Large-Range Switchable Asymmetric Transmission and Circular Conversion Dichroism in a VO₂ Based Metasurface

Abstract

Reconfigurable chiral metasurfaces with a dynamic polarization manipulation capability are highly required in optical integrated systems. In this paper, we simultaneously realized giant and large-range switchable asymmetric transmission (AT) and circular conversion dichroism (CCD) in a vanadium dioxide (VO₂) based metasurface. The AT and CCD of the insulator VO₂ based metasurface reached 0.95 and 0.92, respectively. Utilizing the insulator-to-metallic phase transition of VO₂, the AT and CCD could be continuously switched to near zero. Furthermore, the physics mechanism of the giant and switchable AT and CCD were analyzed. The proposed metasurface with large-range switchable AT and CCD is promising in applications of biochemistry detection, chiral imaging, and biosensing.

1. Introduction

Chirality is defined as the characteristic where an object cannot coincide with its mirror image [1], and has broad potential application prospects in chiral coding [2], analytical chemistry [3], chiral biosensing [4,5,6], polarization optics [7,8,9,10], and nonlinear optics [11], etc. Chiral effects, circular birefringence, and circular dichroism (CD) are first found in three-dimension spiral structures while interacting with circular polarized light (CPL) [12]. However, these chiral effects are relatively weak in natural materials. Differently, chiral metasurfaces, artificially designed subwavelength optical structures, possess excellent performance in enhancing circular birefringence and circular dichroism. With the development of chiral metasurfaces, the two-dimension chiral effects of asymmetric transmission (AT) and circular conversion dichroism (CCD) are also obtained in planar symmetry broken structures [13]. Up to now, metasurfaces have been widely used to realize intrinsic [13] and extrinsic chirality [14] by breaking the symmetry of unit structure and oblique incidence, respectively. Unlike the extrinsic chirality at the expense of oblique incidence, intrinsic chirality could be realized in symmetry broken structures under normal incidence. For instance, Ahsan Sarwar Rana and Inki Kim et al. realized both AT (0.58) and CCD (0.55) in an asymmetric planar structure [15]. Similarly, both AT (0.8) and CD (0.88) were realized in a Z-shaped, germanium metasurface [13]. Although giant 2D chirality has been realized, the chiral effects of these metasurfaces are fixed once the structures have been fabricated [16].

To attain dynamic chiral effects, some active materials including 2D materials of graphene [17,18] and black phosphorus [14], phase change materials Ge₂Sb₂Te₅ (GST) [19,20] and VO₂ [21,22], liquid crystal [23], and ultrathin TiN films [24] have been applied in traditional static metasurfaces. Among them, phase-change material VO₂ exhibits obvious phase transition behavior under the excitations of thermal [25], electrical [26,27], or optical [26] stimuli. With the phase transition from insulator to metallic state, the conductivity of VO₂ changes 4–5 orders of magnitudes. Besides, the lower phase-transition temperature (68 °C) of VO₂ compared to that of GST (160 °C) [19] shows the advantage of lower energy consumption. Benefiting from these advantages, VO₂ has been applied to construct reconfigurable chiral metasurfaces to enable polarization control [27,28], wavefront shaping [29,30], and optical limiting [31,32]. Lv et al. designed a bilayer twisted E-shape structure integrated with VO₂ and achieved strength-switchable AT and CD (0.6/0.45~0) [33]. Meng Liu et al. realized dynamic AT and CCD in a VO₂ based split ring structure where the AT/CCD ranged from 0.12/0.75 to near zero [34]. Though the strength-switchable AT and CCD have been realized, the switching ranges of these chiral effects are not broad enough, which limits the application of these chiral metasurfaces.

In this work, the giant and strength-switchable 2D chiral effects of AT and CCD were simultaneously achieved in a VO₂ based metasurface in the THz region. The metasurface allows the transmission of the RCP wave to propagate along the z-direction (forward) and prevents the propagation of RCP in the z-direction (backward) exhibiting AT effects, as shown in Figure 1a. Meanwhile, the giant difference of cross-polarization reflection of LCP and RCP light indicates the existence of giant CCD as in Figure 1b. The maximum values of AT and CCD of the insulator VO₂ based metasurface are 0.95 and 0.92 at 0.69 THz. Furthermore, by thermally controlling the phase of VO₂, AT/CCD could be dynamically switched from 0.95/0.92 to near zero. The strength and the switching range of AT and CCD are much improved compared with previously reported works. The proposed metasurface with giant and switchable 2D chirality will promote applications in optical imaging and chiral biosensing, etc.

