THz near-field streaking spectroscopy at biased metal nanotips

We demonstrate terahertz (THz) near-field streaking at metal nanotips. In this scheme, photoelectrons are generated at the apex of a nanotip (gold, tungsten) by near-infrared (NIR) femtosecond pulses and accelerated in the THz-induced near-field (cf. Fig. 1a). The high local field strengths and the nanometric spatial confinement of the THz near-field allow electrons to leave the THz electric field within a fraction of an optical cycle, and the kinetic energy of the photoelectrons is determined by the electric field in the moment of photoemission. Such a sub-cycle interaction of the photoelectrons with the streaking field is a characteristic feature of near-field streaking at nanostructures [1]. We record streaking spectrograms (see Fig. 1b) by varying the relative delay between the THz and the NIR pulses to map the temporal evolution of the THz near-field onto the kinetic energy of the photoelectrons [2].