51 attoseconds: the shortest attosecond pulse from near-infrared driving pulse

Pulsed light, like a flash, exposes the darkness for a fleeting instant, allowing us to record fast repeatable processes frame by frame with pump-probe technique. The generation of isolated attosecond XUV pulsed sources opens the new chapter of investigations of ultrafast science. Owing to its ultrashort temporal duration approaching the atomic unit of time, IAP enables the probe of quantum behaviors of electrons in atoms, molecules and solids with unprecedented time resolution. However, two decades later, it is still not trivial to produce ultrashort IAPs routinely. The shortest IAPs reported so far with pulse duration of 43 as and 53 as are driven with mid-infrared (Mid-IR) lasers which require additional broadband frequency conversion techniques and raise the bar for attosecond researches. Furthermore, long wavelength drivers would decrease the conversion efficiency dramatically according to wavelength scaling law of high-order harmonic generation. The generation of ultrashort isolated attosecond pulses (IAPs) with high photon flux is of critical importance to further unleash the power of IAP on the measurements of ultrafast electron dynamics.

Recently, the research team from National University of Defense Technology (NUDT) proposed to apply generalized double optical gating (GDOG) technique on few-cycle NIR driving pulses, which ensured high saturation intensity and generated 51 as IAP. It therefore holds the promise of generating high-flux ultrashort IAPs, so as to popularize and expand the application of attosecond pulses.

By taking advantage of GDOG technique, which preserves both the plateau and cutoff region photons, the sample ionization by the leading edge of the driving field is significantly suppressed, resulting in high saturation intensity and hence high IAP cutoff energy. IAP with pulse duration of 51±4 attoseconds is generated during 1 fs linear polarization gate formed by GDOG. The characterization is done with attosecond streak camera, and phase reconstruction is performed with quick Phase Retrieval by Omega Oscillation Filtering (qPROOF). The demonstrated scheme for IAP generation in principle has much higher conversion efficiency than the long-wave driver scheme.

Another benefit that comes with this new scheme is the needlessness of carrier envelope phases (CEP) stabilization of the driving laser pulses, as experimentally demonstrated previously. However, the underlying mechanisms are never uncovered. In this work, the authors numerically show that narrow linear-polarization gate formed with GDOG allows effective IAP emitting only from specific laser waveforms with some certain CEPs due to the extremely sensitive dependence of IAP generation on the carrier waveform of the driving laser. This makes for passive ultrabroadband IAP gating with CEP unstabilized driving lasers. This work suggests an alternative way to generate ultrashort IAPs by applying GDOG on few-cycle free-CEP NIR driving pulses, and is thereby of great importance to facilitate the development of attosecond science and technology.