Gillian Madden

Intradot dynamics of InAs quantum dot based electroabsorbers

The carrier relaxation and escape dynamics of InAs/GaAs quantum dot waveguide absorbers is studied using heterodyne pump-probe measurements. Under reverse bias conditions, we reveal differences in intradot relaxation dynamics, related to the initial population of the dots’ ground or excited states. These differences can be attributed to phonon-assisted or Auger processes being dominant for initially populated ground or excited states, respectively.

Recovery time scales in a reversed-biased quantum dot absorber

The nonlinear recovery of quantum dot based reverse-biased waveguide absorbers is investigated both experimentally and analytically. We show that the recovery dynamics consists of a fast initial layer followed by a relatively slow decay. The fast recovery stage is completely determined by the intradot properties, while the slow stage depends on the escape from the dot to the wetting layer.

Ultrafast gain and refractive index dynamics in GaInNAsSb semiconductor optical amplifiers

The gain and refractive index dynamics of dilute nitride antimonide semiconductor optical amplifiers are studied using heterodyne pump probe spectroscopy, both in forward and reverse bias regimes. In the forward biased absorption regime, both gain and refractive index relax on the same timescale indicating that both quantities are linked to the same relaxation process, interband recombination. Above transparency, in the forward biased gain regime, the gain and phase exhibit differing timescales resulting in a dynamical alpha factor that varies strongly with time. Reversed bias measurements suggest a recombination dominated absorption recovery where the recovery timescale increases with increasing reversed bias, possibly due to charge separation effects.

The nonlinear absorption and phase recovery of quantum dot based reverse-biased waveguide electro-absorbers

The physics of quantum dot based optical devices has been studied intensively due to their interesting blend of atomic and solid state properties. Recently, attention has been focused on their absorption properties and has led to QD materials finding favour in such applications as monolithic mode-locked lasers, electro-absorption modulators and saturable absorber mirrors. In this study we perform a detailed experimental investigation of the ultrafast absorption and phase dynamics of a QD InAs/GaAs structure under reverse bias conditions using single colour pump-probe measurements. Experimental results reveal the fundamental timescales and underlying dynamical processes occurring in such absorbers. We will also consider the impact of the observed absorption and phase dynamics on some current applications of QD absorbers.

Ultrafast processes in InAs/GaAs quantum dot based electro-absorbers

In this study we perform a detailed investigation of the ultrafast processes which govern the intradot recovery dynamics of a QD InAs/GaAs structure under reverse bias condition by means of the Single and Two Colour Pump-Probe technique. By studying the GS and ES recoveries as a function of reverse bias voltage and fitting the experimental results with a simple rate equation model for the intradot carrier dynamics we have illustrated the dominance of Auger mediated recovery when the ES is initially populated while phonon mediated recovery dominates for the GS case. This provides opportunities for the design of the next generation of electro-absorbing devices based on QD materials.

Recovery time scales in quantum dot devices

The physics of quantum dot (QD) based optical devices has been studied intensively due to their interesting blend of atomic and solid state properties. Recently, attention has focused on their amplification and absorption properties and has led to QD materials finding favour in such applications as monolithic mode-locked lasers, semiconductor optical amplifiers (SOAs), electroabsorption modulators and saturable absorber mirrors. We present experimental results together with an analytical analysis of the nonlinear recovery of QD based SOAs in forward and reversed bias modes. The study reveals the role of capture and escape processes in the formation of two stages of recovery.