Beam‐Steering Metadevices for Intelligent Optical Wireless‐Broadcasting Communications

High‐performance beam‐steering devices play a crucial role for optical wireless communication (OWC), which can meet the demand for increasing number of wireless mobile devices and emerging high‐speed multimedia applications. Conventional beam‐steering optical components like spatial light modulator and digital micromirror device are limited to realize a small steering angle (typ. several degrees), thus dramatically limits the spatial scope of OWC. Herein, a beam‐steering metadevice utilizing an ultracompact metasurface assisted with a spatial light modulator is presented, which can significantly increase the beam‐steering angle without at the cost of complicated optical setup like conventionally used angle magnifier. Based on the actively tunable beam‐steering metadevice, an intelligent bidirectional optical broadcasting communication system is designed and experimentally demonstrated, which exhibits nine broadcasting areas covering a large field‐of‐view of 20° × 20°, with user‐defined dynamic beam‐steering ability in each area, and each user has a data rate of 10 Gbps for both upstream and downstream transmission. The proposed metadevices can meet the demand of intelligent optical wireless communication which requires both high‐performance and low‐cost for practical purpose, under currently available technical architectures.

for precisely controlling the amplitude, phase, and polarization of incident light. [9][10][11][12][13][14][15] Metasurfaces with pushing optical components toward miniaturization of optical components have been competently applied in many areas, ranging from highperformance display [16][17][18][19][20] and metalens imaging [21][22][23][24] to quantum information. [25,26] Due to their excellent ability of wavefront control, the metasurfaces have begun to contribute roles in the field of optical communications. [27][28][29][30][31] For example, a transparent dielectric metasurface was employed for spatial mode multiplexing with a 100 Gbps data transmission in free-space optical communication system over the whole C-band. [27] By combing the geometric phase and the propagation phase, a spatial mode converter based on dielectric metasurface was proposed to transfer the fundamental spatial mode of spin angular momentum into higher order spatial modes in C band, which contributes a new degree for light modulation besides the polarization handedness. [28] A metallic reflective polarization beam splitter based on a gap-surface plasmon metasurface was applied for 1D point to point IR communication with a data rate of 20 Gbps. [29] Through the polarization switching between x-polarized and y-polarized incident beam, the output beam can be discretely redirected in the normal and abnormal reflections. Recently, we demonstrated a reflective dielectric metasurface for a full-duplex optical broadcasting communication system for a downstream and upstream links with 100 and 10 Gbps, which is a PtMP optical wireless communication. By changing the handedness of incident light, the downstream signals can be switched between two locations. [30] Although the signals can be discretely switched between at most two locations by the means of polarization control, actually the metasurfaces in these works operate as passive devices. The transmitted optical signals are not able to be continuously directed to different locations, which cannot satisfy the demands for mobile users.
The actively tunable metasurfaces based on various tunable materials and designs including phase change materials, [32,33] transparent conducting oxide, [34][35][36] tailored plasmonic structure, [37] 2D materials, [38][39][40] and liquid crystals [41][42][43] have been developed for other applications. A 1D phase-only spatial light modulator embedding dielectric nanoantennas into liquid crystals was demonstrated with reduced pixel size and improved deflection angle of 11°at visible range for light detection and ranging (LiDAR) and display technologies. [43] Recently, an all-solid state spatial light modulator consisted of electrically tunable channels with addressable metallic nanoresonators was experimentally proposed with complete phase modulation for LiDAR application. [36] The active metasurfaces reported so far, however, mainly focus on point-to-point beam manipulation, and the embedded nanoresonator cannot support a full-phase manipulation for broadband wavelength operation which the meta-nanoresonator needs to be redesigned and fabricated for different wavelength ranges. How to realize a reconfigurable metadevice for actively beam-steering with a large FOV and PtMP operation is still a main challenge in intelligent OWC system and has not yet been reported.
