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Abstract
Various kinds of metasurfaces have been proposed because they can be tailored to achieve the desired modulations on electromagnetic wave that do not occur in nature. Compared to conventional metamaterials, coding metasurfaces integrated with information science theory possess numerous distinctive advantages - simple design, time-saving and compatibility with digital devices. Here we propose terahertz multifunctional anisotropic reflective metasurfaces with a metal-insulator-metal cavity structure whose top constructional layer consists of a pair of gold arc-rings and a gold cut-wire located between them. Two different functions of narrow-band absorption and broadband polarization conversion are realized based on different coding matrices using the binary codes ‘0’ and ‘1’. Furthermore, we integrate a specific coding metasurface with vanadium dioxide (VO2) to realize a temperature-controlled active metasurface. Through the temperature change, dynamic functionalities switching between a narrow-band polarization converter with a polarization conversion ratio over 94% and an efficient low-pass filter are achieved under the phase transition of VO2, and the active metasurface is polarization independent. The proposed coding metasurfaces are verified numerically and experimentally, and have promising applications in terahertz modulation and functional devices.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Recently, terahertz radiation has attracted increasing attention due to its unique properties, including low photon energy, almost negligible damage on the detected objects, high signal to noise ratio, wide bandwidth and so on, and all these advantages lay a foundation for its various potential applications such as in 6G communications [1], medical scanning [2–4], security screening [5,6], atmosphere investigation [7,8], etc. However, the development of terahertz technology is lagging far behind compared to the microwave electronic and optical photonic technologies due to the lack of miniature and effective sources, modulators and detectors. Metamaterials composed of subwavelength structures are investigated extensively due to their ability to modulate electromagnetic waves arbitrarily. In the past decades, metamaterials have received wider attention with in-depth understanding of their theoretical principles and application scenarios. Meanwhile, metamaterials have played a great role in the design of functional devices to realize desired electromagnetic properties in the microwave [9–11], near infrared [12] and optical [13] ranges. Inspired by the success of applying micro-nano machining to the microwave electronic and optical photonic technologies, with the rapid development of micro-nano machining processing technology and the elaborately designed metamaterial structures, various terahertz functional devices [14,15] have been fabricated, which are not possible with conventional materials.
As early 3D metamaterials rely on the accumulation of phase delay during wave propagation, inevitably there still exist several disadvantages, such as large thickness, fabrication difficulty, challenge in integration, etc. Metasurfaces, 2D planar metamaterials, are much easier to fabricate with lower cost, where the negligible thickness of ultrathin metasurfaces is believed to be suited for integration with other optical components. It is also worth noting that, based on the abrupt electromagnetic changes, most functionalities realized by 3D metamaterials can be achieved with metasurfaces by tailoring the required phase/amplitude distributions of the transmitted and/or reflected waves. Over the past few years, the development of metasurfaces has experienced from initially single-layer designs to multilayer composite structures, from passive to active manipulation, and from microwave to terahertz frequencies. Various kinds of metasurfaces with different functionalities, including phase modulation [15], plasmon induced transparency [16–18], and wireless power transfer [19], have been proposed and even fabricated in experiments. More recently, Cui et al. proposed the digital version of metasurfaces integrated with information science theory [20], which gives more design alternatives. With the coding metasurfaces, one only needs to focus on the design of the coding sequence, and does not have to worry about the physical properties of the materials or the complicated mutual coupling effects among the adjacent structures. Conventional metasurfaces based on the effective medium theory are complicated and time-consuming for the structure designer. On the contrary, coding metasurfaces have overcome the shortcomings of conventional metasurfaces and possess excellent advantages, such as simple design, time saving, and compatibility with digital devices. The operating frequency ranges of coding metasurfaces have been extended from microwaves to terahertz waves very soon, and improved anisotropic coding [21], programmable [22,23] and digital metasurfaces [20,24,25] were subsequently proposed. Based on the generalized Snell’s Law [26,27], different coding matrices can be employed to realize desired wavefront- manipulating devices and impose modulation on the functions of the coding metasurfacs.
