  Quantum dot lasers display many unique dynamic phenomena when optically
injected. Bistability has been predicted in a region of high injection
strength. We show experimentally, rather than a phase-locked bistability, a
square wave phenomenon is observed in this region. The squares can manifest as
a periodic train but also as noise driven Type II excitable events. We
interpret the appearance of the square waves as a thermally induced breaking of
the bistability. Indeed we find experimentally that over the duration of a
square the relative detuning between the master and the slave evolves
deterministically. A relatively simple, physically motivated, rate equation
model is presented and displays excellent agreement with the experiment.