In this work, we propose and experimentally demonstrate an intelligent bidirectional optical wireless-broadcasting communication system based on the active beam-steering metadevice, with which the metacommunication system not only operates the function of downlink and uplink data transmission, but also can transmit signals to many users individually, simultaneously, and dynamically in the coverage area. Figure 1a depicts schematic of the bidirectional reconfigurable meta-broadcasting communication architecture. Through the optical cross connect (OXC) with the government of the central communication controller (CCC), the fiber access network transmits the optical signals on-demand into an active beam-steering metadevice, which plays the key role in the metasystem and intelligently distributes signals to user-defined terminals, such as vehicles, unmanned aerial vehicles, pedestrian, and buildings. The proposed active metadevice consists of an ultracompact metasurface and an SLM based on liquid crystal on silicon (LCoS), which fully utilizes the advantages of precise wave control ability of metasurface and tunability of LCoS simultaneously, as shown in Figure 1b. The dielectric metasurface is enabled by the Pancharatnam-Berry (PB) phase, which acts as a beam splitter and splits the incident beam into a sub-beam array with high efficiency, large beam-steering angles, arbitrary power distribution, and broadband response regardless of polarization. Combined with the LCoS, the metadevice can realize an intelligent and flexible optical signal transmission function with a large FOV. The silicon beam-steering metasurface used in the metadevice can be massively fabricated by the standard semiconductor processing www.advancedsciencenews.com www.adpr-journal.com platform and monolithically integrated with active devices to realize more functions such as LiDAR and AR/VR applications. As the metadevice is optical reciprocal, the metasystem can support bidirectional transmission, which the users can download and upload information with only one system. In this study, we present an active metadevice utilizing the metasurface assisted commercial SLM for an intelligent bidirectional optical broadcasting communication system, which exhibits nine broadcasting mobile users with a 10 Gbps data rate for both downstream and upstream transmission, user-defined dynamic beam-steering individually and simultaneously, and a large FOV of 20°Â 20°.

Design and Characterization of the Metadevice
In this work, the configuration of the proposed metadevice is illustrated in Figure 1b, the incident light passes through the transmitter collimator and illuminates onto the metasurface, and beam spot arrays with different beam-steering angles are generated. Then the spot arrays are actively tuned on the corresponding region of the LCoS device. By loading the phase information into the LCoS, each sub-beam of the spot array can be steered in two dimensions in free space. Therefore, each reflected sub-beam can be precisely controlled by an LCoS and covers its individual area on the ground. Then, the whole ground space intelligent broadcasting communications are expected.
In our research, a c-Si material was employed to design the metasurface, which is composed of nanostructures with the same dimensions (cell size C, length L, width W, and height H) but different orientations (α). Figure 2a shows the schematic of the nanostructure unit cell. By adjusting the four geometric parameters (C, L, W, and H), the nanostructure with high polarization conversion efficiency (PCE) can be optimized for designing phase-modulated metadevices. Here, PCE denotes the cross-polarized transmissivity (T cross ), which can be utilized to characterize the optical efficiency of P-B phase modulation. Hence, our optimization goal was to acquire T cross with high level while compress T co as low as possible.
