  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. The superlattice-type columns resemble bamboo structures
where smaller column sections of gold form junctions sandwiched between larger
silicon column sections ("nano-bamboo"). We perform generalized spectroscopic
ellipsometry measurements and finite element method computations to elucidate
the strongly anisotropic optical properties of the highly-porous nano-bamboo
structures. 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 the nano-bamboo structure. 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 the nano-bamboo
structures as a highly porous plasmonic sensor with optical read out
sensitivity to few parts-per-million solvent levels in water.