2. Design and Simulations

It is widely used to break the structures’ geometric symmetry in the wave propagation direction to generate chirality [35,36]. To break the symmetry of our structure, two Au resonators with different patterns (G-shape and J-shape) are placed on the opposite sides of the VO₂ inserted polymer layer as shown in Figure 2. When VO₂ is the insulator state, the incident CPL could with polarization selectively penetrate and realize a giant chiral effect. In contrast, both incident CPLs will be blocked when the VO₂ phase is changed to the metallic state and shows a very weak chiral effect. Thus, utilizing thermal tuning of the properties of VO₂, the chiral effect of the VO₂ based reconfigurable metasurface could be modulated dynamically. The metasurface with giant chiral effect employs a periodic structure and the period array based on VO₂ is shown in Figure 2a. The structure marked by the dashed frame in Figure 2a is the unit structure. The top layer is the 90° rotated G-shaped Au resonator and the bottom layer is the 180° rotated J-shaped Au resonator which break the rotational symmetry as shown in Figure 2c,d. After optimization, the thickness of the Au resonators in both top and bottom layers is the same as $t_1=18\mathsf{\mu }\mathrm{m}$. The thickness of the separated polymers and inserted VO₂ layer are $t_2=19.7\mathsf{\mu }\mathrm{m}$, $t_3=20\mathsf{\mu }\mathrm{m}$, and $t_4=0.3\mathsf{\mu }\mathrm{m}$ as labeled in Figure 2b.

The optical property of Au substrate could be described by the Drude model, and the conductivity is set as ${\sigma }_{Au}=4.56\times {10}^7\mathrm{S}/\mathrm{m}$ [33]. The polymer spacer is regarded as lossy dielectric material with a permittivity of 3.5 + 0.00945i [37]. With the temperature change from room temperature to phase-transition temperature $T_c\approx 68°\mathrm{C}$, the Drude model is used to characterize the permittivity of VO₂ in the THz region as follows [25]:

$$\epsilon \left(\omega \right)={\epsilon }_{\infty }-\frac{{\omega }_p^2\left(\sigma \right)}{{\omega }^2+i\gamma \omega }$$

(1)

$${\omega }_p^2\left(\sigma \right)=\frac{\sigma }{{\sigma }_0}{\omega }_p^2\left({\sigma }_0\right)$$

(2)

where ${\epsilon }_{\infty }=12$ is permittivity at infinite frequency. The permittivity of the VO₂ $\epsilon \left(\omega \right)$ is determined by the plasma frequency ${\omega }_p$ (${\omega }_{p_0}=1.4\times {10}^{15}\mathrm{rad}/\mathrm{s}$, ${\sigma }_0=3\times {10}^5\mathrm{S}/\mathrm{m}$). Thus, the optical properties of VO₂ under different phases could be described by the conductivities. Especially, with the insulator-to-metallic transition of VO₂, the conductivity of VO₂ increases from 200 s/m to 2 × 10⁵ s/m. In simulation, we numerically calculated the transmission and reflection spectra by using the finite element method (FEM) in the frequency domain in the CST Microwave Studio. The periodic boundary conditions are used in the x- and y-directions and open boundary is applied in the z-direction. Light sources that incident circular polarized light are set in both ports in the z-direction. The mesh is set as adaptive tetrahedral grids. In addition, the experimental feasibility of our structure was well considered. Magnetron sputtering and photolithography could be used to fabricate the designed nanostructure. The phase change of VO₂ could be thermally controlled by heating plates.

