Esta es la versión de autor del artículo publicado en: This is an author produced version of a paper published in:Science 354(6313) (2016): 734-738 DOI: http://dx.doi.org/10.1126/science.aah5188 Copyright: © 2016 American Association for the Advancement of ScienceEl acceso a la versión del editor puede requerir la suscripción del recurso Access to the published version may require subscription However, the rapidity of electron dynamics on the attosecond timescale has precluded their complete measurement in the time domain. Here, we demonstrate that spectrally-resolved electron interferometry reveals the amplitude and phase of a photoelectron wavepacket created through a Fano autoionizing resonance in helium. Replicas obtained by two-photon transitions interfere with reference wavepackets formed through smooth continua, allowing the full temporal reconstruction, purely from experimental data, of the resonant wavepacket released in the continuum. This in turn resolves the buildup of the autoionizing resonance on attosecond timescale. Our results, in excellent agreement with ab initio time-dependent calculations, raise prospects for both detailed investigations of ultrafast photoemission dynamics governed by electron correlation, as well as coherent control over structured electron wave-packets.One Sentence Summary: By monitoring the decay of an excited atom in real time, we reconstruct how photoelectron wavepackets are born and morph into asymmetric Fano profiles. Main Text:Tracking electronic dynamics on the attosecond (as) timescale and Ångström (Å) lengthscale is a key to understanding and controlling the quantum mechanical underpinnings of physical and chemical transformations (1). One of the most fundamental electronic processes in this context is photoelectron emission, the dynamics of which are fully encoded in the released electron wavepacket (EWP) and the final ionic state. The development of broadband coherent sources of attosecond pulses has opened the possibility of investigating these dynamics with attosecond resolution. On such a short timescale, few techniques (2-5) are able to provide access to both spectral amplitude and phase. The spectral derivative of the phase, the group delay, is a practical quantity for describing general wavepacket properties reflecting the ionization dynamics.

Electron dynamics induced by resonant absorption of light is of fundamental importance in nature and has been the subject of countless studies in many scientific areas. Above the ionization threshold of atomic or molecular systems, the presence of discrete states leads to autoionization, which is an interference between two quantum paths: direct ionization and excitation of the discrete state coupled to the continuum. Traditionally studied with synchrotron radiation, the probability for autoionization exhibits a universal Fano intensity profile as a function of excitation energy. However, without additional phase information, the full temporal dynamics cannot be recovered. Here we use tunable attosecond pulses combined with weak infrared radiation in an interferometric setup to measure not only the intensity but also the phase variation of the photoionization amplitude across an autoionization resonance in argon. The phase variation can be used as a fingerprint of the interactions between the discrete state and the ionization continua, indicating a new route towards monitoring electron correlations in time.

Strong field driven electric currents in condensed matter systems open new frontiers in petahertz electronics. In this regime new challenges arise as the role of the band structure and the quantum nature of electron-hole dynamics have yet to be resolved. Here we reveal the underlying attosecond dynamics that dictates the temporal evolution of carriers in multi-band solid state systems, via high harmonic generation (HHG) spectroscopy. We demonstrate that when the electron-hole relative velocity approaches zero, enhanced quantum interference leads to the appearance of spectral caustics in the HHG spectrum. Introducing the role of the dynamical joint density of states (JDOS) we identify its direct mapping into the spectrum, exhibiting singularities at the spectral caustics. By probing these singularities, we visualize the structure of multiple unpopulated high conduction bands. Our results open a new path in the control and study of attosecond quasi-particle interactions within the field dressed band structure of crystals.Induced by the strong field interaction, HHG provides a unique spectroscopic scheme to visualize the coherent evolution of petahertz currents inside solids.Since the first observation [1], solid HHG opened a door into the study of the electronic structure and dynamics in crystals [2,3,4,5,6,7], multiple band dynamics [8,9,10,11] and complex many-body phenomena [12] in crystalline and amorphous systems [9]. For a moderate field strength the electron-hole dynamics are often described semi-classicaly by a single valence and conduction band of

Sub-laser cycle time scale of electronic response to strong laser fields enables attosecond dynamical imaging in atoms, molecules and solids 1-4 . Optical tunneling and high harmonic generation 2, 5-7 are the hallmarks of attosecond imaging in optical domain, including imaging of phase transitions in solids 8, 9 . Topological phase transition yields a state of matter intimately linked with electron dynamics, as manifested via the chiral edge currents in topological insulators 10 . Does topological state of matter leave its mark on optical tunnelling 1 arXiv:1806.11232v2 [physics.optics]

We present a theoretical study of the photoelectron attosecond beating due to interference of two-photon transitions in the presence of autoionizing states. We show that, as a harmonic traverses a resonance, both the phase shift and frequency of the sideband beating significantly vary with photon energy. Furthermore, the beating between two resonant paths persists even when the pump and the probe pulses do not overlap, thus providing a nonholographic interferometric means to reconstruct coherent metastable wave packets. We characterize these phenomena by means of a general analytical model that accounts for the effect of both intermediate and final resonances on two-photon processes. The model predictions are in excellent agreement with those of accurate ab initio calculations for the helium atom in the region of the N=2 doubly excited states.