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Abstract
Circular dichroism (CD) is originally obtained from three-dimensional spiral structures by simultaneously exciting electric and magnetic resonances. To simplify construction, multilayer stacked asymmetric structures and the symmetric structures relying on oblique incidence are proposed for enhancing CD. Herein, we achieved the enhancement of dual-waveband CD by adding a Ge2Sb2Te5 (GST) layer on the top of a Z-shape gold array in a normally incident system. Benefited from the polarization selective excitations of electric and magnetic dipole resonances, the CD in a simple planar structure is immensely enhanced from near zero to 0.73 at 1.58 µm. Furthermore, the CD strengths is dynamically tuned by controlling the phase of GST. With the GST phase transition from amorphous (a-GST) to crystalline state (c-GST), CD magnitudes are switched by about 0.73 and 0.27 at dual wavebands respectively. The enhancement of CD by adding a layer on a simple planar array offers a new method for designing planar metasurfaces with strong chirality.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Chirality is used to describe that an object cannot be overlapped with its mirror image through simple rotation or translation operation. In fact, chirality is widely presented in natural biochemical molecules, including DNA double helix, carbon nanotube and partial viruses [1]. However, the chirality in these natural materials is very weak, which restricts its further application in bio-sensing, imaging, and numerous other areas. Differently, artificially designed three-dimensional (3D) metamaterials [2] could enhance chirality by several orders of magnitude. In the past decade, chiral metamaterials have exhibited strong optical activity [3], asymmetry transmission (AT) [4] and circular dichroism (CD) [5–7]. But the complex-shape construction of these metamaterial devices relies on complicated fabrication technics and the energy loss of 3D nanostructures reduces their working efficiency.
To overcome these challenges in 3D metamaterials, the planar metasurfaces are proposed, such as double- or multi-layer gammadion [8], L-shape [9], U-shape [10] and Z-shape metasurfaces [11–13]. Among them, the transmission-type Z-shapea array exhibits giant chirality by breaking the symmetry of nanostructures [14]. For instance, Ma et al. achieved strong AT effect by exciting magnetic resonance in a Z-shapea Ge array [11]. Hu et al. demonstrated high-efficiency polarization selective transmission in an all-dielectric Z-shape metasurface [12]. Zheng et al. realized AT effect in a Z-shape silicon metasurface and discussed its application in near-field image display [13]. The transmission-type polarization selectivity, behaving as AT effect, is realized through optical path accumulation [15–17], therefore the most transmission-type AT metasurfaces are constructed by multilayer stacked nanostructures.
Unlike AT effect, the CD response, representing the absorption difference between right-handed circularly polarized (RCP) and left-handed circularly polarized (LCP) waves, could be achieved not only in transmission type but also in reflection one. For the reflection-type metsaurface with metal substrates, the CD could be simplified as the reflection difference between LCP and RCP waves. Also, for the transmission-type metsaurface with no reflection, the CD could be simplified as the transmission difference between LCP and RCP waves, which is the same as AT. Differently, CD effect could be achieved not only in transmission type but also in reflection one. Therefore, the reflection-type CD could be obtained by utilizing much simpler structures than the AT ones. In fact, the fewer-layer nanostructures in reflection type in Fig. 1(b) are equivalent to the multilayer stacked transmission-type ones in Fig. 1(c). In summary, the reflection-type nanostructures with fewer layers (Fig. 1(b)) could achieve similar CD as the multilayer stacked transmission-type counterparts (Fig. 1(c)). Thus, to achieve CD with a simplified structure, the fewer-layer Z-shape array in reflection type is adopted in this paper.
