We study the carrier capture and relaxation in self-assembled InAs/ GaAs quantum dots ͑QDs͒ using bleaching rise time measurements as a function of the excitation density, at 5, 77, and 293 K. We observe that the bleaching rise time and the carrier lifetime of the first excited state are longer than the bleaching rise time of the QD ground state, indicating that the excited state does not act as an intermediate state. For high excitation density, we observe a temperature-dependent plateau in the initial bleaching rise time, contradicting an Augerscattering-based relaxation model. Both these experimental results point toward a relaxation through the continuum background, followed by a single LO-phonon emission toward the QD ground state. The relaxation through the continuum background is governed by Coulomb or acoustic phonon coupling between the continuum and the discrete QD energy levels.

Coherent excitation of the quasilongitudinal and quasitransverse acoustic phonon mode in strain engineered InAs∕GaAs quantum dot (QD) clusters grown on (311)B GaAs is monitored by means of time-resolved differential reflection spectroscopy. Carrier capture within the ordered QD clusters initiate coherent acoustic phonon excitation, which induces a transient modulation of the local strain-induced piezoelectric field within the QD clusters. The excited acoustic phonons then modulate the optical properties of the QDs through the quantum-confined Stark effect, causing distinct oscillations of the differential reflection signal.

Strain engineered InAs/GaAs quantum dot (QD) clusters grown on (311)B GaAs are studied by means of time-resolved differential reflection spectroscopy revealing coherent excitation of the quasilongitudinal and quasitransverse acoustic phonon modes. Photogenerated carriers which are captured within the ordered lateral QD clusters initiate coherent acoustic phonon excitation, which induces a transient modulation of the local strain-induced piezoelectric field within the QD clusters. The excited acoustic phonons modulate the optical properties of the QDs through the quantum-confined Stark effect, causing oscillations of the reflection signal.1 Introduction The formation of high quality quantum dots (QDs) in well-defined arrangements, that are QD arrays [1-3] and ordered QD groups [4][5][6][7], by self-organized strain engineering provides a challenge in epitaxial crystal growth. The number of QDs within a single group or QD cluster, formed by using strained-layer superlattice (SL) templates [5,6], is controlled by varying the growth temperature of the SL template and the thickness of the GaAs separation layer between the SL template and the QD layer. A consequence of strained nanostructure growth is the strongly enhanced piezoelectric (PZE) field [8,9], which in turn affects the electro-optical properties of the structure [9][10][11][12].We have studied the differential reflectivity of QDs arranged in small ordered groups of 4 QDs per cluster on average, grown by strain engineering [5,6]. Using pump-probe time-resolved differential reflection spectroscopy (TRDR) [13], we are able to measure the carrier capture and carrier relaxation process, and the carrier recombination process within the QDs. For structures with a random QD distribution, the decay of the TRDR signal is described by a single exponential function. In the case of the QD clusters pronounced oscillations in the transient TRDR signals are observed, due to coherent acoustic phonon excitation [14][15][16] within the QD clusters induced by the screening of the local PZE field governed by carrier capture. The phonons are identified as the quasilongitudinal (QL) and quasitransverse (QT) acoustic phonon modes [17]. In turn, the acoustic phonons modulate the local strain and, hence, the strain-induced PZE-field in the QD clusters. Hereby, the optical properties of the QDs, e.g., the QD reflectivity, is periodically changed through the quantum-confined Stark effect [18,19] causing oscillations in the TRDR signal.