Carrier-envelope-phase insensitivity in high-order harmonic generation driven by few-cycle laser pulses

We present evidence for self-stabilization of the relative spectral phase of high-order harmonic emission against intensity variations of the driving field. Our results demonstrate that, near the laser focus, phase matching of the harmonic field from a macroscopic target can compensate for the intensity dependence of the intrinsic phase of the harmonics emitted by a single radiator. As a consequence, we show experimentally and theoretically the insensitivity of the harmonic spectra produced at the laser focus against variations of the carrier-envelope phase (CEP) of a sub-two-cycle driving field. In addition, the associated attosecond pulse trains exhibit phase locking against CEP changes of the few-cycle driver.The authors acknowledge support from MINECO ( FIS2013-44174-P), from Junta de Castilla y León (Project No. SA116U13) and from Centro de Láseres Pulsados. C.H.-G. acknowledges support from the Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development (2007–2013), under REA grant Agreement No. 328334. W.H. acknowledges ESF support through SILMI Research Networking Programme. B. A. acknowledges Fundação para a Ciência e a Tecnologia (FCT) through grant No. SFRH/BPD/88424/2012. W. H. and I. J. S. also acknowledge support from the Spanish Ministerio de Ciencia e Innovación through the Formación de Personal Investigador and Ramón y Cajal grant programs respectively. This work was partly supported by grant PTDC/FIS/122511/2010 from Fundação para a Ciência e Tecnologia, Portugal, co-funded by COMPETE and FEDER