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Abstract
Perfect absorption and polarization conversion of electromagnetic wave (EM) are of significant importance for numerous optical applications. Vanadium dioxide (VO2), which can be converted from insulating state to metallic state by being exposed to different temperatures, is introduced into a metallic square loop to constitute a switchable bifunctional plasmonic metasurface for perfect absorption and polarization conversion. Combined theoretical analyses and numerical simulations, the results show that at temperature T = 356 K, the metasurface acts as a perfect absorber with nearly 91% absorptance at the wavelength of 1547 nm. When the temperature decreases to T = 292 K, the metasurface expresses as a high efficiency (about 94%) polarization converter with the polarization conversion ratio up to 86% around 1550 nm. The designed bifunctional metasurface has plenty of potential applications such as energy harvesting, optical sensing and imaging. Moreover, it can also provide guidance to research tunable, smart and multifunctional devices.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Electromagnetic waves can be traditionally described by four factors: frequency, amplitude, phase, and polarization. Perfect absorption, which was proposed by Landry et al. [1–3], can be achieved by amplitude modulation of electromagnetic wave and has marvelous practical applications in numerous fields [4–6]. For example, utilizing materials which have property of perfect absorption to cover the fuselage can achieve stealth radar of flight. Traditional electromagnetic absorption materials are limited by selectivity and thickness, thus it is contradicted to the development concept of high efficiency and integration of optical devices. Polarization conversion also plays a critical role in optical communication, sensing, and detection [7,8]. The conventional approaches to manipulate polarization state of electromagnetic wave are based on Brewster effect, Kerr effect or birefringence effect [9,10], which is generally realized by optical grating and birefringent crystal. The thickness of birefringent crystal is larger than the target wavelength thus brings a drawback that the polarization converters are bulky in size. Dielectric metasurface can also form the birefringent effect by writing nanogrooves in the silica substrate [11,12]. Since the concept of metamaterial was proposed [13], as a kind of subwavelength artificial composite materials or structures, metasurfaces have aroused extensive interest among researchers [14,15]. Metasurfaces have some special properties compared to ordinary natural materials, such as little volume, low loss and broadband frequency. Moreover, by choosing the size and array of the unit cell, we can flexibly manipulate electromagnetic wave. These characteristics make them to be potential candidates in investigating light-matter interaction and designing high-performance optical devices, such as meta-lens [16,17], wave plates [18,19], and optical absorbers [20,21].
During the last decades, a tremendous number of plasmonic metasurfaces have enabled the realization of perfect absorption and polarization conversion. However, most of the metasurface devices are static and designed for a single functionality. For the perspective of practical applications, compact devices integrated multiple diversified functionalities show promising prospects. Although numerous researchers demonstrated that metasurface can achieve various functions at different frequency [22–24], up to now, the metasurface devices that can perform the functions of perfect absorption and polarization conversion are still lacking. Tunable materials, such as two-dimensional materials [25], nonlinear material [26] and semiconductor [27], whose characteristics can be tuned by stress [28], electric field [29] or temperature [30], usually have been used to dynamically control the characteristics of electromagnetic wave to realize different functionalities. Recently, phase change materials, such as Ge2Sb2Te5 [31] and VO2 [32–34], have attracted extensive attention. To change the phase state of Ge2Sb2Te5, we need a very high annealing temperature, which is difficult to operate. VO2 has the advantages of easiness of fabrication, high repeatability and low cost. The most important is that when the temperature is higher than the critical temperature TC ≈ 68 °C, VO2 undergoes a change from an insulating state to a metallic state, with almost four orders of magnitude of changes in the conductivity [35]. Therefore, combining metasurface with VO2 will be a promising approach to achieve switchable multi-functional devices at certain frequency.
In this paper, we theoretically analyzed and numerically simulated a bifunctional plasmonic metasurface which contains phase change material VO2. With changing the working temperature, the metasurface can work in two different states. At temperature T = 356 K, VO2 serves as metal, the proposed metasurface yields near-zero reflection around the resonant frequency, with closing to 91% absorptance at 1547 nm. At temperature T = 292 K, VO2 serves as dielectric, the proposed metasurface transforms to a polarization converter, and the efficiency of polarization conversion can reach to 94% at 1531 nm and 86% at 1552 nm. In both cases, we investigated the distributions of the surface induced current on the metasurface and the metallic substrate to illustrate the physical mechanism behind. Through analyzing the results, we make a conclusion that perfect absorption and polarization conversion can be transformed into each other at operating wavelength around 1550 nm with tuning the temperature by introducing VO2 to constitute a metasurface. It will not only enable advanced research in designing active tunable multifunctional devices, but also have plenty of potential applications such as energy harvesting, optical sensing and imaging.
