Integrating tunable characteristics and multiple functions into a single metasurface has become a new scientific and technological undertaking that needs to deal with huge challenges, especially in the terahertz frequency region. The multifunctional design combining the broadband absorption and broadband polarization conversion using a single switchable metasurface is proposed in this paper. The switchable performance can be realized by treating the insulation to metal phase transition properties of vanadium dioxide (VO2). At high temperature (74 °C), the proposed metasurface can be used as a broadband absorber which consists of a VO2 square ring, polyimide (PI) spacer, and VO2 film. Simulated results show that the terahertz wave absorption can reach above 90% with the bandwidth ratio of 75% in the frequency range of 0.74 THz-1.62 THz. This absorber is insensitive to polarization resulted from the symmetry structure and also shows a good performance at large incident angles. Once the temperature is lower than the cooling phase transition temperature (about 62 °C) and VO2 is in insulation state, the metasurface can be transformed into a broadband linear-to-circular polarization converter. Numerical simulation depicts that the ellipticity reaches to -1 and the axis ratio is lower than 3 dB from 1.47 THz to 2.27 THz. The designed switchable metasurface provides the potential to be used in the fields of advanced research and intelligent applications in the terahertz frequency region.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Terahertz wave manipulations have attracted more and more attention due to their broad application potential in the fields of wireless communication, biomedical imaging and radar stealth . The outlook and development plan of 6G communications has been put on the agenda to lay the foundation for the new generation of communication systems to meet the future needs of the 2030s . The critical problem influencing the practical application of the hybrid terahertz/free-space-optical systems integrated in 6G frequency band is the shortage of functional devices with outstanding characteristics. This is mainly due to the shortage of the materials in nature that can directly interact with terahertz wave. In order to solve this problem, metasurfaces, artificially designed electromagnetic composite material, have demonstrated great application prospects in the meta-lens [3–5], perfect absorbers [6–8], and polarization converters [9–11]. Typical metasurfaces consisting of metal or dielectric element are fundamentally different from conventional optical components since the size of the structure is much smaller than the operating wavelength. The behaviors of previously reported metasurfaces are inconvenient to vary once they are manufactured. Recently, many studies about the design of active metasurfaces have been carried out ranging from microwave to optical region. It is important to research switchable metasurfaces for manipulating amplitude, phase, or polarization so as to design active devices, such as filters, modulators, and sensors. As is well known, two shortcomings of terahertz metasurface devices actively discourage actual applications, i.e., unchangeable working frequency range and only one function at the operation frequency.
To date, most of the metasurface structures are mainly designed to perform a single function. Even though various metasurface-based terahertz absorbers and polarization converters have been reported previously, these structures provide similar performances. C. R. D. Galarreta et al.  combined the phase-change material with optical metasurface to form a novel, nonvolatile, reconfigurable near-infrared beam manipulation device. S. G. C. Carrillo et al.  reported the combination of chalcogenide material and absorber to realize the nonvolatile color generating abilities. H. Liu et al.  demonstrated a broadband switchable terahertz absorber on the basis of hybrid vanadium dioxide (VO2) metasurfaces. J. N. Li et al.  treated the insulator-to-metal transition of VO2 to construct a dual broadband terahertz metamaterial absorber with continuous tunability. L. L. Zhang et al.  presented a tunable and triple-band reflective polarization structure converting a linearly polarized mid-infrared wave to its cross-polarized wave based on a periodic ellipse-type graphene patch with a slit. Facilitating the integration of multiple functions into one metasurface device is very desirable.
To realize multiple functions, an alternative method is to integrate metasurfaces with active functional materials, such as phase-change materials, because the characteristics of the phase change materials can be modulated by applying external excitation fields. Thus, the reconfigurable properties can be achieved by constructing the switchable hybrid metasurface devices with graphene , liquid crystal , black phosphorus , perovskite , and VO2. As an important phase change material, VO2 provides the merits of fast response rate, large modulation depth, and various modulation approaches including laser pumping, temperature control and external electric fields . The dramatic variation of the optical and electrical characteristics during the phase transition is resulted from the transformation of the structural properties from an insulation phase state (low temperature) to the metal phase state (high temperature) around 68 °C . Recently, VO2 has been widely researched owing to the large variation of the dielectric constant when the phase state of the VO2 changes from insulation state to metal state . Thus, many novel photonic devices can be designed and fabricated based on the combination of metasurface structures and the large variation of the VO2 dielectric permittivity, such as metasurfaces, optical memory devices, nano-antennas, temperature sensors, and rewritable devices. While phase change material combined with metasurfaces have been previously reported to realize the active design control [21,23–24], metasurfaces providing many different functions in a wide terahertz frequency region have not been fully studied to a large extent.
