  Silicon photonics technology enables compact, low-power and cost-effective
optical microsystems on a chip by leveraging the materials and advanced
fabrication methods developed over decades for integrated silicon electronics.
Silicon foundries now provide many standard building blocks required for
high-performance optical circuits, including passive components such as optical
waveguides, filters and (de-)multiplexors and active optoelectronic components
such as high-speed modulators, switches and photodetectors. However, because
silicon is a poor light emitting material, on-chip light sources are still a
significant challenge for foundry offerings. Current light-source integration
methods are viewed as complex, requiring incompatible and/or expensive
materials and processing steps. Here we report on an ultra-compact silicon
photonic laser consisting of a thulium-silicon hybrid microdisk resonator. The
microdisk design is straightforward and compatible with the fabrication steps
and device dimensions available in all silicon photonics foundries, whereas the
gain medium is added in a backend (final step), room temperature sputter
deposition. This approach allows for low-cost and high-volume wafer-scale
manufacturing and co-integration of light sources with silicon passive and
active devices with no adjustment to standard process flows. The hybrid laser
is pumped at standard telecom wavelengths around 1.6 {\mu}m and emits around
1.9 {\mu}m, which is within an emerging spectral region of significant interest
for communications, nonlinear and quantum optics, and sensing on silicon.
