  Interference between light waves is one of the widely known phenomena in
physics, which is widely used in modern optics, ranging from precise detection
at the nanoscale to gravitational-wave observation. Akin to light, both
classical and quantum interferences between surface plasmon polaritons (SPPs)
have been demonstrated. However, to actively control the SPP interference
within subcycle in time (usually less than several femtoseconds in the visible
range) is still missing, which hinders the ultimate manipulation of SPP
interference on ultrafast time scale. In this paper, the interference between
SPPs launched by a hole dimer, which was excited by a grazing incident free
electron beam without direct contact, was manipulated through both propagation
and initial phase difference control. Particularly, using cathodoluminescence
spectroscopy, the appearance of higher-order interference orders was obtained
through propagation phase control by increasing separation distances of the
dimer. Meanwhile, the peak-valley-peak evolution at a certain wavelength
through changing the accelerating voltages was observed, which originates from
the initial phase difference control of hole launched SPPs. In particular, the
time resolution of this kind of control is shown to be in the ultrafast
attosecond (as) region. Our work suggests that fast electron beams can be an
efficient tool to control polarition interference in subcycle temporal scale,
which can be potentially used in ultrafast optical processing or sensing.
