Correlating stimulated emission phase to the gain spectra in semiconductor lasers

Abstract

In laser physics, the incident electric field and the stimulated field are assumed to have the same frequency, direction of propagation, polarization, and phase —same state. However, no formal proof of the phase identity (zero phase shift) was reported. The existing theories on the phase shift are in conflict with each other and with the phase identity. The phase shift in three semiconductor active media (GaAs, In0.15Ga0.85As, and In0.48Ga0.58As0.9P0.1) has been studied using a semiclassical approach. Contrary to the conventional belief, it has been found that the phase shift is not zero. The phase shift is not even a single value but rather a spectrum corresponding to the gain spectrum. At a carrier concentration of 2.5 × 1024 m −3, the minima of the phase shift spectra are 1.5597, 1.509, and 1.399 radians for GaAs, In0.15Ga0.85As, and In0.48Ga0.58As0.9P0.1, respectively. Stimulated emission is shown to occur whenever the phase shift is positive and lies in the interval between 0 and π/2. Because of radiation reaction of the radiating source and the finite lifetime of excited states, stimulated emission cannot attain the same phase as the incident field —no zero-phase shift. These results reveal that the conventional picture of phase identity is incorrect, and phase matching between the incident field and stimulated is not a necessary condition for stimulated radiation. Despite the fundamentality of the concept of stimulated emission phase, such outcomes are not reported nor discussed in the literature.