Lithium niobate (LN) crystal is a kind of multi-function and multi-purpose artificial crystal material, which has the advantages of good temperature stability, easy optical cold processing, and properties control. As a typical photorefractive (PR) crystal, LN is widely used in research and applications in high-density optical storage, laser physics, information processing, computing, etc. With the rapid development of information science and technology, magnetic tape, disk, and optical disc cannot meet the increasing demand for data storage. The theoretical limit of holographic three-dimensional (3D) storage capacity can reach 1012 bit/cm3, which is much higher than the traditional one-dimensional (1D) and two-dimensional (2D) memory. Meanwhile, real immersion experience makes 3D display a huge market demand. The dreamed 3D display in the future should be viewed without auxiliary wearing devices. Holographic display is one of the techniques to realize 3D display. Then, with the enormous demand and rapid development of massive storage and dynamic holographic display, 3D optical storage and dynamic display based on LN crystals have once again become a research hotspot.
Here, we present an overview of the principles, history, and recent advances in holographic data storage and display based on LN crystal. Compared with the commonly used liquid crystal display materials, LN crystals present many advantages, such as good temperature and chemical stability, high diffraction efficiency, and no need to apply high voltage. The photorefractive response of LN crystal is significantly improved by the doping of cations containing lone-pair electron, and the response time of bismuth and magnesium co-doped LN crystal is reduced to 7.2 ms under the activation of 442 nm laser, which meets the requirements of real-time dynamic holographic display. The crystal has been used to demonstrate a real-time holographic display with a refresh rate of 60 Hz, as that of the high-definition television. It seems that LN∶Bi,Mg is a suitable candidate material for holographic 3D display. Meanwhile, a reasonable calculation is carried out to understand the mechanism of its fast response. The results indicate that the electron mobility while Bi occupying Nb-site is significantly greater than that in Li-site, which directly induces the fast response of LN∶Bi,Mg crystals when the concentration of Mg is beyond its doping threshold. This work provides an ideal candidate material for holographic 3D display and expands the technique for performance control of LN crystals. In addition, doping high valence ions (vanadium and molybdenum) can significantly affect the photorefractive performance of LN crystals. Molybdenum and magnesium co-doped LN crystal can exhibit excellent photorefractive performance in all visible light bands, bringing hopes for real-time refreshable color holographic displays.
Due to its excellent photorefractive properties, LN has become a major candidate material for holographic 3D storage and display. With the rapid development of Internet technology, such as cloud computing, high-density and large-capacity optical holographic 3D storage based on LN crystals provides a solution to the increasingly urgent demand for massive data throughput. However, the improvement of information loading and extraction speed and the extension of fixed and storage life still need further study. With the rise of virtual reality (VR), augmented reality (AR), meta-universe, and other concepts, 3D display has shown great application prospects and economic benefits. Photorefractive holographic display that can be written and read in real time is an important technology for realizing 3D dynamic display. Therefore, it is necessary to continue to carry out in-depth research on the improvement of the response of LN crystal in red, green, and blue bands and to arrange dynamic holographic display devices and technology research in advance.
In addition, thanks to the sub-micron LN single crystal thin film preparation as well as the industrialization of mature semiconductor micro-nano processing technology in preparation of LN micro-nano devices, just after a few years, the research on the new effect of LN on insulator (LNOI) and micro-nano optics has made remarkable achievements. High-performance electro-optical modulators, lasers, amplifiers, waveguides, and other functional devices and transmission devices on the LN chips have been successfully fabricated, and LNOI has become one of the new generations of excellent integrated photonics platforms. With the in-depth research of LNOI-based integrated optics and new effects, photorefractive effect is also shown in the research of on-chip micro and nanodevices. Like bulk LN crystals, photorefractive effect is also a double-edged sword for applying micro and nanodevices of submicron LN films. On the one hand, photorefractive effect can adversely affect the performance of frequency comb and microwave photonics devices, but some micro and nanodevices, on the other hand, also show fast photorefractive response, which provides an opportunity for the research of editable photonics devices. Therefore, the photorefractive effects and applications of LNOI-based micro-nano systems and related integrated optical systems are also worthy of attention.
