materials Review Review of Organic Photorefractive Materials and Their Use for Updateable 3D Display Pierre-Alexandre Blanche 1,* , Jae-Won Ka 2 and Nasser Peyghambarian 1   Citation: Blanche, P.-A.; Ka, J.-W.; Peyghambarian, N. Review of Organic Photorefractive Materials and Their Use for Updateable 3D Display. Materials 2021, 14, 5799. https://doi.org/10.3390/ma14195799 Academic Editor: Alina Pruna Received: 30 August 2021 Accepted: 27 September 2021 Published: 4 October 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 College of Optical Sciences, University of Arizona, Tucson, AZ 85721, USA; nasser@optics.arizona.edu 2 Advanced Functional Polymers Research Center, Korea Research Institute of Chemical Technology, 141 Gajeong-ro, Yuseong-gu, Daejeon 34114, Korea; jwka@krict.re.kr * Correspondence: pablanche@optics.arizona.edu Abstract: Photorefractive materials are capable of reversibly changing their index of refraction upon illumination. That property allows them to dynamically record holograms, which is a key function for developing an updateable holographic 3D display. The transition from inorganic photorefrac- tive crystals to organic polymers meant that large display screens could be made. However, one essential figure of merit that needed to be worked out first was the sensitivity of the material that enables to record bright images in a short amount of time. In this review article, we describe how polymer engineering was able to overcome the problem of the material sensitivity. We highlight the importance of understanding the energy levels of the different species in order to optimize the efficiency and recording speed. We then discuss different photorefractive compounds and the reason for their particular figures of merit. Finally, we consider the technical choices taken to obtain an updateable 3D display using photorefractive polymer. By leveraging the unique properties of this holographic recording material, full color holograms were demonstrated, as well as refreshing rate of 100 hogels/second. Keywords: photorefractive; 3D display; holography; energy levels; polymer; electro optic; birefrin- gence; holographic stereogram 1. Introduction Holograms are known first and foremost for their ability to project 3D images. Holo- graphic 3D images were demonstrated in the early 1960s by Leith, and Upatnieks [1], as well as Denisyuk [2]. What distinguishes holography from other 3D technologies is its abil- ity to reproduce all the human 2D and 3D visual cues: occlusion, parallax, accommodation, and vergence [3–5]. This makes holography a strong contender to obtain the ultimate 3D display that would be able to project images as good as the real world [6,7]. From the early work on holographic imaging, many accomplishments have been made regarding the rendering of 3D images using holograms. The depth of field has been enlarged by using long coherence lasers [8]; for example, full color reproduction has been achieved, thanks to 3 color lasers recording [9,10]; reproductions of large scenes and life subjects have been made possible by using short pulse lasers [11]; and rainbow holograms can be viewed with white light [12]. Today, it is possible to display convincing holographic reproductions of artifacts with exquisite details and in full color [13,14]. After the series of successes experienced in the field of static holographic imaging in the first half of the 1960s, there was a good hope that a holographic display presenting motion would appear soon after. Additionally, indeed, the first holographic motion picture was demonstrated by De Bitetto in 1968 and Jacobson in 1969 [15,16]. Holographic motion pictures use a succession of pre-recorded holograms that are projected fast enough to give the impression of movement. However, motion picture is very different from a refreshable display, such as on a television or computer screen. For these displays, the information could be changed at any Materials 2021, 14, 5799. https://doi.org/10.3390/ma14195799 https://www.mdpi.com/journal/materials