Highly integrated programmable metasurface for multifunctions in reflections and transmissions

Electromagnetic (EM) metasurfaces have been attracting worldwide attention for decades due to their high integration, low loss, and splendid control abilities on the EM waves. Versatile applications based on the metasurfaces have been proposed, such as cloaking,^1,2 imaging,³ and abnormal diffractions.^4,5 To explore the connections between digital information and physical fields, digital coding and programmable metamaterials and metasurfaces have been proposed.^6–8 Based on this concept, various theories and functional designs have been reported, including convolution operation,⁹ programmable hologram,¹⁰ polarization editor,^11,12 amplitude modulator,¹³ non-reciprocal devices,^14,15 smart sensing,^16,17 and self-adaptive modulations.¹⁸ Benefitting from the flexible and digital controls, the digital coding metasurfaces have further led to a variety of new directions, such as temporal modulation,^19,20 space-time-coding,^21,22 reprogrammable plasmonic topological control,²³ light regulation,²⁴ and programmable diffractive neural networks.²⁵ 

Among these numerous applications, EM regulations of multifunctional composites, especially programmable and simultaneous manipulations on the transmission and reflection of EM fields and waves, have been an important research direction.^26–30 For most of passive metasurfaces^26,27,31 for transmission and reflection EM wave controls, multiple layers of metasurfaces are necessary for expanding the phase modulation range. However, these kinds of metasurfaces^26,27,31 usually divide the transmission and reflection functions into two orthogonal polarizations to achieve simultaneous controls on the transmitted and reflective waves, which require different excitations to achieve such functions. Some cascaded designs using chiral structures have also been reported,^32,33 which can realize circular-polarized beam controls for both transmission and reflection modes. In addition to the passive structure based regulations, actively programmable designs have been proposed to achieve more flexible functional controls.^34–39 By tuning the permittivity of indium tin oxide, an all-dielectric tunable metasurface realized the phase modulations on both transmitted and reflected lights.³⁵ A polarization converter integrated with a PIN diode was demonstrated for controlling the transmission and reflection, respectively, in the x and y polarizations.³⁴ In addition, the idea of polarization multiplexing can also be extended to achieve one or two reconfigurable functions by introducing the PIN diodes.^26,36 With the development of miniaturization and integration, multifunctional devices are necessary. However, most of the work mentioned above can only achieve one or two programmable functions in transmitting and reflecting the EM waves.

To further introduce more functions such as wave absorption, some groups have proposed a multi-layer active metasurface composite structure to achieve three functions of transmission, reflection, and wave absorption.^37,38 Wang et al. further superimposed active metasurface layers in two orthogonal directions for tailoring dual polarizations.³⁹ However, these approaches require the introduction of excessive active devices in a single cell and require multi-layer-cascaded metasurfaces, which, in turn, lead to more complicated bias circuit designs and a larger profile.

To overcome the challenge, we propose a highly integrated metasurface to achieve four programmable functions, including the reflected phase to modulate in a programmable way, and to absorb, transmit, and totally reflect the EM waves. The presented method will use a single-pole triple-throw (SP3T) switcher and single-layer multi-channel structure, which can significantly simplify the complexity of the metasurface while enabling multiple functionalities. Experimental results have good agreement with numerical simulations, which further validate the proposed metasurface and its design methodology.

In order to better describe the functional characteristics of the proposed metasurface, we provide a schematic illustration in Fig. 1 in which the metasurface is composed of the programmable unit integrated with a SP3T switcher, which can be switched for different EM responses. These distinct responses include 1-bit phase modulation in the reflection mode, EM wave transmission, absorption, and total reflection. We design three different EM structures for the three switchable ports of the same switcher to achieve the above functions. When the metasurface unit is implemented with a specific digital control-voltage, the SP3T switcher is turned to the related port, establishing the corresponding EM channel to achieve the desired function. Note that the metasurface will perform one function at the same moment since the element is switched to one EM response at a time. The metasurface is excited by a linear-polarized incidence and realizes these functions in the linear polarization.

