  Silicon photonics is being extended from the near-infrared (near-IR) window
of 1.3-1.5 {\mu}m for optical fiber communications to the mid-infrared (mid-IR)
wavelength-band of 2 {\mu}m or longer for satisfying the increasing demands in
many applications. Mid-IR waveguide photodetectors on silicon have attracted
intensive attention as one of the indispensable elements for various photonic
systems. Previously high-performance waveguide photodetectors on silicon were
realized for the near-IR window of 1.3-1.5 {\mu}m by introducing another
semiconductor material (e.g., Ge, and III-V compounds) in the active region.
Unfortunately, these traditional semiconductor materials do not work well for
the wavelength of ~2 {\mu}m or longer because the light absorption becomes very
weak. As an alternative, two-dimensional materials provide a new and promising
option for enabling active photonic devices on silicon. Here black-phosphorus
(BP) thin films with optimized medium thicknesses (~40 nm) are introduced as
the active material for light absorption and silicon/BP hybrid ridge waveguide
photodetectors are demonstrated with a high responsivity at a low bias voltage.
And up to 4.0Gbps data transmission is achieved at 2{\mu}m.
