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Abstract
Intrinsic and extrinsic chiral responses have been widely investigated in metamaterials, however the relationship between them has been seldom discussed. We numerically and experimentally demonstrate angle enhanced chiral dichroism and study the separation between intrinsic and extrinsic chiral responses in metamaterial with asymmetrically split aperture dimers. The metamaterial exhibits triple-band resonant circular dichroism at normal incidence. The oblique incidence leads to giant enhancement of circular dichroism at two low-frequency resonances while yields an obvious resonance split of the circular dichroism in the vicinity of the high-frequency resonance. The whole circular dichroism response results from the balance between intrinsic and extrinsic chirality and the circular dichroism spectra at positive and negative angles of incidence exhibit an asymmetry due to the existence of intrinsic chirality. Importantly, the intrinsic chirality in the metamaterial may be individually investigated since extrinsic chiral response may be removed from the total circular dichroism by superimposing two circular dichroism spectra at positive and negative incident angles. The metamaterial will be promising to achieve enhanced chiral response and also separately utilize intrinsic and extrinsic chirality for manipulating the polarization state of light.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Chiral metamaterial is one of the most important candidates since the advent of metamaterial. An object that can’t be superimposed to its mirror image by in-plane translation and rotation is chiral. Chiral metamaterials have been initially proved to realize negative refraction [1,2] and then achieve polarization manipulation and chiral sensing [3–9]. They can be divided into 3D and 2D (planar) chiral metamaterials, exhibiting optical activity and asymmetric transmission, respectively [10]. 3D chiral metamaterial can be fabricated by top-down methods and self-assembled technology [11]. Reducing fabrication difficulty, various bilayer metamaterials (i.e. quasi-3D structure) fabricated by well-established planar technologies have been widely studied to achieve 3D chiral responses, such as rosettes [12], split rings [13–16], twisted crosses [17], gammadions [18,19] and coupled arcs [20]. Intrinsically, the circular dichroism usually arises from the coupling between two spatially separated resonators lacking two-fold rotation symmetry. This kind of metamaterial has a sense of twist, to some extent equivalent to a helical structure, and it can be regarded as intrinsically chiral one [10]. Moreover, there are several alternative ways to realize chiral responses, such as non-chiral metasurfaces with designed supercell geometries [21] or heteromaterials [22] and extrinsically chiral metamaterials [23,24]. If the whole configuration of one achiral metamaterial together with tilted wave vector cannot be superposed upon its mirror image, it is extrinsically chiral, in which metamolecules usually lack 2-fold rotational symmetry [24]. Extrinsically chiral metamaterials have two important properties. On the one hand, chiral enantiomers may be easily formed by oblique incidence with opposite signs. On the other hand, the strength of chiral response can be tuned by angle of incidence. In recent years, extrinsically chiral metamaterials have been attracted much attention to enhance circular dichrosim resorting to oblique incidence [25–27]. Extrinsic chirality offers a promising way to realize tunable optical manipulation [28,29] and polarization devices, such as circular polarizer [26] and spin-dependent reflector and absorber [30]. A plasmonic metal–insulator–metal metasurface has been demonstrated to be reconfigurable by either changing its intrinsic or extrinsic chirality, in which the magnitude of the circular dichroism is up to 80% [31]. The cantilever based metamaterial has been reported to have dual-band chirality with intrinsic and extrinsic chiral responses and experimentally demonstrated to realize reconfigurable metamolecules by applying bias voltage [32]. However, it is unclear how the intrinsic and extrinsic chiral responses affect the total circular dichroism in one metamaterial since extrinsic chiral responses have been studied in non-chiral metamaterials without intrinsic chiral responses in previous literatures. The individual contributions of intrinsic and extrinsic chiral responses have not been fully investigated and remain somewhat challenging. It is necessary and important to investigate intrinsic and extrinsic chiral response in single metamaterial.
In this work, we report angle enhanced chiral dichroism and study the separation between intrinsic and extrinsic chiral responses in metamaterial with asymmetrically split aperture dimers. The metamaterial exhibits triple-band resonant circular dichroism at normal incidence. The oblique incidence leads to giant enhancement of circular dichroism at two low-frequency resonances while yields an obvious resonance split of the circular dichroism in the vicinity of high resonant frequency. The angle-induced circular dichroism varies from -0.40 to 0.44. In addition, extrinsic chiral response in the metamaterial may be removed from the total response by incorporating two circular dichroism spectra at positive and negative incident angles. The asymmetrically split aperture metamaterial is promising to achieve enhanced chiral response and manipulate the polarization state of light.