3. Giant and Switchable Asymmetric Transmission

The polarization transmission coefficient spectra of CPL propagating forward and backward are numerically calculated in Figure 3a,b. The four transmission coefficients of the CPL propagating forward do not coincide with each other. The right-to-left polarization transmission coefficient (${\overrightarrow{t}}_{-+}$) and right-to-right one (${\overrightarrow{t}}_{++}$) are higher than the other two coefficients (${\overrightarrow{t}}_{+-}$ and ${\overrightarrow{t}}_{--}$) in the band of 0.65–0.8 THz as shown in Figure 3a. Especially, the maximum values of RCP transmission coefficients ${\overrightarrow{t}}_{++}$ and ${\overrightarrow{t}}_{-+}$ reach 0.729 and 0.649 at 0.69 THz, while the LCP ones (${\overrightarrow{t}}_{+-}$ and ${\overrightarrow{t}}_{--}$) approach 0. Since the change of propagation direction will result in the exchange of two enantiomeric arrangements, the subscripts “+” and “−” representing RCP and LCP will be exchanged, namely ${\overrightarrow{t}}_{++}={\overleftarrow{t}}_{--}$, ${\overrightarrow{t}}_{--}={\overleftarrow{t}}_{++}$, ${\overrightarrow{t}}_{+-}={\overleftarrow{t}}_{-+}$ and ${\overrightarrow{t}}_{-+}={\overleftarrow{t}}_{+-}$. The polarization transmission coefficients of CPL propagating in opposite directions are shown in Figure 3b. The directions propagating forward and backward in transmission coefficients are represented by arrows “→” and “←”, respectively. As expected, the co-polarization transmission coefficients propagating backward ${\overleftarrow{t}}_{--}({\overleftarrow{t}}_{++})$ are the same as the forward ones ${\overrightarrow{t}}_{++}({\overrightarrow{t}}_{--})$. Similarly, the cross-polarization transmission coefficients propagating backward ${\overleftarrow{t}}_{-+}({\overleftarrow{t}}_{+-})$ and the forward ones ${\overrightarrow{t}}_{+-}({\overrightarrow{t}}_{-+})$ are also the same.

The transmission difference of the CPL propagating forward and backward is described as an AT phenomenon [12]. To describe the transmission difference propagating in opposite directions, the transmissions of CPL propagating forward and backward are separately calculated in Figure 3c,d. The polarization transmissions are derived from polarization transmission coefficients as ${\overrightarrow{T}}_{\pm }={\left|{\overrightarrow{t}}_{\mp \pm }\right|}^2+{\left|{\overrightarrow{t}}_{\pm \pm }\right|}^2$ and ${\overleftarrow{T}}_{\pm }={\left|{\overleftarrow{t}}_{\mp \pm }\right|}^2+{\left|{\overleftarrow{t}}_{\pm \pm }\right|}^2$. The transmissions of RCP (${\overrightarrow{T}}_{+}$) propagating forward are very high and the value at the peak is about 0.95 at 0.69 THz. However, the transmissions of LCP ${\overrightarrow{T}}_{-}$ maintain low values and the dip values even approach zero as in Figure 3c. As expected, the transmission of RCP/LCP light propagating forward is equal to the transmission of LCP/RCP propagating backward (${\overrightarrow{T}}_{+}={\overleftarrow{T}}_{-}$,${\overrightarrow{T}}_{-}={\overleftarrow{T}}_{+}$) as the curves show in Figure 3d. The transmission difference of RCP light propagating forward and backward in Figure 3c,d means the incident RCP light could transmit our metasurface forward but the RCP light from the opposite direction will be blocked.

To estimate the giant AT performance quantitatively, the AT parameters of RCP and LCP light (${\Delta }_{cir}^{+}$ and ${\Delta }_{cir}^{-}$) are defined below by the polarization transmission differences [38]:

$${\Delta }_{cir}^{+}={\overrightarrow{T}}_{+}-{\overleftarrow{T}}_{+}={\overrightarrow{T}}_{+}-{\overrightarrow{T}}_{-}={\left|{\overrightarrow{t}}_{-+}\right|}^2+{\left|{\overrightarrow{t}}_{++}\right|}^2-{\left|{\overrightarrow{t}}_{+-}\right|}^2-{\left|{\overrightarrow{t}}_{--}\right|}^2$$

(3)

$${\Delta }_{cir}^{-}={\overrightarrow{T}}_{-}-{\overleftarrow{T}}_{-}={\overrightarrow{T}}_{-}-{\overrightarrow{T}}_{+}={\left|{\overrightarrow{t}}_{--}\right|}^2+{\left|{\overrightarrow{t}}_{+-}\right|}^2-{\left|{\overrightarrow{t}}_{-+}\right|}^2-{\left|{\overrightarrow{t}}_{++}\right|}^2$$

(4)

As shown in Figure 3e, the maximum AT parameter of RCP (${\Delta }_{cir}^{+}$) reaches 0.95 at 0.69 THz. Because of the inversion relationship between polarization states and the propagation direction, the AT parameters of LCP and RCP light are negative of each other (${\Delta }_{cir}^{-}=-{\Delta }_{cir}^{+}$). The maximum absolute value of the AT parameter for LCP is $\left|{\Delta }_{cir}^{-}\right|=\left|{\Delta }_{cir}^{+}\right|=0.95$ at 0.69 THz. Thus, the metasurface with giant AT parameter allows near perfect transmission of RCP light propagating forward and prevents that of LCP, which corresponds to the RCP light propagating backward.

