Recent advances in optical dynamic meta-holography Opto-Electronic Advances

Holography, with the capability of recording and reconstructing wavefronts of light, has emerged as an ideal approach for future deep-immersive naked-eye display. However, the shortcomings (e.g., small field of view, twin imaging, multiple orders of diffraction) of traditional dynamic holographic devices bring many challenges to their practical applications. Metasurfaces, planar artificial materials composed of subwavelength unit cells, have shown great potential in light field manipulation, which is useful for overcoming these drawbacks. Here, we review recent progress in the field of dynamic metasurface holography, from realization methods to design strategies, mainly including typical research works on dynamic meta-holography based on tunable metasurfaces and multiplexed metasurfaces. Emerging applications of dynamic meta-holography have been found in 3D display, optical storage, optical encryption, and optical information processing, which may accelerate the development of light field manipulation and micro/nanofabrication with higher dimensions. A number of potential applications and possible development paths are also discussed at the end.


Introduction
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 1,2 , optical encryption 3 , optical information processing 4 and optical manipulation 5,6 . 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 7 . Therefore, traditional optical holography cannot create a holographic reconstruction of a virtual object. In 1966, computergenerated holography (CGH) was invented by Brown and Lohman to overcome this limitation, in which interference patterns are generated by using physical optics theories 8 . 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) 9,10 . 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 11,12 . 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 13,14 of light 15 , such as beam splitters 16,17 , metalenses 18−24 , orbital angular momentum (OAM) devices 25−30 and structural color elements 31−35 . 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 11,12,36 . Meta-holography can be divided into different types based on different classification methods. For example, there are three types of meta-holography according to the principle of the light field modulation component, including phaseonly holography 37−41 , amplitude-only holography 42−44 and complex amplitude holography 45−48 (typical works shown in Fig. 1). Additionally, 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. Dynamic metaholography 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.
There have been some excellent reviews about  meta-holograms 49−51 , which provided big pictures for this research field. Here we focused on the topic of dynamic meta-holography to give a comprehensive review for introducing recent development in this paper. Based on the realization methods, dynamic meta-holography can be mainly divided into two categories: tunable metasurfaces and multiplexed metasurfaces. We investigate these strategies and introduce typical research works on them. Furthermore, we also propose potential applications and possible development paths of dynamic meta-holography in the future.

Tunable meta-holography
The majority of metasurfaces are static and cannot be tuned after being fabricated. However, since the desire for plenty applications requiring active controlling, there are much effort devoted to exploit active materials and tuning methods 52,53 , such as thermo-optic effects (e.g. silicon 54 89,90 , and so on. Also, some have been utilized in the research field of tunable meta-holography.

Phase transitions
In recent years, chalcogenide phase change materials (PCMs) composed of alloys of GST have become popular in the optical storage of commercial DVDs and CDs due to their easy transition between the disordered amorphous state and ordered crystalline state by applying thermal, optical, or electrical stimuli. Moreover, GST has also been widely used in dynamic meta-holography and other metasurface research fields due to its advantages of a large refractive index difference between two states, high switching speed and reliable retention 74−80 . Lee et al. theoretically and experimentally proved the concept of an ITO-GST-ITO (IGI)-based meta-hologram panel 77 . After IGI was fabricated, an excimer pulse laser at an ultraviolet wavelength was used to achieve local crystallization of the GST film with a micrometerscale pixel pitch and a 16k × 16k resolution. Notably, the fabrication of the GST hologram panel required the transfer of the CGH pattern from a Cr mask to an IGI layer, as shown in Fig. 2(a). The large refractive index difference between the amorphous and crystalline states caused a significant change in the reflection coefficient before and after crystallization of the GST. Holographic images could be reconstructed from CGH patterns consisting of local crystallization pixels. Zhang et al. demonstrated a switchable spin-orbit interaction via the combination of plasmonic metasurfaces with PCMs 78 . The designed meta-devices were based on a simple metal-insulator-metal (MIM) configuration in which the insulator layer consisted of GST and MgF 2 films, the bottom layer consisted of a gold ground plane, and the top array consisted of subwavelength plasmonic gold antennas. Three reflective meta-devices were fabricated and measured as a proof of concept that enabled the spin Hall effect, vortex beam generation and a hologram in the amorphous state of GST ("ON" state) separately, which disappeared in the crystalline state of GST ("OFF" state), as shown in Fig. 2(b). Zhou et al. proposed a similar approach to achieve transmissive dynamic meta-holographic encryption based on a structure in which a GST film is sandwiched between a bottom SiO 2 substrate and top gold split-ring resonator (SRR) arrays 76 . The resonance behaviors of the fixed SRR structure were completely different between the two states of GST, resulting in varying phase and amplitude modulation. The holographic reconstruction based on continuous phase modulation in the amorphous state of GST was converted to a binary phase in the crystalline state, as shown in Fig. 2(c).
