Lateral coupling of InxGa1−xAs∕GaAs quantum dots investigated using differential transmission spectroscopy

Abstract

Coupling between InGaAs/ GaAs quantum dots is investigated using differential transmission spectroscopy. Two-color pump-probe techniques are used to spectrally resolve the carrier dynamics, revealing carrier transfer between quantum dots at room temperature. The time constant for this process is shown to increase from 35 ps at room temperature to 130 ps at 230 K.Self-assembled semiconductor quantum dots (QDs) are remarkably flexible structures that are impacting many areas of optoelectronics. At low temperatures, they exhibit atomiclike properties due to their three-dimensional electronic confinement and are promising for such applications as quantum computing and cryptography. Self-assembled QDs have also proven useful when incorporated into conventional devices such as semiconductor lasers and amplifiers. Unfortunately, many of the major device improvements anticipated due to the unique QD density of states have not been realized. This is due in part to the as yet unavoidable size distribution in QD ensembles, but also due to the significant temperature-dependent coupling of individual QD transitions. Temperature-dependent coupling affects the threshold current density in QD lasers 1 and limits the isolation between channels in a QD semiconductor optical amplifier and other multichannel devices. 2,3 Coupling between QDs in optoelectronic devices occurs via both electromagnetic and electronic mechanisms. The former couples all QDs within a homogeneous linewidth of the individual transition. The latter can couple all QDs within the inhomogeneous linewidth as electrons and holes move between QDs with different ground state energies. Each of these effects can result in Fermi level pinning and, therefore, explain the observed single-mode operation of QD lasers at room temperature. 4 In actuality, both of these processes contribute to the collective action of QD ensembles and can be difficult to distinguish. Recently, coupling in self-assembled QD ensembles has been studied extensively. The room temperature homogeneous broadening of the transition has been measured by four wave mixing 5 and single QD emission 6,7 with values ranging from a few nanometers up to more than 10 nanometers. Electronic coupling of QDs via thermal emission and subsequent recapture has been inferred from temperature dependent continuous-wave 8,9 and time-resolved 10,11 photoluminesence (PL) measurements. Unfortunately, this method relies on numerous assumptions and a multivariable fit to a series of data. Results from resonant differential transmission (DT) measurements 12,13 show an initial fast recovery of the QD absorption indicating that carriers are quickly leaving the states that they were initially created in. These experiments cannot determine whether the resonantly excited carriers are moving to other QDs or different states of the same QD.This paper is orginized as follows. Section I contains information on the growth and preparation of the sample as well as the experimental methods employed for this study. Section II contains room ...

Gain and phase dynamics of InAs/GaAs quantum dot semiconductor optical amplifiers

“…We attribute the longest time to interband recombination between electrons and holes, the intermediate time to a combination of electron thermalisation from GS to ES and ES to wetting layer (WL) (similar to Refs. [8,10]), and the short time to a combination of electron decay from ES to GS and hole redistribution (similar to Ref. [10]).…”

“…[8,10]), and the short time to a combination of electron decay from ES to GS and hole redistribution (similar to Ref. [10]). These times are also consistent with Ref.…”

“…Finally, it is important to mention that tuning of the pump/probe wavelength away from ES/GS gain peak results in the appearance of an additional ~35 ps time scale related to dot-to-dot transport and similar to that observed in Ref. [10]. These processes will not be analyzed here.…”

Gain dynamics of quantum dot devices for dual-state operation

“…When pumping and probing the same QD subensemble, a direct intra-QD relaxation from ES to GS is expected to occur which manifests itself in an ultrafast change of the transmitted probe pulse. 32,33 For different pumped and probed QD sub-ensembles, optically generated excitons need to pass through the commonly shared 2D carrier reservoir by up-and down-scattering processes, which are reflected by slower changes of the GS transmission. As mentioned before, the device is optically pumped at the ES laser line, while the probe pulse is tuned from 1230 nm (1.008 eV) to 1310 nm (0.943 eV), covering the GS luminescence energies.…”

Single-exciton energy shell structure in InAs/GaAs quantum dots

“…However, it has been shown using time-resolved spectroscopy that such carrier transfer does not happen at low temperatures for samples with QD densities below 1 ϫ 10 10 cm −2 . 28 Moreover, in samples with strong lateral coupling, the spectral features broaden considerably toward higher energies as the excitation intensity is increased. 29 This is not observed in Fig.…”