NUMERICAL MODELLING OF SEMICONDUCTORQUANTUM DOT LIGHT EMITTERS FOR FIBEROPTIC COMMUNICATION AND SENSING

We present a review of our research work on the modelling of the optical properties of light emitting devices having a semiconductor quantum dot material as active region. The gain region is obtained by a Strasky-Krastanov growth of several layers of quantum dots that are not uniform in size. This causes an inhomogeneous broadening of the gain spectrum that is a peculiar characteristics of these light emitters. The numerical model is based on a multi-population rate equation model used for describing the dynamics of electrons and holes in an inhomogeneous material and in the several energy states confined in the dots. The rate equations of the carriers are also coupled with the rate equations of the photons generated by spontaneous and/or stimulated emission. In this review we provide several examples of simulation results of the optical characteristics of InAs/GaAs quantum dot semiconductor lasers and superluminescent diodes emitting in the near infrared with application in optical communications, sensing and optical coherent tomography. In particular, we show how the inhomogeneous gain broadening and the presence of more than one confined energy state in the dots can influence the laser properties such as the shape of the emitted spectrum, the maximum modulation bandwidth and the frequency fluctuations (chirp) under large signal modulation. The results of this analysis gives useful insights on the meaning, in the quantum dot case, of various parameters (linewidth enhancement factor, differential gain ) that are routinely measured in the lab with the standard characterization techniques for semiconductor quantum well or bulk lasers. We also provide some examples of calculated emission characteristics (light versus current curves and output spectra) of quantum dot superluminescent diodes to highlight the relevant differences respect to the laser case. We also show how the inhomogeneous broadening of the gain, the quantum dot layer composition, and the device geometry can be engineered to get bright sources with broad spectrum useful for medical and sensing applications.