基于二氧化钒和狄拉克半金属的太赫兹传感器/滤波器设计【增强内容出版】
With the continuous advancement of terahertz technology, its applications in wireless communication, medical imaging, and security screening are expanding. Metasurfaces are widely used in terahertz device designs due to their effective modulation properties for terahertz waves. Traditional metallic materials fail to achieve active tuning of terahertz devices. Therefore, the design of terahertz devices with multifunctionality using tunable materials such as vanadium dioxide and Dirac semimetals has become increasingly attractive. In this work, we propose a metasurface with configurable functions based on Dirac semi-metallic mirror-symmetric double-opening rings. By utilizing the phase transition property of the vanadium dioxide, we have realized the switching of the filter and sensor functions in a single device structure. The results not only demonstrate the possibility of implementing the multifunctional metasurface design in the terahertz band but also can promote the application of terahertz technology in the future.
In this study, a metasurface terahertz device with switchable sensor and filter functions was designed by utilizing the Dirac semimetal and vanadium dioxide. When the vanadium dioxide was transformed from the insulating state to the metallic state, the structure could operate as a sensor and a filter, respectively. By using the frequency-domain finite-element method (FEM) based on the commercial software CST Microwave Studio, the performance of the designed device was simulated. The transmission spectra in both functional modes were studied. Based on the three-dimensional electromagnetic simulations, the analysis of the physical mechanism of the device was carried out through the resonance characteristics and the electric field distributions. Moreover, the sensitivity of the device used as a sensor was investigated through simulation by changing the samples with different refractive indices.
By changing the ambient temperature, the vanadium dioxide in the designed device can be transformed from the insulating state to the metallic state, so that the device works in sensor and filter mode, respectively. When the vanadium dioxide is in the insulating state, the device is in the sensor mode, and it achieves sharp transmission dips (Fig. 3). We explain the resonance principle through the transmission spectrum and the electric field distributions (Figs. 4-6). In addition, the resonance can be enhanced as the parameter d increases (Fig. 7), simultaneously causing a change in the Q value at each resonance point. As d increases, the Q value of p3 increases, while other resonance points decrease (Fig. 8). Moreover, the Fermi energy level also influences the resonant frequency and resonant intensity of the sensor (Fig. 9). Simulations show that the sensitivity can be increased with the increase in the sample thickness. When the sample thickness is 10 μm, the sensitivity is 106 GHz/RIU, and when the sample thickness is increased to 45 μm, the sensitivity reaches a saturation value of 226 GHz/RIU (Fig. 14). Moreover, the effect of the distance between the sample and the metasurface on the sensitivity was explored when the sample thickness was fixed at 10 μm. The results show that as the distance increases, the sensitivity increases and reaches a maximum value of 130 GHz/RIU (Fig. 15). When the vanadium dioxide is in the metallic state, the device turns out into the bandpass filter mode. It operates with the insertion loss of 1.3 dB at the center frequency of 0.84 THz while the return loss is 12.7 dB (Fig. 16). The resonance mechanism of the filter was discussed through the transmission curves and electric field distributions. The top vanadium dioxide layer can prevent the terahertz waves from entering the metasurface, thereby suppressing the electric field intensity of the resonance rings (Figs. 17-19). Furthermore, the center frequency can be tuned by adjusting the length of the parameter d (Fig. 20).
A terahertz metasurface based on vanadium dioxide and Dirac semimetal is proposed, which can switch between two functions by adjusting the conductivity of vanadium dioxide. When vanadium dioxide is in the insulating state, the metasurface is a terahertz sensor, and the sensitivity of the sensor is related to the thickness of the sample and the distance between the sample and the sensor. When the vanadium dioxide is in the metallic state, the metasurface is a terahertz bandpass filter, which has a center frequency of 0.84 THz. The insertion loss and return loss at the center frequency are 1.3 dB and 12.7 dB, respectively. The resonance principle is investigated by analyzing the electric field distributions of this metasurface. This work demonstrates the possibility of implementing the multifunctional metasurface design in the terahertz band. The structure proposed in this paper has potential applications in future terahertz sensor, filter, and multifunctional device designs.