Another attractive candidate material for tunable meta-holography is VO 2 . This strongly correlated material exhibits an insulator-to-metal transition (IMT) under thermal, electrical, or optical stimuli 91 . The relatively low transformation temperature (approximately 340 K, 67 °C) and large refractive index contrast of VO 2 from monoclinic (insulating phase) to tetragonal (metallic phase) make it suitable for the realization of switchable metasurface device design. Driscoll et al. demonstrated the first VO 2 -based tunable metasurface device, which achieved dynamic tuning of an infrared hybrid-metasurface resonance in 2008 81 . VO 2 was also used to realize switchable meta-holographic elements. Liu et al. proposed a thermally dependent dynamic meta-holography design using a vanadium dioxide integrated metasurface 83 . The unit cell was a gold split ring with VO 2 filling in the gap, as shown in the illustration of Fig. 2(d). The reconstructed images transformed from "H" to "G" as the temperature increased from 25 °C to 100 °C. Furthermore, a new approach for dynamic meta-holography at optical frequencies by utilizing a cavity incorporating an active VO 2 layer was proposed by Haimov et al. 84 . A Bragg reflector was introduced at the bottom, and a nearly π phase shift could be obtained from the reflected waves before and after the IMT.

Chemical reaction
Some specialized chemical reactions were also used in the dynamic control of meta-holography. For example, magnesium (Mg) has great plasmonic properties in the visible range and undergoes a phase transition from a metal to a dielectric by forming magnesium hydride (MgH 2 ) upon hydrogen loading 92 . By using oxygen, the phase transition is reversible through dehydrogenation.
Li et al. created dynamic meta-hologram devices consisting of plasmonic Mg nanorods that constituted addressable pixels by hydrogenation/dehydrogenation for optical information encryption design 90 . Li et al. pro-posed three different nanorods to demonstrate the capability for optical information processing and encryption. Gold (Au) nanorods (marked as P 2 in ref. 90 and Fig. 3(a)) were stable and irrelevant to chemical reactions. A titanium (Ti) spacer and a palladium (Pd) catalytic layer were capped on Mg nanorods to facilitate hydrogenation and dehydrogenation processes (Mg/Pd, marked as P 1 in ref. 90 ), while a chromium (Cr) capping layer effectively slowed down both the hydrogenation and dehydrogenation rates of the third kind of nanorod (Mg/Pd/Cr, marked as P 3 in ref. 90 ). The three kinds of different nanorods were placed in each unit cell and coded to generate different holographic images to realize information encryption and dynamic control, as shown in Fig. 3(a).   addressable dynamic meta-hologram by utilizing similar hydrogenation/dehydrogenation of Mg 89 . The proposed meta-hologram devices were composed of pure Au Vshaped antennas (PVAs) and composite Mg-Au Vshaped antennas (CVAs). The scattering phase of the CVAs was uniformly shifted by π/3 before and after hydrogenation. To establish a quantified phase relation and calculate the distribution of PVAs and CVAs, an iterative hologram algorithm based on the Fidoc method was developed by the authors. The chemical reaction method can only make a few reconstructed holographic images switchable. The speeds of hydrogenation/dehydrogenation used in these designs were too slow to achieve smooth holographic display. Chemical control methods are more suitable for achieving platforms promising for relatively simple dynamic functionalities that require no quick response, such as optical encryption and smart sensors.
Rewritable meta-holography and stretchable substrate meta-holography In addition to the methods introduced above, there have also been some other particular and inspiring means to achieve tunable meta-holography.
As a two-dimensional material with good electronic and optical properties, graphene is a good choice to be incorporated with metasurfaces to realize tunability. Li et al. demonstrated write-once holograms for wide-angle and full-color three-dimensional images enabled by reduced graphene oxide, as shown in Fig. 4(a) 71 . They discovered that the fs laser intensity of the focal spot was correlated with the refractive index modulation in the reduced graphene oxide. The athermal photoreduction could be confined to a diffraction-limited region since the fs laser pulse removes undesired accumulative heating. Moreover, by precise control of the laser irradiance, reversible reduction and oxidation of graphene oxides could be obtained to achieve updatable reduced graphene oxide holograms.
