Tracking surface plasmon pulses using ultrafast leakage imaging

The ever-growing development of near-field nano-optics, involving in particular surface plasmon excitations in a great variety of configurations and systems, continuously calls for new techniques that can give access to the properties of the specific modes excited in such conditions. One crucial aspect is the capacity to image near-field distributions, both from static and time-dependent perspectives. In this paper, we introduce a new method for performing ultrafast imaging and tracking of surface plasmon wave packets that propagate on metal films. We demonstrate the efficiency of leakage radiation microscopy implemented in the time domain for measuring both group and phase velocities of near-field pulses with a high level of precision. Our methodology involves a spatial-heterodyne interferometer that efficiently resolves dispersion effects associated with an arbitrary plasmonic signal. Demonstrated at the level of single plasmonic wave packets propagating on a planar metal film, our far-field imaging method turns out to be particularly versatile and appealing in the context of ultrafast near-field optics.