1 引言
太赫兹(THz)波是指波长在0.03~3 mm范围内,频率为0.1~10 THz,介于微波与红外线之间的电磁波[1-2],已被广泛应用于无线通信、医学成像和安全检测等领域。由于传统材料对太赫兹波的弱响应阻碍了太赫兹技术的发展,研究者们提出超材料的概念,从而推动了太赫兹技术的进一步发展。超材料是一种复合型人工合成材料,具有奇特的物理结构以及天然材料所不具有的电磁特性[3]。超表面作为二维超材料可以实现对电磁波在振幅、相位、偏振等方面的调控,因此被广泛应用于太赫兹功能器件的设计[4-6]。
太赫兹传感器在环境监测、医学研究、食品安全等方面发挥了重要作用[7-8]。然而,随着研究的深入,研究者们发现基于传统金属材料不能实现介电常数的动态调节,对于设计动态可调的太赫兹传感器存在一定的限制。近年来,石墨烯、狄拉克半金属、二氧化钒(VO2)等可调谐材料的出现为实现超表面传感器提供了新的思路[9-10]。其中,狄拉克半金属被称为三维(3D)石墨烯。与石墨烯相比,狄拉克半金属不容易受到介电常数干扰,没有表面多余电子,容易制备,而且性能稳定。此外,狄拉克半金属具有线性能量色散:在较低频率下,金属响应占主导地位,表现为等离激元;在更高的频率下,介电响应占主导地位[11]。因此,狄拉克半金属在太赫兹传感、太赫兹探测等领域具有重要应用[12]。2022年,Wang等[13]基于狄拉克半金属实现了可调谐高灵敏度超表面传感器,该传感器实现了3个吸收率均大于99.8%的完美吸收峰,传感器的最大灵敏度高达238 GHz/RIU,实现了高灵敏度太赫兹传感。2023年,Hou等[14]基于光学连续域束缚态(BIC)提出一种超表面,实现了无限大品质因子传感器,其折射率灵敏度为158 GHz/RIU。利用狄拉克半金属设计太赫兹超表面器件为超表面以及传感器设计提供了新的思路,但随着实际应用要求的提高,具有固定功能的超表面在一些应用场合中受到限制。
二氧化钒作为一种相变材料,因其在发生相变前后电导率能实现5个数量级的改变而被广泛应用于多功能器件设计[15]。近年来,研究者们将二氧化钒与狄拉克半金属结合,设计多功能太赫兹超表面,有效解决了超表面功能单一的问题。2020年,Wang等[16]将二氧化钒嵌入介质层中,利用二氧化钒的相变特性实现了不对称传输和双向吸收两种功能的切换。2022年,Yi等[17]提出一种基于狄拉克半金属和二氧化钒的新型多功能太赫兹超表面,当二氧化钒为金属态时,出现两个完美的吸收峰,而当二氧化钒为绝缘态时,随着狄拉克半金属费米能级由85 meV增加到180 meV,该超表面可以实现从四分之一波片到半波片的功能切换。2023年,Zhang等[18]提出一种多功能超表面吸收器,当二氧化钒由绝缘态转变为金属态时,该超表面实现了双窄带吸收到宽带吸收响应的切换。上述研究在实现多功能超表面的同时,丰富了狄拉克半金属材料在太赫兹器件中的应用。
太赫兹滤波器作为太赫兹系统中的一种选频器件,由于其可滤除不需要的频段以及应用环境中的噪声,在太赫兹系统中有十分重要的作用[19-21]。然而,在已有的基于狄拉克半金属和二氧化钒的多功能超表面研究中鲜有关于滤波器的设计。
本文提出一种基于狄拉克半金属的镜面对称双开口环的传感器/滤波器多功能超表面,利用具有矩形孔的二氧化钒薄膜对超表面进行功能切换。探究了当二氧化钒处于绝缘态时,超表面工作在传感器模式下的传输特性,并对该超表面的传感性能进行计算。随着样品厚度的增加,传感器灵敏度会增大,当样品厚度为45 μm时,传感器灵敏度为226 GHz/RIU,并且达到灵敏度阈值。当二氧化钒处于金属态时,超表面工作在滤波器模式下,滤波器的中心频率为0.84 THz,中心频率处的插入损耗为1.3 dB,回波损耗为12.7 dB。此外,还结合电场分布图解释了该超表面的工作原理。
2 结构设计及材料说明
图 1. 超表面单元结构示意图。(a)超表面单元结构3D模型;(b)狄拉克半金属层的俯视图;(c)超表面单元结构的俯视图;(d)单元结构的正视图
Fig. 1. Schematic of metasurface unit structure. (a) 3D model of metasurface unit structure; (b) top view of Dirac semimetallic layer; (c) top view of metasurface unit structure; (d) front view of unit structure
根据随机相位近似理论,狄拉克半金属(AlCuFe)的复电导率在长波极限条件下由Kubo公式[11]表示,电导率实部和虚部分别为
