Fund Project:Project supported by the Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 11704075) and the China Postdoctoral Science Foundation.
Received Date:17 July 2019
Accepted Date:08 October 2019
Available Online:01 November 2019
Published Online:20 November 2019
Abstract:Plasmonics plays an important role in the development of nanophotonics, which allows breaking diffraction limit and controlling light in deep-subwavelength scale due to the strong interaction between light and free carriers. Noble metals and 2-dimensional electron gas have been the main platforms for studying plasmonics over the past decade. The metal-based plasmonic devices have exhibited great potential in various applications, including integrated photonic systems, biological sensing, super-resolution imaging and surface-enhanced Raman scattering, etc. Because of the high carrier density, plasmons of noble metals are realized in the near-infrared to visible frequency range. With the rapid development of new materials, many other plasmonic materials are discovered to exhibit new properties. One example is the graphene plasmons working in the mid-infrared and terahertz spectral range, which exhibit strong field confinement and frequency tunability due to the massless Dirac fermions and other exotic electrical and optical properties. Recently, topological materials, the band structures of which are composed of cones with linear dispersion like in graphene, are discovered, such as the topological insulators, Dirac semimetals, Weyl semimetals and nodal line semimetals, providing another platform to study the Dirac plasmons. Such linear dispersion results in small electron mass and unique carrier density dependence of plasmons. In addition, topological materials possess a tremendous amount of exotic electron properties, such as the ultrahigh mobility, topological surface states and chiral anomaly in Weyl semimetals, etc. Many of these electronic properties can be inherited by the collective oscillation of free electrons, promising new possibility for plasmonics. Here, the experimental observations of plasmons in topological insulators and topological semimetals are reviewed, with special focus on the studies based on electron energy loss spectrum and Fourier transform infrared spectroscopy. At the end, other topological materials with potential for hosting 2D plasmons are discussed. This review provides an overview of plasmons in topological semimetals and may stimulate further quest of more exotic features for plasmons. Keywords:topological insulators/ topological semimetals/ plasmons
从石墨烯等离激元的研究中可以知道, 由于光源的限制, 探测远红外(几个至十几个太赫兹)波段的二维等离激元需要大面积的薄膜样品. 意大利的Lupi 组的Pietro等[46]利用分子束外延(MBE)方法在Al2O3衬底上生长了大面积的拓扑绝缘体Bi2Se3薄膜. 随后他们在薄膜上制备了光栅阵列(如图4(b)的插图), 利用傅里叶变换红外光谱仪, 测量了不同宽度的光栅阵列的吸收光谱. 图4(a)展示了在入射光偏振垂直于光栅阵列时的实验结果, 等离激元的信号表现为随着光栅宽度变化而逐渐移动的共振吸收峰. 值得注意的是, 因为与2 THz处Bi2Se3的红外活性声子存在耦合, 等离激元共振峰在声子附近表现出不对称的法诺峰型. 图4(b)展示了实验中得到的等离激元频率和动量的色散关系. 和一般的二维体系一致, 等离激元的频率表现出与动量的1/2次方成正比的关系. 但在拓扑绝缘体中, 等离激元存在两个贡献, 表面态狄拉克费米子和体态的普通费米子. Pietro等[46]经过计算, 发现所测到的等离激元色散必须用狄拉克费米子的情况(图4(b)虚线)才能解释, 以此证明测到的是表面态的二维等离激元. 图 4 拓扑绝缘体等离激元的远红外和可见光远场光谱研究 (a)不同光栅宽度下拓扑绝缘体薄膜等离激元共振模式远红外吸收谱[46]; (b)由(a)中实验得到的等离激元色散关系[46]. 虚线和点线分别为考虑表面态电子和体载流子后计算的等离激元色散; (c)外加垂直于面的磁场后, 拓扑绝缘体薄膜光栅结构中磁性等离激元和回旋共振模式随磁场的变化关系[47]; (d)圆环阵列结构下, 拓扑绝缘体薄膜等离激元色散关系[48]; (e)可见到紫外波段拓扑绝缘体等离激元共振模式[49]; (f)拓扑绝缘体薄膜光栅结构中等离激元共振模式随着薄膜厚度的依赖关系[50] Figure4. Far filed spectroscopic study of the plasmon modes in topological insulator films: (a) Extinction spectra of plasmon resonance modes in topological insulator ribbon arrays with different width; (b) plasmon dispersion extracted from Fig. (a). Fig. (a) and Fig. (b) from Ref. [46]; (c) magnetoplasmon mode and cyclotron resonance in topological insulator ribbon array as a function of external magnetic field[47]; (d) plasmon dispersion in topological insulator microring array[48]; (e) plasmon resonance modes of topological insulator ribbon array from visible to ultraviolet frequency range[49]; (f) plasmon dispersion as a function of film thickness in topological insulator ribbon arrays[50].