To achieve the above idea using the SP3T switcher, we design a programmable element as shown in Fig. 2(a). The designed regular hexagonal metal structure can couple the spatial propagation energy into the SP3T switcher (RC 3373) to realize the programmable functions. We design three functions for the three throw points, as depicted in the enlarged figure in Fig. 2(a). To clearly present the connection of the switch chip, we also provide the top view of the switcher pinout in Fig. 2(a), where the transmission channels are marked as 1–4. The element is composed of three-layer substrates of Arlon450 (ε_r = 4.5, tan δ = 0.003), and the thicknesses from top to bottom are 1.5, 0.5, and 1.5 mm, respectively. There are two layers of metals on both sides of the central substrate to establish the EM ground and simultaneously act as direct-current (DC) positive and negative for the power supply of the SP3T switcher. By implementing specific control voltages in the Pins for voltages 1 and 2, the switcher will be switched to the related Pins, guiding the coupled energies to achieve the corresponding functions. The EM states (on and off) of the switch can be represented as a series RLC equivalent circuit, as given in Fig. 2(a). The digital control Pins of voltages 1 and 2, namely, V1 and V2, are encoded into four functions as listed in Fig. 2(a). The average power supply voltage and current are 5 V and 60 μA, which means that the power consumption per chip is about 0.3 mW. It should be noted that the two control Pins (V1 and V2) are, respectively, connected to two via-holes, which are not connected to any metal layer and directed connected with the peripheral connecting ports. These two control Pins no longer need any DC block circuit since that has been processed in the chip design.

For the phase modulation, the coupled energy is channeled into a metallic delay line, which generates a phase change in the reflection compared to the off state. For the transmission function, the coupled energy enters a symmetrical structure on the backside through a via-hole, which has the same regular hexagonal metal structure, as given in Fig. 2(c). This via-hole for transmission penetrates the three-layer substrate and its two intermediate layers of metal but is not connected to any intermediate layer metal. For the absorption function, the switch is turned to an attenuator to eliminate the energy. The enlarged figure with the equivalent circuit is given between Figs. 2(b) and 2(c), where R₁ = 20 Ω and R₂ = 25 Ω. The two Pins of the attenuator on the upper side are connected to two via-holes to the ground. The front and back views of the detailed element structure are given in Figs. 2(b) and 2(c). The detailed dimensions of the structure are labeled in Figs. 2(b) and 2(c) as follows: a = 17.4 mm, b = 15.07 mm, c = 2 mm, d = 2 mm, e = 0.5 mm, f = 4.2 mm, g = 5 mm, h = 6.65 mm, and P = 30 mm. The element simulation is performed using the commercial software, CST Microwave Studio, in which the periodic boundary condition is applied. For the phase modulation, the amplitude and phase responses in the reflection are given in Figs. 2(d) and 2(e). It can be observed that a 180° phase difference occurs at 4.005 GHz between the on- and off-states of the phase modulation. The amplitude responses of the two states are −0.45 and −3.8 dB, promising a fair reflection efficiency. The simulated result of the EM-wave transmission is given in Fig. 2(f), in which the transmission coefficient is about −3.1 dB at 3.54 GHz, while other states maintain the non-transmitting state. For the absorption function, the largest absorbing point is located at 4.732 GHz, whose absorption magnitude is about 12.6 dB.

To further demonstrate the above element performance of the phase modulation in reflection, we design three coding patterns of dual-beam and perform the full-wave numerical simulations, as exhibited in Figs. 3(a)–3(c). A metasurface composed of 14 × 14 elements is designed and simulated using the CST Microwave Studio. The simulated three-dimensional (3D) far-field patterns are depicted, as well as the related coding patterns are shown in the lower left corners in the sub-figures, where the phase states “0” and “1” are labeled in red and green. Three coding patterns are coded horizontally in the sequence “11001100110011,” “11100011100011,” and “11110000111100,” respectively. The two symmetrical scattering beams are clearly observed at different angles as the coding period changes. According to the simulated results in Figs. 3(a)–3(c), the scattering angles are, respectively, about ±32.1°, ±21.0°, and ±15.0°. In addition, when all control voltages (V1 and V2) are set to 0 V, the metasurface is in the total-reflection state, as shown in Fig. 3(c), where a vertically reflected high energy beam is observed.

