Abstract
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.
Highlights
High-order harmonic generation (HHG) is probably the simplest technique to obtain coherent electromagnetic radiation at the extreme ultraviolet (XUV) [1,2] or soft X-ray regimes [3]
High-order harmonic generation is performed on an argon gas jet (5 bar of backing pressure), coming out from a nozzle of 500 μm diameter
We have analyzed the dependence of HHG with the carrier-envelope phase (CEP) for sub-two cycle driving field aimed to a gas jet target
Summary
High-order harmonic generation (HHG) is probably the simplest technique to obtain coherent electromagnetic radiation at the extreme ultraviolet (XUV) [1,2] or soft X-ray regimes [3]. HHG being a highly nonlinear process, it is very sensitive to changes in the driving field and to changes in the CEP, when working with few-cycle laser pulses. It has been found that, in the few-cycle regime, CEP variations strongly modify the phase of the high-order harmonics, resulting in a spectral shift of the harmonic spectrum [23,24,25,26]. In this paper we study theoretically and experimentally the dependence of HHG with the CEP of the driving field, under different macroscopic phase-matching conditions, in a gas jet target. We present simulations showing that the phase difference between consecutive attosecond pulses becomes insensitive to CEP variations when the gas jet is placed near the focus position
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