The propagation and transport of electrons in crystals is a fundamental process pertaining to the functioning of most electronic devices. Microscopic theories describe this phenomenon as being based on the motion of Bloch wave packets. These wave packets are superpositions of individual Bloch states with the group velocity determined by the dispersion of the electronic band structure near the central wavevector in momentum space. This concept has been verified experimentally in artificial superlattices by the observation of Bloch oscillations—periodic oscillations of electrons in real and momentum space. Here we present a direct observation of electron wave packet motion in a real-space and real-time experiment, on length and time scales shorter than the Bloch oscillation amplitude and period. We show that attosecond metrology (1 as = 10^-18 seconds) now enables quantitative insight into weakly disturbed electron wave packet propagation on the atomic length scale without being hampered by scattering effects, which inevitably occur over macroscopic propagation length scales. We use sub-femtosecond (less than 10^-15 seconds) extreme-ultraviolet light pulses to launch photoelectron wave packets inside a tungsten crystal that is covered by magnesium films of varied, well-defined thicknesses of a few ångströms. Probing the moment of arrival of the wave packets at the surface with attosecond precision reveals free-electron-like, ballistic propagation behaviour inside the magnesium adlayer—constituting the semi-classical limit of Bloch wave packet motion. Real-time access to electron transport through atomic layers and interfaces promises unprecedented insight into phenomena that may enable the scaling of electronic and photonic circuits to atomic dimensions. In addition, this experiment allows us to determine the penetration depth of electrical fields at optical frequencies at solid interfaces on the atomic scal

We investigate the effect of large in-plane strain and vertical electric fields on the binding energies of excitonic complexes confined in single InGaAs/GaAs quantum dots (QDs) and we find that the two independently tunable perturbations modify the interaction energies among electrons and holes in a different manner. By taking advantage of this difference, we frequency-lock the QD fundamental excitation (the neutral exciton) at a predefined value, while the biexciton transition is actively tuned from a binding to an antibinding configuration. Our electrically controlled dual-knob device demonstrates unprecedented control over the electronic properties of the few-particle states in a QD and may be applied to create novel energy-tunable sources of entangled photons using the time-reordering or the time-bin scheme.

We describe an apparatus for attosecond photoelectron spectroscopy of solids and surfaces, which combines the generation of isolated attosecond extreme-ultraviolet (XUV) laser pulses by high harmonic generation in gases with time-resolved photoelectron detection and surface science techniques in an ultrahigh vacuum environment. This versatile setup provides isolated attosecond pulses with photon energies of up to 140 eV and few-cycle near infrared pulses for studying ultrafast electron dynamics in a large variety of surfaces and interfaces. The samples can be prepared and characterized on an atomic scale in a dedicated flexible surface science end station. The extensive possibilities offered by this apparatus are demonstrated by applying attosecond XUV pulses with a central photon energy of ∼125 eV in an attosecond streaking experiment of a xenon multilayer grown on a Re(0001) substrate

INTRODUCTION.Multilayer XUV mirrors serve as key components in the generation of the attosecond pulses from high harmonic radiation 1 . Those pulses pave the way to investigation of dynamics of electronic motion in atoms, molecules and nanostructures with a never before achieved precision and thus allow to draw conclusions on the basic underlying physics 2, 3 . Each experiment requires its perfectly tailored attosecond XUV pulse, fully optimized to perfectly match the experimental requirements as spectral resolution with sufficient signal to noise, temporal resolution and chirp. Until recently, experiments were limited to photon energies below 100 eV due to the available HH intensities as well as X-ray optics. Here, we have performed for the first time characterization of single isolated attosecond pulses at about 130 eV by attosecond electron streaking measurements and applied these pulse to photoionization experiments in Xe atoms. In order to achieve improved spectral resolution and high time resolution simultaneously, as is required for comparing attosecond electron dynamics of two adjacent electronic states, we have spectrally cleaned the attosecond pulse by suppressing unwanted "out-of band" radiation by improved a-periodic multilayer technology, resulting in an improved signal-to noise ratio for time-resolved electron spectroscopy experiments. This opens the way to study electron dynamics of energetically adjacent electronic states with a temporal resolution which is well below the attosecond pulse length itself 4 .. EXPERIMENTAL METHODS.The attosecond experiments were performed at the AS3 beamline at the Max Planck Institute for quantum optics in Garching, Germany. Numerical algorithms as the "needle optimization" 5 have been used to design the aperiodic ternary coatings of La/Mo/B 4 C for normal incidence reflection at about 130 eV and a bandwidth of about 5 eV FWHM... While standard periodic multilayer structures exhibit secondary Kiessig fringes in their a555_1.pdf OSA / UP 2010 PDP8.pdf reflectivity spectrum, these can be efficiently suppressed by advanced a-periodic multilayer structures Two different mirrors, with and without Kiessig fringe suppression, have been produced via ion beam deposition and controlled by insitu-ellipsometry. ., The reflectivity of both mirrors has been characterized by reflectivity measurements at the Advanced Light Source (ALS) 6 . The spectral phase of such mirrors, which controls the chirp of the XUV pulse, is much more difficult to access. Here we use attosecond electron streaking in Ne in a XUV pump/IR probe setup 7 and a FROG/CRAB algorithm 8 to fully retrieve the temporal and spectral intensity of the XUV pulse as well as its phase. Finally, the Xe photoelectron time-of flight spectra have been measured and compared for the two different cases of spectrally improved and non-improved XUV pulses. Figure 1 displays a comparison between the measured photoelectron spectrum of Xe atoms for the periodic mirror designed for maximum reflectivity at 130 eV (left panel) and that...