Isolated attosecond pulses from ionization gating of high-harmonic emission

Combining results from several techniques of attosecond spectroscopy, we show that ionization gating of high-harmonic emission on the leading edge of the driving pulse produces isolated attosecond pulses with a contrast ratio (the energy in the main pulse normalized to the energy in adjacent satellite pulses) c=3.3±0.2. Half-cycle cutoff analysis confirms that harmonic generation proceeds in the ionization-gated regime. The attosecond pulse contrast is measured using the technique of carrier-envelope phase (CEP)-scanning, recently developed by our group, in which photoelectrons generated from Ne atoms by the harmonic pulse are streaked as a function of CEP. Streaking of photoelectrons as a function of attosecond time delay also confirms the isolated nature of the harmonic pulse, which is measured to have a duration of 430±15 as, limited by the bandwidth of the reflective X-ray optics employed. The combined measurements imply that the experimental advantages of the ionization gating technique-tunable X-ray emission, relaxed sensitivity to the CEP and scalability to longer driver pulses-are also conferred on isolated attosecond pulse production.