We used the commercial software CST STUDIO SUITE to simulate the transmissivity of the nanostructure, and swept the geometric parameters (C, L, W, and H) to optimize the performance. More details about the numerical simulations are included in the experimental section. Through the careful design, we finally figured out an optimized nanostructure (C = 800 nm, L = 600 nm, W = 260 nm, and H = 1000 nm) with high PCE (R cross = 70%) at the design wavelength of 1555 nm [ Figure 2b]. Finally, a nanostructured phase modulator with high efficiency was designed. In our design, we recorded a phase-only holographic image (3 Â 3 spot arrays) into the metasurface by employing the Gerschberg-Saxton (GS) algorithm. Due to the opposite phase delays under left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) incidences, the holographic images generated under LCP and RCP incidences are centrosymmetric. Here, the concept of Dammann gratings was employed in our design to generate spot arrays with the maximum beam-steering angles of 20°Â 20°in vertical and horizontal directions in the Fourier space and the www.advancedsciencenews.com www.adpr-journal.com metasurface has period sizes of 400.8 μm Â 400.8 μm. The final phase distribution is shown in Figure 2c. It should be noted that the beam-steering angles are not limited to 20°Â 20°. By adjusting the geometric parameters of the metasurface (e.g., reducing the pixel size), the sub-beams can achieve arbitrary pointing within the half-space. The arrangement of the metasurface into 2 Â 2 periods was done to eliminate laser speckles between adjacent spots. Figure 2d shows the enlarged phase distribution (20 Â 20 pixels) of the metasurface. Subsequently, the orientation distribution of nanobricks can be determined, and a partial view is shown in Figure 2e. We fabricated the silicon-based metasurface with the standard electron beam lithography followed by an inductively coupled plasma etch process and constituted the configurable metadevice with the LCoS. More details about the fabrication are included in Experimental Section. The size of the silicon metasurface is 801.6 Â 801.6 μm 2 . Figure 1b shows the partial view of the scanning electron microscopy (SEM) images for the silicon metasurface, which exhibits a good quality both in the geometric shapes and uniformity.
Next, we characterized its optical performance for actively beam-steering of the metadevice. A laser beam through a collimator illuminated on the silicon metasurface at the operation wavelength of 1555 nm. After modulated by the silicon metasurface, the incident beam was projected into 3 Â 3 sub-beam spots. Then the generated sub-beams enter into the LCoS and be actively steered. Here, we used an IR sensitive card after the LCoS to record the sub-beam spots distribution, as shown in Figure 3a. From the figure, there are 3 Â 3 sub-beam spots with extending angles of 20°Â 20°in vertical and horizontal directions modulated by the LCoS. As the pixel sizes of the metasurface (800 nm Â 800 nm) are much smaller than that of the LCoS and conventional diffractive optical elements, the metadevice can contribute a larger extending angle with eliminating high diffraction orders. One can see that the intensity of the central spot is higher than others; this is due to residual zero-order energy in the hologram computation and the unmodulated beam by the metasurface. Therefore, the average energy of the surrounding eight sub-beams is used as a substitute for the energy of the central sub-beam, and the measured optical efficiency is 11.83% for the current measurement configuration. The measured efficiency of the metadevice is determined by the combined impact of the efficiency of metasurface and the reflectivity of LCoS. In the proposed metadevice, the reflectivity of LCoS is 67% due to its rapid decrease with increasing deflection angles. Therefore, the efficiency of the fabricated metasurface is expected to exceed 17%. The efficiency of the metadevice can be further improved by increasing the size of the metasurface using standard CMOS process and adding the antireflection layer on the metasurface.
According to our design, the projected area is divided into nine subareas and each spot generated by the metasurface belongs to one subarea. With the help of LCoS, each spot can move arbitrarily among its subarea or even be divided into several subspots, corresponding the application scene that multiple terminals might appear. The movements of spots were controlled by changing the phase distribution of the LCoS, mostly like the profiles of a 2D blazed grating. If multiple subspots are required, G-S algorism can help the design.
To clearly observe the beam movement, we used a CMOS beam profiling camera to record the spot movement conducted by the active metadevice system. As an example, we only plan the path and observe the movement and transformation of the www.advancedsciencenews.com www.adpr-journal.com selected spots, as shown in Figure 3a-e. The videos for sub-beam spot movement in different directions recorded by CCD are provided in the Supporting Information. One can clearly see that the spot can be individually steered to different angles at desire by loading different phases into the metadevice. The signal and the power of each sub-beam can be switched between different channels, functioned as the wavelength selective switch (WSS) in the telecommunication industry. The measured channel isolation is around 9 dBm from the channel (0°sub-beam deflection) to channel (1.5°sub-beam deflection), as recorded in Figure S1, Supporting Information. In addition, the intensity of each sub-beam can be continuously tuned from the minimum to maximum values, as shown in Figure S2a, Supporting Information. Specifically, within the height 3 m of the room, the 3 Â 3 output beam spots after modulated by the reconfigurable LCoS can be individually tuned and each beam spot can move freely in a 15 cm Â 15 cm area. In this work, as the limited active window size of the LCoS device, we chose the optimized FOV of 20°Â 20°, which can guarantee the beam project into the LCoS window and the steered beam not be blocked by the metasurface. As strongly wave control ability of the metasurface, the spot numbers and the beam-steering angles can be extended to make this device cover more mobile users and a larger area.

