Since the concept of metasurfaces was proposed, the electromagnetic (EM) wave manipulation technology based on metasurfaces has become one of the hot topics in the field of electromagnetism [1]. Metasurfaces are artificial materials that arrange artificial structural atoms on a two-dimensional surface in a periodic or non-periodic manner, and those material properties are mainly derived from artificial structures rather than the material components that make up their structures [2,3]. Metasurfaces are designed based on the general Snell’s law [4], which introduces abrupt phase shifts on the structure surface. Based on this characteristic, the amplitude, phase, polarization mode, and propagation mode of EM waves can be flexibly and effectively controlled [5–8]. Through changing the effective permittivity and permeability of materials, a new paradigm about customizing EM parameters is established, which provides a new degree of freedom for the free regulation of EM waves [9–11]. As a two-dimensional form of metamaterial, metasurfaces have developed rapidly due to their compatibility with current integrated circuit processes, small size, thin thickness, low loss, simple fabrication, and suitability for miniaturization, and have become one of the largest branches of metamaterial [12]. Metasurfaces are widely used at microwaves [13], terahertz [14,15], and visible light frequencies [16], which can enable many exciting EM functions such as phase modulation [17–19], polarization conversion [20,21], and beam focusing [22,23].

As an essential property of EM waves, polarization is an important basic parameter in addition to amplitude, frequency, and phase, which characterizes the time-varying orientation of the electric field intensity vector at a given point in space [24]. At present, it is an important research direction to effectively control the polarization state of EM waves. Conventional devices to manipulate polarization are mainly achieved through techniques such as the Faraday effect or anisotropic crystals [25,26]. However, these technologies have many limitations, such as low conversion efficiency, limited bandwidth, large size, and poor integration with other miniaturized devices, which seriously limit their practical applications [27]. Excitingly, metasurfaces provide a low-cost, low-loss, and effective strategy for polarization conversion, and their incomparable advantages over natural materials can solve many problems in the field of polarization conversion, which has aroused great interest [28–30]. At present, the polarization conversion functions such as linear-to-linear (LTL) polarization conversion [31], linear-to-circular (LTC) polarization conversion [32], and circular-to-circular (CTC) polarization conversion [33] have been realized. Metasurfaces loaded with active devices such as PIN diodes can achieve different polarization conversion functions under different conditions [34–37], providing a new method. In recent years, the emerging light control method has required the light source to directly illuminate the photosensitive device, thereby exciting the active device in a non-contact manner, which has unique advantages such as simplifying the system device and reducing signal crosstalk [38–40]. The photoresistor is used as an active device, which can further avoid the influence of bias circuit on EM wave transmission [41]. However, this control paradigm forces the metasurface to be placed only according to the light emitter position, and even to need to be integrated on the light emitter, which limits the application scenarios of light-control reconfigurable metasurfaces.

Based on the above research status, this paper proposes a light control method based on optical fiber, with the photoresistor being completely embedded in the meta-atom as an active device. The method is mainly composed of an LED array, optical fiber, and polarization conversion metasurface (PCM), as shown in Fig. 1. Optical fiber transmits the light drive signal emitted by the LED array to the meta-atom, exciting the photoresistor in a non-intrusive way, which can realize the dynamic switching of the polarization conversion function in a broad bandwidth. Because optical fiber can guide and control the light propagation path, the PCM can be placed at any position without being integrated with the light emitter, which breaks through the limitation of physical space and is more in line with practical applications.

In order to verify the concept, a PCM embedded with photoresistors is simulated, fabricated, and measured. Simulation and experimental results show that the optical-fiber-controlled PCM can achieve the LTL polarization conversion in the light environment and the LTC polarization conversion in the dark environment. The EM wave can be switched between LTL polarization conversion and LTC polarization conversion through changing the state of the LED array in a broad bandwidth. This work paves a new way to achieve light-controlled metasurfaces, with strong application in practice. More importantly, it enriches the intelligent metasurface family and has potential application prospects in communication systems, information transmission, EM stealth, and adaptive intelligent perception.