式中:e为电荷量;
图 2. 不同费米能级下狄拉克半金属的动态电导率。(a)实部;(b)虚部
Fig. 2. Dynamic conductivity of Dirac semimetals at different Fermi energy levels. (a) Real part; (b) imaginary part
考虑带间电子传输,狄拉克半金属的介电常数[11]可以表示为
式中:
在太赫兹频率范围内,二氧化钒的相对介电常数可以通过Drude模型[22]表示,即
式中:
式中:
利用电磁仿真软件 CST Microwave Studio 2020中的频域求解器对所设计超表面结构的电磁特性进行建模以及仿真。太赫兹波沿着z轴垂直于超表面单元结构表面入射,并且在x和y方向上均采用周期边界条件(unit cell),在z方向上采用开放边界条件(open add space)。在本文的仿真模拟中,使用TE极化。为了确保仿真的准确性,采用自适应网格设置。经过仿真得到与频率相关的S参数,即|S21|和|S11|,分别表示透射系数和反射系数。
3 仿真结果与分析
3.1 二氧化钒为绝缘态时,超表面为传感器
当二氧化钒的电导率为10 S/m即二氧化钒为绝缘态,且狄拉克半金属的费米能级为170 meV时,所提出的超表面工作在传感器模式。当非对称参数
图 3. 二氧化钒为绝缘态时的传输谱
Fig. 3. Transmission spectra when vanadium dioxide is in the insulating state
接下来,对谐振原理进行分析。当非对称参数
图 4. 当d=15 时,C1-MASRR、C2-MASRR以及MASRR的传输光谱。(a)0~1.3 THz时的传输谱;(b)1.3~2 THz时的传输谱
Fig. 4. Transmission spectra of C1-MASRR, C2-MASRR, and MASRR when d=15 μm. (a) Transmission spectra at 0-1.3 THz; (b) transmission spectra at 1.3-2 THz
图 5. 传输峰p1和p2处的电场分布图。(a)p1处C2-MSRR的电场图;(b)p1处MASRR的电场图;(c)p1处C1-MSRR的电场图;(d)p2处C2-MSRR的电场图;(e)p2处MASRR的电场图;(f)p2处C1-MSRR的电场图
Fig. 5. Electric field distribution at transmission peaks p1 and p2. (a) Electric field diagram of C2-MSRR at p1; (b) electric field diagram of MASRR at p1; (c) electric field diagram of C1-MSRR at p1; (d) electric field diagram of C2-MSRR at p2; (e) electric field diagram of MASRR at p2; (f) electric field diagram of C1-MSRR at p2
为了探究p3的形成原理,对比了当
图 6. 传输峰p3处的电场分布图。(a)d=0 ;(b)d=5 ;(c)d=10 ;(d)d=15
Fig. 6. Electric field distribution at transmission peak p3. (a) d=0 μm; (b) d=5 μm; (c) d=10 μm; (d) d=15 μm
为了探究非对称参数开口中心与圆环中心的距离对传输特性及传感器性能的影响,在保证其他结构参数固定不变的情况下,调整非对称参数
图 7. 非对称参数d从0 μm增加到15 μm的传输谱
Fig. 7. Transmission spectra of asymmetric parameter d increasing from 0 μm to 15 μm
总之,通过增大非对称参数d,传输谱不仅表现为从单谐振到多谐振的可调谐性能,同时共振强度也有所改变,并且谐振频率也会随着d的改变而发生变化,这是因为非对称参数d的大小会影响谐振环非对称分裂口处的电场强度。因此,在实际应用中,可以通过调节超表面的非对称参数d对传感器进行重构。
为进一步讨论非对称参数d对谐振的影响,探讨随着d的增加各谐振点的Q值变化。绘制了
图 8. d从0 μm增加到15 μm时各个谐振点的Q值变化曲线。(a)d1、p1、p2对应的Q值随d的变化;(b)d3、p3、d4对应的Q值随d的变化
