Core–Shell Nanoparticle Resonances in Near-Field\nMicroscopy Revealed by Fourier-Demodulated Full-Wave Simulations

We present a detailed\ninvestigation of the near-field optical response\nof core–shell nanoparticles using Fourier-demodulated full-wave\nsimulations, revealing significant modifications to established contrast\nmechanisms in infrared scattering-type scanning near-field optical\nmicroscopy (s-SNOM). Our work examined the complex interplay of geometrical\nand optical resonances within core–shell structures. Using\na finite element method (FEM) simulation closely aligned with the\nactual s-SNOM measurement processes, we capture the specific near-field\nresponses in these nanostructures. Our findings show that core–shell\nnanoparticles exhibit unexpected distinct resonance shifts and massively\nenhanced scattering driven by both the core and shell properties.\nThis investigation not only advances the understanding of near-field\ninteractions in complex nanosystems but also provides a refined theoretical\nframework to accurately predict the optical signatures of nanostructures\nwith internal heterogeneity.