Abstract:The coupled waveguide-microcavity structure has a wide range of applications in optical filters and optical modulators. The optical transmission properties of structure are mostly determined by the coupling strength of the modes. In the conventional waveguide-microcavity structure, the mode coupling is finished by the form of evanescent field, which is usually achieved by controlling the geometric spacing between waveguide and microcavity. Surface plasmon polaritons are the excitations of the electromagnetic waves coupled to collective oscillations of free electrons in metal. Since the electromagnetic waves are attenuated sharply in the metal, this requires precise control of the spacing between the waveguide and the metal microcavity, and poses a great challenge for controlling the coupling of modes in the metal waveguide-cavity structure. In this paper, we proposed a scheme of using a metal-dielectric-metal waveguide side coupling metal microcavities to overcome this limit. Based on the resonant characteristics of the Fabry–Pérot mode in the metal microcavity, a slit is introduced to connect the waveguide and microcavities. By adjusting the width and the offset location of slits, the leakage rate and coupling strength of the mode in metal microcavity can be controlled. The finite difference frequency domain (FDFD) method was used to numerically simulate the electromagnetic properties of structure. First, we have studied the transmission behaviors of surface plasmon polaritons in the system consisted by metal waveguide and single microcavity. As other microcavity is introduced to the structure and connected the original microcavity by slit, the electromagnetically induced transparency phenomena based on surface plasmon polaritons are demonstrated in the coupled metal waveguide and double microcavities structure. As the width of slit connected the microcavity is increased, the transmission peak of structure and the full width at half maximum of the transparency window also increase accordingly. The change of the geometric parameters of slit will modulate the resonance characteristics of structure, and the corresponding physical mechanism is explained by the temporal coupled mode theory. In our works, the metal waveguide and microcavities are coupled by the energy leakage of microcavities assisted by slits, which breaks the limit of separation distance between metal waveguide and microcavity, and contributes to the manufacture of devices. The results of the paper will have applications in designing the compact photonic devices based on surface plasmon polaritons. Keywords:electromagnetically induced transparency/ surface plasmon polaritons/ finite difference frequency domain method/ temporal coupled mode theory
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2.单个波导-微腔结构分析首先, 分析单个波导-微腔侧边耦合结构. 结构示意图如图1所示, 宽度为wd的MDM波导通过侧边耦合一个矩形金属微腔, 波导与微腔间的间距表示为S. 不同于传统的波导侧边耦合结构[6-8], 这里间距S = 200 nm, 远大于电磁波在金属中的穿透深度. 为了将光波耦合到金属微腔中, 在波导与微腔间引入了一个开口狭缝, 狭缝宽度为C, 狭缝中心与微腔中心的偏移量表示为dsp. 图 1 单个金属波导-微腔侧边耦合结构. 银-空气-银构成一个高局域的MDM波导结构, 空气层厚度为wd, 金属微腔的长度为L, 宽度为D. 波导与微腔间通过开口的狭缝进行耦合, 狭缝的宽度为C, 高度为S, 狭缝中心与微腔中心的偏移量为dsp Figure1. The schematic diagram of single metal waveguide-cavity side-coupled structure. The MDM waveguide is consisted by silver-air-silver. The width of air layer is wd. The length and width of metal microcavity are L and D, respectively. A slit is used to connect the waveguide and microcavity. The width and height of slit are denoted as C and S, respectively. The center-to-center distance between slit and cavity is denoted as dsp.
这里通过FDFD方法来模拟结构的电磁特性. 空气波导层的厚度wd = 200 nm, 在工作波长1500 nm附近, 波导结构仅支持单个SPP模式. SPP模式通过总场-散射场的形式引入到结构中[9]. 模拟中, 离散网格的尺寸为5 nm, 金属银的介电函数来自于实验值[10]. 本文中, 波导与微腔的间距远大于电磁波在金属中的穿透深度. 微腔不是通过波导的倏逝波耦合激发[11], 而是通过开口狭缝的能量泄漏来激发[12,13], 因而狭缝的宽度(C )和位置偏移量(dsp)将直接影响结构的光谱响应. 图2(a)给出了在固定狭缝偏移量(dsp = 120 nm)时, 不同狭缝宽度对应的结构透射谱. 可以看出, 由于存在狭缝, 波导中的电磁能量能有效耦合到金属微腔中, 形成谐振, 从而会在透射谱中形成低谷. 随着狭缝宽度的增加, 微腔中更多的电磁能量会泄漏出来, 使得模式的损耗增加, 因而透射谱的FWHM会展宽. 而且狭缝宽度的增加, 会对微腔中模式谐振频率进行修正, 结构的透射谱的低谷会蓝移. 结构谐振时相应的品质因子(Q值)在图2(b)中给出. 随着金属狭缝宽度的增加, 结构的共振Q值减小. 金属狭缝宽度C = 70 nm, 高度S = 200 nm和偏移量dsp = 120 nm时, 结构谐振时对应磁场的振幅分布(|Hy|)在图2(b)的插图中给出. 可以看出, 微腔中激发一阶FP共振模式. 金属微腔中FP模式的谐振条件表示为[14] 图 2 狭缝偏移位移固定(dsp = 120 nm), 不同狭缝宽度情况下 (a) 结构的透射谱; (b)结构共振Q值的变化情况. 结构谐振时, 对应的磁场振幅分布(|Hy|)也在图(b) 中给出. 狭缝宽度固定(C = 100 nm), 不同狭缝偏移量情况下, (c) 结构的透射谱; (d) 结构共振Q值的变化情况. 微腔的尺寸(长L = 650 nm, 宽度D = 200 nm), 波导的宽度wd = 200 nm Figure2. As the location offset of slit is fixed (dsp = 120 nm), (a) the transmittance spectra of structure with the different width C, (b) the Q factor of structure versus the width C. The amplitude distribution of magnetic field at the resonant wavelength of structure with width C = 100 nm is also shown in the inset of Fig. 2(b). (c) The transmittance spectra of structure with the different location offset dsp; (d) the Q factor of structure versus the dsp. The length and width of microcavity are L = 650 nm and D = 200 nm, respectively. The thickness of waveguide wd = 200 nm.
3.波导-双微腔结构分析利用上述规律以及微腔内FP模式的谐振特性, 在单个微腔的基础上, 再加入一个微腔, 并在两个微腔间通过开口狭缝连接, 结构示意图如图3所示. 由于微腔中, 模式的泄漏率与开口狭缝的宽度和偏移位置有关, 通过调节两个狭缝的宽度和偏移位置, 可以在两个微腔中分别实现高Q和低Q模式共振, 通过两个模式的耦合可以实现SPP波导结构的类EIT现象. 图 3 金属波导-双微腔侧边耦合结构示意图. 在图1的基础上再加入一个谐振腔, 并为两个谐振腔编号为①与②. 靠近波导的为1号微腔, 所有的结构参数的尾数都为1; 远离波导的为2号谐振腔, 所有的结构参数的尾数为2. 空气层厚度为wd Figure3. The schematic diagram of metal waveguide-double microcavities side-coupled structure. A other microcavity is introduced into the structure shown in Fig.1. The two microcavities are numbered as ① and ②, respectively. The width of air layer is wd.