  Flat lens concept based on negative refraction proposed by Veselago in 1968
has been mostly investigated in monochromatic regime. It was recently
recognized that time development of the super-lensing effect discovered in 2000
by Pendry is yet to be assessed and may spring surprises: Time-dependent
illumination could improve the spatial resolution of the focusing. We
investigate dynamics of flexural wave focusing by a 45\degre-tilted square
lattice of circular holes drilled in a Duraluminium plate. Time-resolved
experiments reveal that the focused image shrinks with time below diffraction
limit, with a lateral resolution increasing from 0.8 $\lambda$ to 0.35
$\lambda$, whereas focusing under harmonic excitation remains diffraction
limited. Modal analysis reveals the role in pulse reconstruction of radiating
lens resonances, which repeatedly self-synchronize at the focal spot to shape a
super-oscillating field.
