  Accessing precisely to the phase variation of electronic wave-packet (EWP)
provides unprecedented spatiotemporal information of microworld. A radial
interference pattern at near-zero energy has been widely observed in
experiments of strong-field photoionization. However, the underlying physical
picture of this interference pattern is still under debate. Here we report an
experimental and theoretical investigation of this low-energy interference
structure (LEIS) in mid-infrared laser fields. We clarify that the LEIS arises
due to the soft-recollision mechanism, which was previously found to play a
pivotal role in producing the pronounced low-energy structure. Specifically,
the LEIS is induced by the interference between direct and soft-recollision
EWPs launched within a 1/18 laser-cycle time scale in our experiments.
Moreover, the observation of LEIS is independent of laser wavelength and
specific atomic targets. Our result opens a promising new avenue for retrieving
the structure and dynamics of EWPs in atoms and molecules with attosecond time
resolution.
