近红外磁表面等离激元单向波导 下载: 538次
Unidirectional electromagnetic mode travels along one direction and immunes from backscattering by breaking Lorentz reciprocity. As free from backscattering, it is widely used in laser and optical communication systems. In addition, the unidirectional electromagnetic mode can be realized by introducing a magnetic field to break the time inversion symmetry. The most promising method to realize a unidirectional electromagnetic mode is to use surface magnetoplasmons (SMPs) which exist in the interface between the gyro-electric and dielectric media, and an external magnetic field can be applied to separate the dispersion curves of wave vectors in both directions. When the frequency falls into the gap of the dispersion curves, the electromagnetic wave propagates in one direction. In terahertz bands, achievements on an SMPs-based unidirectional electromagnetic mode are reported in terms of the interface of the semiconductor InSb and the dielectric medium with a normal magnetic field magnitude. While in near-infrared bands, it is difficult to produce SMPs by using magneto-optical materials since the ratio of the non-diagonal term to the diagonal term of the dielectric tensor under the normal magnetic field magnitude is only about 10-3. In order to solve this problem, a meta-material is employed to enhance the ratio of the non-diagonal term to the diagonal term of the dielectric tensor, and SMPs in a 1.1 eV band are achieved at the interface of the proposed meta-material and the dielectric material.
A physical model of a unidirectional waveguide based on SMPs was presented in this paper. The model is an interface consisting of meta-material-based gyro-electric and dielectric materials (PMMA, with a dielectric coefficient of 2.28). According to the continuity condition, a dispersion equation was obtained. The relationship between the dielectric tensor of the gyro-dielectric material and the dielectric coefficient was theoretically analyzed when SMPs existed. Since the non-diagonal term of the dielectric tensor of normal magneto-optical materials is much smaller than the diagonal term, the diagonal term of the dielectric tensor of the meta-material-based gyro-electric materials is close to the dielectric coefficient of the dielectric materials. In order to construct meta-material-based gyro-electric materials, cerium-doped yttrium iron garnet (Ce∶YIG) and silver with a negative dielectric coefficient were employed. Due to the low absorption and large non-diagonal dielectric coefficient under the normal magnitude of magnetic field (0.2 T), Ce∶YIG was widely used in the near-infrared bands. According to the effective dielectric tensor theory, the dielectric tensor of the meta-material-based gyro-electric materials with different proportions could be obtained. With a ratio of 0.108/0.892 (Ag/Ce∶YIG), the dispersion curves of the SMPs were given, from which it is obvious that the unidirectional band exists around a frequency of 1.1 eV.
The characteristics of unidirectional propagation were simulated and analyzed in this paper. The electric field distribution along the interface is shown in Fig. 4 by placing an excitation source in the interface. The electric field rapidly decays in the opposite direction, and there is almost no backscattering. In other words, the structure is immune to backscattering. Since the electric field in the opposite direction is suppressed within a distance of less than half a wavelength, it can be used to design optical isolators in sub-wavelength size, which is of great significance for improving photonic integration. By setting protrusions and depressions on the interface, the robustness of the unidirectional waveguide to interface defects is verified. As shown in Fig. 5, although the electric field amplitude oscillates violently at the setup defect, the surface wave quickly returns to the interface after bypassing the defect and converts back into its original amplitude size with little energy loss. It shows that the unidirectional waveguide has good robustness as the interface defects do not affect the transmission of surface waves or weaken the unidirectional transmission characteristics. This property is helpful to reduce the process requirements during the manufacturing of photonic devices. SMPs-based unidirectional waveguides not only show positive robustness to defects but also have excellent unidirectional characteristics for waveguides with large-angle bending. Fig. 6 shows the transmission characteristics of a structure at four right angles, and the SMPs are still well constrained at the interface after passing through the structure at four right angles without introducing backscattering. This exclusive property is critical for designing optical devices with complex structures.
