index modulation (Δ n ) of photopolymers is a key indicator to determine their holographic performance. ,− Though there are a few photopolymers with Δ n above 0.05, their preparation processes require the use of volatile solvents, which is not favorable for large-scale applications. , So far, with the exception of BayerHX, almost no solvent-free photopolymers with Δ n above 0.05 have been reported . Therefore, the development of solvent-free photopolymers capable of Δ n as high as possible is becoming a hot research topic in this field. ,,, …”