  The ability to generate phase-stabilised trains of ultrafast laser pulses by
mode-locking underpins photonics research in fields such as precision metrology
and spectroscopy. However, the complexity of conventional mode-locked laser
systems, combined with the need for a mechanism to induce active or passive
phase locking between resonator modes, has hindered their realisation at the
nanoscale. Here, we demonstrate that GaAs-AlGaAs nanowire lasers are capable of
emitting pairs of phase-locked picosecond laser pulses when subject to
non-resonant pulsed optical excitation with a repetition frequency up to
~200GHz. By probing the two-pulse interference that emerges within the
homogeneously broadened laser emission, we show that the optical phase is
preserved over timescales extending beyond ~30ps, much longer than the emitted
laser pulse duration (~2ps). Simulations performed by solving the optical Bloch
equations produce good quantitative agreement with experiments, revealing how
the phase information is stored in the gain medium close to transparency. Our
results open the way to applications such as on-chip, ultra-sensitive Ramsey
comb spectroscopy.
