The authors report on room temperature photoluminescence from single CdSe quantum dots. The quantum dots, realized by self-organized epitaxial growth, are embedded in ZnSSe∕MgS barriers. The integrated intensity of the emission drops by less than a factor of 3 between 4K and room temperature. Microphotoluminescence with a spatial resolution of 200nm exhibits single dot emission that remains visible up to 300K. The linewidth of the single dot emission increases thereby from 340μeVto25meV at room temperature, which the authors attribute to the interaction of excitons with optical phonons.

Self-catalysed and self-organized GaN nanowires were grown on c-, a-, m- and r-plane sapphire by metal-organic vapour phase epitaxy. In dependence on the crystallographic orientation of the sapphire substrate, vertical, tilted and in-plane GaN nanowires were achieved. The nanowire orientation is visualized by scanning electron microscopy and analysed by x-ray diffraction. The influence of the sapphire nitridation step on the nanowire formation is investigated. Spatially and spectrally resolved cathodoluminescence studies are carried out on the GaN nanowires to analyse the influence of the GaN nanowire orientation as well as the presence of both N- and Ga-polar sections in a single nanowire on the optical properties.

In this paper we report on recent results concerning conventional edge emitting laser structures containing either quantum wells or quantum dots as region and vertical-cavity surface-emitting lasers. In the first part a series of four similar laser structures containing quantum wells with an emission wavelength of 520 nm was grown by molecular beam epitaxy in order to perform a systematic study of lifetime improvement. They differed in the alternating implementation of an additional 5 nm thick ZnSSe layer with a high sulfur composition of 25 % neighboring the quantum well. A high stability of the CdZnSSe active layer was observed by introducing such a kind of strain compensating layers. Lifetime measurements showed a significant improvement up to one order of magnitude using p-and n-& p-side layers. In the second part electro-optical characteristics of ridge and planar CdSe quantum dot laser diodes were compared. A reduction of the threshold current density by a factor of 4.7 for the ridge structure was obtained. This has to be associated to the reduction of current spreading inside the laser diodes. Furthermore, a significant slower degradation of CdSe quantum dot structures compared to common ZnSe-based QW structures was observed. An operating time over 2700 h in pulsed mode experiments at 50 A/cm 2 in LED mode was achieved. In the third part we report on the realization of an optically pumped monolithic vertical-cavity surface-emitting laser operating at a wavelength of 511 nm. The microresonator has a quality factor of 3200 while the threshold excitation power density for the onset of lasing is 22 kW/cm 2 at room temperature. Micropillars of different diameter fabricated out of this structure show discrete optical modes due to the three dimensional optical confinement of the optical wave.1 Introduction Nowadays for various applications small and compact light sources in kind of semiconductor laser diodes are necessary [1]. They combine the advantages of classical laser, e.g. a coherent light beam at a certain wavelength and the low dimensions which are requirements for new techniques and devices. But many these applications specifically needs light emitters in the green spectra range. Up to now only II-VI-based laser diodes are able to provide such a laser emission. Edge-emitters using CdZnSe or CdZnSSe quantum wells (QWs) as the active part cover the whole wavelength range between 500 nm an 560 nm just by varying the Cd content [2], while a quantum dot (QD) laser diode shows emission at 560 nm [3]. However, the commercialization of ZnSe-based laser diodes is hampered by their limited lifetime [4].A big issue of II-VI LD is the fast degradation caused by the instability of the active region. One problem is the compressive strain of the quantum well due to the Cd content. For an emission in the green-yellow region a Cd content between 20 % and 40 % is necessary. One approach to prevent this kind of degradation is the introduction of additional strain compensating layers. These layers consist out of thin ZnSSe ...

The influence of coherent optical nonlinearities on polariton propagation effects is studied within a theory-experiment comparison. A novel approach that combines a microscopic treatment of the boundary problem in a sample of finite thickness with excitonic and biexcitonic nonlinearities is introduced. Light-polarization dependent spectral changes are analyzed for single-pulse transmission and pump-probe excitation. The propagation of light pulses which are coupled to the excitonic resonances is a fundamental problem in semiconductor optics that has been the subject of intense experimental and theoretical research. The observations are dominated by the strong light-matter interaction and its interplay with the inherent many-particle Coulomb interaction of the electronic system. In the linear optical regime, exciton polaritons give rise to effects like polariton beating in the time-resolved pulse transmission [1] or yield strong modifications of excitonic transmission spectra, see e.g. [2,3,4]. In the nonlinear optical regime, incoherent saturation of the polariton resonances in transmission spectra [5,6] and in the amplitude and phase of the time-resolved transmission [7] as well as transmission changes in pump-probe experiments [8] have been studied.The above investigations are focused on samples where the thickness corresponds to a few exciton Bohr radii where polariton effects are most pronounced in the optical spectra. In this case the interplay of the induced excitonic polarization in the medium with the propagating light field is strongly influenced by sample surfaces which considerably complicates the theoretical description. In a frequently used phenomenological approach [9] the coupling of the exciton relative and center-of-mass (COM) motion at the sample boundaries is neglected. Then the exciton COM motion is subject to quantization effects in the confinement geometry while the exciton relative motion is approximated by the result of the infinitely extended medium. As a result of this approach, the solution of the wave equation for the electromagnetic field requires additional boundary conditions (ABCs) [9,10] which are, however, not uniquely defined. Unfortunately, in many situations the choice of ABCs can influence the theoretical predictions [11]. To avoid these ambiguities, the use of microscopic boundary conditions within a non-local semiconductor response has been discussed in Refs. [12,13] and recently applied to the linear optical regime [3,4,14]. The description of optical nonlinearities combined with a microscopic treatment of boundary conditions has been restricted so far to the quantum-well (QW) limit where propagation effects lead to radiative exciton broadening, which can be modified in radiatively coupled QWs. Furthermore, excitonic nonlinearities in QWs have been studied in connection with microcavity polaritons; for a review see [15]. In the opposite limit of the sample thickness approaching the bulk limit nonlinear pulse propagation effects [16,17] have been analyzed successfully with...

No abstract