Absorption and polarization conversion are two important areas of research in the terahertz region. Unlike the previously proposed coding metasurfaces based on the phase delays accumulated during light propagation, here in this paper we numerically and experimentally demonstrated several anisotropic terahertz coding metasurfaces based on a three-layer unit cell that is equivalent to a metal-insulator-metal (MIM) cavity, which is expected to realize these two functionalities utilizing different coding matrices. More specifically, two different kinds of efficient single-band absorption and broadband polarization conversion were realized with different coding matrices. We further endow the coding metasurfaces with the characteristic of active manipulations by utilizing the phase transition material vanadium dioxide (VO2), which brings about not only more flexibility but also higher operation efficiency. Switching between a highly efficient single-band absorber and a low-pass filter is realized through changing the ambient temperature. The multi-functional coding metasurfaces and the improved active terahertz metasurface provide more alternatives in molding highly-integrated terahertz modulators, which consequently shows a promising and practical prospect in enriching terahertz functional devices and even promoting the development of terahertz science and technology.
2. Structures and methods
The schematic diagram of the designed reflective anisotropic multilayer coding metasurfaces based on a three-layer unit cell that is equivalent to a MIM cavity is shown in Fig. 1. Gold with a conductivity of 4.56 × 107 S/m is employed in the top constructional layer and bottom reflection layer because of its extremely high plasmon frequency (1.37×1016 rad/s), determined by the Drude model. The top layer unit cell is composed of a pair of gold arc-rings and a gold cut-wire located between them. Polyimide with a relative permittivity ɛ = 3.23 and loss tangent δ = 0.044 is employed in the middle dielectric layer for its relatively small loss and excellent chemical stability at terahertz frequencies. The period of the unit cell is p = 100 µm, the inside and outside radii of the arc-rings are r1 = 20 µm and r2 = 40 µm, respectively, both arc angles of the symmetric arc-rings are 90°, and the length and width of the cut-wire are l = 95 µm and w = 20 µm, respectively, as shown in Figs. 1(a) and 1(b). The thicknesses of the top constructional and bottom reflection layers, and of the polyimide dielectric layer are set to be t1 = 0.2 µm and t2 = 25 µm, respectively. Different 2-bit binary codes based on unit cells with different orientations according to their inhibition effects on polarization conversion are arranged (see Figs. 1(c) and 1(d)). Among them, the elements with a deflected angle of 45°/135° achieve the most effective polarization conversion, while their conversion effect would be inhibited due to the mutual coupling with other adjacent elements with different deflected angles. The more effective polarization conversion in the operation frequency band corresponds to larger 2-bit binary codes. The spectra of the polarization conversion ratio shown in Fig. 1(d) are the normalized amplitudes of the received y-polarized THz wave under x-polarized normal incidence. Here, the minimum repeated cell of our proposed anisotropic coding metasurfaces is named as the ‘super unit cell’ (or simply a ‘supercell’). The size of the supercell is set to be 2×2 that provides possibilities to investigate various functionalities and reduce the computation burden as well. On the basis of the elaborately designed anisotropic unit cells, we proposed several 2-bit coding metasurfaces based on different super unit cells to realize diverse functionalities including two different types of narrow-band absorptions and a broadband polarization conversion.
Fig. 1. Illustration of the anisotropic unit cells and the corresponding binary codes. (a) and (b) Schematic diagram of a unit cell and its detailed geometrical parameters: p = 100 µm, t1 = 0.2 µm, t2 = 25 µm, w = 20 µm, l = 95 µm, r1 = 20 µm, and r2 = 40 µm. (c) Unit cells with different orientations and their corresponding binary codes where the binary code before/after the slash represents the coding number of the corresponding unit cell under x/y polarized terahertz wave incidence. (d) The inhibition effects of the unit cells with different binary codes on polarization conversion.
