Electrons in the energy range below 1 keV are strongly scattering probes, providing unique sensitivity to the atomic structure of surfaces and to nanoscale electric fields. Combined with femtosecond temporal resolution, they are ideally suited to study the structural dynamics of 2D crystalline materials, and to probe ultrafast currents and electric fields in nanostructures. Their pronounced dispersion, however, so far prevented their use as femtosecond probes in ultrafast pump-probe techniques. I will present a hybrid setup that was developed for femtosecond point- projection microscopy (fsPPM) and femtosecond low-energy electron diffraction (fsLEED), utilizing sharp metal tips as pulsed low-energy electron source. The strong field enhancement and nanometric size of nanotip photoemitters facilitates ultrashort propagation times and thus minimization of dispersive pulse broadening, as well as unique spatio-temporal control over the photoemission process.
In fsPPM, the point-like shape of nanotips is utilized for imaging in a lens-less projection geometry. As low-energy electrons are easily detected by electrostatic fields, fsPPM is sensitive to the potential distribution at the surface of nanostructures, and allows for imaging their transient changes after photoexcitation. As a proof of concept, ultrafast photocurrents in axially doped semiconductor nanowires (NWs) are investigated by time-resolved imaging of the local surface photovoltage. The results demonstrate the capability of fsPPM to probe ultrafast carrier dynamics in nanoscale systems on femtosecond time and nanometer length scales.
The minimal tip-sample distance and thus the achievable spatio-temporal resolution in fsPPM is limited by the diffraction-limited laser illumination of the tip apex for excitation of the electron probe pulses. To overcome this limitation, a novel type of nanotip femtosecond electron source has been realized, driven non-locally by nanofocused surface plasmon polaritons (SPPs). It is shown that ultra-broadband SPPs of less than 10 fs duration can be nanofocused into the tip apex with high efficiency, inducing the nonlinear ultrafast emission of electrons. The application of such sources for fsPPM is demonstrated by imaging the electric field distribution around a semiconductor NW at a reduced tip-sample distance of 3 μm.