2. Metamaterial design
Fig. 1. (a) Illustration of the extrinsic and intrinsic chirality in metamaterials. The total chiroptical strength refers to the superposition of intrinsic and extrinsic chiral responses that are caused by intrinsic chiral media and oblique incidence, respectively. θ indicates angle of incidence that is achieved by tilting the metamaterial around the y axis. The rotation angle γ is defined as the angle between the mirror lines of the top and back layers. (b) The view of bilayer chiral metamaterial with two orthogonal asymmetry split ring apertures. The asymmetry apertures have different arc slits corresponding to open angle α and β. (c) The picture of the sample fabricated by the photolithography technique.
Generally, intrinsic chiral metamaterials exhibit circular dichroism at normal incidence while extrinsic chiral metamaterials show circular dichroism at oblique incidence [10]. Here, we construct a chiral and anisotropic metamaterial and investigate its extrinsic and intrinsic chirality. The bilayer metamaterial readily creates an intrinsic 3D chirality by two twisted identical metamolecules lacking 2-fold rotation symmetry [33]. At oblique incidence, the bilayer 90°-twisted metamaterial will exhibit extrinsic chiral effect. The total chiroptical strength refers to the superposition of intrinsic and extrinsic chiral responses that are caused by intrinsic chiral structure and oblique incidence, respectively. Therefore, the bilayer metamaterial offers a promising paradigm to study the separation between intrinsic and extrinsic chiral responses, illustrated by Fig. 1(a). The proposed metamaterial is constructed by an array of square asymmetrically split aperture dimer with a period of 15 mm. Each dimer consists of bilayer asymmetrically split ring apertures that are perforated through 35µm-thick copper claddings on either side of F4B PCB substrate. The thickness of the F4B layer is t = 1.5 mm. The two spatially separated asymmetrically split ring apertures are the same, but rotated by γ = 90° around the z axis with respect to each other, as shown in Fig. 1(b). The asymmetric apertures in each layer have two circular slits corresponding to different open angles α and β. The inner radius of the slit is r = 5.6 mm and the outer radius is R = 6.4 mm. Figure 1(c) is the image of the fabricated sample with the overall size of the sample is 300 × 300 mm2. The bilayer 90°-twisted metamaterial with open angles of α = 100° and β = 160° is numerically and experimentally investigated. The simulated results are calculated by using a commercial electromagnetic software, in which the periodic boundary conditions are applied along the x and y directions and the perfect matched layer is used along the z direction. The copper is treated as a perfectly electric conductor in the microwave frequency and the relative permittivity of the lossy dielectric substrate is ε = 2.65 + i 0.003. The experiments were conducted in an anechoic chamber using two broadband linearly polarized horn antennas (Schwarzbeck BBHA 9120D) and a vector network analyzer (Agilent N5230C), in which they were employed to record the transmission coefficients of incident linearly polarized waves. By changing the orientation of the two horn antennas, all four components of incident linearly polarized waves in Jones matrix can be measured. According to Eq. (1), the complex transmission coefficients of circularly polarized waves can be calculated from the linear to circular base.
3. Results and discussions
Figures 2(a) and 2(b) show the simulated and measured transmission spectra of the bilayer 90°-twisted metamaterial for circularly polarized waves at normal incidence. In Fig. 2(a), the cross-polarization transmission coefficients of T+- and T-+ are equal in the frequency range of 5-11 GHz due to the orthogonal arrangement of two identical structures. Importantly, the co-polarization transmission coefficients of T++ and T− are different since the metamaterial has a sense of twist. Obviously, the bilayer 90°-twisted metamaterial has 3D intrinsic chirality. The measured results in Fig. 2(b) well agree with the simulated ones in Fig. 2(a). Besides, the simulated and measured results of the circular dichroism (CD, labelled by Δ) and polarization azimuth angle are presented in Figs. 2(c) and 2(d). The circular dichroism can be calculated according to Eq. (2).
