Coherence buildup in semiconductor lasers
Résumé
Lasers are often perceived as threshold devices, with a rapid all-or-nothing transition around threshold. This approach usually holds for high quality factor (“Class A”) devices or macroscopic ones, but fails to correctly describe the transition in very small components. The long carrier lifetime – compared to the cavity time – typical of semiconductor-based micro- and nanolasers (“Class B” devices) introduces an additional element of complexity to the problem due to the nonlinear dynamical evolution of the coherent fraction of electromagnetic field, and opens questions on coherence buildup.
Below threshold, the characterization of the latter is rendered difficult due to the small number of photons contained in the laser cavity at any given time, thus yielding very low optical intensity levels, close to or below the sensitivity limit of standard photodetectors. Fortunately, recent telecom-based quantum measurement techniques now allow the indirect characterization of the laser’s response through its second order correlation function, g^(2)(τ).