Experiment of Metadevices-Based Intelligent Bidirectional Optical Broadcasting Communication System
We applied the proposed beam-steering metadevice to realize an intelligent bidirectional optical wireless-broadcasting communication system for the proof-of-concept. Figure 4a shows the experimental configurations for the proposed intelligent optical meta-broadcasting communication system. For downlink transmission, we used a light with a wavelength of 1555 nm as optical carrier to simulate signals transmitted to the room. The laser beam was modulated by a Mach-Zehnder modulator with a pseudo-random binary sequence generator from a bit-error-ratio tester (BERT) which provides 10 Gbps on-off-keying (OOK) signals with a length of 2 17 -1. As the LCoS device is polarizationdependent, a polarization controller (PC) was used to align the beam polarization into the polarization which can be responded by the LCoS. The experimental setup was installed on the optical table in a room temperature environment. And multiaxis stages were used for optical alignment of the transmitted and received fiber collimators. The beam power was 20 dBm outputted from a fiber collimator with a beam waist of 0.2 mm which was amplified by an erbium-doped fiber amplifier (EDFA). The measured transmission efficiency of the dielectric metasurface is 11.83%, and splitting the input optical beam into nine sub-beams would introduce another 9.5 dB losses. Therefore, the optical power of the nine sub-beams modulated by the beam-steering metadevice was below 3 dBm, which is under eye-safe operation power. After the laser beam was steered at a desired angle by the active metadevice, signals were transmitted to nine users at a distance of 1.2 m in free space. A variable optical attenuator (VOA) was used to tune the received optical signal for bit-error-ratio measurement. Figure 4b shows the measured BER curves of the sub-beam indicated in Figure 3a as optical carriers for optical wirelessbroadcasting communications at 0°, 1.5°, and 3°deflection angles, respectively. The beam-steering metadevice exhibits the similar performance for different deflection angles. The inset of the receiver eye diagram has been shown with BER at 10 À7 . A BER below the 7% forward error correction threshold (3.8 Â 10 À3 ) for sub-beam at the deflection angles was achieved. As the power of each beam can be continuously tuned from the minimum to maximum value by the metadevice, the BER curve for different power of sub-beam is shown in Figure S2b, Supporting Information.
Data upstream transmission is also important for OWC application. As the metadevice is optical reciprocal, the metasystem can support bidirectional transmission. Here, we took user 1 as an example. Two three-port optical circulars were used to separate and route the downstream and upstream signals. Due to the grating effect of the fabricated metadevice, it is hard to combine the optical beams with different wavelengths for detection. In this scheme, different users only can use the same wavelength for the upstream link. By using the time division modulation (TDM) technique, the individual data stream of the users can be reassembled at the receiving end based on the timing. A full-duplex meta-broadcasting system can be expected which the upstream and downstream data transmission can be done simultaneously in the system. Figure 4c shows the BER curves versus the received optical power for uplinks data transmission at different deflection angles. From the BER curves, we found that the metabroadcasting communication system shows a similar performance for downstream and upstream with the active beamsteering. Therefore, the application of the beam-steering metadevice to realize intelligent optical wireless-broadcasting system is feasible.