Another interesting tunable meta-holography research work was realized by Malek et al. in 2017 87 . The authors demonstrated a reconfigurable meta-hologram with gold nanorods on a stretchable polydimethylsiloxane substrate. Stretchable metasurfaces have also been used in studies of actively tunable structural color 85,86 and zoom metalenses 88 . Stretching the substrate could change the spectrum response of the metasurface 85,86 , focal length of the metalens 88 , and distance of the image display plane from the meta-hologram panel 87 . By coding several reconstructed images at different distances into one piece of metasurface on a stretchable substrate, the displayed holographic images on the same observation plane can be switched by stretching the metasurface device, as shown in Fig. 4   Multiplexed 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 optical communications and optical computing to extremely extend the bandwidths or enormously increase the speed. Additionally, studies on meta-holograms can utilize similar multiplexing technologies to achieve dynamic display.
Wavelength multiplexed meta-holography Traditional optical holography is usually displayed under a single wavelength laser, resulting from the limitation of interference principles. To achieve a colorful holographic display under white light, a rainbow hologram was invented in 1968 by Dr. Stephen A. Benton, who recorded interference patterns using a slit to eliminate vertical parallax and reduce spectral blur in the viewing output image. Furthermore, the concept of rainbow illumination was also introduced into volumetric imaging based on digital dynamic holographic devices (e.g., SLMs) to extend the viewing zone in both the vertical and longitudinal directions 93 but not for the purpose of colorful display. As diffractive elements, these digital dynamic CGH devices possess wavelength-dependent properties that make them not simple to use in recon-structed color imagery. Some special approaches (e.g., frequency filtering 94 and phase shift 95 ) have been developed to realize full-color holographic display using digital dynamic CGH devices with multiwavelength illumination.
The broadband display is of great importance to the practical holographic applications. There are quite a few excellent research works to achieve broadband metaholograms 96−98 . To achieve a colorful meta-holographic display, the abundant degrees of freedom in designing subwavelength structures 99−101 and metasurface elements 102−104 have brought about many novel approaches. In 2015, Huang et al. reported a phase-modulated multicolor meta-hologram element that consisted of a two-dimensional array of pixels 99 , as shown in Fig 5(a). There were four subpixels (two for blue, one for red and one for green) in each pixel. The subpixel had a 4×4 aluminum nanorod pattern. The nanorods in each subpixel representing different colors possessed different reflection spectra due to the various geometrical sizes. The method proposed in this research work was a good idea to set up a corresponding relationship between the geometrical parameters of subwavelength structures and response spectra while not bringing additional fabrication difficulties. However, the orientation directions of all nanorods were the same, and this limitation made the device only obtain a phase difference of 0-π. Wang et al.   5(b)). The response spectra were also modulated by the length and width of the nanoblocks, while the phase delay was generated by the orientation angles of the nanoblocks. The increased degree of freedom in rotation of the nanoblocks made it possible to achieve any desired phase delay. In addition to the various subwavelength structures, other properties of the metasurface have also been used to realize wavelength-multiplexed meta-holograms. Generally, similar to other diffractive elements, the sizes and diffraction angles of reconstructed holographic images are also related to the working wavelengths for normal meta-hologram elements. In 2016, Wan et al. reported a wavelength-multiplexed fullcolor meta-hologram design based on phase-shift methods 103 , as shown in Fig. 5(c). The additional phase shift Δφ along the x-axis was encoded into the metasurface and corresponded to the tilted angle of incident light with wavelength λ. This approach does not require complicated geometrical parameters or special materials of nanostructures to eliminate the color dispersion and is suitable for many-wavelength-multiplexed meta-hologram design. Li et al. demonstrated a sevenwavelength meta-hologram and achieved a 1.39 times larger color gamut than the traditional red/green/blue design (see Fig. 5(d)). Furthermore, Zhang et al. combined a similar off-axis illumination method and polarization-dependent geometric phase to achieve a colorful meta-hologram in both transmission and reflection spaces for the first time 104 .
