Switchable bifunctional metasurface based on VO2 for ultra-broadband polarization conversion and perfect absorption in same infrared waveband
Introduction
Metamaterials, an artificially engineered nanostructure, can manipulate the amplitude, phase, polarization and frequency of electromagnetic waves, which are widely used in anomalous diffraction [1], [2], subwavelength imaging [3], perfect absorption [4], [5] and Chromatic aberration-free device [6]. Metamaterials have drawn particular interest due to their subwavelength size and fascinating optical performance. Among them, the typical metasurface, a 2-D structure of metamaterials, have aroused great research interest due to easy fabrication, low cost and powerful functionality to manipulate the electromagnetic waves [7], [8]. Conventional electromagnetic absorption materials and polarization conversion are bound by specific thickness and bulky configurations, which limit the integration and miniaturization for photonics applications. Much work so far has focused on perfect absorption and polarization conversion realized by metasurfaces. Perfect absorption enables energy harvesting by modulating the amplitude of electromagnetic waves, which is widely used in radiative cooling [9], [10], refractive index sensor [11] and ultra-thin film thermal emitters [12]. Polarization is one an important property of electromagnetic waves. Polarization conversion can be applied in optical communication [13], [14], optical sensing [15] and optical detection [16], [17], [18].
Conventional optical metasurfaces can only realize a single function at a specific wavelength once the structure is fabricated, and it remains challenging to design multifunctional metamaterials. Therefore, the exploitation of tunable metamaterials [19] that can modulate phase, polarization and amplitude is of great importance for active devices such as filters [20], modulators [21], [22], [23], sensors [11] and optical switches [24]. One demonstrable approach is to integrate tunable materials into the metasurfaces, such as liquid crystals [25], [26], phase change materials [27] and graphene [28]. Among them, the phase change material vanadium dioxide (VO2) is widely used because of its unique electrical and optical properties. It can change conductivity by five orders of magnitude in picosecond times when the temperature is below or above 340 K, which undergoes a transition from insulating state to metallic state [29]. And this process is both reversible and hysteretic. VO2 has promising applications in tunable metasurfaces in the optical [30], [31], [32] and terahertz bands [33], [34], [35], [36]. Combining VO2 with metasurface design allows for the tuning of existing electromagnetic effects such as tuning absorption bandwidth [33], optical imaging [37] and electromagnetic induced transparency [38].
However, the existing tunable metasurfaces have been limited by the narrow-band bandwidth due to material loss and fabrication accuracy. Efficient broadband perfect absorption and polarization conversion are difficult to achieve simultaneously. Most of the multifunctional metasurfaces reported so far focus on the visible wavebands and terahertz frequency bands, and few works are available for the switching of two functions in the infrared band. L-shaped resonators [39], [40] are typically used for ultra-broadband polarization conversion based on multiple electromagnetic resonances. Hairong [31] combines VO2 with a metallic square loop structure to accomplish a switchable dual function metasurface with perfect absorption and polarization conversion. In this paper, we propose a VO2 based dual-layer ring structure of the metasurface which enables functional switching of ultra-broadband polarization conversion and perfect absorption in the same waveband. At 303 K, the VO2 behaves as dielectric, and the metasurface achieves ultra-broadband polarization conversion with polarization conversion ratio (PCR) of over 95% in the region from 2500 nm to 3500 nm. At 358 K, the VO2 behaves as metal and the absorptance of metasurface is over 90% in the region from 2900 nm to 3700 nm. So ultra-broadband polarization conversion and perfect absorption are realized in the wavelength band of 2900 nm to 3500 nm. The distributions of surface induced currents on the unit cell and the metallic substrate is investigated to illustrate the physical mechanism. The ultra-broadband performance is the result of multiple electrical and magnetic resonance alternations.
Section snippets
Design and method
The schematic of the proposed broadband bifunctional metasurface is given in Fig. 1, which is composed of three parts: dual-layer ring, a dielectric spacer and a metallic substrate. Fig. 1(a) shows the three-dimensional structure of the unit cell, and Fig. 1(b) shows the top view of unit cell. The upper layer of dual-layer ring structure is a split silver ring and the bottom layer is a continuous VO2 ring. When VO2 is in the insulating state, the split ring destroys the symmetry of the
Polarization converter at T = 303 K
The metasurface is illuminated by x-polarized and y-polarized plane waves propagating in the z direction. When the VO2 is dielectric at T = 303 K, the silver split ring plays a major role. Due to the existence of split ring, the symmetry of the resonant cavity disappears, which can reduce the co-polarization reflection and increase the cross-polarization reflection. After the simulating calculation, high reflectance and polarization conversion ratio of the metasurface are obtained and shown in
Conclusion
In conclusion, we numerically propose a thermally bifunctional and efficient metasurface based on VO2, which consists of the superposition of a metallic split ring and VO2 ring, dielectric layer and metallic substrate. The insulator–metal state switching of VO2 is utilized to obtain ultra-broadband polarization conversion and perfect absorption in the wavelength range of 2900 nm to 3500 nm. Multiple adjacent electrical and magnetic resonances are excited, resulting in the reduction of
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was funded by National Key R&D Program of China (2016YFA0301300); National Natural Science Foundation of China (NSFC) (61875021); Natural Science Foundation of Beijing, China (2192036); The Fundamental Research Funds for the Central Universities, China; Guangxi Key Laboratory of Wireless Wideband Communication and Signal Processing, China .
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