High harmonic generation (HHG) can strongly depend on the carrier envelope phase (CEP) of its driving pulse if that pulse has a short duration of only a few optical cycles (few-cycle pulse)[1,2]. This effect is often used to characterize the CEP of laser sources [3,4], and manifests in observed harmonic cutoff frequency oscillations w.r.t. the CEP, as well as intensity oscillations in between adjacent harmonic peaks from slightly longer pulses . CEP effects in HHG are known to completely vanish when the driving pulse duration is increased beyond a certain threshold [1,6,7], which is also accompanied by the formation of symmetry-allowed discrete harmonics (as opposed to continuous emission) [8,9]. Both these phenomena result from a coherent interference between harmonic photons that are generated by successive pump laser cycles, which behave time-periodically if the pump pulse is long enough. Accordingly, CEP dependence has never been observed in HHG from pulses longer than ~20fs with central wavelength of 800nm.
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