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Direct band gap III-V semiconductors, emitting efficiently in the amber-green region of the visible spectrum, are still missing, causing loss in efficiency in light emitting diodes operating in this region, a phenomenon known as the "green gap". Novel geometries and crystal symmetries however show strong promise in overcoming this limit. Here we develop a novel material system, consisting of wurtzite Al

Group IV semiconductor optoelectronic devices are now possible by using strain-free direct band gap GeSn alloys grown on a Ge/Si virtual substrate with Sn contents above 9%. Here, we demonstrate the growth of Ge/GeSn core/shell nanowire arrays with Sn incorporation up to 13% and without the formation of Sn clusters. The nanowire geometry promotes strain relaxation in the Ge

Thanks to their uniqueness, nanowires allow the realization of novel semiconductor crystal structures with yet unexplored properties, which can be key to overcome current technological limits. Here we develop the growth of wurtzite GaP/In

Wurtzite gallium phosphide (WZ GaP) has been predicted to exhibit a direct bandgap in the green spectral range. Optical transitions, however, are only weakly allowed by the symmetry of the bands. While efficient luminescence has been experimentally shown, the nature of the transitions is not yet clear. Here we apply tensile strain up to 6% and investigate the evolution of the photoluminescence (PL) spectrum of WZ GaP nanowires (NWs). The pressure and polarization dependence of the emission together with a theoretical analysis of strain effects is employed to establish the nature and symmetry of the transitions. We identify the emission lines to be related to localized states with significant admixture of Γ7c symmetry and not exclusively related to the Γ8c conduction band minimum (CBM). The results emphasize the importance of strongly bound state-related emission in the pseudodirect semiconductor WZ GaP and contribute significantly to the understanding of the optoelectronic properties of this novel material.

An experimental and theoretical study of the carrier capture time into a semiconductor quantum well is presented. Oscillations of the phonon emission induced capture time are experimentally observed and good agreement with theory is found. The calculations show that not only the LO‐phonon emission induced capture time (ph‐capture) oscillates as a function of well width, but also the carrier–carrier scattering induced capture time (c‐c capture) oscillates by more than one order of magnitude as a function of the active layer design. Recently it has been shown that the carrier capture time is directly related to the modulation band width in a quantum well laser. As a result, it might be possible to tailor the modulation band width by optimizing the capture efficiency using a proper design of the active layer in a quantum well laser.

The lifetime of the second-order population of a fully inverted inhomogeneously broadened ensemble of quantum dots (QDs) is investigated. It is assumed that a planar structure of two-level QDs is initially non-resonantly excited by a pump laser pulse. After a delay time τ, the QDs are resonantly probed by laser pulse. Calculations using the master equation approach show that the lifetime of the second-order population is proportional to the QD density which is resonant with the probe laser spectrum.

We present a numerical model aiming at a realistic description of light-induced drift of multilevel atoms in a buffer gas, specializing to case of Na atoms in a noble gas. The model is based on the optical Bloch equations in which the effect of collisions is incorporated by introducing Keilson-Storer collision kernels. By marking a connection with kinetic theory all adjustable parameters may be fixed. Bilanzgleichungen lichtinduzierter Drift Wir geben ein numerisches Modell zur realistischen Beschreibung lichtinduzierter Drift von Atomen mit mehreren Zustanden in einem Puffergas. an Als praktisches Beispiel dienen Na-Atome in einem Edelgas. Das Modell basiert auf optischen Blochgleichungen, in denen die Kollision durch Einfuhrung von Keilson-Storer Kollisionskernen eingearbeitet ist. Durch eine Verbindung mit der kinetischen Theorie konnen alle wahlbaren Parameter fixiert werden.

We present photoluminescence (PL) measurements on an ensemble of InAs/GaAs self-assembled quantum dots embedded in GaAs. We observe a transition from an inhomogeneously broadened photoluminescence band under non-resonant excitation into up to five phonon-assisted bands under selective excitation. We interpret the phonon-assisted PL as being indicative of an enhanced electron–phonon interaction. We also perform theoretical calculations of the single-particle energy spectrum of self-assembled quantum dots, in the framework of the single-band effective-mass approximation for electrons and using the Luttinger Hamiltonian for holes. Finally, by taking advantage of the computed wave functions we evaluate the Huang-Rhys parameter: We find an enhancement of the electron–phonon interaction that partially accounts for the experimental results.

Short electrical pulses have conventionally been generated with photoconductive switches on short carrier lifetime material such as low temperature grown GaAs. Another approach is nonuniform illumination of a photoconductive switch.1,2 Both methods yield electrical pulse widths of less than 1 ps. The first method, however, is usually not compatible with active devices on the same wafer. The disadvantage of the second method is that the generated pulse forms are difficult to reproduce.

By a simple technique it is possible to discriminate experimentally between the self-shift and foreign-gas-induced shift of Rydberg levels excited in a heat-pipe system. Illustrative results for two particular rydberg levels of the potassium dimer K2 are presented. The results concern the shift induced by the alkali itself and the shift induced by Ar and He. Earlier reported discrepancies are removed.

The authors have demonstrated the production of molecular Rydberg state in K2 by means of two-step photoexcitation with help of thermionic detection in a disc-shaped heat-pipe oven. Preliminary results concern a spectroscopic investigation of the very high 'Rydberg region'. Results of the pressure shift of Rydberg lines are also presented.