  Silicon nitride (Si3N4), as a complementary metal-oxide-semiconductor (CMOS)
material, finds wide use in modern integrated circuit (IC) technology. The past
decade has witnessed tremendous development of Si3N4 in photonic areas, with
innovations in nonlinear photonics, optical sensing, etc. However, the lack of
an integrated laser with high performance prohibits the large-scale integration
of Si3N4 waveguides into complex photonic integrated circuits (PICs). Here, we
demonstrate a novel III-V/Si/Si3N4 structure to enable efficient electrically
pumped lasing in a Si3N4 based laser external cavity. The laser shows superior
temperature stability and low phase noise compared with lasers purely dependent
on semiconductors. Beyond this, the demonstrated multilayer heterogeneous
integration provides a practical path to incorporate efficient optical gain
with various low-refractive-index materials. Multilayer heterogeneous
integration could extend the capabilities of semiconductor lasers to improve
performance and enable a new class of devices such as integrated optical clocks
and optical gyroscopes.