Previously, great efforts are made to design Z-shape metasurfaces by using various materials for strong CD [18–20]. Despite gaining strong CD, the capability of dynamic control for response wavelength or CD strength is still lacked. In recent years, some active metasurfaces [21] including mechanical reconfiguration and incorporated tuning materials, i.e. graphene [3,22], VO2 [11,23] and Ge2Sb2Te5 (GST) [24] are designed to dynamically tune CD. To date, the switching range of CD strength is not wide enough for application, and the tuning materials of these active metasurfaces suffer from their own deficiencies. For example, the working region of the carrier density tunable graphene is limited in mid-infrared and THz wavebands. The phase transition of VO2 is invertible, but it needs a constant power to maintain its state and the phase-transition temperature is relatively low (68 °C) [25]. In contrast, GST could be switched reversibly between ordered crystalline (c-GST) and disordered amorphous states (a-GST) by thermal [26], optical [27] or electrical [28] stimulus, along with a large change of optical properties in NIR region. Besides, the most important feature of GST is its zero static power, which means that it requires no power to maintain its state [29]. This feature makes a lot of applications possible [30–32]. However, the shape of GST in these active metasurfaces are relatively complex, the switch CD magnitudes exist in a narrow range and the CD effects only response in a single waveband.
In this paper, the CD effect is greatly enhanced by integrating a GST layer without complex shape on the Z-shape reflection-type metasurface. Due to the lack of magnetic resonance, there is no obvious CD to be observed in the Z-shape metasurface without GST layer. After adding a-GST layer, magnetic dipole resonance is excited and the CD is greatly enhanced at dual-band. The CD magnitudes of dual-band are enhanced to 0.73/−0.27 at 1.58/1.40 µm, respectively. The simultaneous excitations of electric and magnetic dipole resonances are verified by analyzing the characteristics of vector electric fields. Furthermore, with the variation from a-GST to c-GST by thermal stimuli, the CD of 0.73/−0.27 are decreased gradually and almost switched to zero at dual near-infrared wavebands, which boost the development of active chiroptical devices.
2. Design and simulations
Essentially, CD results from the superposition of the radiated electric fields from electric and magnetic dipole resonances. The condition for chiral response is described as $p \cdot m \ne 0$, where p and m are the electric and magnetic dipole moments, respectively [8]. However, only the electric dipole resonance could be excited in a Z-shape gold array, and the magnetic dipole resonance is absent. To simultaneously excite the electric and magnetic dipole resonances, a GST layer is added on the top of Z-shape resonators. As shown in Fig. 2(a), a magnetic dipole perpendicular to the Z-shape resonator is excited in the GST layer. Thus, the CD will be enhanced by simultaneously exciting electric and magnetic dipole resonances in a Z-shape array with added GST layer. Figures 2(b)(c) shows the proposed metasurface with added GST layer. In detail, the Z-shape gold resonators are placed on gold substrate with a dielectric spacer. After elaborate optimization, the thicknesses of Au particles (t1), dielectric spacer (t2) and GST layer (t3) are 150, 63 and 295 nm, respectively.
Fig. 2. (a) The model of electric and magnetic resonances. (b) The schematic diagram of the Z-shape array with added GST layer. (c) The top view of a unit structure. Px = 429 nm, Py = 329 nm, l = 264 nm, w = 112 nm, c = 35 nm.
With the illumination of linearly polarized waves, the relationship between incident and reflected waves can expressed by the Jones matrix Rlin:
Polarization spectra are numerically calculated by using the frequency domain finite element method (FEM) in CST Microwave Studio, which is a commercial software. The boundary conditions are unit cell in ± x and ± y directions and open (add space) in ± z directions. The meshes are set as adaptive tetrahedral grids. The refractive index of dielectric spacer is set as CaF2 (n = 1.48). In simulations, the dispersive optical parameters of GST at amorphous and crystalline states are derived from Ref. [27], and the dispersive optical parameters gold are derived from Ref. [33].