In this paper, a four-layer PCM is designed to realize the dynamic polarization conversion function in a broad bandwidth, as shown in Fig. 2(a). The first layer is a PI film (dielectric constant εr=3.4, loss tangent tanδ=0.001), which is used to achieve mechanical support for the metal structure. The second layer is two symmetrical rectangular metal patches plated on the PI film. The GT36516 photoresistor is welded at the gap between the metal patches, whose photosensitive side is facing down to sense the light. The other two layers are an F4B dielectric substrate (dielectric constant εr=2.65, loss tangent tanδ=0.001) and a metal reflective backplane. In order to make the PI film closely adhere to the F4B dielectric substrate, and ensure that the light transmitted through optical fiber can control the photoresistor, the hole is drilled in the dielectric substrate and the metal reflective backplate according to the size and position of the photoresistor, as shown in Fig. 2(b). An optical fiber is inserted into the lower hole for transmitting the excitation light source. The optical fiber is made of polymethyl methacrylate (PMMA) with dielectric constant εr=2.25 and loss tangent tanδ=0.001.

The photoresistor value can reach 0.3 MΩ in the dark environment, when it is in an open state. With the gradual increase of light intensity, it gradually decreases to tens of ohms, when it is in a conductive state. Based on the characteristic, two rectangular metal patches remain as separate parts in the dark environment, and are connected as a whole in the light environment. Through adjusting the light intensity, the resonant characteristics of the metal structure can be changed. As shown in Fig. 2, the period of the meta-atom structure is p=8mm, the length of the metal patch is l1=4.55mm, the width is w=1mm, and the patch gap is l2=0.2mm. The thickness of the F4B dielectric substrate is h1=3mm, the thickness of the metal reflective backplane is h2=0.035mm, and the thickness of the PI film is h3=0.05mm. The thickness of the metal patch is 0.035 mm. The radius and height of the small hole are rbig=2.7mm, hbig=0.8mm, rsmall=1.6mm, and hsmall=2.235mm.

In order to explore the polarization conversion performance, the EM response of the well-designed meta-atom is simulated and optimized in the CST Microwave Studio software. It is considered that the presence of optical fiber may have an effect on the EM response of the meta-atom structure. In order to ensure the accuracy and comprehensiveness of the simulation results, we integrate the optical fiber into the meta-atom structure as an important part of the simulation model for comprehensive simulation analysis. When the photoresistors are in different states, the simulation results for the co- and cross-polarized reflection coefficients are shown in Figs. 3(a) and 3(b). Here, rxx and rxy represent the co- and cross-polarized reflection amplitudes, respectively, φxx and φxy represent the co- and cross-polarized reflection phases, respectively, and the subscripts x and y represent the EM wave polarization. Obviously, the PCM has a controlling effect on the reflection phase and the reflection amplitude. As shown in Fig. 3(a), when the photoresistor is in the light environment, the simulated co-polarized reflection amplitude is low, while the cross-polarized reflection amplitude is relatively high. The cross-polarized reflection amplitude reaches a maximum value of 0.82 at 15 GHz. The reflected wave is mainly a vertically polarized wave, which achieves the purpose of LTL polarization conversion. When the light environment changes to the dark environment, the simulated co- and cross-polarized reflection amplitudes are shown in Fig. 3(b), and it can be seen that the simulated co- and cross-polarized reflection amplitudes are approximately equal, but the reflection phase difference is about 90°. At this time, the reflected wave is a circularly polarized (CP) wave, which achieves the purpose of LTC polarization conversion. See Appendix A for more details on the EM response of the meta-atom structure with and without optical fiber. With the introduction of optical fiber, the EM characteristics of the meta-atom structure are not significantly changed. See Appendix B for more details on the EM response of the meta-atom structure under different resistance values. With the variation of resistance value, the PCM can switch between LTL polarization conversion and LTC polarization conversion.

