Magnesium-Based Metasurfaces for Dual-Function Switching between Dynamic Holography and Dynamic Color Display

Metasurface-based color display and holography have greatly advanced the state of the art display technologies. To further enrich the metasurface functionalities, recently a lot of research endeavors have been made to combine these two display functions within a single device. However, so far such metasurfaces have remained static and lack tunability once the devices are fabricated. In this work, we demonstrate a dynamic dual-function metasurface device at visible frequencies. It allows for switching between dynamic holography and dynamic color display, taking advantage of the reversible phase transition of magnesium through hydrogenation and dehydrogenation. Spatially arranged stepwise nanocavity pixels are employed to accurately control the amplitude and phase of light, enabling the generation of high-quality color prints and holograms. Our work represents a paradigm toward compact and multifunctional optical elements for future display technologies.


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Metasurfaces, composed of spatially arranged metallic or dielectric antennas, enable a generation of optical elements with diverse capabilities. [1][2][3][4] In particular, metasurfaces bring forward interesting design concepts to create light projection and display devices, [5][6][7][8][9][10] which revolutionize the state of the art display technologies. Different from conventional color generation strategies using pigments and dyes, metasurfaces can yield vibrant and brilliant colors based on resonant excitations of optical antennas 8,9,11,12 or nanocavities. 13,14 The resulting color prints possess excellent stability, long durability, as well as high resolution and high data density, ideally suited for data storage and cryptography applications. Apart from color printing, high-quality holograms can also be produced based on metasurfaces. 6,7,[15][16][17] Such optical devices are ultra-thin and ultra-compact, constituting a milestone for modern display technologies.
Often, color printing and holography are separately implemented on metasurface devices.
Very recently, research endeavors have been exerted to combine these two display functions within a single metasurface device. [18][19][20][21][22] For example, monolithic stepwise nanocavity and holographic metasurface layers have been stacked together to achieve low-crosstalk color printing and full-color holography. 19 In addition, single-layer dielectric metasurfaces have been employed to combine color printing and holography. 18,20,22 However, these two strategies suffer from low efficiency and polarization sensitivity, respectively. More crucially, these metasurfaces are intrinsically static, as their optical functions are fixed once the devices are fabricated. This leaves out many opportunities that metasurfaces can offer.
In this work, we demonstrate a metasurface device, which allows for dual-function switching between dynamic holography and dynamic color display. The dynamic functionality is enabled by the phase-transition of magnesium (Mg), which can be transformed to magnesium hydride (MgH2) through hydrogen uptake. [23][24][25][26][27][28][29][30] Importantly, MgH2 can be restored to Mg upon oxygen exposure. Such reversible metal to dielectric transitions are accompanied with dramatic 4 optical response changes, which lay the foundation for building dynamic optical devices in the visible wavelength regime. Figure 1a shows the schematic of the metasurface, which consists of spatially arranged stepwise pixels of subwavelength dimension. The metasurface can work as a holography device, creating high-quality holographic images. The metasurface can also work as a color display device, which produces microprints with vivid colors. These two display functions, dynamic holography and dynamic color display, can be readily switched through hydrogenation and dehydrogenation using hydrogen and oxygen, respectively. can then transmit through the capping layers and experience multiple reflections inside the FP nanocavity, 31 which is formed between the capping layers (TiH2/PdH) and the bottom Ag mirror (see Fig. 2a). In this case, the cavity height is defined by the total thickness of MgH2 + HSQ + SiO2. As shown in the upper panel in Fig. 2b, in contrast to the nearly unity reflectance for different pixels before hydrogenation (black curve), the reflectance intensity exhibits a clear dependence on the cavity height after hydrogenation (red curve). In addition, the associated phase differences become much smaller among different pixels (see red stars in Fig. 2b), when compared to those before hydrogenation.