The polarization azimuth angle can be obtained using Eq. (3)Fig. 2. The transmission and CD spectra of the metamaterial with α = 100° and β = 160° for normally incident circularly polarized waves. (a) and (b) The simulated and measured transmission spectra, (c) and (d) The simulated and measured CD and polarization azimuth rotation angle ϕ. The subscripts + and – refer to right-handed (RCP) and left-handed (LCP) circularly polarized waves, respectively.
The circular dichroism spectra have three intrinsic resonances at around 5.4, 8.2 and 10.2 GHz. At the high frequency, the circular dichroism reaches -0.18. In the broadband frequency range, the polarization azimuth angle is up to 90°. In the frequency range of 10-11 GHz, the polarization azimuth angle can continuously change from -5° to -90°. Except a slight shift, the measured and simulated spectra agree well with each other.
In order to understand the relation between intrinsic and extrinsic chirality, we investigate how the circular dichroism response of the bilayer 90°-twisted metamaterial depends on the angle of incidence. Here, the incident angle θ changes from -40° to 40° by a step of 10°. First, we consider the angular dependence of the circular dichroism in the metamaterial with α = 100° and β = 160° in Figs. 3(a) and 3(b). At normal incidence, the intrinsic circular dichroism is quite weak and both the peak values with different signs are less than 0.025 at 5.4 and 8.2 GHz in Fig. 3(a). When the angle of incidence θ increases from 0° to 40°, both the circular dichroism resonances at 5.4 and 8.2 GHz remarkably experience a giant enhancement by one order of magnitude. The chiral response at 5.4 GHz undergoes its sign change. The value of the circular dichroism decreases from 0.025 to 0 at angle of incidence less than 10° (the detail seen in Fig. 4) and then this sharp resonance is rapidly enhanced to -0.15 at θ = 40°. The negative circular dichroism value at 8.2 GHz is directly enhanced from -0.02 to -0.33. In the range of 0°-40°, the circular dichroism spectrum at 10.2 GHz undergoes a change from single peak to Lorentz resonance, different from two low-frequency resonances. Compared with strong intrinsic circular dichroism at 10.2 GHz, the total chiral response has no enhancement. Contrarily the negative circular dichroism peak weakens with increasing angle of incidence, accompanied by another increasing positive circular dichroism resonance. The sign change of angle of incidence leads to the reversed chiral response in an achiral metamaterial [24]. When the angle of incidence θ alters in the range from 0° to -40°, the two circular dichroism resonances at 5.4 and 8.2 GHz are almost flipped. The circular dichroism value at 5.4 GHz is directly enhanced from 0.025 to 0.2 while the chiral dichroism at 8.2 GHz undergoes its sign change and change from -0.02 to 0.3. The flipped circular dichroism responses at the two resonances exhibit an asymmetry with respect to the zero line for positive and negative angles of incidence due to the existence of intrinsic chirality. However, when the angle of incidence is reversed, the circular dichroism at 10.2 GHz is unpredicted according to the chiral response at positive angle of incidence. Similar to the intrinsic chirality, the circular dichroism spectrum is single resonance and the peak value slightly decreases with increasing angle of incidence. Therefore, the extrinsic chirality as well as the intrinsic chirality in the metamaterial results in the generation of two imperfect chiral enantiomers at angles of incidence with opposite signs. In order to verify the aforementioned chiral response, the angular dependence of the circular dichroism in the metamaterial with α = 100° and β = 160° was measured, shown in Fig. 3(b). The fundamental properties of the metamaterial are observed. Basically, the experimental results agree well with the simulated ones in Fig. 3(a), but the magnitudes of the measured circular dichroism are less than the simulated ones, especially the resonance at 5.4 GHz. The discrepancies may be caused by imperfect sample fabrication and inaccurate incident angle and random noises in the experiment. The asymmetry of the circular dichroism spectra at opposite angles of incidence can be observed, originating from the introduction of the intrinsic chirality.
Fig. 3. Extrinsically enhanced CD spectra at oblique incidence. (a) and (b) The simulated and measured CD spectra of the bilayer 90°-twisted metamaterial with α = 100°, β = 160°.
