Real-space imaging of confined infrared surface plasmon polaritons on doped semiconductors covered with phase-change materials

Surface plasmon polaritons (SPPs) describe the excitation of photons coupled with free charge carriers at the interface of metals (visible) or doped semiconductors (infrared). While SPPs in the mid-infrared spectral range have been demonstrated in 2D materials such as graphene, their short propagation length combined with weak confinement in bulk materials has prevented real-space imaging of those SPPs. Here, we demonstrate real-space imaging of propagating SPPs on the doped semiconductors CdO and InAs with tunable plasma frequencies in the infrared via scattering-type scanning near-field optical microscopy. Adding a thin film of phase-change materials (PCMs) to these doped semiconductors increases the polariton confinement, leading to simplified SPP imaging and SPP resonator fabrication. We investigate optically written circular resonators of the plasmonic PCM In₃SbTe₂ on CdO with near-field spectroscopy and Fabry-Perot resonators of the dielectric PCM Ge₃Sb₂Te₆ on InAs with far-field spectroscopy. Our work enables rapid prototyping of reconfigurable SPP resonators in mid-infrared.