2. Proposed geometry and methods
The schematic diagram of the designed metasurface is shown in Fig. 1, which consists of three parts: a square loop resonator embedded with vanadium dioxide (VO2), a dielectric spacer, and a uniform metallic substrate. The dielectric material employed is high index doped silica-glass (Hydex [36]) with optical refractive index n = 1.7, and the metal considered is silver (Ag) with frequency-dependent effective permittivity given by Johnson and Christy [37]. The conductivity of VO2 is chosen to be δ = 600 S/m and δ = 2.03 × 105 S/m, corresponding to insulating state at temperature T = 292 K and metallic state at temperature T = 356 K, respectively [38]. Figure 1(a) shows the schematic diagram of the designed metasurface. The optimized parameters of the proposed fundamental unit cell are displayed in Fig. 1(b). The period constants are Px = Py = 930 nm. The thickness of the top square loop, dielectric layer and the metallic substrate are t1 = 400 nm, t2 = 320 nm, and t3 = 330 nm. The width of the square ring is w = 80 nm. The lengths of the square loop and the embedded VO2 strips along x- and y-direction are Lx = Ly = 360 nm and lx = ly = 100 nm, respectively. θ is the angle between the x-axis and the symmetric axis of the square loop.
Fig. 1. (a) Schematic diagram of the designed metasurface. (b) Magnified unit cell of the metasurface.
For metallic state of VO2, the building metasurface is an isotropic structure, the Jones matrix can be written as:
where rxx and ryy represent the co-polarization reflection coefficients. Then for a normal incident plane wave propagating along the z axis, the absorptance of the proposed metasurface is calculated by A = 1−|rxx|2 (x- polarized) or A = 1−|ryy|2 (y- polarized).For insulating state of VO2, the building metasurface is an anisotropic structure. By applying the similar analysis method as [39], when at θ = 0, we describe the Jones matrix as:
For the designed structure with a rotated angle of θ, the Jones matrix is θ- dependent:We define the new matrix as:
As can be seen from Fig. 1, the rotated angle of the proposed structure is θ =π/4, thus the Jones matrix reduces to:
3. Results and discussions
A normal incident plane wave with polarization state along x- axis and y- axis were employed to illuminate the proposed metasurface, respectively. We investigated the reflection coefficients Rii (Rii=|rij|, i = x, y and j = x, y) with incident wavelength varying from 1450 nm to 1650 nm. Figure. 2(a) depicts the reflection coefficients when the proposed metasurface is exposed to the environment with temperature of T = 356 K. In this case, VO2 behaves like a conductive metal, which results in an isotropic metasurface. Obviously, the co-polarization reflection coefficients Rxx and Ryy obtain minimum value which is about 20% at resonant wavelength of 1547 nm. Moreover, the maximum cross-polarization reflection coefficient Ryx and Rxy are lower than 20% at 1547 nm. Thus, we can extrapolate that absorption plays a dominant role in this process. As the temperature is tuned to be T = 292 K, the corresponding reflection coefficients are shown in Fig. 2(b). In this case, VO2 transforms into an insulating dielectric state, the building blocks constitute an anisotropic metasurface. When the metasurface is excited by x- axis or y- axis polarized electric-field, symmetric and anti-symmetric resonance modes can be supported [40]. Polarization direction for x/y-polarized incident plane wave will be rotated if the anti-phase condition can be satisfied according to Eqs. (5) and (6). As can be seen from Fig. 2(b), the cross-polarization reflection coefficients are all over 68% from 1530 nm to 1553 nm. The cross-polarization reflection coefficients Rxy and Ryx can be reached up to 71% and 69% at the wavelengths of 1531 nm and 1552 nm, respectively. However, the co-polarization reflection coefficients Rxx and Ryy are suppressed to be 18% and 28% at these two wavelengths, respectively. It is demonstrated that the proposed metasurface has the capability of polarization conversion around 1550 nm.
Fig. 2. (a) The co- and cross-polarization reflection coefficients of the proposed structure at temperature T = 356 K, and (b) T = 292 K.