In this paper, a switchable terahertz metasurface structure with multiple functions based on VO2 is presented. This multifunctional device can realize the transform between the broadband absorber and linear-to-circular polarization converter in terahertz frequency band by treating the insulator-to-metal transition characteristic of VO2. The simulation results show that the multifunctional device can absorb the terahertz wave at high temperature (>68 °C) caused by the metal phase of VO2. And the high absorption is above 90% from 0.74 THz to 1.62 THz. As the temperature decreases, VO2 gradually changes from metal to insulation phase, and the entire structure can be regarded as a linear-to-circular polarization converter with ellipticity close to -1 and axis ratio <3 dB from 1.47 THz to 2.27 THz. Moreover, when the converter is illuminated by transverse electric (TE) and transverse magnetic (TM) polarized incident waves, the reflected waves show the characteristics of right-hand circular polarization and left-hand circular polarization, respectively. Besides the excellent performance at normal incidence, the broadband characteristics of both absorber and converter are analyzed and maintained in a wide range of incident angles. The proposed VO2-based multifunctional metasurface provides a great number of advantages, such as multiple functions and ease of applying to other frequencies.
2. Design and method
As depicted in Fig. 1, the basic unit cell of the proposed tunable multifunctional metasurface is composed of six layers, which from top to bottom are as follows: VO2 square ring, polyimide (PI) spacer, gold strip, VO2 film, PI spacer, and the bottom gold background. In the simulation, a dielectric constant εr = 3.5 and loss tangent δ = 0.0027 are set for the chosen PI material. Considering the terahertz region, the insulator-to-metal transition properties of VO2 can be described by Bruggeman effective medium theory . In this case, the dielectric function εC can be described as [25,26]:
3. Results and discussions
On the basis of the distinct insulator to metal transition characteristics of VO2 material, the designed structure can be treated as a liner-to-circular polarization converter at low temperatures (< 64 °C) caused by the insulation phase of VO2. On the contrary, when the characteristics of VO2 transforms from insulation to metal phase at high temperatures (> 68 °C), the VO2 square ring and VO2 film can be used as the resonant and reflective elements to construct a terahertz absorber. Therefore, when an external temperature field is applied, the terahertz absorption characteristics and polarization conversion performance can be effectively modulated in real time with the help of VO2 metasurfaces.
3.1 Realizing the broadband absorption when VO2 is metal
As illustrated in Fig. 1, the proposed tunable metasurface can be regarded as a representative configuration of terahertz absorber when VO2 is metal. Here, the terahertz metasurface absorber consists of top VO2 square ring, first PI spacer layer, and bottom VO2 film. The geometric parameters shown in Fig. 1 are optimized to perform a broadband absorption by treating the finite element method. The absorption (A) of the absorber can be expressed as A = 1-R-T-R⊥=1-|S11|2-|S21|2-R⊥, where R = |S11|2 (T = |S21|2) is the reflection (transmission). Here, the reflection R⊥ of cross-polarized wave should also be considered. It is worth mentioning that compared with many previously reported terahertz metasurface devices combined with different materials in a single design, the designed metamaterial device structure is easy to be fabricated since the microstructure only use the VO2 as the phase change material. This will effectively reduce the complicacy of the metasurface structure realization.
