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Dipartimento di Fisica - Politecnico di Milano

Investigation of exciton dynamics in 2D materials by high-order harmonic generation

The thesis project aims at investigating nonlinear properties of solids by High-order Harmonic Generation Spectroscopy. The case study will be 2D materials, such as MoS2 investigating the formation, relaxation and decay of a wealth of elemental excitations: excitons, bi-excitons, trions, mixed plasmon-exciton states, highly dressed electronic carriers and more.
High-order Harmonic Generation Spectroscopy can potentially shed light on these phenomena by revealing some of the fundamental processes of quantum physics.

High-order harmonic generation (HHG) is the foundation of Attosecond Science. Since its first demonstration in gases, HHG has played a central role in studying the motion of electrons on the attosecond time scale (1 as = 10-18 s). The process relies on the non-linear interaction of a strong laser field with matter, which triggers a laser-driven motion of the electrons emitting radiation extending up to the extreme ultraviolet and X-ray energy domain. In the time domain, a train of attosecond bursts is produced, which can be used as a source for attosecond pump-probe experiments. Beyond its relevance as a tool in attosecond science, the HHG spectrum also encodes a direct fingerprint on the atomic size scale of the system electronic structure and its changes triggered by the laser on the attosecond timescale.

HHG Spectroscopy allows observing the real-time electronic and atomic dynamics following the excitation induced by a strong and ultrashort laser pulse. These dynamics are connected to the relaxation of a highly excited state through several decay channels like, e.g., multi-electron-hole excitations, collective modes, phonon excitations, charge transfer, and even formation or breaking of chemical bonds and molecular rearrangements. These microscopic mechanisms, which are known to take place on extremely short timescales, are the basis for numerous potential applications, from solar energy harvesting to nanotechnology, biochemistry and life science. During the last decade, the development of new experimental techniques has provided the scientific community with advanced tools allowing HHG spectroscopy to investigate the ultrafast dynamics in realistic and complex materials. The recent shift of laser technology towards the realization of few-cycle strong laser pulses in the mid-IR regime - pioneered by our group - has enabled HHG from bulk crystals, permitting the damage-free exposure of the system to high electric fields. This opened new perspectives for probing the electron dynamics in condensed matter and for generating extreme ultraviolet (EUV) laser sources with high efficiency.
The laser-driven control of ultrafast electronic processes in a solid allows manipulating the non-linear current in the system, which can be the first step for the realization of petahertz solid-state optoelectronic devices. Furthermore, HHG spectroscopy in the condensed matter enables access to the physical properties of the system in an all-optical way, allowing the reconstruction of the band-structure and its out of equilibrium evolution.

 

This thesis work will be performed in the ERC-granted UDynI laboratory:
http://www.udyni.eu



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