Near-infrared optical nano-imaging of waveguide modes in bulk α-RuCl₃

α-RuCl₃ has gained considerable attention as one of candidates for hosting exotic quantum spin liquid (QSL) states. One intriguing aspect of this material is the notable disparity between its bandgap and optical energy. This disparity fuels the debate regarding the origin of its optical resonance around 1.2 eV—whether it arises from intersite d-d transitions or excitonic resonances. In this study, we investigate waveguide modes in α-RuCl₃, which represent electromagnetic modes confined within the bulk material that serve as sensitive probes of collective excitations. By applying advanced scanning near-field optical microscopy (SNOM) techniques, we captured excitonic back-bending signatures of α-RuCl₃, which correlates with results from the far-field Fourier-transform infrared (FTIR) spectroscopy. These findings offer compelling evidence supporting the excitonic origin of the 1.2 eV resonance.