The main goal of the first phase of the project was the establishment of attosecond gated holography as a state-of-the-art experimental scheme. During this stage, we achieved several important breakthroughs in developing this method and applying it to attosecond metrology, thereby uncovering fundamental phenomena in atomic, molecular, and solid-state systems.
The first study focused on establishing attosecond gated holography. We demonstrated this scheme in gas-phase systems and probed one of the most fundamental quantum mechanical phenomena -- field-induced tunneling. This approach enabled us to trace the evolution of an electronic wavefunction under the tunneling barrier and to record the phase acquired by an electron as it propagated through a classically forbidden region. We identified the quantum nature of the electronic wavepacket and captured its evolution in complex time and momentum within a fraction of an optical cycle. The apparatus developed in this study now serves as the main experimental platform for the subsequent phases of the project. In the next step, we established attosecond transient interferometry. Whereas attosecond transient absorption measures the instantaneous response of a quantum system to a laser field by mapping its sub-cycle dynamics onto the absorption spectrum of attosecond pulses, the full quantum dynamics are imprinted in the amplitude, phase, and polarization of these pulses. By applying our interferometric scheme, we introduced attosecond transient interferometry, directly measuring the transient phase and following its evolution within an optical cycle. We demonstrated how such phase-sensitive measurements enable the decoupling of multiple quantum pathways induced in a light-driven system, isolating their coherent contributions and retrieving their temporal evolution.
We further demonstrated an alternative interferometric scheme in molecular systems. By controlling strong-field-driven electron trajectories, we induced an interferometer on a microscopic scale. This approach retrieves the symmetry and structure of molecular orbitals. Focusing on one of the most fundamental strong-field processes—tunnel ionization—we reconstructed the angle at which the electronic wavefunction tunnels through the barrier and followed its evolution with attosecond precision.
The central goal of the proposed research was to extend these techniques to probe sub-cycle dynamics in solids. Intense light–matter interactions induce significant modifications of the electronic and optical properties of materials, reshaping the band structure of light-dressed crystals. Yet, identifying and characterizing these transient modifications remains a major challenge. Using attosecond gating, we addressed this problem and directly probed the laser-induced closing of the band gap between adjacent conduction bands. Our work revealed the link between extreme nonlinear light–matter interactions in strongly driven crystals and the sub-cycle modifications of their effective band structure. Finally, we applied attosecond spectroscopy to resolve the interband Berry phase in solids. The Berry phase underlies a wide variety of quantum phenomena. We introduced and demonstrated a conceptually new manifestation of the Berry phase in light-driven crystals, in which the electronic wavefunction accumulates a geometric phase during a discrete evolution between different bands, while preserving coherence. We experimentally revealed this phase by using a strong laser field to engineer an internal interferometer, formed within less than a single optical cycle, which maps the Berry phase onto the emission of higher-order harmonics.