2.1 Anisotropic coding metasurfaces
To demonstrate the possibility for multiple-functionalities based on our proposed design, we encode the metasurfaces with different supercells, that is, different coding matrices. The commercial software CST Microwave Studio is employed to design and optimize the coding metasurfaces, and unit cell boundary conditions along the x/y directions and open boundary condition along the z direction are used in the simulations. In order for far-field detection, the Floquet port is employed to set the distance between the incidence port and the metasurfaces to greater than two wavelengths. One of the popular applications of metasurfaces is as absorbers, in which electromagnetic waves are localized in the top constructional layer through strong resonance coupling and finally absorbed by the middle dielectric layer. The absorptivity is derived from [28]:
For the metasurface with a coding matrix $M_1^{2 - bit}$ whose coding pattern is shown as Fig. 2(a), the simulation results demonstrate that under x-polarized normal incidence there is a narrow-band absorption around 0.86 THz with a peak absorptivity of more than 93% (see Fig. 3(a)). Theoretically, we can adjust the coding matrix of the supercell to achieve the same absorption effect under y-polarized incidence, and that is also confirmed by simulation. This functionality is realized by encoding the metasurface with the following coding matrix:
Secondly, for the metasurface with a coding matrix $M_2^{2 - bit}$ whose coding pattern is shown as Fig. 2(b), the results show that there is a nearly perfect narrow-band absorption at a center frequency of fc = 0.88 THz under y-polarized incidence, which has a bandwidth of over 0.03 THz. Furthermore, the bandwidth of the narrow band absorptivity exceeding 90% could reach beyond 0.07 THz, as shown in Fig. 3(b). This functionality is realized by encoding the metasurface with the following coding matrix:
Fig. 2. Coding patterns of the 2-bit coding metasurfaces with different coding matrices (a)$M_1^{2 - bit}$ (b)$M_2^{2 - bit}$ (c)$M_3^{2 - bit}$, and (d)$M_4^{2 - bit}$.
Fig. 3. (a) The absorption efficiency Ax of the anisotropic coding metasurface with a coding matrix $M_1^{2 - bit}$ under x-polarized terahertz wave incidence. (b)The absorption efficiency Ay of the anisotropic coding metasurface with a coding matrix $M_2^{2 - bit}$ under y-polarized terahertz wave incidence. (c) The PCR of the anisotropic coding metasurface with a coding matrix $M_3^{2 - bit}$ under x-polarized terahertz wave incidence.
Another successful application of metasurfaces is as polarization converter, and it has been demonstrated that oblique split-ring resonators [10,29] and cut-wire resonators [30] are capable of rotating a linear polarization to its orthogonal direction with low loss. The polarization conversion ratio (PCR) is derived from:
where ${R_x}/{I_x}$ describes the reflection/incidence field amplitude of the x-polarized terahertz waves, and ${R_y}/{I_y}$ represents the reflection/incidence field amplitude of the y-polarized terahertz waves. For the metasurface with a coding matrix $M_3^{2 - bit}$, a broadband polarization conversion (0.76–1.25 THz) with a more than 64% PCR is realized, and the peak can reach up to more than 76.3%, as shown in Fig. 3(c). Here the functionality of polarization conversion on linearly-polarized incident terahertz waves is polarization-independent because the coding pattern is isotropic for linear polarization, and the coding matrix $M_3^{2 - bit}$ is written as follows:2.2 Active metasurface based on VO2
The functionalities of passive metasurfaces are unchangeable as long as their basic structures are fixed, which restrict their use in practical applications. Therefore, various kinds of methods have been proposed to actively manipulate the incident wavefront in the past few decades, including electrical [31–33], optical [34–37] and thermal controls [28,38–41]. Among them, thermally tunable metasurfaces integrated with VO2 films [42] have been widely investigated because the phase transition temperature Tc of VO2 is 68 °C (341 K) which is not a demanding condition. When the ambient temperature is 23°C, the transmission coefficient of the VO2 film is 0.9995 and the conductivity is 10 S/m, indicating that its physical state is in the insulating monoclinic crystal structure, functioning like a transparent dielectric insulator at terahertz frequencies. As the temperature increases and exceeds the phase transition temperature Tc, the transmission coefficient of the VO2 film attenuates to 0.2135 and the conductivity rises to 2.6 × 105 S/m, indicating that its physical state is in the metallic tetragonal system structure, functioning like a metal with high conductivity. Thus, VO2 films have great potential applications in tuning and amplitude modulation scenarios in electronics and photonics for its reversible phase transition effect at such a low phase transition temperature.
