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The electro-optical properties of most semiconductors and insulators oftechnological interest are dominated by the presence of electron-holequasiparticles called excitons. The manipulation of these hydrogen-likequasi-particles in dielectrics, has received great interest under the nameexcitonics that is expected to be of great potential for a variety ofapplications, including optoelectronics and photonics. A crucial step for suchexploitation of excitons in advanced technological applications is a detailedunderstanding of their dynamical nature. However, the ultrafast processesunfolding on few-femtosecond and attosecond time scales, of primary relevancein view of the desired extension of electronic devices towards the petahertzregime, remain largely unexplored. Here we apply attosecond transientreflection spectroscopy in a sequential two-foci geometry and observesub-femtosecond dynamics of a core-level exciton in bulk MgF$_2$ singlecrystals. With our unique setup, we can access absolute phase delays whichallow for an unambiguous comparison with theoretical calculations based on theWannier-Mott model. Our results show that excitons surprisingly exhibit a dualatomic- and solid-like character which manifests itself on different timescales. While the former is responsible for a femtosecond optical Stark effect,the latter dominates the attosecond excitonic response and originates by theinteraction with the crystal. Further investigation of the role of excitonlocalization proves that the bulk character persists also for stronglylocalised quasi-particles and allows us to envision a new route to controlexciton dynamics in the close-to-petahertz regime.

Unravelling the intertwined atomic and bulk nature of localised excitons by attosecond spectroscopy