Notably, most colorful meta-holography research works were focused not on achieving dynamic display but on full color display, data storage and information encryption. However, the reconstructed images could be switched by changing the wavelengths of the incident light, so these research works conform to the definition of dynamic meta-holograms.
Polarization multiplexed meta-holography As a transverse wave, light possesses a polarization property that specifies the orientation of the oscillation, namely, the direction of the electric field. Traditional CGH devices are polarization-insensitive (e.g., diffractive optical elements and freeform optics) or can only work at specific polarization states (e.g., liquid crystal SLMs). Meta-hologram elements consisting of anisotropic subwavelength structures can offer the capability to respond variously according to the polarization state of the interacting light. This characteristic makes them appropriate for polarization-multiplexed holography. In 2013, Montelongo and Chen et al., from two independent research groups, proposed and theoretically and experimentally demonstrated the concept of polarization-multiplexed meta-holograms separately 105,106 . The two independent images for each polarization were coded into independent diffraction patterns, and the two patterns were merged to obtain one metasurface element. The reconstructed images could be switched by changing the polarization state of incident light from one direction to the other orthogonal direction (from 0° to 90°). The merged metasurface produced the two images simultaneously with half the intensity at 45°. Although the results from the two groups were similar, Montelongo et al. designed metasurface elements by the amplitude modulation method, while Chen et al. implemented a four phase level design to achieve this purpose. All these basic research works provided the possibility of achieving switchable meta-holographic display by changing the polarization state of incident light. Besides linear polarization states, spin angular momentum (circular polarization states) also can be regarded as a fundamental degree of freedom to encode the meta-holography 107−109 . In 2015, Wen et al. experiment-ally demonstrated a helicity-multiplexed meta-hologram with two symmetrically distributed off-axis images that were interchangeable by switching the helicity of the incident light beam 110 . Arbabi et al. demonstrated metasurface devices with complete control of the polarization and phase, including polarization-switchable phase holograms 111 . Compared with Chen's work based on four phase levels in 2013, Arbabi's design could realize arbitrary phase distributions for two orthogonal polarization states by anisotropic dielectric metasurfaces. The input (E in ) and output waves (E out ) at each pixel could be related by a general Jones matrix, as E out =TE in . In this way, any desired polarization and phase control can be achieved by implementing any unitary and symmetric Jones matrix at each pixel. Furthermore, Mueller et al. developed and advanced the method and combined the geometric phase and propagation phase to achieve two independent and arbitrary phase distributions for any pair of orthogonal polarization states (linear, elliptical or circular) 112 . Mueller et al. demonstrated this method by two independent reconstructed images for left-hand and right-hand circular polarization states separately, as shown in Fig. 6(a) 113 . Similarly, two switchable independent images could be achieved for left-hand and right-hand circular polarization states separately, as shown in Fig. 6(b). This device also demonstrated the advantages of broadband operation (from the visible to the near-infrared) and high efficiency (as high as 75%). These methods utilizing the propagation phase, detour phase and geometric phase provided more degrees of freedom for metasurface polarization optics. However, most research on polarization-dependent meta-holograms has focused on two orthogonal states. In 2018, Zhao et al. integrated twelve polarization manipulation channels for various phase profiles into a single birefringent vectorial meta-hologram 114 . In 2019, by using structurally birefringent dielectric metasurfaces, Arbabi et al. proposed and demonstrated vectorial holograms with arbitrary polarization states in which the color information was converted to Stokes parameters to be stored and reconstructed as a monochromatic hologram image 115 . To realize the reconstructed image, as shown in Fig. 6(c), a modified Gerchberg-Saxton algorithm was developed. To code 8-bit (0−255) color images, the Stokes parameters S i were coded as S i =(X−128)/128, where i represents 1, 2, or 3 and X represents the value for the R, G, or B channel.
The success of the advances in metasurfaces researches in recent years, to a great extent, should be attributed to the development of classical optical theories, e.g., the generalized Snell's law. Currently, the developments still continuously provide us with interesting and instructive ideas. For example, Deng et al. noted the orientation degeneracy implied in Malus's law and found a one-to-M mapping between the light irradiance and Pancharatnam -Berry phase, which could be applied to design advanced geometric metasurfaces. They proposed a Malus-metasurface-assisted polarization multiplexing method and generated a near-field grayscale pattern as well as an independent far-field holographic image simultaneously with one sample, as shown in Fig. 6(d) 116 .
