Large and uniform optical emission shifts in quantum dots externally strained along their growth axis

We introduce a method which enables to directly compare the impact of elastic strain on the optical properties of distinct quantum dots (QDs). Specifically, the QDs are integrated in a cross-section of a semiconductor core wire which is surrounded by an amorphous straining shell. Detailed numerical simulations show that, thanks to the mechanical isotropy of the shell, the strain field in a core section is homogeneous. Furthermore, we use the core material as an in situ strain gauge, yielding reliable values for the emitter energy tuning slope. This calibration technique is applied to self-assembled InAs QDs submitted to incremental tensile strain along their growth axis. In contrast to recent studies conducted on similar QDs stressed perpendicularly to their growth axis, optical spectroscopy reveals 5-10 times larger tuning slopes, with a moderate dispersion. These results highlight the importance of the stress direction to optimise QD response to applied strain, with implications both in static and dynamic regimes. As such, they are in particular relevant for the development of wavelength-tunable single photon sources or hybrid QD opto-mechanical systems

Generation of ultrashort (~10ps) spontaneous emission pulses by quantum dots in a switched optical microcavity

We report on the generation of few-ps long spontaneous emission pulses by quantum dots (QDs) in a switched optical microcavity. We use a pulsed optical injection of free charge carriers to induce a large frequency shift of the fundamental mode of a GaAs/AlAs micropillar. We track in real time by time-resolved photoluminescence its fundamental mode during its relaxation, using the emission of the QD ensemble as a broadband internal light source. Sub-ensembles of QDs emitting at a given frequency, interact transiently with the mode and emit an ultrashort spontaneous emission pulse into it. By playing with switching parameters and with the emission frequency of the QDs, selected by spectral filtering, pulse durations ranging from 300 ps down to 6 ps have been obtained. These pulses display a very small coherence length, which opens potential applications in the field of ultrafast imaging. The control of QD-mode coupling on ps-time scales establishes also cavity switching as a key resource for quantum photonics.Comment: 11 pages, 8 figures; includes supplemental materia

Determination of the valence band offset at cubic CdSe/ZnTe type II heterojunctions: A combined experimental and theoretical approach

We present a combined experimental and theoretical approach for the determination of the low-temperature valence band offset (VBO) at CdSe/ZnTe heterojunctions with underlying zincblende crystal structure. On the experimental side, the optical transition of the type II interface allows for a precise measurement of the type II band gap. We show how the excitation-power dependent shift of this photoluminescence (PL) signal can be used for any type II system for a precise determination of the VBO. On the theoretical side, we use a refined empirical tight-binding parametrization in order to accurately reproduce the band structure and density of states around the band gap region of cubic CdSe and ZnTe and then calculate the branch point energy (also known as charge neutrality level) for both materials. Because of the cubic crystal structure and the small lattice mismatch across the interface, the VBO for the material system under consideration can then be obtained from a charge neutrality condition, in good agreement with the PL measurements.Comment: 11 pages, 5 figure

Residual strain and piezoelectric effects in passivated GaAs/AlGaAs core-shell nanowires

International audienceWe observe a systematic red shift of the band-edge of passivated GaAs/Al0.35Ga0.65As core-shell nanowires with increasing shell thickness up to 100 nm. The shift is detected both in emission and absorption experiments, reaching values up to 14 meV for the thickest shell nanowires. Part of this red shift is accounted for by the small tensile strain imposed to the GaAs core by the AlGaAs shell, in line with theoretical calculations. An additional contribution to this red shift arises from axial piezoelectric fields which develop inside the nanowire core due to Al fluctuations in the shell

Surface effects in a semiconductor photonic nanowire and spectral stability of an embedded single quantum dot

We evidence the influence of surface effects for InAs quantum dots embedded into GaAs photonic nanowires used as efficient single photon sources. We observe a continuous temporal drift of the emission energy that is an obstacle to resonant quantum optics experiments at the single photon level. We attribute the drift to the sticking of oxygen molecules onto the wire, which modifies the surface charge and hence the electric field seen by the quantum dot. The influence of temperature and excitation laser power on this phenomenon is studied. Most importantly, we demonstrate a proper treatment of the nanowire surface to suppress the drift