By controlling the phase change of VO₂, the strength of the AT effect could be continuously switched. As shown in Figure 3f, when VO₂ is in the insulator state (200 S/m), the metasurface exhibits giant AT effect (0.95) at 0.69 THz but the AT will disappear when VO₂ is transformed into the metallic state (2 × 10⁵ S/m). The strength of AT could be dynamically switched in a broad switching range from 0.95 to 0.05. The VO₂ based chiral metasurface with giant and large-range switchable AT shows great potential in the field of the optical integrated system.

4. Giant and Switchable Circular Conversion Dichroism

Not only is there a giant AT effect but the same nanostructure also shows great polarization selectivity for the reflection CPL. The reflection coefficient spectra of the CPL propagating forward and backward are displayed in Figure 4a,b. The co-polarized refection coefficients of both CPL propagating in two opposite directions are equal (${\overrightarrow{r}}_{++}={\overrightarrow{r}}_{--}$ and ${\overleftarrow{r}}_{++}={\overleftarrow{r}}_{--}$) at 0.5–1.0 THz. Conversely, the cross-polarized reflection coefficients are totally different. In detail, the left-to-right polarization reflection coefficient ${\overrightarrow{r}}_{+-}$ is much larger than the one of the right-to-left one ${\overrightarrow{r}}_{-+}$ in Figure 4a. Especially, the ${\overrightarrow{r}}_{+-}$ is as large as 0.96 at 0.69 THz while ${\overrightarrow{r}}_{-+}$ approaches zero at the same frequency, which indicates the giant polarization selectivity of reflected light. In contrast, for the reflected light incident backward, the difference between cross-polarized reflection coefficients is not remarkable as shown in Figure 4b.

To quantitatively characterize the giant difference of the cross-polarization reflection coefficients, CCD was calculated, as shown in Figure 4c, which is defined by the difference of ${\left|{\mathrm{r}}_{+-}\right|}^2$ and ${\left|r_{-+}\right|}^2$ as $CCD={\left|{\overrightarrow{r}}_{+-}\right|}^2-{\left|{\overrightarrow{r}}_{-+}\right|}^2$ [34]. The curves of ${\left|{\overrightarrow{r}}_{+-}\right|}^2$ and ${\left|{\overrightarrow{r}}_{-+}\right|}^2$ do not coincide in the whole frequency band and the maximum difference occurs at 0.69 THz with the values of ${\left|{\overrightarrow{r}}_{+-}\right|}^2=0.92$ and ${\left|{\overrightarrow{r}}_{-+}\right|}^2\approx 0$, respectively. The maximum of CCD reaches 0.92 at 0.69 THz. The high CCD represents the giant difference between the cross-polarization conversion of incident LCP and RCP light.

The strength of CCD could be dynamically switched in the VO₂ based active metasurface. Utilizing the phase-transition of VO₂, the strength of the CCD of the metasurface was continuously controlled from 0.92 to ~0 at 0.69 THz, as in Figure 4d. In detail, with the conductivity increasing from 200 S/m (insulator VO₂) to 2 × 10⁵ S/m (metallic VO₂), the CCD value gradually decreased in a broad range from ~1 to ~0 at 0.69 THz. The broad-range and continuous switching for CCD is a major advance when compared with previously reported works.

5. The Principle of Chiral Effects

To explore the physical mechanism of AT and CCD, the electric field and surface current distributions of the chiral metasurface at 0.69 THz are presented in Figure 5. Here, electric field distribution corresponds to the absolute value (|E|) of electric field distributions. As mentioned, the metasurface with insulator VO₂ exhibits different polarization responses for RCP and LCP light. The incident RCP light could penetrate the metasurface as a homogeneous electric field distributed above and below the metasurface, as in Figure 5a. In contrast, the incident LCP light is reflected which leads to obvious differences of electric field distributions above and below the metasurface as shown in Figure 5b. The huge differences in transmission and reflection of RCP and LCP light exhibit remarkable AT and CCD effects.