These polarization-multiplexed meta-hologram works implemented many modulation approaches and implied potential applications in dual-channel display, anticounterfeiting, encryption, security and data storage.

Angle multiplexed meta-holography
The angular response characteristics are significant for diffractive optics, including gratings, diffractive optical elements, and metasurfaces. The corresponding input and output angles are correlated by physical optics theories, which make each pair of them an independent channel. This optical property provides a new degree of freedom to achieve multiplexed meta-holography.
In 2017, Kamali et al. introduced an angle-multiplexed metasurface design composed of dielectric Ushaped units with reflective high contrast in which linear momentum depending on the incident angle was added to display different images at different tilted angles of incidence 117 . The geometrical sizes of the U-shapes in different dimensions were optimized to achieve a full 2π phase shift. The structure provided independent phase control under a TE-polarized light beam at 0° and 30° incident angles. Two different images were coded as different phase profiles of the metasurface corresponding to different angles, so the designed angle-multiplexed metahologram element created two independent projected images under 0° and 30° separately, as shown in Fig. 7(a). In this way, Kamali et al. created two independent channels corresponding to different incident angles to display two images. In 2020, Wang et al. proposed an anglemultiplexed multichannel metasurface to achieve arbitrary spatially varying light fields 118 . The designed anglemultiplexed metasurfaces had the capability to independently control the handedness and azimuth and possessed four channels, as shown in Fig. 7(b). The proposed methods enable realization of full Poincaré beams, dual-way switching and vectorial print images, and optical information communication. Furthermore, the number of channels could be expanded by combining angle multiplexing methods with other multiplexing methods. As we discussed above in section wavelength multiplexed meta-holography, an additional phase shift along a selected axis could be encoded into the metasurface to achieve wavelength-multiplexed meta-holography 102−104 . To correct the phase shift for different wavelengths and display the reconstructed images at desired positions, the incident light beams should possess the right tilted angles corresponding to the wavelengths. Therefore, the reconstructed images could be switched by changing the wavelengths of incident light beams or incident angles. Strictly speaking, these research works utilized wavelength multiplexing methods and angle multiplexing methods simultaneously. In addition to achieving a multichannel meta-hologram, other types of dynamic meta-holograms can also be realized by angle multiplexing methods. In 2017, Zhang et al. proposed an ultrahigh-capacity meta-hologram design by encoding information in nanohole arrays 119 . The imaging information capacity could be increased 11.5 times compared with the usual metasurface element, and the compressed image information could be reconstructed by changing the tilted incident angle, as shown in Fig. 7(c). The high capacity of the designed metasurface was achieved by transferring the evanescent wave to the propagating wave. At normal incidence, much imaging information could not be reconstructed in the far field since it was hidden in the evanescent wave region. However, hidden information could be exhibited via off-axis illumination. The reconstructed meta-holographic image appeared like a dynamically varying scroll painting when changing the incident angle of the light Intensity (a.u.)  beam. This method could also be used in large capacity optical storage and lithography technologies.

OAM multiplexed meta-holography
As a fundamental property of photons, the orbital angular momentum (OAM) plays a significant role in many applications of light, including optical communication, stimulated emission depletion microscopy, and optical tweezers, due to the special helical wavefront, doughnutshaped intensity distribution and unbounded set of orthogonal helical modes (named topological charge l).
Additionally, recently, many approaches have been proposed to achieve dynamic meta-holograms by considering OAM as a degree of freedom. Ren et al. proposed OAM-multiplexed meta-holography designs for the first time in 2019 and achieved dynamic holographic display, as shown in Fig. 8(a) 120 . In this paper, Ren et al. demonstrated a design of a 10-bit OAM-multiplexed meta-hologram that could achieve 2 10 different OAM-dependent holographic images. In addition to the multiplexed design, two other kinds of metasurface OAM holograms, including OAM-conserving and OAM-selective meta-holograms, were also demonstrated theoretically and experimentally. Fang et al. demonstrated similar OAM-preserved, OAM-selective and OAM-multiplexed holograms based on the SLM platform and proved the potential applications of OAM holography for high-security encryption 121  Complex-amplitude OAM multiplexing metasurface hologram independent OAM channels, as shown in Fig. 8(b) 122 .
The out-of-plane height of the nanopillar offered amplitude control, while its in-plane rotation generated the phase response. A 3D metasurface sample was fabricated by a femtosecond laser, which provided possibilities for large-area fabrication. Moreover, the demonstrated reconstructed images were shown as two independent smooth holographic videos at two different planes, which suggested the capability for realizing 3D holography.
