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Design of tunable circular dichroism extrinsic chiral metasurface based on phase change material GST
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摘要:
本文提出了一种基于相变材料Ge2Sb2Te5 (GST)的圆二色性可调谐外在手征超表面，该超表面由两层对称的银(Ag)方形开口谐振环和GST中间层单元周期排列而成。结合斜入射光线，该超表面能实现与手征结构相同的电磁特性。数值模拟结果表明：该超表面在50 THz~300 THz的频率范围内，GST为非晶态时，圆二色性(CD)值最大为0.85；GST为晶态时，CD值最大为0.52。当GST在两种相态(非晶态-晶态)之间切换时，实现了70 THz左右的频率调谐。通过研究电场分布，解释了圆二色性产生的原因；还研究了入射角和结构参数对该超表面圆二色性的影响。这项研究在光频段高效偏振调制器件、圆偏振器和偏振滤光器等方面有潜在的应用价值。
Abstract:We propose a metasurface with tunable circular dichroism extrinsic chiral based on the phase change material Ge2Sb2Te5 (GST). The metasurface is composed of two symmetrical square silver split ring resonators and a GST intermediate layer arranged periodically. Combined with the oblique incident light, this metasurface is capable of achieving electromagnetic properties similar to that of the chiral structure. Numerical simulation results show that in the frequency range of 50 THz~300 THz, the maximum circular dichroism (CD) of the metasurface is 0.85 in the amorphous state of GST and 0.52 in the crystalline state of GST. When the GST switches between two states (amorphous-crystalline), a tunable frequency of about 70 THz is achieved. Compared with the reported works, the CD of this metasurface is larger and the tuning range is wider. By studying the electric field distribution, the origin of the circular dichroism is explained; and the effects of incidence angles and structural parameters on the CD of this metasurface are investigated. This research will have potential applications in efficient polarization modulation devices, circular polarizers, and polarization filters in the optical frequency band.
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Key words:
- metasurface /
- extrinsic chirality /
- circular dichroism /
- tunable
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Overview: Chiral metasurfaces have physical properties that are difficult to achieve with natural materials, such as strong circular dichroism (CD) and optical activity (OA), asymmetric transmission (AT), and negative refractive index. Unlike the conventional chiral metasurfaces, the structure of extrinsic chiral metasurface is symmetrical. Its chiral electromagnetic response is generated by the effect of oblique incidence of light combined with the non-chiral structure, which reduces the common problems of complex structure, single function, and narrow band width of some metasurfaces. However, once a conventional metasurface is manufactured, its function is also fixed. Therefore, many scholars have been working on tunable extrinsic chiral metasurfaces. By adding tunable materials such as phase change material (PCM), graphene, liquid crystals, etc. in the design of the metasurfaces, together with the action of external excitation, the electromagnetic properties of the metasurfaces can be tuned to achieve wavefront tuning of electromagnetic waves, which has potential applications in various optical devices such as sensors, polarizers, and detectors.
Phase change material Ge2Sb2Te5 can perform non-volatile, fast, and reversible switching between amorphous and crystalline states by electrical or optical excitation. The metasurface based on phase change materials is one of the current research hotspots, but there is no research has been reported on tunable metasurface with external chiral characteristics in the optical frequency band.
In this paper, a tunable extrinsic chiral metasurface with giant circular dichroism in the optical frequency band is proposed. The unit cell of the metasurface consists of two symmetrical square silver split rings and a GST film sandwiched between them. Compared with the works reported in the existing literature, the CD of the metasurface is larger and the tuning range is wider. In the frequency range of 50 THz~300 THz, the CD of this extrinsic chiral metasurface is up to 0.85 when the GST is amorphous. Due to the large loss of the crystalline GST, the extrinsic chiral response is weakened and the maximum CD is 0.52 in the crystalline state. The GST switches between two phase states (amorphous-crystalline) and enables the frequency tuning range to reach about 70 THz. Further studies have shown that the CD can be tuned by changing the incident angle and the geometric parameters of the GST layer. When θ = 0° and φ = 0°, the CD is zero in all frequency bands, which means when the wave is incident normally, there is no intrinsic chiral characteristics when the wave is incident normally. The electric field distributions at the resonance point in different phase states are also investigated. This work provides a way to realize devices such as efficient polarization modulation devices, circular polarizers, and polarization filters in the optical frequency band.
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图 1 设计的外在手征超表面结构阵列及其单元结构示意图。 (a) 外在手征原理和斜入射方向；(b) 单元结构；(c) 具体结构参数
Figure 1. Schematic diagram of the designed extrinsic chiral metasurface structure array and its unit cell structure. (a) The principle of extrinsic chirality and the direction of oblique incidence; (b) Unit cell structure; (c) The specific structural parameters
图 3 GST为(a)晶态和(b)非晶态时的透射系数
Figure 3. Transmission coefficients for (a) crystalline and (b) amorphous states of GST
图 4 在θ = 50°，φ = 0°时，GST为非晶态和晶态时的CD谱
Figure 4. The CD spectra for amorphous and crystalline states of GST at θ = 50° and φ = 0°
图 5 当φ = 0°，(a) GST为非晶态和(b)晶态时不同入射角θ的CD参数曲线；当θ = 50°，(c) GST为非晶态和(d)晶态时不同入射投影角φ的CD参数曲线
Figure 5. The curve of CD parameters with different incidence angles θ in GST (a) amorphous and (b) crystalline states when φ = 0°; The curve of CD parameters with different incidence projection angles φ in GST (c) amorphous and (d) crystalline states when θ = 50°
图 6 GST为非晶态时，不同(a)周期p和(b)厚度t的CD谱
Figure 6. The CD spectra with different (a) period p and (b) thickness t in the amorphous state of GST layer
图 7 当θ = 0°，φ = 0°时的电场分布图。 (a) 晶态GST，LCP入射，f = 130 THz；(b) 晶态GST，RCP入射， f = 130 THz；(c) 非晶态GST，LCP入射，f = 200 THz；(d) 非晶态GST，RCP入射，f = 200 THz
Figure 7. Electric field distribution when θ = 0°，φ = 0°. (a) Crystalline GST, LCP incidence, f =130 THz; (b) Crystalline GST, RCP incidence, f =130 THz; (c) Amorphous GST, LCP incidence, f = 200 THz; (d) Amorphous GST, RCP incidence, f = 200 THz
图 8 当θ = 50°，φ = 0°时的电场分布图。 (a) 晶态GST，LCP入射，f = 130 THz；(b) 晶态GST，RCP入射， f = 130 THz；(c) 非晶态GST，LCP入射， f = 200 THz；(d) 非晶态GST，RCP入射，f = 200 THz
Figure 8. Electric field distribution when θ = 50°，φ = 0°. (a) Crystalline GST, LCP incidence, f =130 THz; (b) Crystalline GST, RCP incidence, f =130 THz; (c) Amorphous GST, LCP incidence, f = 200 THz; (d) Amorphous GST, RCP incidence, f = 200 THz
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