To explain the mechanism of different responses for RCP and LCP light, the surface current distributions in the top and bottom layers are shown in Figure 5e,f,i,j. Under the excitation of RCP light, there are enhanced parallel and circular surface currents in the top layer as shown in Figure 5e. In detail, the parallel surface current points in the y-direction and the circular surface current flows clockwise. As known, electric dipole resonance is characterized by a parallel surface current flowing from one pole to another pole. In contrast, when the surface current distributes circularly, magnetic dipole resonance will be generated [39]. Thus, with incidence of RCP light, the electric current marked by arrows shows excitations of electric ($E_t$) and magnetic ($M_t$) dipole resonances in the top layer in Figure 5e. Similarly, electric and magnetic dipole resonances could be simultaneously excited in the bottom layer (Figure 5i).

In contrast, with LCP light incidence, the electric and magnetic dipole resonances could also be excited in Figure 5f,j, but with the electric moment pointing in opposite directions as the black arrows marked in both layers for comparison (Figure 5e,f,i,j). Conversely, the directions of electric moment are reversed at the same geometric position in the top and bottom layers under LCP excitation compared with RCP excitation. It is worth noting that the intensity of the surface current under LCP excitation is generally enhanced compared to that of RCP excitation. This is because the variation of resonance modes and surface current intensity result in different responses for RCP and LCP and generate giant AT and CCD in the metasurface with the insulator VO₂.

By changing the phase of VO₂, both the values of AT and CCD could be continuously switched to near zero. When VO₂ undergoes insulator-to-metallic transition, the incident CPL could hardly penetrate the metallic VO₂ and excites a very weak electric field and surface current in the bottom layer as shown in Figure 5g,h,k,l. The difference of electric and magnetic resonance modes and surface current are unremarkable under the excitations of RCP and LCP light, which result in the weak response difference between RCP and LCP light. Thus, when the VO₂ is changed to the metallic state, the 2D chiral effect including AT and CCD could hardly be excited in the same nanostructure and achieves large-range dynamic switching.

Here, we compare the chiral performances of our chiral structure with previously reported works in

Table 1. The comparison with reported chiral metasurfaces.

Nano-Structures	Bandwidths	AT	CCD/CD	Switching Capability	Active Materials	Years
Twisted S-shaped structure [23]	0.4–2.0 THz	0.73	CD = 0.28	No	/	2020
split rings [40]	0.5–4 THz	0.36	CCD = 0.36	Yes	graphene	2020
multilayered structure [41]	7.5–10.7 THz	0.34	CD = 0.42	Yes	VO2–graphene	2021
split rectangular annulus [18]	0.2–0.8 THz	0.77	CD = 0.53	Yes	graphene	2021
Split rings [22]	0.4–0.8 THz	0.12	CCD = 0.75	Yes	VO2	2020
“G”-and “J”-shaped hybrid structure	0.5–1.0 THz	0.95	CCD = 0.92	Yes	VO2	This work

. The first works realized prominent but fixed chiral effects. The latter four works realized active manipulation for AT and CCD/CD by integrating VO₂, but the switching ranges were not large enough. In contrast, not only the stronger AT (0.95)/CCD (0.92) is obtained but also the larger-range switching for 2D chirality from ~1 to ~0 are realized by controlling the phase states of VO₂. The proposed chiral metasurfaces with simultaneous giant AT/CCD and broad range switching capability for chirality are essential for the applications of polarization filtering, polarization optical isolating, etc.

6. Conclusions

In conclusion, we numerically achieved giant and strength-switchable AT and CCD in a VO₂ based chiral metasurface by breaking the geometric symmetry of the structures in the wave propagation direction. The giant AT parameter (0.95) and CCD (0.92) are realized in the THz region, which is much improved compared to the proposed chiral metasurfaces. Furthermore, the strength of the AT and CCD effects are large-range switched from ~1 to ~0 by thermally controlling VO₂. The physical mechanism of the giant and reconfigurable AT and CCD are analyzed by surface current distributions. The giant chirality originates from the polarization selective excitations of the electric and magnetic dipole resonances. The proposed active metasurface with broad switchable range for both AT and CCD will promote the development of integration chiral devices.