Fig. 8. Plots of Q variation corresponding to each resonance point as d increases from 0 μm to 15 μm. (a) Q variation with d for d1, p1, and p2; (b) Q variation with d for d3, p3, and d4
为探究狄拉克半金属费米能级对传感器谐振频率的影响,研究了
图 9. 费米能级(EF)从90 meV增加到170 meV时的传输谱
Fig. 9. Transmission spectra when the Fermi energy level (EF) increases from 90 meV to 170 meV
接下来对所设计的超表面传感器进行性能分析。传感性能的优劣主要通过品质因数Q、灵敏度S等性能参数来衡量。品质因数Q的定义为谐振的中心频率
综合考虑谐振点的Q值和谐振强度这两个因素。在接下来关于传感性能的讨论中,选择非对称参数
为研究该超表面传感器对外加样品的折射率灵敏度,在谐振结构上方覆盖一层折射率不同的样品,并通过调整样品厚度t和折射率n来研究其对谐振频率的影响。首先,固定非对称参数
图 10. 样品覆盖整个谐振结构(200 μm×100 μm)示意图。(a)3D图;(b)正视图
Fig. 10. Schematic of the sample covering whole resonant structure (200 μm×100 μm). (a) 3D view; (b) front view
首先,将样品折射率n从1增加到2(步长为0.2),并进行仿真分析。
图 11. 传输谱及谐振频率偏移量随折射率的变化。(a)当覆盖样品折射率n从1增加到2时谐振结构的传输谱;(b)当覆盖样品折射率n从1增加到2时d3谐振点处的频率偏移量拟合曲线
Fig. 11. Transmission spectra and resonance frequency shift as a function of refractive index. (a) Transmission spectra of resonance structure when refractive index n of covered sample is increased from 1 to 2; (b) fitting curve of frequency shift at the resonance point of d3 when refractive index n of covered sample increases from 1 to 2
为了更加直观地展示样品折射率对谐振频率的影响,绘制了谐振频率偏移量随样品折射率n的变化。
为了深入探究外加样品对谐振频率产生影响的机制,利用等效电路思想分析谐振频率。狄拉克半金属谐振层被分裂口分成两个弧形谐振器,将它们分别等效为电感L1和L2,而分裂口可以等效为电容C,狄拉克半金属谐振层为镜面对称型结构,所以等效电路也呈现镜面对称,从而形成LC谐振模型[24],如
式中:
大部分物质都是非磁性的,即
综合考虑各谐振点处的Q值和灵敏度S,选择谐振波谷d3作为所提传感器的最佳观测点,并进一步研究样品厚度对频率偏移量的影响。首先,将样品折射率n设为1.6,并将样品厚度t设为从5 μm增加到50 μm(步长为5 μm)。
图 13. 频率偏移量随样品厚度的变化。(a)通过改变样品厚度t从5 μm增加到50 μm时d3处的传输谱;(b)d3处的频率偏移量随样品厚度t的变化
Fig. 13. Variation of frequency shift with sample thickness. (a) Transmission spectra at d3 when increasing sample thickness t from 5 μm to 50 μm ; (b) variation of frequency shift of d3 with sample thickness t
为了更加直观地表示频率偏移量随样品厚度的变化,进一步绘制了d3处的频率偏移量与样品厚度之间的数值关系,如
为进一步探讨样品厚度t对传感器灵敏度S的影响,对d3处传感灵敏度随样品厚度的变化情况进行分析,结果如
图 14. d3处灵敏度随样品厚度t的变化曲线
Fig. 14. Variation curve of sensitivity at d3 with sample thickness t
