Strong amplitude-phase coupling in submonolayer quantum dots

Abstract

Submonolayer quantum dots promise to combine the beneficial features of zero- and two-dimensional carrier confinement. To explore their potential with respect to all-optical signal processing, we investigate the amplitude-phase coupling (α-parameter) in semiconductor optical amplifiers based on InAs/GaAs submonolayer quantum dots in ultrafast pump-probe experiments. Lateral coupling provides an efficient carrier reservoir and gives rise to a large α-parameter. Combined with a high modal gain and an ultrafast gain recovery, this makes the submonolayer quantum dots an attractive gain medium for nonlinear optical signal processing.

Mode-locking Instabilities for High-Gain Semiconductor Disk Lasers Based on Active Submonolayer Quantum Dots

“…At higher pump levels, the laser becomes unstable with intermittent transitions from cw modelocking to irregular spikings and multi-pulse fluctuations. This noisy behavior can be explained by the strong amplitude-phase coupling in SML QDs typically characterized by the linewidth enhancement factor (α factor) [34,35]. With this result, we present experimental verification of theoretical predictions by models of passive modelocking in semiconductor lasers from more than 10 years ago [36].…”

“…Furthermore, the efficient lateral coupling with an optically inactive but fast accessible carrier reservoir can provide a very fast gain recovery, but can also induce significant refractive index changes after an optical excitation [44]. Recent measurements revealed linewidth enhancement factors for SML QDs up to one order of magnitude higher compared to SK QDs and QWs [34]. The numerical model for passive modelocking in semiconductor lasers developed by Vladimirov et al [36] states that if the linewidth enhancement factor of the gain α g exceeds the one of the absorber α a , the modelocking becomes unstable and chaotic.…”

“…Later publications by Lingnau et al concluded, however, that for fast carrier scattering the QD laser behavior can be reasonably well characterized by an α factor [47]. Furthermore, Herzog et al used the α factor concept to describe the amplitudephase coupling in SML QDs [34]. For these reasons, we also use the α factor for our description of the carrier induced refractive index changes.…”

Sideband pump-probe technique resolves nonlinear modulation response of PbS/CdS quantum dots on a silicon nitride waveguide

“…This confirms the hypothesis that the fast process is a thermalization process, as hot carriers and unpaired electrons and holes cause intraband transitions with a significant contribution to the refractive index. 46 The time constant of the process differs significantly from the relaxation time of carriers created with a large excess energy in the absorption continuum of the PbS/CdS QD. For these conditions, relaxation times in the QD GS as short as 2 ps have been reported.…”

Determining the linewidth enhancement factor via optical feedback in quantum dot micropillar lasers

“…Furthermore, conventional methods usually give only a single scalar value as outcome for α, not being able to track its dependence on key laser parameters like the pump current. These are important limitations for the generic application of those methods, given the fact that α depends on the device characteristics [15] and on its operating conditions [16][17][18].…”