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摘要:
为满足太赫兹器件对多功能偏振调控与动态可调特性的需求，提出了一种基于石墨烯载流子调控的双功能可切换太赫兹手性超表面。通过改变入射波偏振态，并利用外加栅压连续调节石墨烯费米能级，该结构可分别表现出圆二色性(CD)和线二色性(LD)响应，从而实现不同偏振选择性吸收特性之间的切换。仿真结果表明，当石墨烯费米能级为1 eV时，超表面在2.65 THz处对左右旋圆偏振光产生显著选择性吸收，CD幅值达到0.89，在1.97至3.44 THz频段内CD幅值保持在0.6以上；当费米能级降至0.2 eV时，结构在1.91 THz处表现出明显的线二色性响应，LD幅值达到0.75。电场与表面电流分布分析表明，不同偏振态在谐振条件下激发的电共振强度差异是产生偏振选择性吸收的主要原因。此外，该结构对入射角和结构参数变化具有较好的稳定性，在圆/线二色性探测、偏振调控以及太赫兹光学器件等方面具有潜在应用价值。
Abstract:To meet the demand for multifunctional polarization manipulation and dynamic tunability in terahertz devices, a dual-functional switchable chiral metasurface based on graphene carrier modulation is proposed. By changing the polarization state of the incident wave and continuously tuning the Fermi level of graphene through an external gate voltage, the structure can exhibit circular dichroism (CD) and linear dichroism (LD) responses, enabling switching between different polarization-selective absorption characteristics. Simulation results show that when the graphene Fermi level is 1 eV, the metasurface exhibits pronounced selective absorption for left- and right-handed circularly polarized waves at 2.65 THz, with a CD value reaching 0.89, and maintaining CD values above 0.6 within the frequency range of 1.97–3.44 THz. When the Fermi level decreases to 0.2 eV, a significant linear dichroism response appears at 1.91 THz, with an LD value of 0.75. Analysis of the electric field and surface current distributions reveals that the difference in the strength of electric resonances excited under different polarization states is the primary mechanism responsible for polarization-selective absorption. In addition, the proposed structure demonstrates good robustness against variations in the incident angle and structural parameters, indicating its potential applications in circular/linear dichroism detection, polarization control, and terahertz photonic devices.
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Key words:
- metasurfaces /
- circular dichroism /
- linear dichroism /
- tunability
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图 1 手性超表面单元结构示意图。(a)三维原理图；(b)俯视图及几何参数定义
Figure 1. Schematic diagram of the chiral metasurface unit cell. (a) Three-dimensional schematic view; (b) top view with the definition of geometric parameters
图 2 超表面对两种圆偏振光的电磁响应曲线。(a)反射系数；(b)吸收谱及CD谱
Figure 2. Electromagnetic response curves of the metasurface to two types of circularly polarized light. (a) Reflection coefficients; (b) absorption spectrum and CD spectrum
图 3 超表面对两种线偏振光的电磁响应曲线。(a)反射系数；(b)吸收谱及LD谱
Figure 3. Electromagnetic response curves of the metasurface to two orthogonal linearly polarized light. (a) Reflection coefficients; (b) absorption spectra and LD spectra
图 4 两种偏振条件下的动态调控特性。(a)圆偏振光入射时的CD光谱；(b)线偏振光入射时的LD光谱
Figure 4. Dynamically tunable characteristics of the metasurface under two polarization conditions. (a) CD spectra under circularly polarized incidence; (b) LD spectra under linearly polarized incidence
图 5 石墨烯费米能级分别为1 eV与0.2 eV时，对应谐振频率的电场与电流分布。(a)(e) LCP光在2.65 THz入射；(b)(f) RCP光在2.65 THz入射；(c)(g) x偏振光在1.91 THz入射；(d)(h)y偏振光在1.91 THz入射
Figure 5. Electric field and surface current distributions at the corresponding resonance frequencies for graphene Fermi levels of 1 eV and 0.2 eV. (a)(e) LCP incidence at 2.65 THz; (b)(f) RCP incidence at 2.65 THz; (c)(g) x-polarized incidence at 1.91 THz; (d)(h) y-polarized incidence at 1.91 THz
图 6 改变结构参数对CD的影响。(a)晶胞周期P；(b)金开口环外径r；(c)金开口环宽度w1；(d)石墨烯圆弧宽度w2
Figure 6. The effect of structural parameter variations on CD. (a) Unit-cell period P; (b) outer radius r of the gold split ring; (c) width w1 of the gold split ring; (d) width w2 of the graphene arc
图 7 改变结构参数对LD的影响。(a)晶胞周期P；(b)金开口环外径r；(c)金开口环宽度w1；(d)石墨烯圆弧宽度w2
Figure 7. The effect of structural parameter variations on LD. (a) Unit-cell period P; (b) outer radius r of the gold split ring; (c) width w1 of the gold split ring; (d) width w2 of the graphene arc
图 8 入射角对CD与LD影响。(a)CD响应的等值线分布；(b)LD响应的等值线分布
Figure 8. Influence of the incidence angle on the CD and LD. (a) Contour map of the CD response; (b) Contour map of the LD response
表 1 与近年来报道的超表面的比较
Table 1. Comparison with recently reported metasurfaces
Ref. Working band Tunability Modulation method Device structure Response bandwidth CD LD [45] Near-infrared No Fixed geometrical design Au rectangular-hole array / SiO2 spacer / Au mirror CD peak near 880 nm 0.76 - [46] THz Yes VO2 phase-transition modulation VO2-integrated double-aperture metasurface Switching at 2.68 THz; dual-band polarization-selective absorption - 0.86 [47] THz Yes Temperature-tunable InSb Double-layer InSb chiral resonators / dielectric spacer / metal ground plane CD peaks at 2.31 and 5.68 THz 0.89 - [48] Microwave No Fixed geometrical design G-shaped split-ring resonator metasurface CD peaks at 7.7, 8.3, 11.5 GHz;
LD peak at 17.3 GHz0.90 0.52 [49] THz Yes Thermally driven VO2
phase-transition modulationVO2-filled Au split-ring resonator / polyimide spacer / Au ground plane CD > 0.40: 0.60–0.72 THz;
LD > 0.30: 0.61–0.74 THz0.64 0.54 [50] THz Yes VO2 temperature modulation and graphene electrical modulation Two-layer graphene metasurfaces / VO2 film / Au substrate PCR > 0.9: 2.89–4.02 THz;
ellipticity > 0.9: 2.32–2.69 THz0.71 0.60 This work THz Yes Electrical modulation by tuning
the graphene Fermi levelGraphene arc–Au split-ring resonator / Topas spacer / Au ground plane CD > 0.6: 1.97–3.44 THz;
LD peak at 1.91 THz0.89 0.75 
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