Speaker
Description
Approaching the attosecond time scale is crucial for advancing petahertz opto-electronics, that is the capability to coherently control and manipulate the optical properties of solid-state materials when excited by PHz fields, offering processing speed much beyond current limits.
With this respect, Attosecond Transient Reflection Spectroscopy (ATRS) in solids has emerged as a reliable tool to access the attosecond electron dynamics driven by few-femtosecond infrared (IR) pulses. Based on a pump-probe scheme, it relies on high-harmonic generation (HHG) to produce attosecond extreme-ultraviolet (XUV) pulses that serve as broad-band probing radiation, enabling a sub-femtosecond temporal resolution. The reflection geometry used in ATRS offers several advantages over more conventional absorption-based methods, including enhanced surface sensitivity and improved heat dissipation. In addition, it allows for high-intensity IR excitation (up to 10¹⁴ W/cm²), unveiling highly nonlinear processes that have remained so far unexplored.
In my discussion, I will first present the attosecond beamline at the Attosecond Research Centre at Politecnico di Milano, specifically developed to study the attosecond charge dynamics in solids. This setup features a unique two-foci geometry that allows for the measurement of transient reflectivity changes with an absolute pump-probe delay calibration. I will then present key results obtained using this scheme, including the investigation of attosecond photo-injection dynamics in germanium, the motion of virtual charges in diamond, and the ultrafast optical response of core excitons in magnesium fluoride. This will lead me to discuss our recent findings in lithium fluoride, where we observed a much more intricate interplay that results from the simultaneous excitation of virtual charge dynamics and a never-observed hybrid exciton state, arising from the interaction between a hole in the valence band and an electron in the deep conduction band.
In conclusion, ATRS allows us to study the non-linear processes that stem from coherent light-matter interaction when a material is strongly driven out of equilibrium by a few-fs IR pulse. The attosecond resolution reveals transient reflectivity changes that give direct access to the complex electron dynamics occurring in the medium, offering a deeper understanding of the strengths and limitations of such materials in the framework of petahertz opto-electronics and expanding our knowledge of fundamental solid-state physics.