  Black phosphorus is a very promising material for telecommunication due to
its direct bandgap and strong resonant absorption in near-infrared wavelength
range. However, ultrafast nonlinear photonic applications relying on the
ultrafast photo-carrier dynamics as well as optical nonlinearity in black
phosphorus remain unexplored. In this work, we investigate nonlinear optical
properties of solution exfoliated BP and demonstrate the usage of BP as a new
saturable absorber for high energy pulse generation in fiber laser. In order to
avoid the oxidization and degradation of BP, we encapsulated BP by polymer
matrix which is optically transparent in the spectrum range of interest to form
a composite. Two fabrication approaches were demonstrated to produce BP-polymer
composite films which were further incorporated into fiber laser cavity as
nonlinear media. BP shows very fast carrier dynamics and BP-polymer composite
has a modulation depth of 10.6%. A highly stable Q-switched pulse generation
was achieved and the single pulse energy of ~194 nJ was demonstrated. The ease
of handling of such black phosphorus-polymer composite thin films affords new
opportunities for wider applications such as optical sensing, signal processing
and light modulation.