最后,进一步探究样品与超表面的距离对灵敏度的影响,固定样品厚度为10 μm,当样品与超表面之间的距离从10 μm增加到90 μm时,超表面的传感灵敏度会先快速增大后略微减小,当样品距离超表面70 μm时,灵敏度达到最大值130 GHz/RIU,如
图 15. 灵敏度随样品与超表面距离的变化
Fig. 15. Variation of sensitivity with distance between sample and metasurface
3.2 二氧化钒为金属态时,超表面为带通滤波器
当二氧化钒的电导率为200000 S/m,即二氧化钒为金属态,费米能级为
图 16. 二氧化钒为金属态时的传输谱和反射谱
Fig. 16. Transmission and reflection spectra of vanadium dioxide in the metallic state
接下来对该超表面工作在滤波器模式时的谐振原理进行研究。为了更好地解释带通滤波器的通带形成原理,绘制了当二氧化钒为绝缘态和金属态且
图 17. 二氧化钒为绝缘态和金属态时的传输谱
Fig. 17. Transmission spectra of vanadium dioxide in the insulating and metallic states
图 18. p2、p3处的电场图。(a)二氧化钒为绝缘态时p2处的电场图;(b)二氧化钒为金属态时p2处的电场图;(c)二氧化钒为绝缘态时p3处的电场图;(d)二氧化钒为金属态时p3处的电场图
Fig. 18. Electric field diagrams at p2 and p3. (a) Electric field at p2 when vanadium dioxide is in the insulating state; (b) electric field at p2 when vanadium dioxide is in the metallic state; (c) electric field at p3 when vanadium dioxide is in the insulating state; (d) electric field at p3 when vanadium dioxide is in the metallic state
图 19. 电场图。(a)二氧化钒为绝缘态时d1处的电场图;(b)二氧化钒为金属态时d1处的电场图;(c)二氧化钒为绝缘态时滤波器中心频率处的电场图;(d)二氧化钒为金属态时滤波器中心频率处的电场图
Fig. 19. Electric field diagrams. (a) Electric field diagram at d1 when vanadium dioxide is in the insulating state; (b) electric field diagram at d1 when vanadium dioxide is in the metallic state; (c) electric field diagram at the center frequency of the filter when vanadium dioxide is in the insulating state; (d) electric field diagram at the center frequency of the filter when vanadium dioxide is in the metallic state
随后分析了非对称参数d的改变对带通滤波器传输特性的影响。
图 20. 二氧化钒为金属态时不同非对称参数d对应的传输谱
Fig. 20. Transmission spectra of vanadium dioxide in the metallic state with different asymmetric parameters
为了探索狄拉克半金属费米能级对滤波器性能的影响,研究了
4 结论
提出一种基于狄拉克半金属及二氧化钒的多功能太赫兹超表面。该超表面由四层结构组成,其中狄拉克半金属层由两个镜面对称的开口谐振环组成。功能的切换通过调节具有矩形凹槽的二氧化钒薄膜的电导率实现。当二氧化钒处于绝缘态时,该超表面实现了高Q值传输波谷,并且随着非对称量d的增大谐振强度会增大。从传感器的应用角度来看,当样品厚度为10
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