  We propose a scheme for ultrafast real-time imaging of laser-induced
collective electron oscillations (Mie plasmons) in gas phase metal clusters by
interferometrically stable scanning of two intense few-cycle optical laser
pulses. The feasibility of our nonlinear spectral interferometry method with
experimentally accessible observables is tested in a theoretical case study on
simple-metal clusters (Na$_{147}$). The results show that the plasmon period
and lifetime as well as the phase and relative amplitude of the collective
electron motion can be extracted with sub-fs resolution. The access to
nonlinear response effects, as the demonstrated increase of the plasmon
lifetime with laser intensity due to ionization-induced contraction of the
electron cloud, opens up vast opportunities for interrogating ultrafast
many-particle dynamics in nanosystems under strong laser fields with
unprecedented resolution.
