Abstract

Currently, an isolated attosecond pulse (IAP), particularly in the water window region, from the high-order harmonic generation (HHG) driven by a few-cycle infrared (IR) laser source has been one of the fastest advancing frontiers in ultrafast laser science. According to the “cut-off” law of HHG, to extend the cut-off photon energy of the HHG spectrum, the essential requirement is to employ the driving laser with a longer wavelength. On the other hand, with the use of a few-cycle laser field, the electron recombination in HHG was confined to a monocycle pulse, resulting in IAP emission [1]. It is evident that the driving laser with a less-cycle can generate a broader HHG continuous spectrum, which gives the potential of creating an isolated attosecond pulse with a shorter pulse duration. In the reported results, most of the demonstrated generation of IAP, especially in the water window region, are achieved with the output energy around sub-pJ class because of the limitation of the driving energy of a few-cycle IR laser pulses. Moreover, the continuous region of the generated HHG spectrum is relatively limited by the pulse duration of the driving IR laser source, and the corresponding pulse duration of IAP is generally 50 attoseconds [2]. Given the above considerations, a high-energy, single-cycle, CEP-stable IR laser source is strongly desirable for achieving high photon flux, high photon energy, and short pulse duration for IAP. In this paper, we extended the amplified spectral bandwidth over one octave by adding MgO:LiNbO3 (MgO) crystals to cooperate with BiB3O6 (BiBO) crystals. After compression, the middle IR (MIR) laser pulse is down to 8.58 fs (1.05 cycle @ 2.44 μm) with a pulse energy of 53 mJ.

© 2023 IEEE