Chirality, as a geometric property, means a structure cannot be overlapped with its mirror image through the operations of rotation or translation [1]. Chiral objects are ubiquitous in nature, such as various amino acids and proteins [2]. Chiral nanostructures could exhibit different optical responses to right circular polarization (RCP) and left circular polarization (LCP) waves, which could be described by CD [3]. In recent years, CD has received great attention due to the wide use in the fields of chiral molecular sensing, chiral catalysis, polarization control, nonlinear imaging and so on. However, the CD effect of natural chiral materials is very weak, which limits the further applications in practice. Differently, not only three-dimensional chiral structures [4] but also the artificially designed symmetry broken planar nanostructures [5,6], such as L-shaped [7], Z-shaped [8], double-rectangle array [9], could enhance CD extremely.

As a traditional planar chiral structure, 卍-shaped resonators are usually used to realize chirality [[10], [11], [12], [13], [14], [15]]. However, 卍-shaped metasurfaces with C4 rotation symmetry could show obvious optical activity [[11], [12], [13], [14]] but very weak CD with the order of a few percent [15]. So, the 卍-shaped based resonators with C4 rotation symmetry are not typically used for gaining CD. Surprisingly, in 2018, Alexander Y Zhu et al. enhanced CD in planar 卍-shaped nanostructures by exciting the higher-order multipole responses in an all-dielectric metasurface [14], which enlightens us that the possibility of planar 卍-shaped nanostructure for strong CD.

On the other hand, the properties and functions of most metasurfaces are usually fixed after manufacturing, which cannot meet the requirements of dynamic polarization modulation in the fields of biosensing, dynamic display, spectroscopy, and so on. In order to achieve dynamic electromagnetic manipulation, some reconfigurable metasurfaces are proposed by integrating various active materials, such as electrically controlled graphene [16], thermally controlled phase change materials GST [17], VO2 [18], and mechanically stretchable PDMS, etc. Among them, PDMS is a flexible material with low optical loss and excellent elasticity. By stretching or bending the flexible PDMS substrate of reconfigurable metasurface, the dynamic electromagnetic manipulation could be realized. To date, PDMS has been widely used in many micro/nano-structures [[19], [20], [21], [22]]. For example, Atwater et al. achieved the tuning of the resonant frequency of a gold split ring structure by applying mechanical stress to PDMS substrate [19]. Gutruf et al. demonstrated the tunable dielectric metasurfaces by mechanically controlling PDMS [20]. Li et al. proposed a multifunctional tunable metasurface, which is assisted by elastic PDMS substrate [21]. Also, Zou et al. designed a nanostructure using temperature sensitive PDMS to actively modulate plasmonic signals [22].

Here, to gain the strength-switchable CD, the all-dielectric 卍-shaped resonators integrated with PDMS is proposed. By breaking the period symmetry of the C4 rotation symmetric 卍-shaped resonator array in x- and y-directions, the giant CD (0.99) is realized. By mechanically stretching PDMS substrate in x-direction, the magnitude of CD is switched to 0 from 0.99, which indicates the dynamic switching for CD strength. The giant CD originates from magnetic dipole and electric quadrupole resonances, which is demonstrated by quantitative multipole decomposition and electric and magnetic field analysis.

Fig. 1(a) shows the schematic diagram of the C4 rotation symmetric 卍-shaped Ge resonators, which are periodically placed on PDMS substrate. The distance of the adjacent 卍-shaped resonators in x- and y-directions are different. So, the C4 rotation symmetry of the 卍-shaped array is essentially broken by the different periods in x- and y-directions. As the top view of the unit structure shown in Fig. 1(b), the 卍-shaped Ge resonator consists of two orthogonally arranged Z-shaped sub-structures with the same geometric parameters. The width and the length of Z-shaped arms are w = 155 nm and l = 480 nm. The width and length for the middle part of the Z-shaped arms are k = 157 nm and d = 303 nm. The periods of the 卍-shaped Ge array in x- and y-directions are Px = 960 nm and Py = 1000 nm (unstretched state). Fig. 1(c) illustrate the sectional view of the 卍-shaped unit structure, the thickness of 卍-shaped Ge resonator is t1 = 380 nm, and the thickness of PDMS substrate is t2 = 1940 nm.

In simulations, the refractive index of Ge and PDMS are set as 4.25 [23] and 1.41 [21] respectively. All spectra are simulated by using a finite difference time domain (FDTD) method with periodic boundary conditions in x- and y-directions. Perfectly matched layer (PML) is used in the ±z directions. Circularly polarized light (CPL) is normal incident from the top of the metasurface along -z direction. In FDTD simulation, the index of background material is set as 1. Moreover, for the mesh setting, the mesh type is set to “auto non-uniform” and the mesh accuracy is set to 2.