  Black phosphorus has been recently rediscovered as a new and interesting
two-dimensional material due to its unique electronic and optical properties.
Here, we study the linear and nonlinear optical properties of black phosphorus
thin films, indicating that both linear and nonlinear optical properties are
anisotropic and can be tuned by the film thickness. Then we employ the
nonlinear optical property of black phosphorus for ultrafast (pulse duration
down to ~786 fs in mode-locking) and large-energy (pulse energy up to >18 nJ in
Q-switching) pulse generation in fiber lasers at the near-infrared
telecommunication band ~1.5 {\mu}m. Our results underscore relatively large
optical nonlinearity in black phosphorus and its prospective for ultrafast
pulse generation, paving the way to black phosphorus based nonlinear and
ultrafast photonics applications (e.g., ultrafast all-optical
switches/modulators, frequency converters etc.).
