Metamaterial absorbers can achieve high-efficiency electromagnetic absorption in a specific band, which have been used in biochemical sensing, photoelectric detection, imaging and other fields. Tunable metamaterial absorbers provide more possibilities for the development of multifunctional electromagnetic absorption devices. Here we propose a tunable and polarization-dependent terahertz metamaterial absorber which can work for both linearly and circularly polarized waves. By introducing single layer graphene and vanadium dioxide (VO2), switching between the two working states and wide-range tuning of the absorption efficiency are realized. When VO2 is in insulating state, the absorber shows different tunable absorption performance for the x- or y-polarized terahertz waves, in which the maximum absorption rate is close to 100%. When VO2 is in metallic state, the metasurface behaves as a chiral absorber, and the maximum absorption difference between the two circular polarizations is about 0.45, while the tuning efficiency reaches 86.3%. Under the two working conditions, the absorber can maintain efficient absorption with a large incident angle. In addition, as an application exploration of the absorber, we demonstrated its application in tunable and polarization multiplexed near-field image display.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
As two-dimensional derivative of three-dimensional electromagnetic metamaterials, metasurfaces have shown great application possibilities in control of spatial beam, sensing, optical imaging, and quantum science [1–4], and they are very likely to develop into mainstream optical devices in the future. Utilizing the sub-wavelength artificial meta-atoms arranged in an array, metasurfaces can realize electromagnetic manipulation on the sub-wavelength scale, and has great advantages such as high efficiency, low loss, small size, and easy to process . The working band of electromagnetic metasurfaces currently covers microwave, terahertz, infrared, visible and even ultraviolet. They have evolved from a single-function metasurface to a multifunctional and tunable meta-device . Among them, the tunable metasurface is the most studied in terahertz band [7–9].
Electromagnetic absorber means electromagnetic medium with both low reflectivity and transmittance. Traditional absorbers attenuate and absorb electromagnetic waves based on resistance loss, dielectric loss or hysteresis loss of the base material and medium . With the deepening of metasurface research, various types of absorbers based on metasurfaces continue to be reported, which are used in sensing [11,12], photoelectric detection [13,14] and polarization imaging [15,16]. In terahertz band, broadband or multiband absorbers, polarization-dependent absorbers and tunable absorbers have been reported [17–22]. The absorber working in the linear polarization is mainly realized by using the polarized electromagnetic responses of the anisotropic resonators . The circular polarization related absorbers need to use asymmetric chiral meta-atoms, or the extrinsic chirality under oblique incidence [24,25]. Tunable terahertz absorbers have been widely studied, and the active materials have been used including single-layer graphene, VO2, liquid crystal, Dirac semi-metal [26–29] and photosensitive semiconductor [8,9]. The electric bias of graphene or liquid crystal has the advantage of easy integration , while the phase change material VO2 is simple to prepare and can provide giant electrical conductivity . Photosensitive semiconductors can realize ultra-fast modulation in optical pumping . However, most of these absorbers adjust the absorption efficiency or operating frequency in a single function, or switch the absorption bandwidth. There are few reports about terahertz absorbers that can switch the absorption polarization state and absorption efficiency at the same time.
In this paper, we propose a new tunable absorber that can target both linearly and circularly polarized terahertz waves. Using the hybrid reflective structure of graphene, VO2 and gold, a wide range of continuous adjustment of absorption efficiency can be achieved in two working conditions. When the VO2 is in insulating state, the meta-atoms are anisotropic achiral structures and show tunable and selective absorption of linearly polarized terahertz waves. When the VO2 is in metallic state, the absorber is a chiral metasurface which shows tunable circular polarization absorption. In addition, as a potential application of this polarization-dependent terahertz absorber, we used the array of different meta-toms to demonstrate absorber-based tunable and polarization multiplexing near-field imaging.