3. Results and discussion
3.1 Enhanced CD
To illustrate the enhanced CD by adding GST layer, the polarization spectra of the Z-shape array without GST layer are calculated first for comparison. Figure 3(a) shows the reflection coefficient spectra of Z-shape gold array. Not only the reflection coefficients of the co-polarized waves are overlapped, but also the reflection coefficients of the cross-polarized waves are almost the same over the whole waveband. Since the incident circularly polarized light (CPL) are completely prohibited by gold substrate ($T = 0$), the absorption could be simplified into ${A_ \pm } = 1 - {R_ \pm } = 1 - ({{{|{{r_{ {\mp}{\pm} }}} |}^2} + {{|{{r_{ {\pm}{\pm} }}} |}^2}} )$, where “+” and “-” represent the right- and left-handed circular polarization states of incident electromagnetic waves respectively. Consequently, the absorptions of LCP and RCP waves are almost the same in Fig. 3(b). Only a slight absorption difference between LCP and RCP waves appeared for the Z-shape array without GST.
Fig. 3. The weak CD of the metasurface without GST layer (a) The reflection coefficients of the metasurface without GST layer. (b) The polarization dependent absorptions of the metasurface without GST layer.
After integrating a GST layer into the Z-shape array, the CD are greatly enhanced. Figure 4(a) shows the polarization reflection coefficients of the metasurfaces with a-GST top layer. Due to ${C_2}$ rotational symmetry of Z-shape array, the co-polarized reflection coefficients are equal (${r_{ +{+} }} = {r_{ -{-} }}$) [14]. Differently, the reflection coefficients of the cross-polarized waves are totally different. The cross-polarized reflection coefficient ${r_{\textrm{ + } - }}$ are much higher than ${r_ - }_ + $ near 1.58 µm. Oppositely, the cross-polarized reflection coefficient ${r_ - }_ + $ are much higher than ${r_ - }_ + $ near 1.40 µm. The differences of cross-polarized reflection coefficients at dual wavebands predict the opposite polarization selectivity for incident CPL. Furthermore, the polarization reflections are calculated. According to the formula ${R_ \pm }\textrm{ = }{|{{r_{ {\mp}{\pm} }}} |^2} + {|{{r_{ {\pm}{\pm} }}} |^2}$, the polarization reflection spectra are calculated. As expected, the reflection of LCP wave ${R_ - }$ is much higher than the RCP ones (${R_ + }$) near 1.58 µm and the reflection of LCP wave ${R_ - }$ is much lower than the RCP ones (${R_ + }$) near 1.40 µm as shown in Fig. 4(b).
Fig. 4. Enhancing CD in the GST integrated metasurface. The (a) reflection coefficients, (b) reflections (c) absorptions and (d) CD spectra under the excitations of CPL at amorphous state of GST.
Figure 4(c) verifies the dual-waveband absorption differences between LCP and RCP waves. Specifically, the absorption of incident LCP wave ${A_ - }$ reaches 0.23, but the RCP one ${A_ + }$ is about 0.96 at 1.58 µm. CD represents the absorption difference between RCP and LCP waves, which is described by cross-polarization reflection coefficients as $CD = {A_ + } - {A_ - }$. The CD at dip and peak reach −0.27 and 0.73 at 1.40 and 1.58 µm respectively in Fig. 4(d). Compared the chirality of the Z-shape arrays with and without GST layer in Fig. 3(b) and Fig. 4(c), CD is enhanced greatly by adding a-GST layer on the top of Z-shape gold array.
For comparing the polarization characteristics between the a-GST integrated metasurface and the one without GST, the electric field vector distributions excited by LCP and RCP waves are plotted in Fig. 5. For the metasurface without GST layer, the vector electric fields excited by RCP and LCP waves at 1.58 µm are very weak as shown in Figs. 5(a)(b)(g)(h). The weak electric field is in accordance with our expectation since most of the LCP and RCP are directly reflected. Differently, after adding GST layer, the electric fields of LCP and RCP in Figs. 5(c)(e)(i)(k) are much stronger and the distribution characteristics are totally different.
Fig. 5. (a), (b), (g), (h) Vector electric field distributions at 1.58 µm for the metasurface without GST. The vector electric and magnetic field distributions for metasurfaces integrated with (c), (d), (i), (j) a-GST layer and (e), (f), (k), (l) c-GST layer.