In order to describe the LTL polarization conversion efficiency, the polarization conversion ratio is defined as shown in Eq. (1), which characterizes the conversion efficiency of one polarization to another polarization: PCR=rxy2/(rxx2+rxy2).(1)The axial ratio is defined as shown in Eq. (2), which is used to analyze the CP conversion: AR=|10log10(rxycosa−rxxcosΔφsina)2+rxx2sin2Δφsin2a(rxysina−rxxcosΔφcosa)2+rxx2sin2Δφcos2a|,(2)where a=12arctan(2rxyrxxcosΔφrxy2−rxx2),(3)Δφ=φxx−φxy.(4)The PCR and AR of the photoresistor in different states are calculated through MATLAB, as shown in Figs. 3(c) and 3(d). According to the calculation results, when the photoresistor is in the light environment, the PCR is higher than 0.8 at 11.73–19.85 GHz, and the maximum is 0.955 at 15.72 GHz. When the photoresistor is in the dark environment, the AR is less than 3 dB at 10.97–19.22 GHz, and the AR is close to 0 dB at 12.10 and 16.81 GHz, that is, the polarization conversion efficiency is close to 100%.

In order to explain the polarization conversion principle, the x-polarized wave is taken as an example and decomposed into two vertical components, u- and v-polarized waves, which are 45° and −45°, respectively, in the x direction. Thus, the incident wave electric field can be expressed as Ei=xE0eikzz=(u+v)E0eikzz.(5)Assuming that the reflection coefficients of the u- and v-polarized incident waves are |ruu|ejφu and |rvv|ejφv, respectively, the reflected wave can be expressed as Er=(u|ruu|e−i(kzz+φu)+v|rvv|e−i(kzz+φv))E0.(6)When the photoresistor is in the light environment, the simulated reflection amplitude and reflection phase under u- polarized and v-polarized waves incidence are shown in Figs. 4(a) and 4(b). As shown in Fig. 4(a), the co-polarized reflection amplitude ruu is greater than 0.8, and rvv is close to 1 at 9.9–16.4 GHz, which can be considered approximately equal. As shown in Fig. 4(b), the reflection phase difference is around the π, i.e., |φvv−φuu|=π. According to Eq. (6), it can be derived as $$\begin{gathered} Er=(u|ruu|e−i(kzz+φu)+v|rvv|e−i(kzz+φv))E0 \\ =(ue−i(kzz+φu)+ve−i(kzz+φu+π))|ruu|E0 \\ =(u−v)e−i(kzz+φu)|ruu|E0 \\ =ye−i(kzz+φu)|ruu|E0. \end{gathered}$$(7)According to Eq. (7), it can be seen that the u- and v-polarized waves can be combined into the y-polarized wave, which can achieve the purpose of LTL polarization conversion. Due to the existence of the photoresistor value, a certain loss will be generated under u-polarized wave incidence, resulting in the decrease of the co-polarized reflection amplitude ruu. The surface current distribution and analysis of the meta-atom in the light environment are shown in Figs. 5(a) and 5(b). According to Fig. 5(a), when the u-polarized wave is incident, the reflected wave is in the same phase direction as the incident wave, which is equivalent to an electrical conductor. When the v-polarized wave is incident, the reflected wave is in the opposite phase direction to the incident wave, which is equivalent to a magnetic conductor. After the two reflected waves are synthesized, the EM wave is rotated by 90°, which can achieve polarization rotation, as shown in Fig. 5(b).

When the photoresistor is in the dark environment, the simulated reflection amplitude and reflection phase under u- and v-polarized waves incidence are shown in Figs. 4(c) and 4(d). It can be seen that the co-polarized reflection amplitudes ruu and rvv are almost equal, close to one, i.e., ruu=rvv, while the reflection phase difference is π/2, i.e., |φvv−φuu|=π/2. According to Eq. (6), it can be derived as $$\begin{gathered} Er=(u|ruu|e−i(kzz+φu)+v|rvv|e−i(kzz+φv))E0 \\ =(ue−i(kzz+φu)+ve−i(kzz+φu+π/2))|ruu|E0 \\ =(u−vi)e−i(kzz+φu)|ruu|E0. \end{gathered}$$(8)In this case, the reflected wave is a CP wave. The surface current distributions and analysis of the meta-atom in the dark environment are shown in Figs. 5(c)–5(f). According to Fig. 5(c), the reflected wave is almost zero when the u-polarized wave is incident, and is inverted to the incident wave when the v-polarized wave is incident. The reflected wave polarization points to the v negative half axis, as shown in Fig. 5(d). According to Fig. 5(e), when the u-polarized wave is incident, the reflected wave is in the same phase direction as the incident wave, which is equivalent to an electrical conductor. When the v-polarized wave is incident, the reflected wave is in the opposite phase direction to the incident wave, which is equivalent to a magnetic conductor. The reflected wave polarization points to the y positive half axis, as shown in Fig. 5(f). According to the analysis results, the polarization direction can change along with time, which can achieve the LTC polarization conversion.

