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Dipartimento di Fisica - Politecnico di Milano

Attosecond source in the water window (300 - 550 eV) based on high-order harmonic generation in chip

This thesis project aims at the generation and characterization of isolated attosecond pulses in the water window (between 300 eV and 550 eV). This source is based on high-order harmonic generation driven by a mid-IR ultrafast laser source in a miniaturized lab on chip realized by Femtosecond Laser Micromachining. Isolated attosecond pulses will be generated by state-of-the-art polarization gating technique and characterized by proper photon and electron spectroscopy approach.

Attosecond science studies the motion of electrons on atomic and molecular scale, which typically occurs on the timescale of attoseconds (1 as = 10-18s). The generation of attosecond pulses relies on the high-order harmonic generation (HHG) process. High-order harmonics are generated when an intense laser pulse is focused on a gas medium: due to the strong nonlinear interaction, very high odd harmonics of the driving pulse optical frequency can be generated. This strong non-linear interaction leads to the production of a train of light bursts with attosecond duration in the XUV spectral region. Such bursts are separated by half of the period of the driving electric field. Thus, for probing electron dynamics, it is often necessary to isolate a single burst within the train that can then be synchronized with another optical pulse. Different reliable schemes have been demonstrated for achieving the generation of isolated attosecond pulses, namely by spectral or temporal gating. One of the most simple and versatile of them is the polarization gating. Since the maximum photon energy in the harmonic spectrum - and the spectral bandwidth of the attosecond pulse - increases as the square of the driving field wavelength, longer driving wavelengths extend the HHG cutoff and allow the generation of isolated attosecond pulses up to the water window (300 - 500 eV) and beyond.

Attosecond pulse train and isolated attosecond pulses can nowadays be generated in the water window and beyond. However, the photon flux of attosecond sources is in general extremely low. For this reason, we proposed and demonstrated a new regime of HHG in a micromachined chip platform to increase the process efficiency and generate bright soft-X radiation. The demonstration and characterization of bright isolated attosecond pulses in the water window will be extremely beneficial for the study of ultrafast electron dynamics in molecules and materials.

This thesis work will be performed in the ERC-granted UDynI laboratory:

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