ABSTRACT
Extreme ultraviolet (XUV) light is notoriously difficult to control due to its strong interaction cross section with media. We demonstrate a method to overcome this problem by using opto-optical modulation guided by a geometrical model to shape XUV light. A bell-shaped infrared light pulse is shown to imprint a trace of its intensity profile onto the XUV light in the far-field, such that a change in the intensity profile of the infrared pulse leads to a change in the shape of the far-field XUV light. The geometrical model assists the user in predicting the effect of a specific intensity profile of the infrared pulse, thus enabling a deterministic process.
ABSTRACT
Attosecond pulses are central to the investigation of valence- and core-electron dynamics on their natural timescales1-3. The reproducible generation and characterization of attosecond waveforms has been demonstrated so far only through the process of high-order harmonic generation4-7. Several methods for shaping attosecond waveforms have been proposed, including the use of metallic filters8,9, multilayer mirrors10 and manipulation of the driving field11. However, none of these approaches allows the flexible manipulation of the temporal characteristics of the attosecond waveforms, and they suffer from the low conversion efficiency of the high-order harmonic generation process. Free-electron lasers, by contrast, deliver femtosecond, extreme-ultraviolet and X-ray pulses with energies ranging from tens of microjoules to a few millijoules12,13. Recent experiments have shown that they can generate subfemtosecond spikes, but with temporal characteristics that change shot-to-shot14-16. Here we report reproducible generation of high-energy (microjoule level) attosecond waveforms using a seeded free-electron laser17. We demonstrate amplitude and phase manipulation of the harmonic components of an attosecond pulse train in combination with an approach for its temporal reconstruction. The results presented here open the way to performing attosecond time-resolved experiments with free-electron lasers.
ABSTRACT
Nitrogen molecules in ambient air exposed to an intense near-infrared femtosecond laser pulse give rise to cavity-free superradiant emission at 391.4 and 427.8 nm. An unexpected pulse duration-dependent cyclic variation of the superradiance intensity is observed when the central wavelength of the femtosecond pump laser pulse is finely tuned between 780 and 820 nm, and no signal occurs at the resonant wavelength of 782.8 nm (2ω_{782.8 nm}=ω_{391.4 nm}). On the basis of a semiclassical recollision model, we show that an interference of dipolar moments of excited ions created by electron recollisions explains this behavior.