Fig. 4. The simulated dependence of the CD on the incident angle θ in the range of -88° to 88°. The open angles of asymmetric spilt ring apertures correspond to α = 100°, β = 160°. 1st, 2nd and 3rd denote the resonances at around 5.4, 8.2 and 10.2 GHz.
In order to fully illustrate the angular dependence of the circular dichroism, the chiral response of the bilayer 90°-twisted metamaterial with α = 100° and β = 160° is investigated in the angle range of -88° to 88°. In Fig. 4, there are three resonances located at 5.4, 8 and 10 GHz, which is in consistent with Fig. 2. It is obvious that the oblique incidence leads to giant enhancement of the circular dichroism at two low-frequency resonances and there is a weak asymmetry between two positive and negative circular dichroism spectra. The total circular dichroism changes continuously with the angle of incidence and the circular dichroism reaches 0.40 and -0.44 at around 8.2 GHz for θ = -63° and 63°, respectively. The first resonance is kept unmoved while the second resonance shows a pronounced red shift when the angle of incidence increases. It is unambiguously concluded that the circular dichroism is greatly enhanced by extrinsic chirality. At the third resonance, the intrinsic chirality dominates the circular dichroism spectrum at small incident angle while the extrinsic chirality dominates the circular dichroism at large angle of incidence. The whole circular dichroism response results from the balance between intrinsic and extrinsic chirality.
Undoubtedly, the chiral response of the bilayer 90°-twisted metamaterial can be regarded as the superposition of the intrinsic and extrinsic chirality. Due to the orthogonal arrangement of two identical structures, the metamaterials at positive and negative angle of incidence are geometrically chiral enantiomers, but the circular dichroism spectra are not exactly opposite due to intrinsic circular dichroism. The angular dependence of the intrinsic chirality in the metamaterial with α = 100° and β = 160° can be derived from removing the extrinsic chirality by Δint = (Δ++ Δ-)/2, in which Δint, Δ+ and Δ- denote the intrinsic circular dichroism, circular dichroism at positive and negative angles of incidence, respectively. When the angle of incidence increases, the circular dichroism gradually decreases at three resonant frequencies while the first and second resonances are kept unmoved and the third resonance slightly shifts to red in Fig. 5(a). The signs of the circular dichroism at two low-frequency resonances remain unchanged, while the sign of the circular dichroism at the third resonance is reversed at about θ = 25°, which is attributed to different angle dispersion of RCP and LCP waves. Limited by the experiment, the intrinsic circular dichroism spectra at four angles of θ = 0°, 10°, 20° and 40° are shown in Fig. 5(b). The measured and simulated results are in consistent with each other. Therefore, the bilayer 90°-twisted metamaterial may offer a possible way to study the intrinsic and extrinsic chiral responses as well as angular dependence.
Fig. 5. The angular dependence of the intrinsic chirality in the bilayer 90°-twisted metamaterial. (a) The calculated intrinsic circular dichroism in the angle ranged of 0°-88°. (b) The calculated and measured intrinsic circular dichroism for θ = 0°, 10°, 20° and 40°, marked by the dashed lines in panel a. The solid and dashed lines represent the simulation and experiment results.
