Speaker: Matteo Lucchini, Politecnico di Milano
Program Description:
The possibility to manipulate the electrical properties of matter with very short optical pulses is a fascinating field of research with possible far reaching applications in many relevant technological fields. The first step towards the realization of this goal is to understand the physical processes at the basis of light-matter interaction. In turn, this requires light pulses with a duration comparable to the natural time scale of electronic motion: the attosecond. Despite the great effort made by the scientific community, the flux of common attosecond sources is rather limited. Thus typical pump-probe schemes combine attosecond pulses with phase-locked few-cycle IR pulses (Fig. 1), as it is done in attosecond transient absorption spectroscopy (ATAS)1. This technique has been employed to investigate electron dynamics in solids, especially in dielectrics, where a deeper understanding of the electronic properties is of great importance for the development of high power electronics and signal processing2. Pioneering experiments conducted on bulk insulators and semiconductors made use of a resonant core transition to disclose ultrafast phenomena like reversible photodoping in and core-exciton dynamics SiO23,4, permanent electron injection in Si by tunneling5, virtual electron dynamics in diamond6, ultrafast charge injection in GaAs7, and a variety of other important ultrafast mechanisms 8.
Fig. 1 Cartoon of a typical ATAS setup for the investigation of ultrafast electron dynamics in solids.
Unfortunately, the low photon flux of standard attosecond sources in combination with the high absorption coefficient in the XUV spectral range limit the applicability of ATAS to thin samples (typical thickness < 200 nm). For bulk samples equivalent information can be obtained with attosecond transient reflection spectroscopy (ATRS) by looking at transient changes in the sample reflectivity rather than absorption. Compare to ATAS, ATRS is still in its infancy with only few known examples reported in literature9,10. In this seminar I will introduce ATAS and ATRS, reviewing the most important experiments performed with these techniques and discussing non-equilibrium ultrafast electron and exciton dynamics in dielectrics driven by few-cycle PHz pulses.
References
1. Gallmann, L. et al. Resolving intra-atomic electron dynamics with attosecond transient absorption spectroscopy. Mol. Phys. 111, 2243–2250 (2013).
2. Krausz, F. & Stockman, M. I. Attosecond metrology: from electron capture to future signal processing. Nat. Photonics 8, 205–213 (2014).
3. Schultze, M. et al. Controlling dielectrics with the electric field of light. Nature 493, 75–78 (2012).
4. Moulet, A., Bertrand, J. B., Klostermann, T., Guggenmos, A. & Karpowicz, N. Soft x-ray excitonics. Science. 1138, 1134–1138 (2017).
5. Schultze, M. et al. Attosecond band-gap dynamics in silicon. Science. 346, 1348–1352 (2014).
6. Lucchini, M. et al. Attosecond dynamical Franz-Keldysh effect in polycrystalline diamond. Science. 353, 916–919 (2016).
7. Schlaepfer, F. et al. Attosecond optical-field-enhanced carrier injection into the gaas conduction band. Nat. Phys. 14, 560–564 (2018).
8. Geneaux, R., Marroux, H. J. B., Guggenmos, A., Neumark, D. M. & Leone, S. R. Transient absorption spectroscopy using high harmonic generation: A review of ultrafast X-ray dynamics in molecules and solids. Philos. Trans. R. Soc. A Math. Phys. Eng. Sci. 377, (2019).
9. Géneaux, R. et al. Attosecond Time-Domain Measurement of Core-Level-Exciton Decay in Magnesium Oxide. Phys. Rev. Lett. 124, 207401 (2020).
10. Lucchini, M. et al. Unravelling the intertwined atomic and bulk nature of localised excitons by attosecond spectroscopy. Nat. Commun. 12, 1021 (2021).
