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Shi Z L, He J L, Wang J J, et al. Design of tunable circular dichroism extrinsic chiral metasurface based on phase change material GST[J]. Opto-Electron Eng, 2022, 49(10): 220092. doi: 10.12086/oee.2022.220092
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Design of tunable circular dichroism extrinsic chiral metasurface based on phase change material GST
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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.-
Keywords:
- metasurface /
- extrinsic chirality /
- circular dichroism /
- tunable
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References
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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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Shi Z L, He J L, Wang J J, et al. Design of tunable circular dichroism extrinsic chiral metasurface based on phase change material GST[J]. Opto-Electron Eng, 2022, 49(10): 220092. doi: 10.12086/oee.2022.220092
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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
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Figure 2.
(a) Dielectric constants of amorphous and crystalline GST [27-28]; (b) Sketch of phase transitions between crystalline and amorphous phases of GST [8]
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Figure 3.
Transmission coefficients for (a) crystalline and (b) amorphous states of GST
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Figure 4.
The CD spectra for amorphous and crystalline states of GST at θ = 50° and φ = 0°
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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°
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Figure 6.
The CD spectra with different (a) period p and (b) thickness t in the amorphous state of GST layer
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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
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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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