  Strong coupling with single molecules in plasmonic picocavities has emerged
as a resource for room-temperature quantum control with nanoscale light.
Tip-based nanoprobes can measure the local dynamics of individual molecular
picocavities, but the overhead associated with sampling an inhomogeneous
picocavity distribution can be challenging for scalability. We propose a
macroscopic approach in which an ensemble of molecular picocavities acts as a
nonlinear plasmonic metamaterial. Using a quantum optics perspective, we study
theoretical performance limits for optical cross-phase modulation in the
system, taking into account realistic distributions of picocavity volumes and
molecular transition frequencies. The medium nonlinearity is mediated by local
strong coupling with the lowest vibronic emission sideband of individual
organic chomophores. The local vacua change the refractive index of the medium
at the frequency of a weak probe field $\omega_p$, set to drive the bare
zero-phonon absorption band. Refractive index variations $\Delta n/n$ of a few
percent, relative to a molecule-free scenario, are feasible with dilute
ensembles. The probe phase evolution can be switched off in the presence of a
signal field at a higher frequency $\omega_s$, for intensities as low as 10
kW/cm$^2$. The mechanism for optical switching involves a novel
($\omega_p+\omega_s$) two-photon absorption channel, assisted by local vacuum
fields. Our work paves the way for future studies of plasmonic metamaterials
that exploit strong light-molecule interactions, for applications in optical
state preparation and control.
