Metasurfaces, artificially designed two-dimensional arrays on subwavelength scale, exhibit fantastic electromagnetic (EM) properties. Their ability to arbitrarily engineer the amplitude, phase, polarization state of EM waves has generated various exotic responses, such as negative refraction [1], sub-diffraction imaging [2], invisibility cloak [3] and so on. However, these static metasurfaces only show single function of EM manipulation and work in fixed wavelengths. To gain the capability of dynamic control, some tuning materials are incorporated into the design of active metasurfaces in the broad wavebands from visible to terahertz (THz) [4], [5], [6], [7]. Among them, THz metasurfaces are highly required, because THz technology plays an important role in biochemical sensing, wireless communication, and security monitoring [8], [9]. Currently, the constitute materials for THz active metasurfaces include but not limited to two-dimensional (2D) material graphene [10], [11], liquid crystal [12], photo-excited semiconducting silicon [13], phase change materials Ge2Sb2Te5 (GST) [14], [15], and VO2 [16], [17], [18], [19], [20], [21].

Despite gaining the dynamic control for THz waves, these tuning materials still have their own disadvantages. The Fermi energy of graphene could be electrically tuned [10], [11] but the preparation of atomic-layer thick 2D material in a large area is difficult. The orientations of liquid-crystal molecules could be controlled via an external electric field [12]. Nevertheless, the molecules are unstable and difficult to align. The optical property of semiconductor silicon could be tuned through laser pulses [13], but the technology for carrier injection is not mature enough. Ge2Sb2Te5 could be switched between crystalline and amorphous states but requires relatively high annealing temperature (100-150 °C) [14]. Differently, VO2 exhibits a volatile insulator-to-metal transition at around 68 °C [18]. Compared to the switching temperature of GST, the lower phase transition temperature of VO2 is more attractive for real-world applications. Besides, VO2 has the advantage of the drastic change in optical properties between metallic and insulator phases [22]. The phase transition of VO2 can be excited by thermal, electrical, and optical stimuli. Compared with the other two excitation modes, photoexcitation can stimuli VO2 phase transition rapidly. When optical stimulus is applied, phase transition of VO2 can occur at an ultra-fast time scale [23]. It shows strong dependence on temperature or electric field. M. F. Becker et al. excite insulator–metal transition (IMT) in VO2 by a 500-fs pulse in less than 10 ps [24]. M. R. Otto et al. photoexcite the polycrystalline VO2 sample is by a 35-fs 800-nm optical pump pulse [25]. The phase transition of VO2 leads to the change of conductivity by five orders of magnitudes.

At present, some VO2 based metasurfaces have realized active manipulation for THz waves. For example, G. Wu et al. proposed a VO2 based three hollow rings array with high absorption (>90%), and the absorption could be switched from 0.04 to ∼1 [26]. An active switch from absorption to transmission is realized through changing the phase of VO2 [27]. H. Zhang et al. sed the phase transition of VO2 resonators to achieve the regulation of single-band to ultra-wideband absorption [28]. But most of these adjustable absorbers are polarization-independent. Except for amplitude manipulation, polarization state could also be tuned dynamically. S. Wang et al. achieved the dynamic polarization control for THz waves using a VO2 based metamaterial [29]. Unfortunately, the above-mentioned metasurfaces focus on single modulation for absorption strength or polarization state. More recently, the VO2 based metasurfaces achieved the dual functions of absorption together with spin Hall effect [30], or polarization conversion [31], [32]. In addition, some recent works achieve good polarization selectivity. For instance, H. Huang et al. proposed a metasurface that achieves high polarization selectivity: linear dichroism (0.81) and circular dichroism (0.86) [33]. Notably, very few active metasurfaces could simultaneously exhibit strength-switchable absorption and polarization selectivity in a nanostructure.

In this paper, we proposed an active metasurface with the dual functions of high-efficiency optical switch and strength-switchable polarization selective absorption by integrating phase change material VO2 in THz frequencies. In practical applications, VO2 phase transition can be achieved by imposing thermal, electrical, or optical stimuli. In this work, compare with using thermal or electrical stimuli using optical stimuli can realize the phase transition of VO2, which is more rapidly. Refer to Ref. [25], we could use this regime with a pump pulse laser generating trains of a controlled number of 500 fs pulse at a wavelength of 800 nm to switch the state of VO2. When the room temperature (RT) rises from 25 °C to above 68 °C, with VO2 phase transition from insulator to metallic phase, the absorption is high-efficiency switched from ∼0 to ∼1 at 2.68 THz, which behaves as an optical switch as shown in Fig. 1(a). Benefited from the polarization selective excitations of electric and magnetic resonances, the same metasurface also shows the strength-switchable polarization selectivity by optical excitation controlling the phase of VO2. Fig. 1(b) shows the opposite and switchable polarization selectivity in dual THz wavebands. At 1–2.32 THz, TE polarized waves are absorbed and TM polarized waves are reflected. At 2.32–3 THz, the metasurface filters TM polarized waves and reflects TE polarized waves. The proposed active metasurface with dual functions has great potential for the practical applications in biosensing, information processing and so on.