Introduction

Sustainable society is a long-term goal pursued by human society1, and the concept of sustainability has been integrated into the development of various fields. In recent years, sustainable wireless communications have become the focus of academic and industrial attention. In the current fifth-generation communication era, various energy-saving methods have been proposed to pursue sustainability, such as always-on reference signal removal, broadcast synchronization signal interval expansion, and base station sleeping2. However, as network deployment density increases and the scale of connected Internet of Things (IoT) devices expands, the foreseeable growth in hardware costs, power consumption, and carbon emissions remains big challenges for rapidly evolving wireless communications to progress toward a more sustainable furture3,4. Programmable metasurface, also known as reconfigurable intelligent surface, has attracted tremendous attentions in wireless communications because of its capability to actively reconfigure electromagnetic (EM) wave propagation in real time5,6,7,8,9. Programmable metasurface can not only be used as passive relaying surfaces to significantly improve the signal strength and reduce the impact of obstacles10,11,12, but also have broad potential in promoting the sustainable development of wireless communications. Compared with traditional relaying systems and transmission systems, programmable metasurface has lower cost, complexity, and smaller carbon footprint because it typically does not require numbers of amplifiers, analog-to-digital converters, mixers, and other radio frequency (RF) components13,14,15,16,17,18,19,20,21. In addition, the programmable metasurface can be deployed along with the base station to significantly improve the system energy efficiency by performing joint beamforming22,23,24. Furthermore, programmable metasurface can power numbers of IoT devices through wireless RF energy transfer without frequent battery replacement, thus contributing to the development of energy-sustainable IoT25,26.

However, the seamless operation of most programmable metasurfaces relies on external power supplies to energize large numbers of tunable components loaded thereon as well as the integrated control and driving circuits27. This reliance not only escalates the overall system cost, bulkiness, and complexity, but also significantly constrains their deployment flexibility in special scenarios that have insufficient or no power supply. Recently, harvesting wireless RF energy through integrated rectifier circuits has emerged as a viable approach for achieving self-powered programmable metasurface28,29. However, this approach has difficulty in obtaining sufficient energy from ambient environments with low RF energy intensity, and the use of a dedicated wireless energy transfer source will result in additional hardware costs and power consumption. Moreover, additional frequency/polarization channels required for wireless energy transfer will exacerbate communication spectrum scarcity and make the EM environment more complex. In contrast, acquiring energy from renewable energy sources such as sunlight, wind, and vibration is a more environmentally friendly, energy-efficient, and EM-interference-free solution for autonomous energy supply. Providing programmable metasurface with self-sufficient renewable energy supply is expected to accelerate the development of sustainable wireless communications but remains rarely reported.

Moreover, a narrow focus on reducing costs, power consumption and carbon emissions is insufficient for truly sustainable wireless communications, and there is also the necessity to ensure communication equity for minorities in sparse, developing, and rural areas by enabling ubiquitous connectivity30,31,32. To realize this purpose, combined applications of RF and optical wireless technologies for multi-domain information transmission is required to extend the communication network to non-terrestrial (e.g., space and underwater) environments33. However, information transmission between optical and RF domains typically relies on hybrid relaying systems composed of distributed optical and RF components, whose complicated working mechanism and high hardware overhead hinder their sustainability. Recently, several photodiode-based programmable metasurfaces have been proposed to remotely modulate microwaves through light illumination, providing the opportunity to enable light-microwave interactions through a simple physical platform34,35,36,37. However, these works roughly integrate photodiodes into microwave programmable metasurface without constructing photocurrent loops, resulting in slow light modulation rates. Later, a hybrid programmable metasurface that integrates photodiode-based photodetection circuit has been further developed for direct light-to-microwave information transmission38. However, this design only implemented the simple binary frequency-shift keying scheme, and its high sensitivity to light intensity makes it vulnerable to external environments (e.g., sunlight interference and inclement weather). More importantly, it lacks energy self-sufficiency and still relies on external power supplies like most conventional programmable metasurfaces.