The dispersion equation of the dielectric/gyro-electric model of the SMPs was theoretically analyzed, and the relationship requirement between the dielectric tensor and the dielectric coefficient was obtained, so as to achieve unidirectional transmission of the SMPs. According to the effective tensor theory, a meta-material with the combination of Ce∶YIG/Ag was proposed to construct a gyro-electric material, so as to satisfy the requirement. The dispersion characteristics of the SMPs were analyzed, and the transmission characteristics were simulated by a finite element method. An SMPs-based unidirectional waveguide operating in the near-infrared band was constructed by two materials of Ce∶YIG/Ag with a magnetic field with normal magnitude (0.2 T). The unidirectional waveguide structure with defects was also simulated, and the results show that the SMPs-based unidirectional waveguide has good robustness.
1 引言
单向电磁模式是通过打破洛伦兹互易性实现沿一个方向传输而不会沿原路径返回的电磁模式,其具有无反向散射的特性,在激光和光通信系统[1-2]、隔离器[3]、环行器[4]等方面有广泛应用。在由磁光材料构成的波导系统中,通过引入磁场可打破时间反演对称,进而可实现单向电磁模式。一种方法是利用光子晶体中类似量子霍尔边缘态构造单向电磁模式[5-8],内部光子禁带使得电磁模式只能在边缘传输,引入外磁场可打破时间反演对称性,实现单向电磁模式,这一现象在磁光光子晶体中得到实验验证[9-10]。另一种方法是利用磁表面等离激元(SMPs)实现单向模式,该模式存在于旋电介质和电介质之间的界面,施加外磁场使得正反两个方向波矢的色散曲线分离[11],当频率处于色散曲线频率间隙内时电磁波可以单向传播,单向频带范围可通过调谐磁场来实现单向电磁模式[12]。相比前一种方法,基于SMPs的单向模式结构更加简单,且能够将光束约束在表面,并具有较好的鲁棒性[13],对器件小型化以及光通信领域应用[14]有重要意义。
利用SMPs实现光波段和微波段单向电磁模式一直是研究的热点[15-16]。2008年,Fan团队[15]利用光子晶体的禁带抑制体模,在光子晶体和金的界面上实现光波段的单向表面波,但由于在光波段使金产生旋电效应所需的磁场非常强(约104 T),从而限制了其应用。2015年,Shen等[16]将目光转向太赫兹波段,利用半导体材料InSb和电介质构建了三明治结构,该结构在常规磁场大小情况下实现了单向SMPs。单向SMPs为功能器件提供了传播方向和频率范围等方面的更多设计自由度,如可通过构建紧凑圆形腔实现环形器[17]。在光波段且在常规大小磁场下,一般磁光材料的磁场导致的介电张量非对角项与对角项比值仅约为10-3[18],产生具有单向传输特性的边缘态或SMPs均较为困难。为解决这一问题,Kempa团队[19]利用金属和旋电介质组合形成超构材料,并在其中构造光子晶体禁带以实现光波段的边缘态单向传输。
本文在介电材料和旋电材料构成的模型中,根据表面波存在的条件,在常规磁场大小(0.2 T)情况下构造超构材料并得到满足表面波条件的介电张量,分析了SMPs的色散条件,针对1.1 eV波段仿真实现了单向表面波,通过在界面上引入缺陷仿真分析了单向传输的鲁棒性。
2 基本模型和理论
2.1 基本模型
考虑一个由旋电材料和介电材料构成的界面,如
式中:i为虚数单位;在没有外磁场的情况下,
图 1. 单向SMPs波导模型。(a) x-y平面结构图;(b) y-z平面结构图
Fig. 1. Physical models of unidirectional waveguide based on SMPs. (a) Structural diagram in x-y plane; (b) structural diagram in y-z plane
2.2 理论条件
SMPs中的电磁模式为横磁(TM)模,由界面处电磁场分量的连续性条件可以得到色散方程[11]:
式中:
将上述系数表达式代入色散方程[
由于SMPs要求旋电材料介电张量的对角项
