  Double ionization in intense laser fields can comprise electron correlations,
which manifest in the non-independent emission of two electrons from an atom or
molecule. However, experimental methods that directly access the electron
emission times have been scarce. Here, we explore the application of an
all-optical streaking technique to strong-field double ionization both
theoretically and experimentally. We show that both sequential and
non-sequential double ionization processes lead to streaking delays that are
distinct from each other and single ionization. Moreover, coincidence detection
of ions and electrons provides access to the emission time difference, which is
encoded in the two-electron momentum distributions. The experimental data agree
very well with simulations of sequential double ionization. We further test and
discuss the application of this method to non-sequential double ionization,
which is strongly affected by the presence of the streaking field.
