Nanometric probing with a femtosecond, intra‐cavity standing wave

Abstract Optical standing waves are intrinsically nanometric, spatially fixed interference‐field patterns. At a commensurate scale, metallic nanotips serve as coherent, atomic‐sized electron sources. Here, we explore the localized photofield emission from a tungsten nanotip with a transient standing wave. It is generated within an optical cavity with counter‐propagating femtosecond pulses from a near‐infrared, 100‐MHz frequency comb. Shifting the phase of the standing wave at the tip reveals its nodes and anti‐nodes through a strong periodic modulation of the emission current. We find the emission angles to be distinct from those of a traveling wave, and attribute this to the ensuing localization of emission from different crystallographic planes. Supported by a simulation, we find that the angle of maximum field enhancement is controlled by the phase of the standing wave. Intra‐cavity nanotip interaction not only provides higher intensities than in free‐space propagation, but also allows for structuring the light field even in the transverse direction by selection of high‐order cavity modes.