In order to understand the transmission properties of the bilayer metamaterial, next we study the dependence of the chiral response on the asymmetry of the asymmetrically split aperture and the rotation angle between the top and back layers when the incident angles are θ = 0° and 30°, respectively. Considering the asymmetry, one open angle is fixed at β = 160° while the other open angle α is the only variable parameter, thus the metamaterial is still a 90°-twisted structure. Under normal incidence, the asymmetry of the asymmetrically split aperture strongly affects the circular dichroism spectra as the open angle α varies from 100° to 160° in Fig. 6(a). For increasing the open angle α, each resonance exhibits an obvious red shift, marked by 1st, 2nd and 3rd (i.e. the same as ones in Fig. 4). As the open angle α varies from 100° to 150°, the circular dichroism at resonance 2nd is greatly enhanced while it is obviously suppressed at resonance 3rd until the positive circular dichroism occurs. Importantly, the circular dichroism is inaccessible in the metamaterial with symmetric split apertures, i.e. α = β = 160°. At oblique incidence of θ = 30°, the circular dichroism at resonance 2nd is several times enhanced in the metamaterial with asymmetrically split apertures (compared with normal incidence) and the signs of the circular dichroism at resonances 1st and 3rd are reversed, as illustrated in Fig. 6(c). Although the oblique incidence is considered, the circular dichroism is also inaccessible in the metamaterial with symmetric split apertures. As a result, the metamolecules lacking 2-fold rotational symmetry are necessary. Next, we discuss how the rotation angle γ affects the circular dichroism of the metamaterial with α = 100° and β = 160°, in which all other parameters are kept unchanged. When the rotation angle decreases from γ = 90°, the circular dichroism spectrum has an obvious shift and the strength of the circular dichroism is strongly affected by the rotation angle under normal incidence in Fig. 6(b). Once the rotation angle is γ = 0°, the circular dichroism of the bilayer metamaterial completely vanishes. At oblique incidence of θ = 30°, typical characteristics of the circular dichroism evolution are similar to the case in Fig. 6(c). The circular dichroism at resonance 2nd is several times enhanced (compared with normal incidence) since there is a rotation angle between two layers, while the signs of the circular dichroism at resonances 1st and 3rd are changed, as illustrated in Fig. 6(d). Although the metamaterial is illuminated by oblique incident wave, the circular dichroism is not allowed in the metamaterial without any rotation angle. Therefore, the metamolecules lacking 2-fold rotational symmetry and the rotation angle are required in the bilayer metamaterial for achieving circular dichroism.
Fig. 6. The dependence of the chiral response on the asymmetry of asymmetrically split aperture and the rotation angle in the bilayer metamaterial. (a) The circular dichroism spectra of the metamaterial for different asymmetries at θ = 0°. (b) The circular dichroism spectra of the metamaterial for different rotation angles at θ = 0°. (c) The circular dichroism spectra of the metamaterial for different asymmetries at θ = 30°. (d) The circular dichroism spectra of the metamaterial for different rotation angles at θ = 30°.
The surface current distributions of the metamaterial are presented in Fig. 7 to better explain why the bilayer 90°-twisted metamaterial has three transmission bands, in which the open angles of α = 100° and β = 160° are kept unchanged. Obviously, three transmission resonances arise from different excited modes of the asymmetrically split apertures. Excitations of both short and long apertures contribute to the occurrence of resonance 1st, while excitations of long and short apertures individually dominate the advent of resonances 2nd and 3rd. When the oblique incidence occurs, the magnetic component of incident wave will pass through the aperture of the metamaterial, thus the chiral response of the metamaterial will be modulated by angles of incidence.
Fig. 7. The surface current distributions of the top and back layers in the bilayer 90°-twisted metamaterial at resonances 1st, 2nd and 3rd under normal incidence of θ = 0°. 1st, 2nd and 3rd are also marked in Fig. 4. The open angles are α = 100° and β = 160°.
4. Conclusion
In conclusion, we have numerically and experimentally demonstrated angle enhanced chiral dichroism and investigated intrinsic and extrinsic chirality in bilayer metamaterial with asymmetrically split aperture dimers. The metamaterial exhibits triple-band resonant circular dichroism at normal incidence. The oblique incidence leads to the enhancement of circular dichroism at two low-frequency resonances by one order of magnitude while yields an obvious resonance split of the circular dichroism in the vicinity of high-frequency resonance. The angle-induced circular dichroism varies from -0.40 to 0.44. The whole circular dichroism response results from the balance between intrinsic and extrinsic chirality and the circular dichroism spectra at positive and negative angles of incidence exhibit an asymmetry due to the existence of intrinsic chirality. Importantly, the intrinsic chirality may be individually investigated since extrinsic chiral response in the bilayer 90°-twisted metamaterial may be removed from the total circular dichroism by superimposing two circular dichroism spectra at positive and negative incident angles. The bilayer 90°-twisted metamaterial will be promising to achieve enhanced chiral response and also separately utilize intrinsic and extrinsic chirality for manipulating the polarization state of light.
Funding
National Natural Science Foundation of China (61675054, 91750107, U1931121); Natural Science Foundation of Heilongjiang Province (ZD2018015); Fundamental Research Funds for the Central Universities; 111 Project Harbin Engineering University (B13015).
Disclosures
The authors declare no conflicts of interest.
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