Light-matter interaction with electron nanoemitters
Femtosecond electron source
In this project we develop and use a novel electron source that enables us to perform fundamental research with free electrons. Our approach for such a source is the following: We focus light pulses consisting of a few optical cycles (duration of 6 fs) onto a nanometric metal tip (typical radii are about 30nm). Owing to the high intensity of up to 10^(11)W/cm^2 electrons are emitted on a very short time scale by highly non-linear photon absorption. Due to plasmonic effects the actual laser intensity is further increased by a factor of up to 50 at the tip's apex. The unprecedented temporal and spatial control over the emission process will be used in further applications.
As a first result we have found that single electrons absorb the energy of up to six photons more than actually needed to escape from the metal, a phenomenon that is called above-treshold-photoemission (ATP). Furthermore, at intensities of a few 10^(11)W/cm^2 strong-field effects occur which cannot be explained in a simple photon picture. In general, those effects start to be observable in the electron spectra once the oscillation energy (ponderomotive energy) of the emitted electron in the light field is comparable to the photon energy. We have for the first time observed these effects in ATP (see Fig. and publication).
In addition, we have evidence that electrons are influenced by the light field so strongly that they return after emission and recollide/rescatter at the tip's surface. This is usually a requisite to generate high harmonics (HHG) in atomic gases at high intensities. Here we would have the possibilty to use the full laser repetition rate of typically around 100 MHz. By using a nanometric metal tip other problems occurring with atomic gases can be circumvented (e.g. focal averaging, gas depletion/saturation)
Due to the relative simple experimental setup and the high control on the atomic level is such a nanometric metal tip an ideal model system to study light-matter interactions in the strong-field regime. The wider applicability of such a source is also investigated in our group in the context of electron acceleration or time-resolved electron diffraction.
C-E-phase sensitive electron emission
In extremely short laser pulses the electric field can be controlled within the envelope with the so-called frequency comb technique (Nobel Prize to T. W. Hänsch). With these so-called C-E-phase stabilized pulses, we could show that electron emission is sensitive on the C-E-phase. The spectra show a strong dependence on C-E-phase, see animation on the right. From theoretical models we infer that electrons are laser emitted, return in the laser field back to the parent tip, are elastically scattered in the tip, gain more energy in the laser field, and are only then detected. Depending on the C-E-phase this can happen once or twice per pulse. If it happens twice, interference arises, which leads to the strong maxima and minima that can be seen in the animation for CEP ~ pi. This is the time-energy analog of the famous Young's double slit experiment, where interference also only arises if particles can fly through two slits simultaneously.