2. Design and method
The proposed bi-functional and tunable metasurface is shown in Fig. 1. In order to achieve polarization-dependent terahertz absorption, we use two anisotropic elliptical resonators (long axis is 30 μm, short axis is 10 μm) made of gold and VO2, respectively. The period of the meta-atoms is P=50 μm, the dielectric spacer is a polyimide (PI) film with thickness of H=40 μm. The VO2 resonator is placed in the middle of the PI layer, and the long axis is 45 degrees with that of the upper gold film. The electrical doping of graphene is achieved through the ionic gel technology , with an external bias voltage of Vg. The photoinduced phase change of VO2 resonators can be excited by obliquely incident visible light, while graphene is almost transparent in the visible band. When the VO2 is in insulating state, it shows almost no effect on the resonance of the metasurface. After its insulator-metal phase transition, it will resonate with the upper resonator to form a chiral structure, which shows selective absorption for the circularly polarized terahertz waves. In both working conditions, the absorption properties of the metasurface can be affected by the biased graphene.
Possible solution for the sample preparation mainly includes the following steps. First, titanium/gold film are deposited on a solid silicon substrate, then a polyimide layer is spin-coated on the gold film and heated to cure. Next, the pulsed laser deposition(PLD) technology can be used to produce the VO2 film, then the elliptical VO2 resonators are obtained by using photolithography and inductively coupled plasma etching (ICPE) . Then the upper polyimide film is spin-coated, and UV photolithography and magnetron sputtering are performed to form the metal resonators. Next, single-layer graphene is transferred on top of the gold resonators and patterned using e-beam lithography . Finally, the conductive glue is used as electrode when cover the ion gel film.
In Eq. (3), the Fermi level can be determined by changing the carrier concentration via doping, chemical surface modification, voltage or static magnetic field.
On the other hand, the dispersion relationship of the VO2 layer after satisfies the Drude form 
3. Results and discussions
3.1 Tunable absorber for linearly polarized terahertz waves
Using the above metasurface, we obtain selective absorption of linearly and circularly polarized terahertz waves, respectively. First, we show the absorption performance of the absorber under linear polarization working state. When the VO2 is in insulating state (σ=20S/m), the meta-atoms in Fig. 1 can be considered as an achiral structure because it has a mirror symmetry axis. We use finite element method (FEM) to calculate absorption efficiency of the metasurface when the graphene Fermi level is EF = 0.05 eV, where the x- or y-polarized terahertz waves are incident perpendicularly, as shown in Fig. 2(a). The x-polarized wave has two absorption peaks at 2.85 THz and 3.57 THz (point A and point C), while the y-polarized wave shows only a weak absorption peak at 3.25 THz (point B). This is because the absorption of the metasurface is mainly caused by the upper elliptical metal sheet when the Fermi level of graphene is very small. When the incident electric field is along long axis of the ellipse, it first induces a strong electric polarization and forms electric dipole resonance. Then the incident wave enters into the dielectric layer, the bottom metal plate causes destructive interference similar to F-P (Fabry-Perot) resonance, resulting in a large energy loss of the incident wave. From Fig. 2(b), we can see that a strong magnetic field along y axis is formed in the dielectric layer at point C, which means an impedance matching of the entire absorber, leads to the highest absorption efficiency. On the other hand, when the incident electric field is consistent with the short axis of the ellipse, the polarization caused by the metal sheet is weak, and the generated magnetic field in the dielectric layer is also very small. So most of the energy is reflected and the absorption efficiency is very low.
Next, we study the tunable absorption performance for linearly polarized wave. The x and y polarization absorption rates when the Fermi level increases from 0.05 eV to 1 eV are shown in Figs. 2(c) and 2(d), respectively. It can be seen that when the Fermi level increases, the absorption peak of x-polarized wave at point A gradually increases. When the Fermi level is about 0.4 eV, the resonant modes of metal and graphene are coupled and impedance matching is achieved. After that, a slight decrease occurs in absorption. The absorption at point C shows a slow downward trend, perhaps because the increase in the conductivity of graphene disrupted its initial impedance matching. The peak absorption of y-polarized wave (point B) is monotonically increasing. Under this condition, the resonance of the metal sheet is weak. When the Fermi level increases, the absorption caused by graphene gradually increases and plays a leading role. When EF = 1 eV, the absorption rate close to 100% (point D), the inset of Fig. 2(d) shows the magnetic field at two points (yoz plane). In addition, the working frequencies of points A, B, and C all show red shifts when the Fermi level increases.