RESULTS AND DISCUSSION
A holographic image of a musical note is designed based on the Gerchberg-Saxton algorithm and its phase distribution is shown in Fig. 2c. Fig. 2d presents the scanning electron microscopy (SEM) image of the fabricated sample. A tilted view is shown as inset in Fig. 2d.
A high-quality hologram of a musical note with 58% efficiency is experimentally reconstructed at 633 nm (see Fig. 2e), demonstrating the precise control of the stepwise pixel heights and thus the phase distribution. The incident laser light is linearly polarized along one lateral side of the stepwise pixels in the experiment. However, the reconstruction of the hologram is independent on the light polarization. It is noteworthy that the hologram is detected at an 11.5° off-axis angle along only one direction. Upon hydrogen exposure (5% in nitrogen), Mg is transformed to MgH2. The phase differences among different stepwise pixels significantly decrease, resulting in the disappearance of the hologram. When MgH2 is restored to Mg upon oxygen exposure (10 % in nitrogen), the hologram comes into existence again. As a result, the phase-transition between Mg and MgH2 enables dynamic holography that can be reversibly switched on/off by loading O2/H2. 6 Meanwhile, through hydrogenation and dehydrogenation, the stepwise pixels also experience abrupt changes in reflectance, rendering dynamic color generation possible. As revealed by the optical microscopy images in Fig. 3a, before hydrogenation the pixels with different HSQ heights exhibit blank color when interacting with unpolarized white light, owing to their high reflectance in the visible wavelength range. This is also confirmed by the measured reflectance spectra of these pixels, which are characterized by the black curves in Fig. 3b

CONCLUSIONS
In conclusion, we have realized a dual-function metasurface for switching between dynamic holography and dynamic color display. The spatially-arranged stepwise nanocavity pixels allow for accurate control over the amplitude and phase of light, enabling the facile design of metasurface color prints and holograms. In addition, the reversible phase-transition between Mg and MgH2 enables the dynamic functionality, leading to switchable holography and color display at visible frequencies. Our work represents a paradigm to realize multi-tasking optical elements, which broads the realm of light projection devices and display technologies. 8 The substrate was prepared by electron beam evaporation of Ag (120 nm) and SiO2 (190 nm) on a Si wafer. The multiple overlay patterning of the stepwise metasurface was performed using a Raith electron-beam lithography system with an accelerating voltage of 30 kV and beam current of 890 pA. The spin-coated HSQ film was directly utilized without prebaking to avoid thermally induced cross-linking. Before carrying out the overlay fabrication, gold alignment markers were defined by a standard "Sketch and Peel" lithography process. To exactly define the heights of the stepwise pillars, the spinning speed and the concentration of the HSQ resist were finely tuned in each overlay. The details of the spin coating and exposures are shown in Table S1. The development time was 15 s to avoid the damage of the pillars fabricated in the previous overlays. Subsequently, Mg (50 nm)/Ti (2 nm)/Pd (3 nm) films were successively deposited on the patterned sample by electron beam evaporation.

Optical setups
The holograms were recorded by an optical setup shown in Figure S1. The light beam was generated from a laser diode source (633 nm). A linear polarizer (LP) was employed to obtain linearly polarized light. An optical lens and an aperture were utilized to reshape the light beam to a similar size as the sample area in order to avoid undesired reflected light. The light beam was incident on the metasurface sample placed in a homemade gas cell. The reflective hologram was projected onto the screen in the far field. All the experiments were carried out at 80°C to facilitate the switching upon H2 or O2 loading. The hydrogenation and dehydrogenation experiments were carried out in a homebuilt stainless steel chamber. Ultrahigh-purity hydrogen, oxygen, and nitrogen were used with mass flow controllers to adjust the flow rates and gas concentrations in the chamber. The flow rate of the hydrogen and oxygen gases was both 1.0 L/min.
The color microprint was revealed using a NT&C bright-field reflection microscopy set-up (using a Nikon ECLIPSE LV100ND microscope) illuminated by a white light source (Energetiq