One of the most exciting advances in attosecond science is high-harmonic generation (HHG) spectroscopy. It combines sub-angstrom spatial resolution with attosecond temporal resolution, allowing us to dynamically resolve the structure of electronic wavefunctions as they evolve. During the past two decades, HHG spectroscopy has been successfully applied to resolve fundamental strong-field phenomena such as field-induced tunneling in atoms, hole dynamics, charge migration in molecules and field-induced currents in solids. An important aspect of HHG spectroscopy lies in its coherent nature. The strong-field interaction directly transfers the coherence of the laser field into the coherent properties of the electronic wavefunction that interacts with the field and then back into the optical properties of the emitted harmonics. Resolving the internal coherence is key to reconstructing the internal dynamics, being one of the primary challenges in HHG spectroscopy. As in many other branches of physics, the presence of coherence is determined through interferometry.
In this talk, I will describe advanced schemes for attosecond interferometry. Applying time domain interferometry, provided a direct insights into one of the most fundamental strong field phenomena – field induced tunneling, resolving its fundamental properties. Inducing spatial interferometers, by shaping 2D electron trajectories, we probed the spatial properties of tunneling in molecular systems. An alternative scheme applies attosecond interferometry based on an all-optical approach. By using two interfering phased-locked attosecond sources, we resolved the scattering phase associated with the basic mechanism underlying photoionization. Going from gas phase into solid state systems opens new frontiers in attosecond science. Strong field phenomena in solids raise new challenges as the role of the band structure and the quantum nature of ultrafast electron-hole dynamics have yet to be resolved. Attosecond interferometry enabled us to obtaine a new insight into the origin of multiband strong-field currents. We probed the underlying attosecond dynamics that dictates the temporal evolution of carriers in large bandgap dielectrics via HHG spectroscopy and visualized the structure of multiple unpopulated high-energy conduction bands.