Nano Lett. 2018 Jul 11;18(7):4576-4583. doi: 10.1021/acs.nanolett.8b01840. Epub 2018 Jun 13.
Nano-optical studies of confined modes in planar waveguides have attracted significant interest as a means to probe exciton-polaritons and other hybrid light-matter quasiparticles in layered semiconductors, such as transition metal dichalcogenides or boron nitride. There is a need to broaden such studies to other materials and to identify alternatives to scanning near-field optical microscopy for exciting and measuring confined waveguide modes. Here, we establish an approach for probing the dispersion of traveling waveguide modes by cathodoluminescence spectroscopy excited by the focused electron beam in scanning transmission electron microscopy (STEM-CL) and apply it to solid-state resonators consisting of mesoscale monocrystalline prisms and plates composed of GeS, an anisotropic layered semiconductor with direct bandgap in the near-infrared spectral range. Structure, crystallography, and chemical composition of the mesostructures are analyzed by analytical electron microscopy. STEM-CL maps and spectra show pronounced interference effects and sharp emission peaks at photon energies below the fundamental bandgap of GeS. Our analysis demonstrates that locally excited light emission in STEM-CL launches in-plane waveguide modes in the mesoscale GeS structures, which are internally reflected by highly specular GeS edges to cause interference of the waveguide modes. Reabsorption and secondary luminescence give rise to the intensity modulations detected in the far field. Our results highlight avenues for probing light-matter interactions below the diffraction limit in a wide range of quantum materials and open up the possibility of tuning light emission geometrically using interference rather than by the conventional bandgap engineering.
平面波导中的受限模式的纳米光学研究作为探测层状半导体（如过渡金属二卤化物或氮化硼）中的激子极化激元和其他混合光物质准粒子的一种手段，引起了极大的关注。需要将这些研究扩展到其他材料，并确定替代扫描近场光学显微镜的方法来激发和测量受限波导模式。在这里，我们通过扫描透射电子显微镜（STEM-CL）中的聚焦电子束激发的阴极荧光光谱法建立了一种探测传播波导模式色散的方法，并将其应用于由介观单晶棱镜和由 GeS 组成的平板组成的固态谐振器，GeS 是一种各向异性的层状半导体，具有近红外光谱范围内的直接带隙。介观结构的结构、晶体学和化学成分通过分析电子显微镜进行了分析。STEM-CL 图谱和光谱显示出明显的干涉效应和在光子能量低于 GeS 的基本带隙时的尖锐发射峰。我们的分析表明，在 STEM-CL 中局部激发的光发射在介观 GeS 结构中产生了平面波导模式，这些波导模式被高度镜面反射的 GeS 边缘内部反射，从而导致波导模式的干涉。再吸收和二次发光导致在远场中检测到的强度调制。我们的结果突出了在广泛的量子材料中在衍射极限以下探测光物质相互作用的途径，并开辟了通过干涉而不是传统的能带工程来几何地调谐光发射的可能性。
