  With the proposal of dual-wavelength pumping (DWP) scheme, DWP Er:ZBLAN fiber
lasers at 3.5 um have become a fascinating area of research. However, limited
by the absence of suitable saturable absorber, passively Q-switched and
mode-locked fiber lasers have not been realized in this spectral region. Based
on the layer-dependent bandgap and excellent photoelectric characteristics of
black phosphorus (BP), BP is a promising candidate for saturable absorber near
3.5 um. Here, we fabricated a 3.5-um saturable absorber mirror (SAM) by
transferring liquid-phase exfoliated BP flakes onto a gold-coated mirror. With
the as-prepared BP SAM, we realized stable Q-switching and continuous-wave
mode-locking operations in the DWP Er:ZBLAN fiber lasers at 3.5 um. To the best
of our knowledge, it is the first time to achieve passively Q-switched and
mode-locked pulses in 3.5 um spectral region. The research results will not
only promote the development of 3.5-um pulsed fiber lasers but also open the
photonic application of two-dimensional materials in this spectral region