In order to further verify the practicability of the optical-fiber-controlled PCM, different materials are processed to carefully fabricate samples composed of 16×16 meta-atoms, where the photoresistor is welded at the gap between the rectangular metal patches, as shown in Fig. 6(a). Depending on the size and position of the photoresistor and the thickness of the optical fiber, holes are drilled in the dielectric substrate and the metal reflective backplate. Optical fibers are inserted on the back of the sample to control the light propagation path and transmit the light source driving signal, so as to realize non-invasive excitation of photosensitive devices. Optical fiber can bend the light propagation path, without linear propagation in the traditional way, as shown in Fig. 6(b).

In order to reduce the noise effects, the sample is tested in a microwave anechoic chamber. In the experimental measurement setup, two broadband horn antennas are used, one as the transmitting antenna (signal source) and the other as the receiving antenna, which are connected to two ports of the vector network analyzer (VNA). The experimental setup is shown in Fig. 6(c), with two antennas symmetrically placed close to each other and far enough away from the sample. The height of the transmitting and the receiving antennas, as well as the sample, is consistent to ensure that the EM wave emitted by the transmitting antenna can be effectively reflected by the sample and received by the receiving antenna. Optical fibers are inserted on the back of the sample to transmit an optical signal, which is used to control the photoresistor value and adjust the resonant response.

First, the lamp beads on the LED array are supplied with power. The light source emitted by the lamp beads is transmitted to the photoresistor’s photosensitive surface through the optical fiber. At this time, two rectangular metal patches are connected as a whole for LTL polarization conversion measurement, and the measurement results are shown in Fig. 6(d). According to the measurement results, it can be seen that the co-polarized reflection amplitude is relatively low, while the cross-polarized amplitude is relatively high. The PCM converts the linearly polarized (LP) wave into an orthogonal LP wave, which realizes the purpose of LTL polarization conversion. When the power supply is disconnected, two rectangular metal patches remain as separate parts for LTC polarization conversion measurement, and the measurement results are shown in Fig. 6(e). According to the measured results, the reflection amplitudes rxx and rxy are approximately equal, and the reflection phase difference between the co- and cross-polarized is about 90°. The PCM converts the LP wave into a CP wave, which realizes the purpose of LTC polarization conversion.

However, comparing the simulation data with the experimental data, it was found that there is an error. The main reason is that when the horn antennas are placed side by side, they are not completely facing the experimental sample, and there will inevitably be a small angle, which will introduce oblique incidence. In addition, the measurement data shift towards low frequencies. The main reason is that the photoresistor will have its own capacitance and inductance after packaging, which is not considered in the design simulation model, and there are dimensional errors in the manufactured samples.

In order to excite photosensitive devices, the traditional light control method will require the metasurface and the LED array to be placed in a fixed position, and even to need to be integrated together, which is not conducive to the actual application situation. In this paper, a light control method based on optical fiber is proposed, the basic idea of which is that the light source is guided to the meta-atom through optical fiber, so as to realize non-intrusive excitation of photosensitive devices and change the resonant response of the rectangular metal patch. This control method uses optical fiber as the transmission medium, which can control the light propagation path, so the metasurface can be freely placed in any position according to the actual application scenario, which has strong applicability.

The simulation results show that the PCM can reflect LP waves into orthogonal LP waves at 11.73–19.85 GHz in the light environment, and CP waves at 10.97–19.22 GHz in the dark environment. According to the experimental results, the designed metasurface can realize the mutual conversion between LTL polarization conversion and LTC polarization conversion in a broad bandwidth, which verifies the feasibility and effectiveness of the light control method based on optical fiber. This work provides a new idea for the design of light-controlled metasurfaces, which can use the characteristic that optical fibers can change the light propagation path to excite photosensitive devices and tailor EM wave functions. More importantly, it enriches the intelligent metasurface family and has potential application prospects in communication systems, information transmission, EM stealth, and adaptive intelligent perception.