Modeling differential transmission spectroscopy experiments in quantum dot optical amplifiers and saturable absorbers

Abstract

We present a numerical model based on a time-domain travelling-wave approach to describe pulse propagation in InAs Quantum-Dot (QD) based Semiconductor Optical Amplifiers (SOA) and Saturable Absorbers (SA). The onedimensional field propagation equation is solved in the time domain, in the slowly varying envelope approximation and it is coupled to a set of multi-population rate-equations modeling carrier dynamics in the QD layers in each longitudinal section of the waveguide. The optical response of the QD active medium is introduced in the field equation via a proper polarization term described as a set of infinite impulse response numerical filters. The inhomogeneous broadening of the density of states of the QD system induced by the QD size dispersion is properly taken into account. The influence of a static electric field on carrier dynamics in a reversely biased SA is described in the rate equations via bias dependent thermionic escape rates and tunneling processes. Absorption dynamics in a QD SA shows an initial ultrafast, bias independent recovery, followed by a slower recovery strongly dependent on the applied reverse bias. The QD SOA shows instead a dominant ultrafast gain recovery on a subpicosecond time scale at both QD Ground-States (GS) and first Excited-States (ES 1 ) wavelengths. Direct capture/escape processes between quantum well states and deeply confined QD states slightly influence the gain and absorption dynamics.