Therefore, developing a programmable metasurface with self-sufficient renewable energy supply and flexible and reliable multi-domain information transmission capability is highly desired but remains a challenging goal. Herein, we propose an optically-transparent microwave programmable metasurface (OTMPM) and a photovoltaic module (PVM) as two main units to construct a solar-powered light-modulated microwave programmable metasurface (SLMPM). The OTMPM is designed to selectively exhibit high reflectivity and excellent real-time programmability to microwaves and good transmittance characteristics to light frequency bands. The PVM is utilized to convert rapidly changing intensities of modulated light into different control voltages for the OTMPM, as well as harvest energy from sunlight at the same time. By assembling OTMPM on the top of PVM, both modulated light reception, sunlight energy harvesting, and microwave reflection and modulation can be realized through a shared physical aperture. After further integrating a filtering circuit, a signal amplifying circuit, and an energy managing system, the SLMPM can eventually realize direct, flexible, and reliable information transmission from light to microwave domains under direct sunlight exposure and meantime achieve fully autonomous supply by using its harvested sunlight energy. Together with other outstanding features such as compact design, simple hardware structure, and low cost, SLMPM can promote comprehensive sustainable development of wireless communications by improving cost-effectiveness and energy efficiency, reducing carbon emission, and facilitating ubiquitous connectivity.

Results

The conceptual illustration of the SLMPM is shown in Fig. 1. Benefit from the shared-aperture multitasking capability of SLMPM, it can simultaneously acquire the information from the modulated light and the energy from the sunlight and respectively use them for light-to-microwave information transmission and self-sufficient renewable energy supply. On one hand, the SLMPM can modulate either phase or amplitude of the fundamental reflection microwave (e.g., fc1 or fc2) by directly encoding the intensity of the modulated light. Furthermore, by applying different time-domain coding strategies to the intensity of the modulated light, the SLMPM can achieve arbitrary phase and amplitude modulation of the reflection microwave harmonic (e.g., fc1 − f0). Therefore, digital information (e.g., logo image of Southeast University) mapped onto the light intensity can be seamlessly mapped onto amplitude and phase of the reflection microwave for transmission in various modulation schemes, including binary amplitude shift keying (BASK), binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), and eight quadrature amplitude modulation (8QAM) and so on. On the other hand, the SLMPM can use its entire aperture area to effectively harvest energy from sunlight. In addition, by designing the OTMPM structure based on varactors, the power consumption of unit cells can be close to zero, thus reducing the overall power consumption of the SLMPM. Due to both on-board sunlight energy harvesting capability and low power consumption feature, autonomous energy supply can be easily achieved by the SLMPM, and the surplus energy can be stored as backup. When there is insufficient sunlight irradiation (e.g., at night), the SLMPM can continue to operate using the stored energy, thus reducing susceptibility to uncontrollable weather changes.

Fig. 1: Conceptual illustration of SLMPM for simultaneous light-to-microwave information transmission and self-sufficient sunlight energy supply.
figure 1

 The SLMPM is an integrated structure of OTMPM and PVM, sharing the same physical aperture, and equipped with a filtering circuit, a signal amplifying circuit, and an energy managing system. On one hand, SLMPM can seamlessly map the digital information carried by light intensity onto reflected microwaves and transmit them in various modulation schemes. On the other hand, SLMPM can utilize its entire aperture area to effectively harvest sunlight energy, achieving 24-h uninterrupted operation with 8 h of sole sunlight energy input.

Working mechanism of SLMPM

The working mechanism of the SLMPM is shown in Fig. 2a. When modulated light and sunlight both illuminate at the SLMPM, they transmit through the OTMPM and are then converted by the PVM into the photocurrent containing the information-carrying component iph(t) and the energy-carrying component Iph. Then, the photocurrent is input into the filtering circuit composed of a capacitor C0 and an inductor L0 to separate iph(t) and Iph for control and supply purpose respectively. However, iph(t) cannot be utilized immediately as the control bias for the OTMPM because it changes sensitively with variations in solar irradiance. This phenomenon is fundamentally caused by the reduced light receiving performance of the PVM in sunlight (see Supplementary Note 1 for details). When the sunlight irradiation becomes significant, the generated iph(t) may be too weak and lost in the background noise. To solve this problem, the iph(t) branch is connected to the signal amplifying circuit consisting of a transimpedance amplifier, a voltage signal amplifier, and a comparator. Especially, the transimpedance amplifier forms a current feedback loop that greatly improves the information receiving bandwidth and gain of the PVM under direct sunlight exposure (see Supplementary Note 2 for details). Regardless of changes in sunlight irradiance, the signal amplifying circuit always outputs a square-wave voltage vo(t) with fixed amplitude for the OTMPM to accurately modulate reflection microwaves. Therefore, even if the illumination intensity of the modulated light is much lower than that of sunlight, the information it carries can be stably received and directly transmitted through reflection microwaves. On the other hand, the Iph branch is connected to an energy managing system that includes a solar charging management circuit, a lithium battery and two voltage boost circuits. The solar charging management circuit automatically tracks the maximum power point of the PVM based on changes in Iph induced by uncontrollable sunlight irradiation to maximize the harvested energy. The harvested energy is then stored in the lithium battery, whose output voltage is converted by two voltage boost circuits into suitable supply voltages for the signal amplifying circuit. Since no additional RF and optical components are used and only the signal amplifying circuit requires low driving energy, the SLMPM can achieve fully autonomous energy supply using sunlight as the sole energy input.

