  The main interest of group-III-nitride nanophotonic circuits is the
integration of active structures and laser sources. A photonic platform of
group-III-nitride microdisk lasers integrated on silicon and emitting in the
blue spectral range is demonstrated. The active microdisks are side-coupled to
suspended bus waveguides, and the coupled emission is guided and outcoupled to
free space using grating couplers. A small gap size of less than 100 nm between
the disk and the waveguide is required in the blue spectral range for optimal
evanescent coupling. To avoid reabsorption of the microdisk emission in the
waveguide, the quantum wells are etched away from the waveguide. Under
continuous-wave excitation, loaded quality factors greater than 2000 are
observed for the whispering gallery modes for devices with small gaps and large
waveguide bending angles. Under pulsed excitation conditions, lasing is
evidenced for 3 $\mu$m diameter microdisks integrated in a full photonic
circuit. We thus present a first demonstration of a III-nitride microlaser
coupled to a nanophotonic circuit.
