  On-chip microlaser sources in the blue constitute an important building block
for complex integrated photonic circuits on silicon. We have developed photonic
circuits operating in the blue spectral range based on microdisks and bus
waveguides in III-nitride on silicon. We report on the interplay between
microdisk-waveguide coupling and its optical properties. We observe critical
coupling and phase matching, i.e. the most efficient energy transfer scheme,
for very short gap sizes and thin waveguides (g = 45 nm and w = 170 nm) in the
spontaneous emission regime. Whispering gallery mode lasing is demonstrated for
a wide range of parameters with a strong dependence of the threshold on the
loaded quality factor. We show the dependence and high sensitivity of the
output signal on the coupling. Lastly, we observe the impact of processing on
the tuning of mode resonances due to the very short coupling distances. Such
small footprint on-chip integrated microlasers providing maximum energy
transfer into a photonic circuit have important potential applications for
visible-light communication and lab-on-chip bio-sensors.