Discussion and Conclusion
In this work, we demonstrated an active beam-steering metadevice for bidirectional optical wireless-broadcasting communications. In virtue of the proposed actively metadevice, we realized a bidirectional optical broadcasting communication system with a 10 Gbps for downstream and upstream data transmission. The signals can be dynamically transmitted to nine users with movement over 20°Â 20°FOV range. By loading the phase information into the metadevice, sub-beams can be actively controlled within a millisecond response individually and simultaneously. The data rate can be increased to a high data rate and the receiver sensitivity can be improved by upgrading intensity modulation/direct detection into coherent detection. Due to the powerful light control ability of the metasurface, the broadcasting coverage can be enlarged by increasing the number of the sub-beams. In addition, the actively steering angle for each user can be enlarged by using an LCoS device with smaller pixel sizes. The demonstrated beam-steering metadevice is not necessarily limited to the IR range in this work, but also can be extend to other wavelength range from visible to terahertz range, and applications such as LiDAR for autonomous vehicles, digital holographic display for augmented reality. Compared with the conventional phase control component, such as diffractive optical element (DOE) which requires various etch depths for multiphase level modulation, the fabricated silicon transmissive metasurface continuously controls the phase in only two-leveldepth ultrathin planar layer with subwavelength resolution, which can efficiently and precisely control the wavefront of light with eliminated higher diffraction orders and integrate with the active optical components.
Our method is a feasible and robust approach to empower the optical performance and functions of the commercial SLM, which avoids the complex fabrication of metastructure inside the tunable materials. In addition, using the fiber array and recent emerging multicore fiber with the space-division multiplexing techniques (SDM), a more flexible high-capacity optical wireless-broadcasting communication system with a wide fieldof-view and full-duplex function is expected.
In conclusion, we realized a beam-steering metadevice which utilizes the precise wave control ability and tunability simultaneously to realize bidirectional optical broadcasting communication. Our method is a feasible and robust approach to empower . Experimental configuration and results for bidirectional reconfigurable optical meta-broadcasting communication system. a) Optical layout for the setup of metadevices-based intelligent bidirectional OWC system. b,c) Downlink and uplink BER curves versus received optical power for sub-beam 1 at three different deflection angles. The operation wavelength is 1555 nm. The inset of (b) shows the receiver eye diagram with BER at 10 À7 when the system provides downlink communication.
www.advancedsciencenews.com www.adpr-journal.com the optical performance and functions of conventional SLM, which avoids the complex fabrication of metastructure inside the tunable materials. And this work extends the concept of tunable metadevice into intelligent optical wireless communications which is promising for future 6 G application, and opens the way for metasurface applications.

Experimental Section
Numerical Simulations: Numerical simulations of the metasurface unit cell were conducted by the CST STUDIO SUITE software. Periodic boundary conditions were set around the nanostructure, and the orientation angle was set to be 0°. A CP plane wave was normally incident onto the nanostructure. The cross-polarized transmissivity and copolarized transmissivity were collected by the transmission field port. If a CP light beam illuminates the nanostructure, the cross-polarized component of output light carries a geometric phase shift of AE2α, which is positive for LCP incidence while negative for RCP incidence. Thus, a desired phase profile can be stored in the orientation distribution of a metasurface, which helps to design a phase-modulated device. On the other hand, the copolarized component possesses no phase modulation, and it finally contributes to the zero-order light in the phase-modulated device. The PCE of the optimized nanostructure was 70% at the design wavelength of 1555 nm.
Device Fabrication: Samples were fabricated with a double side polished silicon wafer. First, the wafer was cleaned with successive acetone, isopropyl alcohol (IPA), and deionized water (DIW) rinses, followed by dry nitrogen gun. Then, the wafer was evenly coated with 120 nm-thick positive photoresist (AR-P 672.06, Allresist) and baked on a hot plate of 150°C for 3 min. Metasurface patterns were transferred onto the photoresist using a standard electron beam lithography process (eLINE Plus, Raith). After the development process, a 30 nm-thick chromium (Cr) layer was deposited with thermal evaporation and lift-offed. The pattern was transferred from the Cr mask to the amorphous silicon by inductively coupled plasma etcher (PlasmaPro100 Cobra300, Oxford). Finally, the Cr masks were removed by Cr etchant.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.