  Few-layer black phosphorus (BP), as the most alluring graphene analogue owing
to its similar structure as graphene and thickness dependent direct band-gap,
has now triggered a new wave of research on two-dimensional (2D) materials
based photonics and optoelectronics. However, a major obstacle of practical
applications for few-layer BPs comes from their instabilities of laser-induced
optical damage. Herein, we demonstrate that, few-layer BPs, fabricated through
the liquid exfoliation approach, can be developed as a new and practical
saturable absorber (SA) by depositing few-layer BPs with microfiber. The
saturable absorption property of few-layer BPs had been verified through an
open-aperture z-scan measurement at the telecommunication band and the
microfiber-based BP device had been found to show a saturable average power of
~4.5 mW and a modulation depth of 10.9%, which is further confirmed through a
balanced twin detection measurement. By further integrating this optical SA
device into an erbium-doped fiber laser, it was found that it can deliver the
mode-locked pulse with duration down to 940 fs with central wavelength tunable
from 1532 nm to 1570 nm. The prevention of BP from oxidation through the
'lateral interaction scheme' owing to this microfiber-based few-layer BP SA
device might partially mitigate the optical damage problem of BP. Our results
not only demonstrate that black phosphorus might be another promising SA
material for ultrafast photonics, but also provide a practical solution to
solve the optical damage problem of black phosphorus by assembling with
waveguide structures such as microfiber.
