In science fiction movies (e.g., Star Wars), naked-eye three-dimensional (3D) display scenes are so fantastic that they have attracted much attention to develop related technologies. Via recording and reconstructing wavefronts of light, holography is an ideal technology to achieve naked-eye 3D display as well as many optical applications, including optical storage, optical encryption, optical information processing and optical manipulation. Holography technology would provide a wonderful naked-eye display platform to greatly enhance visual immersion and reality, change the methods of human-computer interaction and human-human communication, and revolutionize our daily lives. Traditional optical holography requires a complicated shooting process to record the interference pattern of light beams from target objects and a reference path. Therefore, traditional optical holography cannot create a holographic reconstruction of a virtual object. In 1966, computer-generated holography (CGH) was invented by Brown and Lohman to overcome this limitation, in which interference patterns are generated by using physical optics theories. Moreover, CGH can also provide dynamic holographic display by using digital light field modulators, such as spatial light modulators (SLMs) and digital micromirror devices (DMDs). However, there are many shortcomings resulting from the large pixel sizes and limited modulation principle that hinder further development of holographic technology, such as the small field of view (FOV), twin imaging, narrow bandwidth and multiple orders of diffraction.
In recent years, with the enormous development of nanofabrication technologies, metasurfaces consisting of subwavelength nanostructures have attracted much attention in many optical research fields due to their powerful capabilities in modulating the amplitude, phase, and polarization of light, such as beam splitters, metalenses, orbital angular momentum (OAM) devices and structural color elements. Holograms require complicated light field modulation capabilities, and therefore, metasurfaces can be utilized to achieve holographic display. The target CGH patterns for holographic reconstructions can be calculated by physical and mathematical theories, and various nanostructure arrays are arranged according to the designed distribution to compose target CGH patterns. Metasurfaces possess more powerful light modulation abilities that provide much more degrees of freedom to design holograms than conventional CGH devices. In addition, meta-holography has several advantages compared with conventional CGH, such as a higher spatial resolution, lower noise, a larger working frequency bandwidth and elimination of undesired diffraction orders.
Meta-holography can be classified into the two categories of static meta-holography and dynamic meta-holography based on the number of optical images reconstructed from a single piece of a metasurface element. Static meta-holography means that only one fixed image can be reconstructed by meta-hologram elements, while dynamic meta-hologram elements can reconstruct more than one image.
Typical research works in dynamic meta-hologram
Dynamic meta-holography is more suitable for optical display and information processing applications than static meta-holography. For example, to achieve fantastic naked-eye 3D display scenes as shown in science fiction movies or to realize optical camouflage in military reconnaissance, dynamic display is a fundamental and essential capability.
The research groups of Prof. Wei Xiong from Huazhong University of Science and Technology and Prof. Minghui Hong from National University of Singapore focused on the topic of dynamic meta-holography to give a comprehensive review for introducing recent development. Based on the realization methods, dynamic meta-holography can be mainly divided into two categories: tunable metasurfaces and multiplexed metasurfaces. They investigated these strategies and introduced typical research works on them.
First method is tunable metasurface. The majority of metasurfaces are static and cannot be tuned after being fabricated. However, since the desire for dynamic meta-holography applications requiring active controlling, there are much effort has been devoted to exploit active materials and tuning methods, such as thermo-optic effects, free-carrier effects, phase transitions, stretchable structures, chemical reaction, and so on.
And multiplexed metasurface is another method to achieve dynamic meta-holography. Many fundamental properties of light act as independent dimensions, such as the propagation direction, wavelength (frequency), polarization, and OAM, which enables multiplexing technologies. Multiplexing technologies have been widely used in the research fields of dynamic meta-holographic display.
Also, authors shared their opinions about the development of meta-holography. For smooth holographic display, it is essentially required to achieve infinite numbers of vivid frames at a considerable frame rate. In this respect, some progress has been made through multiple methods. However, these methods are still far from achieving the ultimate holographic display, as shown in science-fiction films. Recently, several groups reported their research progress in electrically tunable metasurface-based SLMs. Although these works were based on different materials and methods, all of these designs were one-dimensional (1D) metasurface-based SLMs. 1D SLMs are suitable for the applications of beam steering, 1D focusing and lidar but not for holographic display. 2D metasurface-based SLMs in the visible range are still challenging and difficult to achieve due to the limitations of fabrication technologies. Therefore, OAM, space channel and reprogrammable methods provide promising and alternative approaches to achieve dynamic meta-holography in specific application scenarios due to their excellent performance in terms of frame number and frame rate. With the rapid development of nanofabrication technologies and creative design methods, we believe that ideal dynamic meta-holography will appear in the near future.
The first authors of this paper are Dr. Hui Gao and Mr. Xuhao Fan, and corresponding author is Prof. Wei Xiong. The review paper was published on Opto-Electronic Advances recently, and received positive interactivity on multiple social platforms and academic press release channelshortly after the publication, including EurekAlert! and AlphaGalileo.
Introduction of the research group
The Micro-Nnano Optoelectronics Laboratory of Huazhong University of Science and Technology headed by Prof. Xiong Wei, a national-level overseas high-level talent, mainly focuses on micro nano scale laser 3D / 4D printing, laser-induced synthesis and assembly of nano functional materials, ultrafast laser imaging and characterization, metasurface micro nano optical devices, etc. Based on Wuhan National Laboratory for Optoelectronics, the team has carried out a series of pioneering work in the interdisciplinary fields of ultra-fast laser micro nano extreme manufacturing technology and equipment. The team has undertaken a number of projects, such as the National Key R&D Program of China, the general projects of the National Natural Science Foundation of China. In recent years, the group has published more than 50 papers in international well-known journals such as Science Advances, Nature Communications, Advanced Materials, Light: Science & Application, Nano Letters, etc., and applied for more than 20 authorized and public invention patents, and being cited more than 1000 times. Wei Xiong has presented more than 20 reports at international conferences in this field, such as Photonics West, MRS, ICALEO, etc. He has won the best paper award of ICALEO. He has served as the chairman of ICALEO laser nano processing and manufacturing branch of American Laser Association, CO chairman of laser branch of POEM International Conference, and vice president of Wuhan Laser Society of Hubei Province, Member of extreme manufacturing Committee of Chinese society of mechanical engineering.