There were also other approaches to achieve OAMmultiplexed meta-holography. In 2019, Jin et al. proposed a dielectric multimomentum meta-transformer design that could reconstruct different OAM beams with different topological charges into distinct on-axis images, as shown in Fig. 8(c) 123 . The meta-transformer possessed fixed phase profiles itself but could synergize the intrinsic properties (e.g., OAM and linear momentum) of light to achieve a dynamic meta-hologram. The total phase profile of the transmitted beam was , where was fixed after fabrication and defined by the orientation of the nanostructure array and was the phase profile of incident OAM beams, which was changeable by switching the topological charge l. Vivid "R ", "G " and "B " reconstructed patterns were demonstrated under red, green, and blue illumination separately to prove the possibilities in color holographic display applications.
The OAM-multiplexed meta-hologram methods could also be combined with other modulation methods to achieve richer dynamic modulation functions. For example, in 2020, Zhou et al. proposed a holographic encryption design based on OAM-multiplexed meta-holograms in different polarization channels 124 . In this way, the selective holographic image could only be reconstructed under the exact light beam with the right topological charge and a specific polarization state, as shown in Fig. 8(d). Another interesting research work was reported by Yu et al. in 2019 125 . In section Chemical reaction, we introduced the dynamic meta-hologram that could be achieved by chemical reactions of Mg due to its phase transition property between a metal and a dielectric. In 2019, Yu et al. proposed a dynamic meta-holography design based on two metasurface elements. The first metasurface was composed of Mg nanorods, which could generate switchable vortex beams with different topological charges (l=m and l= -m) by hydrogenation and dehydrogenation reactions. The second metasurface could generate different reconstructed images in response to the OAM beams with designed topological charges m and -m. By using these two cascaded metasurfaces, a dynamic meta-hologram was achieved, as shown in Fig. 8(e).
Space channel multiplexed meta-holography One can obtain some inspiration for designing dynamic meta-holograms by comparing dynamic meta-holograms and common 2D display technologies in daily life. There are several approaches to achieve good 2D display apart from pixel display screens. One practical approach is the cinematographic method, which projects different images from a continuous video at different times; the other way is dividing whole images into many subgraphs and displaying them in different combinations at different times, e.g., the digital tube of an electronic scoreboard.
In 2020, Izumi et al. demonstrated a meta-holographic movie by the cinematographic approach 126 . Forty-eight meta-hologram channels were arrayed on a substrate, and the substrate was fixed on a two-axis electric moving stage and moved in a designed sequence at a designed speed. Each hologram channel was sequentially illuminated and each holographic frame was reconstructed on a screen, as shown in Fig. 9(a). A smooth movie could be achieved with a frame rate of 30 frames per second. In the same year, Gao et al. proposed dynamic 3D meta-hologram designs with large frame number and high frame rate based on space channel methods 127 . The meta-hologram samples were divided into many space channels and illuminated by a high-speed coded structured light beam. In addition to a cinematographic metaholographic video with twenty frames, a space channel multiplexed meta-hologram of 28 bits was also demonstrated, which could display 2 28 different frames, as shown in Fig. 9(b). To generate a high precision coded structured light beam, a dynamic space beam coding module consisting of a DMD, a tube lens and an objective lens was built, which made the high frame rate of the meta-hologram system achieve 9523 frames per second.

Nonlinear wavelength multiplexed meta-holography
Most research works on metasurfaces were operated in the linear optics regime, including meta-holography. However, researchers have extended metasurface research to the nonlinear optics regime in recent years 128−130 . For example, in 2015, Li et al. were inspired by the concept of spin-rotation coupling, experimentally demonstrated nonlinear metasurfaces containing plasmonic antennas and theoretically proved that the concept could be applied to dielectric and bulk-like metamaterials 128 .
In 2016, Ye et al. combined nonlinear metasurfaces and several multiplexing methods to achieve spin-and wavelength-multiplexed nonlinear meta-holography 131 . Since these metasurfaces are still static elements, nonlinear meta-holography could also be regarded as a special multiplexing method. The split-ring resonator was selected as the meta-atom to design the nonlinear metasurface due to its strong polarization properties in linear optics and high efficiency of second-harmonic generation. The reported design allowed the construction of multiple independent holographic images by fundamental and second harmonic waves with different spins separately, as shown in Fig. 10(a). Nonlinear meta-holography provides independent and crosstalk-free channels for holographic reconstruction, which is suitable for multichannel holographic display, optical data storage and optical encryption.