Fig. 2: Working mechanism and performance of SLMPM.
figure 2

a Working mechanism of SLMPM. b Geometry of OTMPM unit cell. c Simulated and measured reflection amplitude and phase of OTMPM. d Frequency responses of PVM with and without connecting the signal amplifying circuit. e Recovered reflection phase and amplitude waveforms at different sunlight illumination intensities. f Theoretically calculated and measured normalized reflection amplitude of the −1st harmonic (fc1 − f0 = 4.5999 GHz) versus different duty cycles at different sunlight illumination intensities. g Theoretically calculated and measured reflection phase of the −1st harmonic (fc1 − f0 = 4.5999 GHz) versus different duty cycles at different sunlight illumination intensities. h, Harvested energy measured at different sunlight illumination intensities. SAC signal amplifying circuit, TIA transimpedance amplifier, VSA voltage signal amplifier, COMP comparator, FC filtering circuit, EMS energy managing system, SCMC solar charging management circuit, VBC voltage boost circuit, PET polyethylene terephthalate, PC polycarbonate.

Implementation of SLMPM

Figure 2b shows the geometry of a basic OTMPM unit cell, whose patch layer and ground layer are both composed of mesh metal grids. In addition, optically-transparent polyethylene terephthalate (PET) and polycarbonate (PC) are used as supporting substrates. This design approach enables OTMPM to have both good light transmittance and high microwave reflectivity. To achieve reconfigurable control of reflection microwaves, each unit cell incorporates a varactor diode soldered between the gap of two mesh patches. By adjusting the bias voltage applied to the varactor, its capacitance will change and leads to reflection phase variation at a fixed frequency. Based on the proposed unit cell, we fabricated an OTMPM array and experimentally characterized its microwave and light responses (see Methods for sample design and fabrication). Under bias voltages of 0 and 5 V, the OTMPM exhibits two distinct microwave reflection states, which are respectively defined as 0-state and 1-state (Fig. 2c). At the fundamental frequency fc1 = 4.6 GHz, the two states have high reflection amplitudes and a reflection phase difference of 180°, which can be used for 1-bit phase modulation. While at another fundamental frequency fc2 = 3.2 GHz, the two states have an amplitude difference of more than 10 dB and almost no phase difference, which can be used for 1-bit amplitude modulation. In addition, full-wave simulation results are provided for comparison, which are in good agreement with the measured ones. In addition, we used a light transmittance meter to measure the light transmittance of the OTMPM array at different positions on its aperture and obtained an average value of 45.8%, demonstrating good light transmittance. See Supplementary Note 3 for more details on the OTMPM design.