The stimulated terahertz absorption of the designed tunable VO2 metasurface absorber at different temperatures during heating process is depicted in Fig. 2(a). The red and purple solid curves show the absorption spectra at low (54 °C) and high (74 °C) temperatures, respectively. The results show that the terahertz absorption is blow 50% in the entire frequency range at 54 °C, and the absorption will reach above 90% in the range of 0.74 - 1.62 THz at 74 °C. The absorption does not show a significant change with the increase of temperature. When the temperature increases to 74 °C which is higher than the heating point temperature 68 °C, a very high absorption (> 90%) in the the frequency range of 0.74 - 1.62 THz can be obtained. In this frequency range, the bandwidth ratio (fmax-fmin)/[(fmax+fmin)/2] is 75% . Absorption greater than 50% is able to be maintained in the range of 0.48 - 2.01 THz, and the corresponding bandwidth ratio is 123%. In addition, the absorption spectrum of the terahertz absorber using the metal square ring instead of VO2 square ring is also simulated and depicted in Fig. 2(a). In this case, the absorption spectrum shows two discrete narrow absorption peaks around 0.6 THz and 2.02 THz with the corresponding absorption around 47% and 77.8%, respectively. The square ring with metal material in the first layer will cause the narrow bandwidth absorption peaks as demonstrated in our previous work . On the contrary, for the VO2 square ring design with the same geometric parameters, the bandwidth and efficiency of the terahertz wave absorption is greatly enhanced. The broadband and high absorption is mainly caused by the metal-like characteristics of the VO2 square ring. Although the maximum photoconductivity of VO2 is up to 105 S/m orders of magnitude, there is still a big difference compared with metal material (107 S/m). Therefore, the absorption characteristics are different when patterned VO2 without metal and metal are used as a terahertz absorber , respectively. The terahertz absorption spectra for the cooling process are calculated and depicted in Fig. 2(b), too. The results show that the terahertz absorption does not decrease significantly with decrease of temperature. This reveals that the absorption characteristics of the cooling process is less sensitive to the decrease of temperature. This interesting result is mainly caused by the hysteresis effects of insulator to metal transition characteristics. The VO2 materials gradually change from insulation to metal phase with the increase of temperature, so the transmission of terahertz wave can not transmit through the VO2 film ground plane. Thus, the square ring resonant microstructure set in the top surface of the designed device still experts the effect and the incident terahertz waves are gradually coupled into the metal-like VO2 structure, leading to the efficient increase of the absorption. When the temperature rises to 74 °C, VO2 fully transforms into the metallic phase with a conductivity only two orders of magnitude lower the conductively of gold. Therefore, the incident terahertz wave is almost impossible to penetrate the VO2 film. And the square ring microstructure provides the maximum resonance response, so that the reflection of the absorber is also greatly reduced, resulting in the strong absorptive capacity of the designed metasurface absorber. On the other hand, the conductivity of the VO2 film and square ring cannot achieve the truly reversible conversion and the similar temperature hysteresis loops related to the heating and cooling processes will case the difference in resonance responses, leading to the sensitivity of the devices to the external thermal field .
In order to better analyze the designed absorber characteristics, the effects of geometric parameters are studied and shown in Fig. 3. The results depicted in Fig. 3(a) reveals that the amplitude of the absorption augments with the increase of b2, which the bandwidth deceases with the increase of b2. The absorption can reach above 99% in the range of 1.12 - 1.35 THz when b2 = 40.5 μm. In Fig. 3(b), the amplitude of absorption illustrates a slight increase when the frequency region is below the central frequency (1.23 THz) and depicts a slight decrease when the frequency region is above the central frequency with the increase of h3. The whole intensity shows a slight red-shift with the increase of h3. When the width of square ring b2 is 38.5 μm and the thickness h3 of PI dielectric spacer is 32 μm, a better flat absorption spectrum is achieved.
The relationships between the absorption and the polarization angle and incident angle of the incident linear terahertz wave are also studied. Figure 4(a) depicts the color map of the absorption with different polarization angles ranging from 0° to 90° with a step of 5°. The results show that the absorption is totally insensitive to the polarization when the incident angle of the terahertz wave is 0. The symmetrical property of the designed structure guarantees the polarization-insensitive characteristics, which can be widely used in various practical fields. Figure 4(b) presents the relationship between the absorption and the frequency with different incident angles. The incident terahertz wave is TE-polarized wave and the electric field is perpendicular to the incident plane all the time. The proposed absorber achieves remarkable characteristics with stable absorption (greater than 80%) and bandwidth for TE polarized wave when the incident angle changes from 0° to 55°. The absorption begins to weaken when the terahertz wave is incident at an angle greater than 55°. These results demonstrate that the proposed metasurface absorber shows a better absorption performance at larger incident angles. The performances related to the incident angle and polarization angle will lead to many potential applications in energy measurement and optical sensing.
