  Metamaterials are fascinating tools that can structure not only surface
plasmons and electromagnetic waves but also electromagnetic vacuum
fluctuations. The possibility of shaping the quantum vacuum is a powerful
concept that ultimately allows engineering the interaction between macroscopic
surfaces and quantum emitters such as atoms, molecules or quantum dots. The
long-range atom-surface interaction, known as Casimir-Polder interaction, is of
fundamental importance in quantum electrodynamics but also attracts a
significant interest for platforms that interface atoms with nanophotonic
devices. Here we perform a spectroscopic selective reflection measurement of
the Casimir-Polder interaction between a Cs(6P_{3/2}) atom and a nanostructured
metallic planar metamaterial. We show that by engineering the near-field
plasmonic resonances of the metamaterial, we can successfully tune the
Casimir-Polder interaction, demonstrating both a strong enhancement and
reduction with respect to its non-resonant value. We also show an enhancement
of the atomic spontaneous emission rate due to its coupling with the evanescent
modes of the nanostructure. Probing excited state atoms next to nontrivial
tailored surfaces is a rigorous test of quantum electrodynamics. Engineering
Casimir-Polder interactions represents a significant step towards atom trapping
in the extreme near field, possibly without the use of external fields.
