Sutter Peter, Khorashad Larousse Khosravi, Argyropoulos Christos, Sutter Eli
Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln, NE, 68588, USA.
Department of Mechanical and Materials Engineering, University of Nebraska-Lincoln, Lincoln, NE, 68588, USA.
Adv Mater. 2021 Jan;33(3):e2006649. doi: 10.1002/adma.202006649. Epub 2020 Dec 6.
Ultrathin van der Waals semiconductors have shown extraordinary optoelectronic and photonic properties. Propagating photonic modes make layered crystal waveguides attractive for photonic circuitry and for studying hybrid light-matter states. Accessing guided modes by conventional optics is challenging due to the limited spatial resolution and poor out-of-plane far-field coupling. Scanning near-field optical microscopy can overcome these issues and can characterize waveguide modes down to a resolution of tens of nanometers, albeit for planar samples or nanostructures with moderate height variations. Electron microscopy provides atomic-scale localization also for more complex geometries, and recent advances have extended the accessible excitations from interband transitions to phonons. Here, bottom-up synthesized layered semiconductor (Ge Sn S) nanoribbons with an axial twist and deep subwavelength thickness are demonstrated as a platform for realizing waveguide modes, and cathodoluminescence spectroscopy is introduced as a tool to characterize them. Combined experiments and simulations show the excitation of guided modes by the electron beam and their efficient detection via photons emitted in the ribbon plane, which enables the measurement of key properties such as the evanescent field into the vacuum cladding with nanometer resolution. The results identify van der Waals waveguides operating in the infrared and highlight an electron-microscopy-based approach for probing complex-shaped nanophotonic structures.
超薄范德华半导体已展现出非凡的光电和光子特性。传播的光子模式使层状晶体波导对光子电路以及研究混合光-物质态具有吸引力。由于空间分辨率有限和面外远场耦合较差，通过传统光学方法获取导模具有挑战性。扫描近场光学显微镜可以克服这些问题，并且能够将波导模式表征到几十纳米的分辨率，尽管这仅适用于平面样品或高度变化适中的纳米结构。电子显微镜还能为更复杂的几何结构提供原子尺度的定位，并且最近的进展已将可获取的激发从带间跃迁扩展到了声子。在此，展示了具有轴向扭曲和深亚波长厚度的自下而上合成的层状半导体（GeSnS）纳米带作为实现波导模式的平台，并引入阴极发光光谱作为表征它们的工具。结合实验和模拟表明，电子束激发了导模，并通过纳米带平面内发射的光子对其进行了有效检测，这使得能够以纳米分辨率测量诸如进入真空包层的倏逝场等关键特性。结果确定了在红外波段工作的范德华波导，并突出了一种基于电子显微镜的方法来探测复杂形状的纳米光子结构。