We also customize a piece of cadmium telluride (CdTe) PVM with an active area nearly the same size as the OTMPM array (see Methods for sample design and fabrication). We choose the CdTe PVM because of its low temperature coefficient and high stability, making it robust in outdoor applications. We characterize the light receiving performance of CdTe PVM by measuring its frequency response under the irradiation of modulated light provided by a costumed white light emitting diode (LED) transmitter (see Supplementary Note 4 for details). When the LED transmitter irradiates a square-wave modulated light with the illumination intensity of 0.15 mW/cm2, the CdTe PVM exhibits a −3 dB cut-off frequency of 800 Hz (Fig. 2d). To further evaluate the light receiving performance of CdTe PVM under ambient sunlight interference, a full-spectrum LED-based solar simulator is customized to provide artificial sunlight (see Supplementary Note 4 for details). However, as a weak sunlight with the illumination intensity of 3.2 mW/cm2 is additionally illuminated at the CdTe PVM, its frequency response is sharply attenuated by more than 28 dB, indicating a significant performance deterioration. We designed a signal amplifying circuit to solve the problem (see Supplementary Note 5 for details). After connecting the signal amplifying circuit, the upper cut-off frequency of the CdTe PVM is significantly broadened to 1.65 MHz under no sunlight condition. It is worth noting that the ambient sunlight is influenced by numerous factors such as daytime/season, latitude, and weather phenomena, resulting in significant fluctuations in the actual illumination intensity. Hence, it is essential to comprehensively assess the light receiving performance of the CdTe PVM under diverse sunlight illumination intensities. Considering that the energy harvesting performance of PVM is typically measured under the nominal illumination intensity of 100 mW/cm2, herein, we employ the solar simulator to supply sunlight above and below this nominal illuminance to simulate various ambient sunlight interference scenarios. It is observed conspicuously from Fig. 2d that the upper cut-off frequency can respectively reach 1 MHz and 550 kHz when the illumination intensity of sunlight changes to 65.4 mW/cm2 and 116.7 mW/cm2 (436 and 778 times the illumination intensity of the modulated light), thereby demonstrating remarkable light receiving reliability. We also notice here that the frequency response of CdTe PVM after connecting the filtering circuit and the signal amplifying circuit has a bandpass characteristic. Since the lower cut-off frequency remains below 450 Hz at different illumination intensities, this bandpass characteristic will not affect the efficient information reception from a high-speed modulated light.

Light-to-microwave information transmission performance of SLMPM

With the OTMPM array and CdTe PVM ready, we integrate them together to construct the SLMPM. To verify the light-to-microwave information transmission performance, we first placed the SLMPM under the illumination of a direct-intensity-coding modulated light carrying specific bit sequences and measured microwave waveforms reflected by it (Fig. 2e). At a transmission rate of 250 kbps, the SLMPM can accurately receive the light-carried bit sequence ‘00110101’ and map it onto the 1-bit reflection phase at the fundamental frequency fc1 = 4.6 GHz. Similarly, another bit sequence ‘11010100’ can also be mapped onto the 1-bit reflection amplitude at the fundamental frequency fc2 = 3.2 GHz. Then, we fixed the illumination intensity of the modulated light to 0.15 mW/cm2 and gradually increased the sunlight illumination intensity from 0 to 65.4 mW/cm2 and 116.7 mW/cm2. The consistency of the recovered phase and amplitude waveforms indicates the reliability of SLMPM for high-speed light-to-microwave information transmission under direct sunlight exposure.

Furthermore, SLMPM can generate reflection microwave harmonics with arbitrary amplitude and phase for more flexible information transmission by applying different time-domain coding strategies to the light intensity. Under the illumination of the modulated light composed of periodic square-wave light waveforms with a time period of T0, SLMPM will rapidly alternate between the 0-state and 1-state, thus generating a series of reflection harmonics with frequency shifts of mf0 (m is the harmonic order, f0 = 1/T0 is the frequency interval). Specifically, the reflection amplitude Am and phase φm of the mth harmonic can be expressed as:

$$\left\{\begin{array}{c}{A}^{m}=|{\Gamma }_{{{{\rm{s}}}}0}-{\Gamma }_{{{{\rm{s}}}}1}|D{{\mbox{Sa}}}(m\pi D),{{{\rm{0}}}}\le D\le \frac{1}{2|m|}\\ {\varphi }^{m}=\angle ({\Gamma }_{{{{\rm{s}}}}0}-{\Gamma }_{{{{\rm{s}}}}1})-\frac{2m\pi {t}_{0}}{{T}_{0}},{{{\rm{0}}}}\le {t}_{0}\le \frac{{T}_{0}}{|m|}\end{array}\right.$$
(1)