3.3 Realizing broadband linear-to-circular polarization conversion when VO2 is insulator
When VO2 is in the insulation state, only the bottom structure consisting of symmetrical double L-shaped gold strips, PI dielectric layer and gold background works. Here, this structure design can achieve low absorptive capacity and wideband linear-to-circular polarization conversion capacity. The reflected wave can be described as Er = Exrex+Eyrex = rMEexp(jφxy)Eyiex+rEEexp(jφyy)Eyiey , where rME and rEE respectively define the reflection coefficient of TE-to-TM and TE-to-TE (y-to-x and y-to-y) polarization conversion, and φME and φEE denote the corresponding phases. The amplitude and phase of Exr and Eyr may vary with the anisotropy characteristics of the metasurface. If rME = rEE and Δφ = φME-φEE = 2mπ±π/2 (m is a whole number), the perfect linear-to-circular polarization conversion can be achieved, and “-” and “+” respectively denote the left-hand circular polarization and right-hand circular polarization [31,32]. In order to obtain the efficient polarization conversion in the design, the reflection coefficients should be at the highest level they can. As shown in Figs. 5(a) and (b), in the frequency range of 1.47 - 2.27 THz, the reflective coefficient shows the approximate same strength, and phase difference is about 90° or -270°. Based on the above analysis, the reflected wave can be recognized as a right-hand circular polarization wave. The normalized ellipticity of E = 2|rME||rEE|sin(Δφ)/(|rME|2+|rEE|2) is also used to describe the effects of the polarization converter. It is particularly indicating that the reflected wave is a right-hand circular polarization wave with E = -1 and a left-hand circular polarization wave with E = +1, respectively. The variation of the ellipticity with operation terahertz frequency is shown in Fig. 5(c). The result shows that the ellipticity is almost equal to -1 within the frequency range of 1.47 THz to 2.27, furtherly confirming that the reflected wave can be regarded as the right-hand circular polarization wave. It should be noted that when the incident wave is TM polarization, the reflected wave still demonstrates the circularly polarized characteristics with almost the same bandwidth but reverse rotation. The axis ratio AR = 10log(tan(0.5×arcsin(2|rME||rEE|sin(Δφ)/(|rME|2+|rEE|2)))) is introduced to verify the circular polarization capability . As illustrated in Fig. 5(d), the axis ratio is lower than 3 dB from 1.47 THz to 2.27 THz, demonstrating that the designed converter achieves good performance in linear-to-circular polarization conversion.
The relationship between the simulated ellipticity and axis ratio of the designed VO2-assisted metasurface converter and the various temperatures during the heating process is illustrated in Fig. 6(a). When the temperature is below 66 °C, the ellipticity shows a slight change and close to -1 in the range of 1.47 THz to 2.27 THz, and the corresponding axis ratio is under 3 dB in the same range. This demonstrates that the proposed device can be used as linear-to-circular converter when the VO2 is under insulator state. When the temperature increases to 70 °C, the characteristic of the VO2 film and square ring changes from insulator to metal state, the ellipticity changes from -1 to 0, with the corresponding axis ratio increases significantly. Thus, when the temperature is above metal state temperature (68 °C), the device can be used as a broadband terahertz metasurface absorber, as shown in Fig. 1. In addition, the ellipticity and axis ratio of the device change with the variation of temperature during the cooling process is depicted in Fig. 6(b). Similar to Fig. 2(b), the ellipticity and axis ratio are not reduced significantly, indicating that the decrease of the temperature has little effect on the cooling process.