Next, we show the absorption performance of the absorber for obliquely incident terahertz waves. The incident angles in xoz and yoz planes are increased from 0 to 80 degrees, respectively, while the Fermi level of graphene is 1 eV. In Fig. 3(a), a x-polarized incident wave increasing the incident angle in xoz plane is equivalent to TM polarization. At small angles such as 0-20 degrees, the resonance does not change significantly. The resonance itself is over-coupled when EF is 1 eV, while the angle continues to increase to a larger value, the electric field along the horizontal plane will decrease sharply. Then it is closer to the optimal resonance condition, so that the absorption rate increasing obviously. In Fig. 3(b), TE polarization is obtained when a y-polarized beam changing the incident angle in xoz plane. Similar to TM wave, the maximum absorption does not change quickly in the range of about 20 degrees, but the width of the absorption spectrum decreases sharply at the subsequent incident angle and a significant red shift occurs. However, the absorption peak does not change significantly during the whole process. This is because the electric field is always parallel to the upper surface of the metasurface, and the absorption caused by the isotropic graphene hole plays a leading role, so the absorption peak is not sensitive to the incident angle.In Fig. 3(c), the absorption is more sensitive to incident angle when the x-polarized wave is incident obliquely in yoz plane. When it is greater than 40 degrees, the main absorption peak drops sharply and new resonance peaks appear. At this time, TE polarized wave with large incident angles may excite high-order resonance modes. The situation in Fig. 3(d) is equivalent to TM polarization, so the change of the absorption spectrum is similar to Fig. 3(a). The absorption interval shows a small broadening, the reduction in absorption efficiency is also relatively slow because the absorption is mainly caused by graphene.
On the other hand, machining error is unavoidable in the actual sample processing. In order to ensure the feasibility of our proposed scheme, the influence of processing error on the absorption performance is analyzed. we believe that the links most likely to cause errors may be the etching of the graphene hole and the spin coating of the polyimide films. Without loss of generality, we simulated the metasurfaces with different hole diameters of graphene and thicknesses of the upper polyimide film and obtain the absorption performance of the absorbers. Figure 4 shows the effect of hole radius on absorption performance. As has been already analyzed, when the graphene Fermi level is 0.05 eV, the absorption of x-polarized wave is mainly caused by the metal resonators, so the radius change has little effect on it in Fig. 4 (a). However, when the Fermi level is 1 eV, the absorption of x-polarized waves is mainly caused by the coupling resonance between the metal resonators and the single-layer graphene. Therefore, the absorption performance at this time is more sensitive to the radius, as shown in Fig. 4(c). On the other hand, the absorption of the y-polarized wave by the absorber is mainly caused by graphene, so it is very sensitive to the Fermi level. But the change of the radius has little effect on the absorption performance, as shown in Figs. 4(b) and 4(d). Figure 5 shows the influence caused by the thickness of upper polyimide layer on absorption performance. We can find that whether it is x- or y-polarized waves, including high or low Fermi levels, the effect of thickness H1 is mainly manifested as frequency shift. The absorption efficiency does not change significantly.
3.2 Tunable absorber for circularly polarized terahertz waves
Next, we analyze the circular polarization absorption of the metamaterial absorber. When the VO2 layer is in metallic state (σ=200000S/m), the upper and lower elliptical plates with an included angle of 45 degrees form a chiral meta-atom, and the absorption rate for two circularly polarized incident waves (LCP and RCP waves) will be significantly different. When the Fermi level of graphene is 0.05 eV, the four reflection coefficients are shown in Fig. 6(a). The co-polarization reflection coefficient of LCP (rLL) has a low trough close to zero at 3.1 THz, while the RCP component (rRR) is about 0.7 at this time (the rest is mainly consumed by the intrinsic absorption of graphene). The absorption rate and circular dichroism (CD, absorption difference) of the two circular polarizations are shown in Fig. 6(b), in which the peak value reaches about 0.45. Where the CD is calculated as
The mechanism for the selective absorption of circular polarized waves can be explained by the electromagnetic field distributions at the absorption peak, as shown in Figs. 6(c) and 6(d). From the field in xoy (z=40.2 μm) plane, we can know that the difference in resonance is mainly caused by the elliptical layer of VO2. The upper metal resonator generates a strong polarized electric field and magnetic field for both LCP and RCP incident waves, resulting in broad spectrum absorption without polarization selectivity in Fig. 6(b). On the other hand, the VO2 elliptical sheet generates strong electric and magnetic fields only when LCP wave is incident, the local electromagnetic field excited by RCP wave is relatively weak.
