Active control of plasmon interference in nanohole dimers by free electrons

Interference among surface plasmon polaritons (SPPs) is a well-established phenomenon and has been experimentally demonstrated across both classical and quantum realms. However, actively controlling SPP interference within subcycle time intervals (typically less than several femtoseconds in the visible range) remains challenging, hindering the ultimate manipulation of SPP interference on an ultrafast timescale. Here, we manipulated the interference between surface plasmon polaritons (SPPs) launched by a hole dimer excited by a grazing incident free-electron beam without direct contact. Specifically, using cathodoluminescence spectroscopy, the emergence of higher-order interference orders was obtained by increasing separation distances of the dimer. Meanwhile, the observation of peak-valley-peak evolution at a specific wavelength by varying the accelerating voltages is attributed to the controlled initial phase difference of hole-launched SPPs. Our findings suggest that fast electron beams can serve as an efficient tool for controlling polariton interference on a subcycle temporal scale, potentially enabling applications in ultrafast optical processing or sensing.