where \({\Gamma }_{{{{\rm{s}}}}0}={A}_{{{{\rm{s}}}}0}{e}^{{\varphi }_{{{{\rm{s}}}}0}}\) and \({\Gamma }_{{{{\rm{s1}}}}}={A}_{{{{\rm{s1}}}}}{e}^{{\varphi }_{{{{\rm{s1}}}}}}\) are reflectivity of 0-state and 1-state, Sa(x) = sin(x)/x is the sampling function, D and t0 are the duty ratio and time delay of the square-wave light waveform respectively (See Supplementary Note 6 for the detailed derivation). Especially at the fundamental frequency fc1 that As0 and As1 approach 1 and φs0φs1 = 180°, Am and φm can be modulated almost independently by tuning D and t0, respectively. Since ±1st harmonics have the highest conversion efficiency than other harmonics, the −1st harmonic is chosen for further demonstration. Without loss of generality, here we set T0 = 10 us and measured the reflection amplitude and phase of the −1st harmonic with frequency of fc1 − f0 = 4.5999 GHz (Fig. 2f, g). By tuning D from 5% to 50%, the normalized amplitude can cover 0.16 to 1. While by tuning t0 from 0 to T0, full 360° phase coverage can be achieved. When the sunlight illumination intensity varied from 0 to 162.59 mW/cm2, the relationships between the harmonic amplitude versus D and between the harmonic phase versus t0 always maintain consistent with the theoretical calculations (See Supplementary Note 7 for details of measured reflection harmonic spectra).

Sunlight energy harvesting and autonomous supply performance of SLMPM

Different from most programmable metasurfaces that require external energy supplies, the SLMPM is powered directly on-site by its harvested sunlight energy with the assistance of an energy managing system (see Supplementary Note 8 for details). As a verification, we measured the harvested energy versus different sunlight illumination intensities (Fig. 2h). Benefit from the integrated PVM with an active area of 266 × 164 mm2, SLMPM can harvest energy of 545 mW under the nominal sunlight illumination intensity of 100 mW/cm2. As the sunlight illumination intensity further increases to 160 mW/cm2, over 720 mW of the sunlight energy can be harvested. It is worth mentioning that these results are measured under the maximum power transfer condition, which means the internal resistance of the filtering inductance L0 and the CdTe PVM consumes half of the harvested energy. In addition, we measured the power consumption of SLMPM during light-to-microwave information transmission and observed that it is stable at 176.4 mW, and the power consumption per unit area of SLMPM can be further obtained at 0.39 mW/cm2. This stability is maintained regardless of whether the modulated light adopts direct or time-domain coding strategies and is not affected by changes in the intensity of both sunlight and modulated light. It is notable that the power consumption mainly comes from the signal amplification circuit, whereas the power consumption of OTMPM is negligible because the varactors loaded on it are reversely biased and allow only a tiny current to flow.

Subsequently, we consider the measured power consumption as the threshold for judging whether SLMPM can achieve fully autonomous energy supply. Benefit from the above outstanding sunlight energy harvesting capability and low power consumption feature, the SLMPM can achieve autonomous energy supply when the illumination intensity exceeds 40 mW/cm2. Especially under the nominal illumination intensity, the energy harvested by the SLMPM is more than three times its own power consumption. Apart from the energy used for instant energy supply, the surplus energy can be stored in the lithium battery of the energy managing system as a backup to drive the SLMPM when sufficient sunlight irradiation is not available. A straightforward estimation reveals that exposing SLMPM to nominal sunlight illumination for 8 h enables it to carry out light-to-microwave information transmission uninterruptedly for 24 h. In practical deployment, the required battery capacity needs to be further determined in accordance with the usage duration of SLMPM and the sunlight conditions in the deployment area. Supposing that SLMPM operates on average 6 h per day, its average daily power consumption is 176.4 mW × 6 h = 1058.4 mWh. In the event of abundant sunlight, for instance, when the average nominal illumination time per day exceeds 2 h, the required battery capacity merely needs to reach the average daily power consumption of SLMPM. Nevertheless, if the deployment area experiences adverse weather conditions such as continuous raining, the battery capacity needs to be more adequate. Therefore, we employed a lithium battery with the capacity of 3.7 V × 850 mAh = 3145 mWh, which can guarantee SLMPM to operate for approximately 3 days without any sunlight input.