We have also studied the tunable absorption performance of the metasurface for circular polarization. As shown in Fig. 7(a), when the Fermi level of graphene increases from 0.05 eV to 1 eV, the difference in absorption rate (ACD) decreases from 0.46 to about 0.06, and the peak frequency shows a slight red shift. The specific peak absorption and frequency shift are shown in Fig. 7(b). The modulation efficiency for CD can be calculated as η=(Amax- Amin)/Amax=(0.46-0.063)/0.46 = 86.3%.
Next, we also study the circular polarization absorption performance in the case of oblique incidence. The difference in absorption in xoz (yoz) plane is shown in Fig. 8, where the Fermi level is 0.05 eV. It can be found that the absorber is more sensitive to the incident angle under circular polarization. The absorption peak splits at about 20 degrees, and there are multiple peaks in the absorption spectrum when the incident angle is continued to increase. This may be due to the complex high-order resonance mode generated by the asymmetric structure when metasurface irradiated by a large angle incident wave.
3.3 Polarization multiplexed near-field image display
As an exploration of the application for above-mentioned absorber, we show its application in the polarization multiplexed near field image display here. Without loss of generality, we only show the working state of linear polarization, and the circular polarization situation has been reported in our previous work . We first sample the Tai Chi pattern in Fig. 9(a) into 30×30 pixels, and then place the two types of units in the white and black areas of the picture, as shown in Fig. 9(b). From the results in Fig. 2, we can see that when the Fermi level of graphene is 0.05 eV, the linear polarization absorber has a great difference in the absorption of x and y polarized waves at point C (3.57 THz), where the x polarized wave is almost perfectly absorbed but y polarized wave is only absorbed less than 30%. In this way, when we combine the two kinds of units in Fig. 9(b), the metasurface will show obvious reflection differences in the black and white areas. When the Fermi level is 1 eV, the absorption difference between x- and y-polarized waves at point C for the two kind of units is sharply reduced, so the near-field display pattern of the will become blurred. To verify this function, we simulated the linearly polarized terahertz responses of the entire metasurface using the finite element method, and set the graphene Fermi levels to be 0.05 eV and 1 eV, respectively. The simulation results are shown in Figs. 9 (c) and (d). The working frequency is 3.57 THz, and the observation distance is 20 μm from the upper surface of the absorber. It can be seen that when the Fermi level is 0.05 eV, the x and y polarized waves show complementary images, and we define it as the “ON” state. When EF = 1 eV, the patterns under both polarizations almost disappeared, and we define it as the “OFF” state. These simulation results are consistent with our design. Due to the limited number of elements in the metasurface, the image can only be observed at a very close position. If the number of elements is increased, the image can be observed further away.
Finally, in order to further demonstrate the innovation of this work, we compare the absorption performance of the proposed tunable absorber with related reports. As shown in Table 1, we compared several typical reports on terahertz absorbers, including absorbers for circularly or linearly polarized waves. We mainly focus on the working band, absorption bandwidth and working mode. In general, although our proposed metasurface absorption bandwidth is not the widest, it is wider than most reports, and the peak absorption is also large. Most importantly, there are almost no absorbers in these reports that can work in two polarization states which are independently tunable.
In summary, we have shown a new type of tunable terahertz absorber based on anisotropic metasurface, which has polarization-dependent high-efficiency absorption performance for both linearly and circularly polarized terahertz waves. We use the insulator-metal phase transition of VO2 to realize the state switching of the metasurface absorber, and then dynamically tune the absorption efficiency of the absorber via a single-layer graphene containing circular holes. We calculate the tunable absorption efficiency of the absorber in two working conditions and analyze the absorption mechanism. The highest absorption efficiency in both states is close to 100%, and the tunable efficiency of circular dichroism in the case of circular polarization absorption reaches 86.3%. We also study the absorption performance of the metasurface under oblique incidence, the results show that high-efficiency absorption can be achieved in a wide range of angles. Finally, we applied the proposed metamaterial absorber to near-field image display and achieved polarization multiplexing and dynamic tuning of the image. These results provide new ideas for the design of new multifunctional terahertz absorption devices.
Basic Research Program of Shenzhen (JCYJ20170412154447469); National Key Research and Development Program of China (2017YFA0700202); National Natural Science Foundation of China (61675147, 61735010, 91838301).
The authors declare no conflicts of interest.
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