Figure 3(a) illustrated the absorptivity (A), reflectivity (R) and transmissivity (T) of the proposed metasurface when T = 356 K. Here, we only considered an x- polarized plane wave irradiating the proposed metasurface by considering its C2 symmetry. The absorptivity of the proposed metasurface is expressed as A = 1 − R − T. As the thickness of the metal substrate is larger than the penetrable depth of the incident plane wave, the incident plane wave can hardly penetrate the structure, resulting in the transmissivity T = 0. While for the reflectivity R, when the incident light is an x- polarized, it can be defined as R = Rxx2 +Ryx2 according to the previous analysis. From the spectrum, one can see that at temperature T = 356 K, the metasurface integrated with VO2 can efficiently absorb incident plane wave and the absorption peak occurs at 1547 nm with the total absorptance reaching about 91% for normal incidence. From 1532 nm to 1567 nm, the simulated absorptances are all above 60%. When the temperature turns to be T = 292 K, we calculated the polarization conversion ratio (PCR) to better elucidate the polarization converter. Here, we define the polarization conversion ratio as PCR = Ryx2 / (Rxx2 + Ryx2), and the corresponding curve is presented in Fig. 3(b). As can be seen, the two peaks of PCR are located at the wavelengths of 1531 nm and 1552 nm with the efficiency of polarization conversion reaching to 94% and 86%, respectively. Even at 1540 nm (valley point between the two PCR peaks), the efficiency of polarization conversion can also reach to 67%. The results show that our proposed metasurface performances well polarization conversion function around 1550 nm.
Fig. 3. (a) The calculated absorptivity (A), reflectivity (R) and transmissivity (T) of the proposed metasurface at temperature T = 356 K and (b) the polarization conversion ratio (PCR) at temperature T = 292 K.
To get deeper insight into the physical mechanism of the absorption effect in the designed structure, in the following, we give the induced current distributions on the metasurface and metallic substrate at 1547 nm which were showed in Figs. 4(a) and 4(b). It can be seen from Fig. 4(a) that the induced current mainly located at the top and bottom stripes with the direction along x direction on the metesurface, which can be strongly couple with its mirrored current distributions in the metal substrate [Fig. 4(b)]. This antiparallel current distribution pattern can induce a strong magnetic resonance. Figure 4(c) depicts the magnetic field (Hx) distributions in x-z plane under temperature T = 356 K. It can be clearly seen that the magnetic field is strongly confined in the doped silicon glass region, resulting in low reflectivity.
Fig. 4. (a) The induced current distributions of one unit cell on the metasurface and (b) metal substrate at 1547 nm, and (c) the magnetic field (Hx) distributions in x-z plane when the proposed metasurface serve as a perfect absorber at T = 356 K.
The induced current distributions of the upper surface of the metasurface and metal substrate also are calculated at the wavelengths of 1531 nm and 1552 nm to further reveal the underlying mechanisms responsible for the characteristics of polarization conversion when T = 292 K. For 1531 nm, the induced currents are mainly located at the up and left stripes [ Fig. 5(a)]. The (corresponding area) induced current patterns on the metal substrate has the same flow direction with currents in metasurface [Fig. 5(b)]. Therefore, the magnetic resonance cannot be induced under this situation. Fortunately, the left stripes can be regarded as an electric dipole with its orientation along + y direction [Fig. 5(a)]. Obviously, it has the ability to convert the power of incident light from x direction to y direction [41]. Similar electromagnetic distribution exists when the incident wavelength is 1552 nm [Figs. 5(c) and 5(d)], which also has the effect of polarization conversion.
Fig. 5. The induced current distributions of one unit cell on the metasurface (a), (c); and (b), (d) on the metal substrate at 1531 nm and 1552 nm when the proposed metasurface serves as a polarization converter at T = 292 K.
4. Conclusions
In conclusion, a bifunctional plasmonic metasurface integrated with perfect absorption and polarization conversion working near 1550 nm wavelength based on phase change material VO2, which is temperature controllable, was proposed. By tuning its working temperature, the metasurface can be transferred from a perfect absorber to a polarization converter. Through theoretical analyses and numerical simulations, we confirm that at temperature T = 356 K, the metasurface acts as an efficient perfect absorber with nearly 91% absorptance at 1547 nm. Once the temperature was decreased to T = 292 K, due to the change in the state of VO2, the metasurface switched to a polarization converter and the efficiency of polarization conversion can reach to 94% at 1531 nm and 86% at 1552 nm. The proposed bifunctional metasurface reveals the practicability of using phase-change materials to build tunable multifunctional metasurface, enabling novel applications such as energy harvesting, optical sensing and imaging.
Funding
Natural Science Foundation of Hunan Province (2019JJ50671); Education Department of Hunan Province (18C0204, 18C0214, 18C0239).
Disclosures
The authors declare no conflicts of interest.
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