  Modulation of the optical index by means of atomic and material resonances
provides a basis for controlling light propagation in natural and artificially
fabricated materials. In addition, recent advances in the tuning of
spatiotemporal couplings of ultrashort laser pulses have enabled almost
arbitrary control over the group velocity of light. Here, using femtosecond
time-resolved microscopy and numerical calculations, we investigate the
spatiotemporal dynamics of a surface plasmon polariton wave packet (SPP WP)
that interacts with a plasmonic nanocavity. The nanocavity consists of
metal-insulator-metal multilayer films that function as subwavelength meta-atom
possessing tunable discretized eigenmodes. When a chirp-induced femtosecond SPP
WP is incident on a nanocavity, only the spectral component matching the
resonance energy is transmitted. This spectral clipping effect is accompanied
by a spatial shift of the WP. The shift can be adjusted in either the positive
or negative direction by controlling the resonance energy or the chirp. If this
spatial shift is regarded as a modulation of the apparent group velocity in the
nanocavity, the range of modulation includes superluminal, subluminal, and
negative group velocities.