Working mechanism of a hybrid wireless communication system based on SLMPM

Since SLMPM enables arbitrary amplitude and phase modulations on harmonics, we further utilized it to construct a hybrid light-to-microwave wireless communication system that can flexibly realize different modulation schemes. Without loss of generality, here we illustrate the light-to-microwave information transmission process based on the 8QAM scheme (Fig. 3a). First, the original information to be transmitted is translated into the binary bit stream (e.g., ‘000001011010…’). Subsequently, the bit stream is divided into the 3-bit form and mapped to a series of 8QAM symbols (S0–S7). According to the amplitude and phase required by each 8QAM symbol, the symbol stream can be further mapped to the modulated light composed of a series of square-wave light waveforms (W0–W7) featuring distinct D and t0. It is worth noticing that each square-wave light waveform repeats for two cycles to ensure accurate microwave amplitude and phase are produced. To better illustrate the mapping process from standard 8QAM symbols to square-wave light waveforms, here we provide I-Q constellation diagrams (Fig. 3b) and D-t0 scattering diagrams (Fig. 3c) of four different high-order schemes (e.g, QPSK, 8PSK, 8QAM, 16QAM). It can be observed that constellation points with the same distance from the origin correspond to scattering points on a line perpendicular to the D axis, whereas constellation points with different azimuth angles correspond to scattering points with different t0. Therefore, it is anticipated that any high-order modulation scheme can be implemented using the proposed mapping method. However, it is noteworthy that high-order modulation schemes that rely on both amplitude and phase for information transmission can only be realized at harmonic frequencies, since our SLMPM is only a conceptual design that implements 1-bit phase modulation at the fundamental frequency fc1. Extending the phase modulation capability of SLMPM at the fundamental frequency to 2 bits or more can further facilitate the realization of any modulation scheme at any desired frequency39.

Fig. 3: Working mechanism of the hybrid wireless communication system based on SLMPM.
figure 3

a Design process of the modulated light required to generate the reflection microwave carrying 8QAM symbols. b I-Q constellation diagrams of QPSK, 8PSK, 8QAM and 16QAM schemes. c D-t0 scattering diagrams of QPSK, 8PSK, 8QAM and 16QAM schemes. d Block diagram of the light-to-microwave information transmission process. SAC signal amplifying circuit, EMS energy managing system, FC filtering circuit, LED light emitting diode, CSG carrier signal generator, SDR software-defined radio.

The light-to-microwave information transmission process of the hybrid wireless communication system is then described as follows (Fig. 3d). On the light transmitting side, the LED controller drives the LED transmitter to transmit the square-wave modulated light. Then, when the SLMPM receives the light-carried information, it maps such information directly onto the −1st reflection microwave harmonic carrying 8QAM symbols under the incidence of a monochromatic microwave generated by a microwave carrier signal generator (CSG). It is worth noting that such information transferring process from light to microwave domains is accomplished solely by the SLMPM, eliminating the need for a complex hybrid relaying system composed of distributed optical and RF components to perform multiple signal processing operations. Meantime, the SLMPM harvests sunlight energy through the same physical aperture to self-power its light-to-microwave information transferring process. On the microwave receiving side, the −1st microwave harmonic signal is received by a software-defined radio (SDR) platform. The received reflection microwave harmonic is first processed as a baseband signal, and then transformed into the frequency spectrum by using the fast Fourier transform. Digital symbols can be further detected from the spectrum information and recovered as the bit stream, which can be finally transformed into the digital information. In addition, the constellation diagram can also be obtained in real time.

Implementation of the hybrid wireless communication system and its performance

To demonstrate the above designs and methods, we built a realistic hybrid wireless communication system in the indoor environment (Fig. 4a, see Methods for measurement set-ups). We firstly implement six modulation schemes without sunlight, and the deployed SLMPM (Fig. 4b, c) is driven by the energy pre-stored in its energy managing system. At fundamental frequencies fc1 = 4.6 GHz and fc2 = 3.2 GHz respectively, we implemented the BPSK scheme and BASK scheme at the same symbol rate of 500 kS/s (bit rate of 500 kbps). Then by setting a frequency interval f0 = 312.5 kHz, we implemented another four high-order modulation schemes (QPSK, 8PSK, 8QAM and 16QAM) at the −1st harmonic frequency fc1 − f0 = 4.5996875 GHz. These four modulation schemes share the same symbol rate of 156.25 kS/s, and the corresponding bit rates are 312.5 kbps, 468.75 kbps, 468.75 kbps, and 625 kbps, respectively. Measured constellation diagrams of these modulation schemes are all quite standard (Fig. 4d–i), demonstrating the modulation scheme flexibility empowered by the SLMPM. To quantitatively characterize the communication performance of the system under different modulation schemes, we calculated error vector magnitude (EVM) of each implemented modulation scheme, which are 4.42%, 4.37%, 6.45%, 6.78%, 9.62% and 12.12% respectively. The EVM deteriorates with increasing complexity of the modulation scheme, which can be mitigated by further improving accuracy of SLMPM in modulating the amplitude and phase of reflection microwaves. We further evaluate the hybrid information modulation performance of SLMPM by varying the incidence angles of microwave carrier and modulated light. Both simulated and measured results demonstrate the robustness and practicality of SLMPM under oblique microwave and light incidences (see Supplementary Note 9 for more details).

