Compressing Few-Cycle Optical Near Fields in the Tip-Sample Junction of a Scanning Probe Microscope.

Plasmonic nanogaps confine light to dimensions in the nanometer or even subnanometer range while simultaneously enhancing the local electromagnetic field strength. This spatial light confinement has been key for the development of nano-optics. So far, the temporal dynamics of such nanoconfined fields has received comparatively little attention, in particular in the visible spectral range. Here we measure the amplitude and phase of the electric field of visible to near-infrared light pulses scattered from the gap between a sharp gold tip and a metal surface. We retrieve the time structure of the field with subcycle precision. We provide evidence for a complex-valued local near-field enhancement and demonstrate that the spatial confinement of a few-cycle pulse in the investigated nanogap is correlated with a substantial reduction in its pulse duration. Our results pave the way for probing the linear and nonlinear electric field dynamics of single quantum emitters in nanogaps.