  It attracts wide interest to seek universe saturable absorber covering
wavelengths from near infrared to mid-infrared band. Multilayer black
phosphorus, with variable direct bandgap (0.3-2 eV) depending on the layer
number, becomes a good alternative as a universe saturable absorber for pulsed
lasers. In this contribution, we first experimentally demonstrated broadband
saturable absorption of multilayer black phosphorus from 1 {\mu}m to 2.7 {\mu}m
wavelength. With the as-fabricated black phosphorus nanoflakes as saturable
absorber, stable Q-switching operation of bulk lasers at 1.03 {\mu}m, 1.93
{\mu}m, 2.72 {\mu}m were realized, respectively. In contrast with large-bandgap
semiconducting transition metal dichalcogenides, such as MoS2, MoSe2,
multilayer black phosphorus shows particular advantage at the long wavelength
regime thanks to its narrow direct bandgap. This work will open promising
optoelectronic applications of black phosphorus in mid-infrared spectral region
and further demonstrate that BP may fill the gap of between zero-bandgap
graphene and large-bandgap TMDs.
