  Ultraviolet microdisk lasers are integrated monolithically into photonic
circuits using a III-nitride on silicon platform with gallium nitride (GaN) as
the main waveguiding layer. The photonic circuits consist of a microdisk and a
pulley waveguide terminated by out-coupling gratings. We measure quality
factors up to 3500 under continuous-wave excitation. Lasing is observed from
374 nm to 399 nm under pulsed excitation, achieving low threshold energies of
$0.14 ~\text{mJ/cm}^2$ per pulse (threshold peak powers of $35
~\text{kW/cm}^2$). A large peak to background dynamic of around 200 is observed
at the out-coupling grating for small gaps of 50 nm between the disk and
waveguide. These devices operate at the limit of what can be achieved with GaN
in terms of operation wavelength.
