  Diatoms are microscopic algae found in all of Earths water courses. They
produce frustules, porous silica exoskeletons, grown by precipitation of
silicic acid from water. Frustule components, known as girdles, from some
diatom species also feature highly periodic pore arrays with properties on the
same scale and quality of ordering as manufactured photonic crystals. Here, the
girdles of two diatom species with various lattice constants are used as
bio-silica scaffolds for plasmonic crystals, combined with metals chosen for
spectral agreement between permittivity and photonic crystal properties. The
use of frustules as scaffolds for plasmonic application has been tested
earlier, as nano-porous features can enhance scattering and local hotspots,
useful for sensing application or SERS. However, both the use of girdles and of
continuous metal films are new approaches offering previously unseen plasmonic
effects. Angularly resolved dispersions from reflection Fourier spectroscopy
are provided for Coscinodiscus granii girdles, forming well-ordered square
lattices, and Coscinodiscus wailesii, forming ordered hexagonal lattices,
covered with a 40 nm film of silver or with 40 nm of gold, respectively. These
were confirmed with optical modelling of the 3D systems, including both metal
surface layers and internal frustule structures, allowing for the calculation
of dispersions and electric fields. Measurements are further compared with
structures produced via Focused Ion Beam microscopy, demonstrating the
advantages of girdles as photonic crystal scaffolds and as an alternative to
conventional plasmonic crystal fabrication. The results demonstrate that
high-quality plasmonic crystal scaffolds from diatoms are available in nature,
and their natural diversity can be applied in a controlled way to obtain
plasmonic crystals with a wide range of behaviours and spectral responses.
