Enhanced multi-colour gating for the generation of high-power isolated attosecond pulses

Abstract : Isolated attosecond pulses (IAP) generated by high-order harmonic generation are valuable tools that enable dynamics to be studied on the attosecond time scale. The applicability of these IAP would be widened drastically by increasing their energy. Here we analyze the potential of using multi-colour driving pulses for temporally gating the attosecond pulse generation process. We devise how this approach can enable the generation of IAP with the available high-energy kHz-repetition-rate Ytterbium-based laser amplifiers (delivering 180-fs, 1030-nm pulses). We show theoretically that this requires a three-colour field composed of the fundamental and its second harmonic as well as a lower-frequency auxiliary component. We present pulse characterization measurements of such auxiliary pulses generated directly by white-light seeded OPA with the required significantly shorter pulse duration than that of the fundamental. This, combined with our recent experimental results on three-colour waveform synthesis, proves that the theoretically considered multi-colour drivers for IAP generation can be realized with existing high-power laser technology. The high-energy driver pulses, combined with the strongly enhanced single-atom-level conversion efficiency we observe in our calculations, thus make multi-colour drivers prime candidates for the development of unprecedented high-energy IAP sources in the near future. More than a decade ago, the first observations of isolated attosecond light pulses 1 (IAP) or attosecond pulse trains (APT) 1,2 launched the field of " attosecond physics " , for which these optical tools became the technological base. The high-order harmonic generation (HHG) process in atoms, at the origin of these attosecond light pulses, can be described by a semi-classical three-step model 3,4 , where an electron is first released by tunnel ionization, then accelerated by the strong laser field and finally driven back to the core. The recombination leads to the emission of an ultrashort burst of XUV light. In a multi-cycle laser pulse, this process is repeated coherently every half cycle, leading to the emission of an APT. In order to isolate a single attosecond pulse and thus greatly facilitate the implementation of traditional pump-probe schemes for time-resolving processes on the sub-femtosecond time scale, it is necessary to temporally gate the HHG process and limit it to a single laser half-cycle 5,6
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Stefan Haessler, T Balčiūnas, G Fan, L Chipperfield, A. Baltuska. Enhanced multi-colour gating for the generation of high-power isolated attosecond pulses. Scientific Reports, Nature Publishing Group, 2015, 5, pp.10084. ⟨10.1038/srep10084⟩. ⟨hal-01164321⟩

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