  Light-matter interactions can be dramatically modified by the surrounding
environment. Here we report on the first experimental observation of molecular
spontaneous emission inside a highly nonlocal metamaterial based on a plasmonic
nanorod assembly. We show that the emission process is dominated not only by
the topology of its local effective medium dispersion, but also by the nonlocal
response of the composite, so that metamaterials with different geometric
parameters but the same local effective medium properties exhibit different
Purcell factors. A record-high enhancement of a decay rate is observed, in
agreement with the developed quantitative description of the Purcell effect in
a nonlocal medium. An engineered material nonlocality introduces an additional
degree of freedom into quantum electrodynamics, enabling new applications in
quantum information processing, photo-chemistry, imaging, and sensing.
