  The local density of optical states governs an emitters lifetime and quantum
yield through the Purcell effect. It can be modified by a surface plasmon
electromagnetic field, but such a field has a spatial extension limited to a
few hundreds of nanometers, which complicates the use of optical methods to
spatially probe the emitter-plasmon coupling. Here we show that a combination
of electron-based imaging, spectroscopies and photon-based correlation
spectroscopy enables measurement of the Purcell effect with nanometer and
nanosecond spatio-temporal resolutions. Due to the large variability of
radiative lifetimes of emitters embedded in nanoparticles with inhomogeneous
sizes we relied on a statistical approach to unambiguously probe the coupling
between nitrogen-vacancy centers (NV^0) in nanodiamonds and surface plasmons in
silver nanocubes. We quantified the Purcell effect by measuring the NV^0
excited state lifetimes in a large number of either isolated nanodiamonds or
nanodiamond-nanocube dimers and demonstrated a statistically significant
lifetime reduction for dimers.