To endow the coding metasurface with more flexibility and the ability of active manipulation, here we extract a specific coding supercell with a coding matrix $M_4^{2 - bit}$ whose coding pattern is shown as Fig. 2(d), and integrate the supercell with VO2 to realize an active multifunctional coding metasurface. Its structure is sketched as Fig. 4(a), and the corresponding coding matrix $M_4^{2 - bit}$ is written as follows:
Similarly, CST Microwave Studio is employed to design and optimize the active metasurface. A plane wave excitation source with x/y polarization is used in the simulations, and a field probe is set to function as the terahertz wave receiver. The distance between the filed probe and the metasurface is set to be more than two wavelengths for far-field detection. The relative permittivities of the materials applied in simulation are listed as follows: ɛVO2 = 9 and ɛSapphire = 9.67. We adjust the conductivity of VO2 to simulate the variation of temperature in the experiments, i.e. 20 S/m and $2.6 \times {10^5}$ S/m in the fully insulating and metallic states corresponding to the temperatures of 298 K (room temperature) and 358 K.
Fig. 4. (a) Illustration of the active metasurface supercell derived from the coding metasurface with a coding matrix $M_4^{2 - bit}$. The thickness of the sapphire layer is h = 2 mm. (b) The PCR of the active metasurface when VO2 stays in the dielectric state (σ = 10 S/m). (c) The absorption efficiency of the active metasurface when VO2 stays in the metallic state (σ = 2.6×106 S/m).
In the simulation results shown in Fig. 4, when VO2 stays in the fully insulating state (10 S/m), the active metasurface integrated with VO2 functions as an effective narrow-band polarization converter with the PCR peak exceeding 94% at 0.53 THz. With the increase of conductivity, the PCR of the active metasurface gets smaller until almost negligible. As the VO2 film transforms into the fully metallic state ($2.6 \times {10^5}$ S/m), the active metasurface acts as an effective low-pass filter, that is, for incident terahertz waves over 1.12 THz, the absorption reaches more than 80%, and becomes even perfect at 1.27 THz, while most incident waves at lower frequencies reflect back with lower loss. Note that the designed active metasurface is polarization independent for normally incident linearly-polarized terahertz waves, as the supercell is isotropic for linear polarization, which has also been verified in simulations and experiments.
3. Results and discussion
To experimentally verify the performance of the designed metasurfaces, we fabricated the above-mentioned metasurface samples using evaporation, photolithography and other micro/nano processing technologies, and the optical images of the fabricated samples are shown as Figs. 5(a)–5(d). A reflection-type terahertz time-domain spectroscopy system was employed to characterize the samples and verify their performances, and the schematic diagram of the experimental set-up is shown in Fig. 5(e). In order to reduce the strong absorption of air moisture on terahertz waves, the ambient humidity was reduced to below 3.0%.
Fig. 5. The optical microscopy images of the fabricated samples with coding matrices (a)$M_1^{2 - bit}$, (b)$M_2^{2 - bit}$,(c)$M_3^{2 - bit}$, and (d)$M_4^{2 - bit}$. (e) Schematic diagram of the reflection-type terahertz time-domain spectroscopy system employed in this work.
Here we briefly describe the fabrication process. What needs to be made clear in advance is that the first three coding metasurfaces and the improved active metasurface have different fabrication procedures. Firstly, for the three basic coding metasurfaces, the insulator layer between the two golden layers was a 25 µm-thick polymide sheet fabricated by a spin coater, and a 0.2 µm-thick gold reflection layer was deposited on the polymide sheet by electron beam evaporation, and thereafter another 0.2 µm-thick gold layer was carved into the desired pattern using photolithography after being deposited on the other surface by evaporation. For the active metasurface, a 200 nm-thick VO2 film was first deposited on a 2 mm-thick sapphire substrate, and the constructional patterns of VO2 and gold were etched and grown separately based on laser direct writing technology. At the same time, a 25 µm-thick polyimide film was fabricated on a silicon wafer via spin coating and curing, then a 10 nm-thick titanium layer and a 200 nm-thick gold layer were successively deposited onto the polyimide film by evaporation, Here the titanium layer was employed to enhance the adhesion between the gold reflection layer and polymide dielectric layer. So far, we had fabricated the constructional and reflection layers. The sample fabrication was finally completed after bonding the constructional and reflection layers together. All the above metasurface samples containing 50×50 supercells are of 1 cm × 1 cm, much larger than the incident terahertz spot size, thus making reasonable the use of infinite periodic boundary conditions in the simulations.
