title: Interferometric autocorrelation of an attosecond pulse train in the single-cycle regime.
abstract: We report on the direct observation of the phase locking of the attosecond pulse train (APT) via interferometric autocorrelation in the extreme ultraviolet region. APT is formed with Fourier synthesis of high-order harmonic fields of a femtosecond laser pulse. Time-of-flight mass spectra of N+, resulting from the Coulomb explosion of N2 absorbing two photons of APT, efficiently yield correlated signals of APT. The measured autocorrelation trace exhibits that the duration of the pulse should be only 1.3 periods of the extreme ultraviolet carrier frequency. A few interference fringes within the short pulse duration clearly show two types of symmetry, which ensure the phase locking between pulses in APT.
authors.affiliation: Laser Technology Laboratory, RIKEN, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan.
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Nabekawa
authors.forename: Toshihiko
authors.initials: T
authors.lastname: Shimizu
authors.forename: Tomoya
authors.initials: T
authors.lastname: Okino
authors.forename: Kentaro
authors.initials: K
authors.lastname: Furusawa
authors.forename: Hirokazu
authors.initials: H
authors.lastname: Hasegawa
authors.forename: Kaoru
authors.initials: K
authors.lastname: Yamanouchi
authors.forename: Katsumi
authors.initials: K
authors.lastname: Midorikawa