  The missing piece in the jigsaw of silicon photonics is a light source that
can be easily incorporated into the standard silicon fabrication process.
Recent advances in the development of atomically thin layers of semiconducting
transition metal dichalogenides (TMDs), with direct bandgaps in the
near-infrared region, have opened up new possibilities for addressing this
need. Here, we report a unique silicon laser source that employs molybdenum
ditelluride (MoTe2) as a gain material in a photonic crystal nanocavity
resonator, fabricated in silicon-on-insulator. We demonstrate optically pumped
MoTe2-on-silicon devices lasing at 1305 nm, i.e. in the centre of the O-band
used in optical communications, operating in the continuous-wave (CW) regime,
at room temperature and with a threshold power density as low as 1.5 kW/cm2.
This 2D-on-silicon geometry offers the promise of an integrated low-cost
electrically pumped nanoscale silicon light source, thereby adding an essential
building block to the silicon photonics platform.