Fig. 4: Implementation of the hybrid wireless communication system and its performance.
figure 4

a Indoor measurement set-up. b Schematic view of the SLMPM. c Zoomed view of the SLMPM unit cell. di Measured constellation diagrams of BPSK, BASK, QPSK, 8PSK, 8QAM and 16QAM schemes under no sunlight condition. j Original logo image of State Key Laboratory of Millimeter Waves. kp Recovered images and constellation diagrams measured when the system implemented the QPSK scheme at different sunlight illumination intensities (23.1 mW/cm2, 116.7 mW/cm2, 162.6 mW/cm2). q Original logo image of Southeast University. rw Recovered images and constellation diagrams measured when the system implemented the 8QAM scheme at different sunlight illumination intensities (23.1 mW/cm2, 65.4 mW/cm2, 143.7 mW/cm2). x Measured signal-to-noise ratio versus different sunlight illumination intensities. y Measured harvested energy versus different sunlight illumination intensities. LED light emitting diode, CSG carrier signal generator, SDR software-defined radio.

We further utilized such system for real-time image transmission under direct sunlight exposure, where the SLMPM was driven by the sunlight energy it harvested instantly. During the measurement, both QPSK and 8QAM schemes were chosen for comprehensive verification. We considered a frequency interval f0 = 156.25 kHz, resulting in the −1st harmonic frequency fc1 − f0 = 4.59984375 GHz, with bit rates of 156.25 kbps and 234.375 kbps for QPSK and 8QAM schemes, respectively. Then, we transmitted logo images of State Key Laboratory of Millimeter Waves (Fig. 4j) and Southeast University (Fig. 4q) respectively based on the two schemes and measured recovered images and real-time constellation diagrams at different sunlight illumination intensities (Fig. 4k–p, r–w). We also used the structural similarity index measure (SSIM) to quantify the degree of distortion of recovered images compared to original ones. When the QPSK scheme was implemented, the system enabled distortion-free image transmission (SSIM = 100%) even when the sunlight illumination intensity reaches 162.6 mW/cm2. While when the 8QAM scheme was implemented, the recovered image begun to be slightly distorted (SSIM = 99.5%) when the sunlight illumination intensity reaches 143.7 mW/cm2. The slight performance degradation is mainly attributed to increased phase noise caused by sunlight exposure, which can be observed from the spread of constellation points in the measured constellation diagrams. To further illustrate the information transmission quality, we measured the signal-to-noise ratio (SNR) of the system at different sunlight illumination intensities (Fig. 4x). It is observed that the system can maintain high SNR of more than 20 dB using both modulation schemes when the sunlight illumination intensity stays below 116.7 mW/cm2. Finally, we measured the sunlight energy harvested by the SLMPM during the real-time image transmission process (Fig. 4y). It is observed that SLMPM harvested nearly the same energy when implementing both modulation schemes, verifying the high independence between the light-to-microwave information transmission process and the sunlight energy harvesting process (see Supplementary Video 1 and Video 2 for more details). We also deployed the system in the outdoor environment for real-time image transmission, further demonstrating its practicality and reliability under the natural sunlight exposure (see Supplementary Note 10 and Video 3 for more details).

Discussion

We reported an advanced solar-powered hybrid programmable metasurface that fully exploits the capability of PVM to simultaneously acquire information from modulated light and energy from sunlight and highly integrates it with OTMPM to achieve reliable and flexible light-to-microwave information transmission and fully autonomous sunlight energy supply. By further integrating a filtering circuit, a signal amplifying circuit and an energy managing system, the SLMPM can reliably receive information from a weak modulated light under strong sunlight exposure and seamlessly transmit such information through its reflected microwave with any specified modulation scheme. In addition, the SLMPM can achieve all-day solar-powered light-to-microwave information transmission by benefiting from its low power consumption feature and excellent sunlight energy harvesting capability. To demonstrate the above intriguing functions of the SLMPM, we built a hybrid wireless communication system and conducted experiments on real-time image transmission from light to microwave domains. Compared to previous programmable metasurfaces, the proposed SLMPM can reduce the cost, complexity, and size of hardware, and enhance its robustness and practicability in remote areas and harsh environments by employing self-harvested green energy for on-board power supply rather than using external dedicated power supply. Meanwhile, SLMPM can further unleash the potential of programmable metasurface in expanding communication coverage with its flexible and reliable multi-domain information transmission capability, thereby providing a simplified architecture hardware solution for practical application requirements such as space-air-ground-sea integrated networks and full-spectrum coverage in future 6G communications.