In detail, the electric field of the metasurface with GST layer excited by RCP wave is very strong and confined near Z-shape gold resonators. The interaction of metal Z-shape resonator with CPL waves can excite surface plasmon resonators, behaving as the electric dipole resonance mode marked by black arrows in Fig. 5(c) at the corner of Z-shape resonator. Also, the vector electric field in the y-z plane is investigated. The circled electric fields marked by the half-circled curve in Fig. 5(i) is consistent with the characteristic of magnetic dipole resonance, and the magnetic field in Fig. 5(j) is pointing outward in the GST layer. The half of circled electric fields in the x-y plane and the magnetic fields perpendicular to x-y plane demonstrate the excitations of magnetic dipole resonance in GST layer by incident RCP wave. In summary, after integrating the GST layer, the electric and magnetic dipole resonances are simultaneously excited by RCP wave, leading to the high absorption. However, the electromagnetic fields excited by LCP wave in Figs. 5(e)(f)(k)(l) are much weaker than the RCP ones. There is no obvious characteristics supporting the excitations of electric and magnetic dipole resonances under the incidence of LCP wave.
3.2 Switchable CD
Owing to the stable characteristic of disordered amorphous GST (a-GST) and ordered crystalline GST (c-GST), the CD strength of the Z-shape metasurface with GST layer could be dynamically tuned by switching the phase of GST between amorphous and crystalline states. For the Z-shape array with crystalline GST layer, the both absorptions of LCP and RCP waves are above 0.6 (Fig. 6(a)) because of the high imaginary part of c-GST. The CD of the c-GST based active matesurface is very weak in the whole band ($CD \approx 0$) in Fig. 6(b).
Fig. 6. Switchable CD. The (a) absorption spectra of the proposed metasurface with c-GST. (b) The switchable CD spectra with different-degree crystalline GST.
As expected, our proposed metasurface exhibits totally different CD spectra with the GST phase variation from amorphous to crystalline state as shown in Fig. 6(b). Specifically, with the phase change of GST from amorphous to crystalline states, the CD is switched to near zero from 0.73 at 1.58 µm. To quantitively estimate the dynamic switching capability of CD magnitudes, the CD difference expressed as $\Delta CD = C{D_{\textrm{a - GST}}} - C{D_{\textrm{c - GST}}}$ is also calculated. The CD differences ($\Delta CD$) are 0.27 and 0.73 at 1.40 and 1.58 µm, respectively, which represent the strong switchable capability for CD magnitudes. By elaborately controlling the crystallization degree of GST, the CD could be continuously switched. When the GST becomes into partial crystalline state, the CD peak would redshift from 1.58 um to 1.60 um and the CD value at the wavelength of 1.58 nm is between 0.73 and 0. Actually, the electric and magnetic resonances for the metasurface with partial crystalline GST are weaker than that of a-GST, but stronger than the c-GST one. As a result, the CD strength at a certain wavelength could be dynamically switched in a large range.
4. Summary
In this paper, the CD effect is enhanced by integrating a Ge2Sb2Te5 layer on the top of Z-shape array. The absorptions of LCP and RCP waves in the metasurface without top GST layer are slightly different, representing the weak CD effect. After integrating the GST layer, with the illumination of RCP waves, not only the electric dipole resonance in Z-shape array is excited, but the magnetic dipole also exists in the GST layer. The polarization selective excitations of electric and magnetic dipole resonances result in the great enhancement of CD. Furthermore, the excellent switching capability brought by phase change material GST is demonstrated. Before and after the phase change of GST, CD magnitudes change by 0.73/0.27 at dual wavebands. The novel design of enhancing CD by adding a simple GST layer paves a new way to improve the performance of chiroptical meta-devices.
Funding
National Natural Science Foundation of China (12004080, 61705046); Special Fund for Application, Science and Technology Planning Projects of Guangdong Province of China (2017B010127002).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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