We report evidence for the first generation of XUV spectra from relativistic surface high-harmonic generation (SHHG) on plasma mirrors at a kilohertz repetition rate, emitted simultaneously and correlated to the emission of energetic electrons. We present measurements of SHHG spectra and electron angular distributions as a function of the experimentally controlled plasma density gradient scale length L for three increasingly short and intense driving pulses: 24 fs (9 optical cycles) and a0 = 1.1, 9 fs (3.5 optical cycles) and a0 = 1.8, and finally 4 fs (1.7 optical cycles) and a0 ≈ 2.0. For all driver pulses, we observe relativistic SHHG in the range L ∈ [λ/25, λ/10], with an optimum gradient scale length of L ≈ λ/15.
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