The metasurface is fabricated using the printed circuit board technology, and the elements are controlled in a column manner, that is to say, each column of elements shares the control voltages. For the experimental demonstration of these programmable functions, we firstly implement the measurements of the transmission and reflection coefficients, based on the lens antenna platform presented in Fig. 4(a). A pair of focusing lenses working in the C-band is used, which are connected to a vector network analyzer, to measure the S-parameters. The lens can focus energy on the central area of the sample to reduce the unwanted diffraction and scattering interference. A time-domain gating method is applied to measure the accurate EM responses of the metasurface. The measured transmission is given in Fig. 4(b), where the simulation result is also provided for comparison. We observe that the frequency point slightly deviates to 3.558 GHz and the transmission coefficient reduces about 0.7 dB, compared to the numerical simulations. In Fig. 4(c), the measured S11 shows an obvious absorption peak at 4.76 GHz, which is deviated about 0.03 GHz, and the lowest S11 is −10.24 dB, corresponding to the maximum absorption point. In the reflection measurement, we set both control voltages (V1 and V2) to 0, and the reflected intensity is slightly lower about 0.5 dB in overall than in the simulation, as exhibited in Fig. 4(d). From 3 to 4.5 GHz, we observe that the metasurface achieves high reflection intensity. Although there are small deviations between the simulation and measurement results, overall agreement is good. The main source of errors is the difference between the equivalent circuit fitted in simulations and the realistic component. The component itself and the soldering process may also introduce additional parasitic inductance or capacitance effects, resulting in errors. In addition, the slight errors in the dielectric constant and thickness shift of the substrate in fabrication will affect the resonance characteristics.

To experimentally demonstrate the phase modulations in the reflection mode, we perform the far-field measurements for the dual-beam patterns. The experiment is carried out in a standard microwave chamber room, as shown in Fig. 5(a). The metasurface and the feed source are fixed on the rotatable table. To imitate the homogeneous incident distributions on the metasurface, the horn antenna transmitter is set at about 1.5 m away from the metasurfaces. Three dual-beam patterns with different scattering directions are measured in which the corresponding coding sequences are “11001100110011,” “11100011100011,” and “11110000111100,” respectively. To clearly show the performance, we present both simulation and measurement results in blue and red colors, respectively, as illustrated in Figs. 5(b)–5(d). The scattering angle of three patterns in measurements is about 33.4°, 22.8°, and 15.1°, respectively, showing slight errors compared to the simulation results of 32.1°, 21.0°, and 15.0°. The measured energies of three patterns are about 9.8, 10.3, and 10.6 dB, respectively, which are about 0.6 dB lower than the simulation results on average. The deviation in the scattering angles are mainly resulted from the imperfectly incident plane wave from the broadband rectangular horn antenna, the error in the equivalent circuit of the switcher, the fabrication error, as well as the manual operation. In addition, the slight loss in the main radiation lobes is due to non-ideal factors, such as chip device, dielectric, and solder processing. Overall, the experimental results are in good agreement with the simulations and theoretical designs. It should be noted that the switching time is about 20 μs, which is the time for the chip to switch to any functional states. Considering the time for execution instructions of FPGA and other devices, the overall switching time of the metasurface can be kept above 1 MHz, suggesting good potential for time-varying applications. In addition, the presented design holds promise for applications under extreme temperature conditions, if high temperature-stability chips and substrates are used.

We propose and experimentally demonstrate a highly integrated metasurface based on the SP3T switcher to alternatively realize four programmable functions: reflection-phase modulations in a programmable way, transmission, absorption, and reflection. We design the metasurface element embedded in the SP3T switcher and an attenuator based on the single-layer metasurface layout, instead of multi-layer cascaded structures. In addition, the highly integrated package of the switcher eliminates the need for complicated DC isolations and biasing circuits. Compared to the previous studies on multifunctional metasurfaces, the proposed method significantly improves the integration while reducing the design complexity. The experimental results are in good agreement with the simulations and theoretical designs. For higher frequency applications, we believe that the presented design method is possible to apply based on more advanced chips with much smaller packages. We expect that this work can expand the idea of designing multifunctional metasurfaces and promote the development of highly integrated smart information devices and systems.

This work was supported, in part, by the National Key Research and Development Program of China (Grant Nos. 2017YFA0700201, 2017YFA0700202, and 2017YFA0700203), the Major Project of the Natural Science Foundation of Jiangsu Province (Grant No. BK20212002), the National Natural Science Foundation of China (Grant Nos. 61871127, 61735010, 61731010, 61890544, 61801117, 61722106, 61701107, 61701108, and 61701246), the State Key Laboratory of Millimeter Waves, Southeast University, China (Grant No. K201924), the Fundamental Research Funds for the Central Universities (Grant No. 2242018R30001), the 111 Project (Grant No. 111-2-05), and the China Postdoctoral Science Foundation (Grant No. 2021M700761).

The authors have no conflicts to disclose.

The data that support the findings of this study are available from the corresponding author upon reasonable request.