The relations between ellipticity and structure parameters (h2 and l) are also investigated and discussed at normal incidence. Figure 7(a) depicts the effect of the lower PI dielectric thickness h2 on the ellipticity with other parameters unchanged. The bandwidth will become narrower and shows a slight red-shift with the thickness h2 increasing from 14 μm to 18 μm. And the intensity shows a slight increase. As one of the important geometric parameters affecting the performances of the polarization conversion, the impact of the arm’s length l of the symmetric L-shaped gold strip on the ellipticity is calculated and discussed. The ellipticity curves as a function of frequency with different l are illustrated in Fig. 7(b). When the l increases from 26 μm to 30 μm, the bandwidth of the ellipticity is broadened gradually and the intensity changes slightly.
The influences of polarization and incident angle on the performance of the linear-to-circular polarization conversion are also investigated. The results in Figs. 8(a) and (b) are ellipticity and axis ratio as a function of incident angle (θ) for incident TE-polarized terahertz wave and frequency (f) when φ = 0°. As illustrated in Figs. 8(a) and (b), the designed converter can achieve the excellent linear-to-circular polarization operation in a broad bandwidth when the incident angle changes from 0° to about 50°. The lowest intensity of ellipticity and axis ratio is mainly resulted from the diffraction which is caused by the two PI spacer in the design structure. This phenomenon is also obviously depicted in Fig. 4(b). Due to the characteristics of the double symmetrical L-shaped metal-type resonant structure, the converter proposed here shows less sensitive to the polarization of the incident terahertz wave. As depicted in Figs. 8(c) and (d), when the incident terahertz wave is TM-polarized terahertz wave (φ = 90°), the incident polarized terahertz wave in the region of 1.47 THz to 2.27 THz can be converted into left-hand circular polarization wave (E = 1) with the axis ratio below 3 dB. Only one narrowband circular polarized wave can be observed when the incident angle is above ≥50°.
The recently reported VO2-based experiments have been successfully performed which can provide the practical guidance to our design. And the dielectric material PI can achieve stable properties with different temperature operation . The VO2-based devices sample preparation has been previously reported by researchers [34,35]. First, a thin VO2 film is deposited on a PI substrate by molecular beam epitaxy. Then, the gold structures are fabricated on top of the VO2 film by conventional lithography and metallization. Next, a thick PI is coated on top of gold structure. Finally, another VO2 layer is deposited on the thick PI by molecular beam epitaxy and etched using reactive ion etching and conventional lithography, forming the VO2 square ring. The previously reported work demonstrates that the excitation of the phase transition of VO2 can be easily realized by applying external temperature field. The design approach proposed in this paper opens up new avenues for the research of tunable metasurface structure to perform totally different functions using only one metasurface structure.
The innovation and performance of the proposed VO2 based multifunctional metasurface device has been highlighted by comparison with some of the best results on terahertz broadband absorbers and linear-to-circular converters, as listed in Table 1. The main advantages of our design are in utilization of a simple structure, realizing the efficient absorption and linear-to-circular conversion at the same time.
In conclusion, a multifunctional metasurface structure with tunable properties is proposed by treating the phase transition characteristics of VO2 material. This design can realize the transformation between the broadband absorption and broadband linear-to-circular polarization conversion by changing the operating temperature. At high temperature (74 °C), a broadband absorber based on the VO2 square ring and film is realized in the terahertz frequency range. By optimizing structure sizes, the absorption greater than 90% ranging from 0.74 - 1.62 THz is obtained. The absorption curves are independent of polarization angle of the incident terahertz wave. With the decreasing temperature from 74 °C to 50 °C, the VO2 film and square ring gradually transform from the metal to insulation phase. Thus, the designed structure can be used as a broadband terahertz linear-to-circular polarization converter. The linear polarization terahertz wave can be converted into circular polarized terahertz wave with ellipticity close to -1 and the axis ratio below 3 dB between 1.47 THz and 2.27 THz. The broadband characteristics stay the same when the incident angle reaches to 50°. Based on the above results and discussions, in this design, the broadband absorption and linear-to-circular polarization conversion can be transformed between each other by exciting the insulator-metal transition of VO2. This structure can enable advanced applications, for example, fronthaul of 6G communication system, stealth technology, temperature-controlled metasurface and other fields.
National Natural Science Foundation of China (62001444, 61831012, 61871355); Natural Science Foundation of Zhejiang Province (LQ20F010009, LY18F010016); State Key Laboratory of Crystal Materials Shandong University (KF1909).
The authors declare no conflicts of interest.
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