图 2. 存在SMPs的范围( ),插图为虚线圆圈内的放大图,插图内的圆点为不同比例时所对应的 和 值(比例因子 从0.1076至0.1083的取值间隔0.0001)
Fig. 2. Range for existence of SMPs ( )[inset is enlarged view of region in dashed circle, and dots in inset correspond different ratios (scaling factor is from 0.1076 to 0.1083 at interval of 0.0001)]
3 模型构造与特性分析
3.1 模型有效介电张量
根据
根据有效介电张量理论[22],引入介电系数为负数的金属,将其与Ce∶YIG交替组成超构材料,如
Ag和Ce∶YIG组成的超构材料的介电张量元素[22]可表示为
式中:下标m和y分别表示Ag和Ce∶YIG,g表示组合得到的超构旋电材料;
以
超构材料的介电张量会随着组分比例的变化而变化,即使在组分比例确定的情况下,由于组成超构材料的Ce∶YIG[21]和Ag[23]在近红外波段具有色散特性,因此超构材料的介电张量也会相应地发生变化,PMMA的介电系数可视作不变。在
图 3. 光子能量为1.1 eV附近的SMPs色散曲线(圆点对应的光子能量为1.1 eV)
Fig. 3. SMPs dispersion curve for photon energy around 1.1 eV( dot corresponds to photon energy of 1.1 eV)
3.2 SMPs传输特性
使用有限元仿真软件COMSOL Multiphysics对
图 4. SMPs电场强度分布。(a)磁场方向为-z方向;(b)磁场方向为+z方向;(c)激励源放置在界面下方20 nm(五角星处);(d)图4(a)虚线处的归一化电场强度分布;(e)图4(a)中沿着界面的归一化电场强度分布
Fig. 4. Distributions of electric field amplitudes of SMPs. (a) Direction of magnetic field towards -z direction; (b) direction of magnetic field towards +z direction; (c) source is placed inside gyroelectric material, 20 nm below interface (at position of star); (d) normalized electric field amplitude at dashed line in Fig. 4(a); (e) normalized electric field amplitude along interface in Fig. 4(a)
为验证基于SMPs的单向波导对于界面缺陷具有鲁棒性,在界面上设置突起与凹陷。在
图 5. SMPs的鲁棒性。(a)传输界面上存在突起;(b)传输界面上存在凹陷;(c)与图5(a)对应的传输方向上归一化电场强度;(d)与图5(b)对应的传输方向上归一化电场幅度
Fig. 5. Robustness of SMPs. (a) There is protrusion on transport interface; (b) there is depression on transport interface; (c) normalized electric field amplitude in transmission direction corresponding to Fig. 5(a); (d) normalized electric field amplitude in transmission direction corresponding to Fig. 5(b)
基于SMPs的单向波导不仅对于缺陷具有很好的鲁棒性,对于大角度弯曲波导也有非常好的通过性。考虑SMPs通过一直角波导,如
图 6. SMPs在经过90°波导时电场幅度分布图
Fig. 6. Distribution of electric field amplitude of SMPs passing through 90° waveguide
4 结论
从电介质/旋电材料界面的SMPs色散方程出发,分析探讨了方程有解即表面波存在的条件,得到存在SMPs时旋电介质介电张量应满足的条件。根据这一条件和有效介电张量理论,提出利用Ag/Ce∶YIG复合材料构建满足条件的旋电介质的介电张量,基于这一方法可以构造在常规磁场大小情况下工作于近红外波段的SMPs单向波导,并分析了其色散特性,色散曲线表明这种结构存在单向传输频率带隙。在近红外单向频段内,仿真计算了单向传输特性, SMPs在经过界面缺陷时能很快恢复到原来的电场强度且几乎没有能量损失,这说明基于该结构模型的单向表面波还具有很好的鲁棒性,界面缺陷不影响表面波的传输和单向特性。该结构可在亚波长尺寸内完全抑制反向散射,可以用于设计亚波长尺寸的光隔离器。
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