Square wave excitability in quantum dot lasers under optical injection

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.