Incident light field multiplexed meta-holography
The coded information for holographic reconstruction of all the research works we discussed above was fabricated on the metasurface elements and not embedded in the incident light beams. Even for methods with modulation of incident light, e.g., OAM multiplexing methods 120,122,123 , the OAM beam with a specific topological charge was only used to select and open the target channel and did not contain any information for holographic reconstruction.
In 2020, Qu et al. proposed an interesting research work about incident light field-multiplexed meta-holography (named the "reprogrammable meta-hologram" in ref. 132 ), which divided the hologram information into two matrices 132 . One information matrix was coded into the metasurface as usual; however, the other matrix was coded into the incident light beam. The phase matrix of incident light was an additional degree of freedom to design the meta-hologram. This work paved a new way to optical information encryption. In this way, the method could achieve infinite arbitrary holographic images and videos by a static metasurface and dynamic incident light, as shown in Fig. 10(b).

Multi-dimensions multiplexed meta-holography
In practical applications, to increase frame numbers and degrees of freedom to encode holograms, multiple methods can be utilized to design a meta-hologram element simultaneously. As we discussed above, Li et al. demonstrated a colorful meta-hologram by making use of wavelength and angle multiplexed method at same time. And Yu et al. proposed a dynamic meta-holography design based on two metasurface elements, in which first element was achieved by chemical reaction method and second one utilized OAM multiplexed approach. Moreover, in 2018, Jin et al. demonstrated (2 6 -1) spinand wavelength-encoded holograms by manipulating six bases of incident photons simultaneously to reconstruct 6-bit wavelength-and spin-dependent multicolor images as shown in Fig. 11(a) 133 . Furthermore, in 2020, Deng et al. proposed a multi -freedom metasurface, which combined geometric Pancharatnam -Berry phase and detour phase. The method allowed us to control the complex amplitude and polarization of the wavefront simultaneously with totally decoupled wavelength and incident angle dependence, and ultimately to achieve full -color complex -amplitude vectorial meta -hologram as shown in Fig. 11(b) 134 . Besides dynamic holographic display, there were also some interesting research works about combining metahologram and other imaging methods recently. In 2020, Li et al. experimentally demonstrated a three -channel metasurface which can simultaneously record a continuous grayscale nanoprinting image in the near field and project two independent holographic images in the far field 135 . In the same year, Dai et al. proposed a "three-inone" metasurface device which simultaneously acted as a structural -color nanoprint element, a polarization -controlled grayscale metasurface image displayer and a phase-modulated meta-hologram 136 . These excellent works inspired researchers to explore more possibilities of meta-holograms.

Discussion and outlook
In summary, we have discussed recent advances in dynamic meta-holography. Compared with traditional CGH devices, metasurfaces have many advantages for holographic display, such as a large FOV, a high  resolution, and low noise. Benefiting from the powerful capabilities in modulating the amplitude, phase, and polarization of light, metasurfaces provide multiple degrees of freedom for dynamic hologram design. In this paper, we reviewed typical research works on dynamic metaholography based on various methods, including tunable metasurfaces, multiplexed metasurfaces and multidimensions multiplexed metasurfaces. From the discussion above, it can be found that dynamic meta-holography has many applications in optical storage, anticounterfeiting, security, lithography, optical encryption and holographic display. 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. One ideal and general approach to achieve a dynamic meta-hologram is to control the interaction between waves and each nanostructure of the metasurface at high speed, similar to the display strategy of LED pixel arrays or LCD screen in showing 2D images in our daily life. In other words, we need a metasurface-based SLM with good performance in terms of the refresh rate, modulation efficiency and broadband response in the visible range.
Recently, several groups reported their research progress in electrically tunable metasurface-based SLMs 68,72,137 . Although these works were based on different materials and methods, all of these designs were onedimensional (1D) metasurface-based SLMs. 1D SLMs are suitable for the applications of beam steering, 1D focusing and lidar but not for holographic display. 2D metas-urface-based SLMs in the visible range are still challenging and difficult to achieve due to the limitations of fabrication technologies. Therefore, OAM 122 , space channel 127 and reprogrammable 132 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.