Tunable plasmonic resonances in Si-Au slanted columnar heterostructure thin films

We report on fabrication of spatially-coherent columnar plasmonic nanostructure superlattice-type thin films with high porosity and strong optical anisotropy using glancing angle deposition. Subsequent and repeated depositions of silicon and gold lead to nanometer-dimension subcolumns with controlled lengths. We perform generalized spectroscopic ellipsometry measurements and finite element method computations to elucidate the strongly anisotropic optical properties of the highly-porous Si-Au slanted columnar heterostructures. The occurrence of a strongly localized plasmonic mode with displacement pattern reminiscent of a dark quadrupole mode is observed in the vicinity of the gold subcolumns. We demonstrate tuning of this quadrupole-like mode frequency within the near-infrared spectral range by varying the geometry of Si-Au slanted columnar heterostructures. In addition, coupled-plasmon-like and inter-band transition-like modes occur in the visible and ultra-violet spectral regions, respectively. We elucidate an example for the potential use of Si-Au slanted columnar heterostructures as a highly porous plasmonic sensor with optical read out sensitivity to few parts-per-million solvent levels in water.
Funding Agencies|National Science Foundation (NSF) through the Nebraska Materials Research Science and Engineering Center (MRSEC) [DMR-1420645, CMMI 1337856, EAR 1521428, DMR 1808715]; German Research Foundation [FE 1532/1-1]; Air Force Research Office [FA9550-18-1-0360]; American Chemical Society [ACS PRF 59374-ND5]; University of Nebraska-Lincoln; J.A. Woollam Co., Inc.; J.A. Woollam Foundation