Abstract
All-semiconductor plasmonic gratings are investigated by spectroscopic nanoimaging in the vicinity of the plasma frequency, where the material behaves as an epsilon near-zero (ENZ) material. Both phase-sensitive scattering type nanoscopy (s-SNOM) and photoinduced force microscopy (PiFM) are carried out on this structure. The obtained data and models reveal that PiFM, as for s-SNOM, can have a mostly dispersive line shape, in contrast with recent near-field spectra obtained with photothermal AFM nanoscopic imaging on ENZ material where absorption maxima are observed. On the obtained result, PiFM signal exhibited better sensitivity to the dielectric function variation while interferometric s-SNOM can provide additional phase information. Localized surface plasmon resonances (LSPR), highly confined on the structure edges were also observed with both techniques. A higher sensitivity was observed with PiFM for both dielectric contrast imaging and LSPR observation. In addition, for both microscopies, the near-field response is phenomenologically described using a similar formalism based on dipole-image dipole approach. In this model, the sensitivity difference between both techniques is mostly accounted for by probes having different polarizabilities.