The experimentally obtained absorption efficiencies and PCR values of the fabricated samples are provided in Fig. 6, and the measured results agree well with the simulations. Now we discuss the experimental results for these samples. Firstly, for the coding metasurface with the coding matrix $M_1^{2 - bit}$, the peak absorptivity of the narrow-band absorption can reach 83% at 0.83 THz. For the coding metasurface with the coding matrix $M_2^{2 - bit}$, a narrow-band absorption with an average efficiency of more than 72% is realized, and the corresponding frequency range shows a small blue shift. As for the last coding metasurface with the coding matrix $M_3^{2 - bit}$, the PCR peak can reach 71% at 0.80 THz, and the bandwidth can reach 0.67 THz with a PCR over 34%. For the active metasurface with the coding matrix $M_4^{2 - bit}$, the PCR peak can reach up to 87% at 0.52 THz when the VO2 film stays in the insulator phase with a conductivity of 10 S/m, as shown in Fig. 6(d). After that, with an electronic thermostat, we heated the temperature of the sample to more than 80 °C at which the phase transition of VO2 occurs, and the VO2 film transformed into the fully metallic state with a conductivity of $2.6{\; } \times {\; }{10^6}{\; }$S/m. At this time, the active metasurface acts as a low-pass filter to absorb over 73% for incident waves above 1.2 THz, while the absorption at 1.25 THz can reach over 96%. Here, slight reductions occured in the efficiency of the metasurfaces with different functionalities, while the red/blue shifts of the center operation frequencies are negligible, which provides a promising prospect for practical application in the future. In addition, increasing the size of the supercell may be a promising way to improve the efficiency of the polarization conversion and/or absorption, broaden the operation bandwidth, and even bring new practical functionalities to the anisotropic coding metasurfaces.
Fig. 6. The absorption efficiencies of the coding metasurfaces with the coding matrix (a) $M_1^{2 - bit}$ and (b) $M_2^{2 - bit}$. (c) PCR of the coding metasurface with the coding matrix $M_3^{2 - bit}$. (d) PCR of the coding metasurface with the coding matrix $M_4^{2 - bit}$ when VO2 stays in the insulator state (σ = 10 S/m). (e) The absorption efficiency of the coding metasurface with the coding matrix $M_4^{2 - bit}$ when VO2 stays in the fully metallic state (σ =2.6 ×106 S/m).
4. Conclusions
Unlike previously proposed coding metasurfaces based on the phase delay accumulated during light propagation, the proposed anisotropic coding metasurfaces and the improved active metasurface are dependent on the strength of mutual coupling between neighboring unit cells. The multiple functionalities of the anisotropic coding metasurfaces composed of arc-ring pairs and cut-wires are demonstrated numerically and experimentally in the terahertz regime. The coding metasurfaces with different coding matrices present diverse functionalities, including two different types of narrow-band absorptions and broadband polarization conversion, based on the initial unit cells. Among them, for the coding metasurface with the coding matrix $M_1^{2 - bit}$ the absorptivity peak of narrow-band absorption can exceed 93%, while perfect absorption can even be achieved for the coding metasurface with the coding matrix $M_2^{2 - bit}$, and the bandwidth of polarization conversion reaches over 0.5 THz with a PCR peak of 76.3% at 0.83 THz for the coding metasurface with the coding matrix $M_3^{2 - bit}$. Furthermore, a coding metasurface with the coding matrix $M_4^{2 - bit}$ is designed and integrated with VO2 to realize active manipulation. This active metasurface can switch between a highly efficient polarization converter and a low-pass filter by changing the ambient temperature. When the temperature of the sample is 23°C, the active metasurface acts as a highly effective narrow-band polarization converter, and the PCR peak reaches over 94% at 0.53 THz. While at a higher temperature (>68°C), the active metasurface acts as a low-pass filter. The proposed coding metasurfaces and active metasurface are expected to provide more choices to enrich terahertz modulation devices and promote the development of terahertz science and technology.
Funding
National Natural Science Foundation of China (62175180, 62005193, 61875150, 61805129); National Key Research and Development Program of China (2017YFA0701004); China Postdoctoral Science Foundation (2020M680877, 2020TQ0224).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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