It is noteworthy that in the current proof-of-concept, the realized SLMPM is polarization-independent in light frequency, single-polarized in microwave, and globally reconfigurable. Beyond this verification, the function and performance of SLMPM can be further expanded to accommodate more application requirements. For instance, by introducing space-domain modulation to microwaves besides the time-domain modulation, SLMPM can achieve wavefront shaping and information modulation concurrently, thereby facilitating the channel quality enhancement and communication coverage expansion (see Supplementary Note 11 for more details). By designing a dual-polarized OTMPM and employing multiple highly collimated lasers to expand the modulated light channels, SLMPM can further possess polarization diversity, thereby offering advantages such as communication capacity enhancement and interference reduction (see Supplementary Note 12 for more details). Furthermore, by exploring more low-power hardware solutions such as using low-power RF switches to replace the currently used varactors, lower reflection loss can be anticipated while maintaining low power consumption, ultimately improving the overall performance of the SLMPM (see Supplementary Note 13 for more details). We believe this compact, green, and powerful SLMPM can comprehensively promote the sustainable development of future wireless communications from multiple aspects, including improving cost-effectiveness by reducing hardware usage and power consumption, reducing the carbon emissions by highly integrating renewable energy harvesting technologies, and promoting ubiquitous connectivity through multi-domain information transmission.

Methods

Sample design and fabrication

The OTMPM array consists of 8 × 8 unit cells, with an overall size of 276 mm × 164 mm × 3.312 mm. Metal mesh patches and mesh ground are etched onto two PET films (dielectric constant εr = 3.5, loss tangent tanσ = 0.03, thickness 0.188 mm) by using the flexible printed circuit board technic. The two PET films are then adhered to different sides of a PC substrate (dielectric constant εr = 2.9, loss tangent tanσ = 0.007, thickness 2.9 mm). A ‘MA46H120’ varactor diode is soldered on each OTMPM unit cell. All direct-current (DC) bias lines converge on the electrodes (width 5 mm) on both sides of the OTMPM array, enabling all unit cells to share the same control voltage output from the signal amplifying circuit. Additionally, a piece of CdTe PVM is customized with the overall dimensions of 266 mm × 164 mm × 3.2 mm. Under the standard test condition (sunlight irradiance of 100 mW/cm2, temperature of 25 °C and air mass 1.5), the customized CdTe PVM yields a peak power voltage of 10.6 V, a peak power current of 0.4 A, and a photoelectric conversion efficiency of 9.62 %. Then, the CdTe PVM is attached to the back side of the OTMPM array in a center-aligned manner. A photograph of the assembly of OTMPM and PVM is presented in the inset of Fig. 4a.

Measurement setups

The proposed SLMPM-based hybrid wireless system mainly consists of the light transmitter, the SLMPM, and the microwave receiver. At the light transmitter side, a white LED transmitter is used to irradiate the modulated light and is placed 30 cm away from the SLMPM. To drive the white LED transmitter, a portable test and measurement instrument (ADALM2000, ADI) with a built-in field programmable gate array and digital-to-analog converter is utilized as the LED controller. In all measurements, the illumination intensity of the modulated light is held constant at 0.15 mW/cm2. A microwave CSG consisting of ‘NI PXIe-1083’, ‘NI PXIe-5654’, and ‘NI PXIe-5696’ is connected to the transmitting antenna to deliver a monochromatic microwave carrier to SLMPM. At the microwave receiver side, a receiving antenna and a SDR platform ‘NI USRP-2954’ are used to receive microwave signals reflected by SLMPM and recover the information. In the indoor measurement, the distance between the transmitting antenna and SLMPM is 0.6 m, and the distance between the receiving antenna and SLMPM is 3 m. A customed solar simulator is placed 15 cm in front of the SLMPM to irradiate artificial sunlight with controllable illumination intensity. In addition, a programmable electronic load ‘UTL8512’ is connected to the SLMPM to record its instantaneously harvested sunlight energy.