因为高能电子具有比较大的动量, 所以利用电子能量损失谱来探测等离激元不需要额外制作光栅结构来补偿动量. 随着技术的不断提高, 电子能量损失谱的能量探测范围和探测精度也在不断地进步. 从开始的探测高能激发(几个至十几个eV)到目前可以探测十几个meV的低能激发, 使得这项技术更加有利于探测等离激元模式, 尤其是在载流子浓度比较低的材料中. 最早, Nascimento等[52]利用电子能量损失谱测量到, 在16.5 eV左右Bi2Se3存在体等离激元模式. 随后Liou等[53]利用结合了(透射)扫描电子显微镜的EELS来对Bi2Se3进行具有空间分辨的探测. 如图5(a)所示, 当测量位置远离样品的边缘时, 测量到的结果主要是7 和17 eV两个体等离激元的峰. 但当探测位置逐渐移向样品边界时, 会在5.5和10 eV处出现两个新的峰. 作者认为这是来自表面等离激元的贡献. Politano等[54]使用了更高能量探测精度的EELS. 通过高分辨率的测量, 他们测量到了两支等离激元色散(图5(b)), 分别指认为表面态的二维等离激元(${\omega _{{\rm{2 D}}}}$)和体载流子的表面等离激元(${\omega _{{\rm{SP}}}}$)模式. Kogar等[55]做了类似的测量, 发现在这个频段研究表面态的等离激元必须综合考虑表面态载流子和体载流子的贡献. 2015年, 中国科学院物理研究所的Zhu等[56]开发出了世界首台具有能量-动量二维解析能力的高分辨电子能量损失谱仪. 随后在2017年, Jia等[57]对Bi2Se3进行了超低能的电子能量损失谱测量, 并观测到了反常的声学支等离激元模式(如图5(c)); 这个色散模式在很大的动量范围内都呈现出线性色散的特性. 而当采用磁性掺杂来破坏材料的拓扑表面态时, 这个线性色散的特性也同时消失, 证明该色散特性与表面态电子密切相关. 另外, 与普通电子的等离激元不同, 这个特殊的色散模式即使在高动量区间仍表现出弱衰减的特性, 进而作者推测这个性质和表面态的拓扑保护性质有关. 图 5 拓扑绝缘体等离激元电子能量损失谱研究 (a)距离晶体边界不同位置处的能量损失谱(上图)和由上图黑线导出的介电常数(下图)[53]; (b)高能量分辨EELS测量到的拓扑绝缘体等离激元色散模式[54]; (c)高能量分辨EELS测量到的具有线性色散的声学支等离激元模式[57] Figure5. EELS study of plasmon modes in topological insulator: (a) EELS spectra at different spots from the edge (upper panel). Calculated permittivity from the black line (lower panel)[53]; (b) plasmon dispersion derived from the EELS spectra with high energy resolution[54]; (c) unusual acoustic plasmon modes with linear dispersion measured with high energy resolution EELS[57].