Observation of interband Berry phase in laser driven crystals

Abstract Ever since its discovery, the notion of Berry phase has permeated through all branches of physics, being at the heart of a large variety of quantum phenomena. However, so far all its realizations were based on a continuous evolution of the quantum state, following a cyclic path. Here, we introduce and demonstrate a conceptually new manifestation of the Berry phase in light driven crystals, where the electronic wavefunction accumulates a geometric phase during discrete evolution between different bands, while preserving the coherence of the process. We experimentally demonstrate this phase by using a strong laser field to engineer an internal interferometer, induced during less than one cycle of the driving field, which maps the phase onto the emission of high order harmonics. Our work opens a new branch in the study of geometric phases, leading to a variety of new observations in light driven topological phenomena and attosecond solid state physics.