An analysis of the threshold behavior of long-wavelength ( lambda =1.55 mu m) multiquantum well separate-confinement lasers with InGaAs wells and quaternary ( lambda g=1.3 mu m) barriers is presented. Using the effective mass approximation and Fermi statistics for carriers, an approximately logarithmic dependence of optical gain on carrier density for quantum well lasers with one confined electron state is predicted theoretically. This prediction is verified by measured threshold currents of broad-area lasers of various cavity lengths and different numbers of quantum wells. Moreover, the characteristic parameters, such as transparency current density, gain constant, and absorption outside the active region, are determined

Emission and absorption spectra are measured of several ZnO-crystals in the region of bound-exciton complexes near the band gap. From Zeeman-effect studies up to 8 T a t several angles between the c-axis of the crystal and the magnetic field most of the observed spectral lines are identified. The electron g-factor is found to be g, = 1.8, if the exciton is formed with a hole from the A-band, and g, = 2.1, if the exciton is formed with a hole from the B-band. The mass of the R-hole seems to be nearly equal to the mass of the A-hole.Emissions-und Absorptionsspektren von ZnO-Kristaikn werden im Gebiet der gebundenen Exzitonen in der Nahe der Bandkante gemessen. Aus Zeemaneffekt-Untersuchungen bis 8 T bei verschiedenen Winkeln zwischen Magnetfcld und c-Achse der Kristalle werden die mersten der beobachteten Spektrallinien identifiziert. Der g-Faktor des Elektrons ist g, = 1,8, wenii das Exziton mit einem Loch aus dem *%-Band gebildet wird, bzw. g , = 2,1, wenn das Loch des Exzitons ansdem B-Band stammt. Das B-Loch hat nngefahr die gleiche Masse wic das A-Loch.

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I n honour of Prof. Dr. F. ST~CEMANN'S 60th birthday A careful quantitative analysis of the reflectance of CdS crystals in the spectral range of the A-exciton is presented. Measured spectra for the isotropic and the mixed-mode orientation of the crystal are fitted by calculations based on the model of excitonic polaritons. Spatial dispersion and Pekar's abc, damping, exciton-free surface layers, and oblique incidence of light are considered simultaneously. The good agreement between theory and experiment confirms the model, the still existing discrepancies are due to the simplifying assumption of a homogeneous surface layer. The formulae needed are summarized, and the influence of the different polariton parameters on the calculated reflectivity spectra are discussed. Es wird eine sorgfiiltige Analyse des Reflexionsvermogens von CdS-Kristallen im Spektralbereich des A-Exzitons durchgefiihrt. Die gemessenen Spektren fur die isotrope Anordnung und die mixed-mode-Anordnung des Kristalls werden durc h Rechnungen auf der Grundlage desModells der Exzitonenpolaritonen angepalt. Dabei werden riiumliche Dispersion und Pekars abc, Diimpfung, exzitonenfreie Oberflachenschicht und schriiger Lichteinfall beriicksichtigt. Die gute Ubereinstimmung zwischen Theorie und Experiment bestiitigt das Modell, die noch existierenden Abweichungen beruhen auf der vereinfachenden Annahme einer homogenen Oberfliichenschicht. Die benotigten FormeLn werden zusammengefaBt und der Einflul der verschiedenen Polaritonenparameter auf die berechneten Reflexionsspektren diskutiert.

Recently B r o s e r e t al. /I/ reported that simultaneous measurements of the reflectivity and transmission of a CdS sample near the excitonic polariton resonance allow them to find out the correct damping function r ( 0 ) . F r o m Maxwell boundary conditions and the additional boundary condition (abc) a t both surfaces of the sample one gets a s e t of s i x equations which allow one to calculate the transmitted and reflected electric fields and also the electrics fields of the two polariton waves in the sample. Conversely, as B r o s e r et al. pointed out, it is possible to express the ratio A of the two electric fields close to the f i r s t interface as a function of the amplitude reflexion coefficient r. The calculation is straightforward. One finds 1 -r -n , ( l + r )This is valid only when the sample is thick enough so that one can assume that the two polariton waves reflected at the rear surface of the sample are absorbed before reaching back the front surface.In c a s e one uses'an exciton free l a y e r t o describe thesample surface, A becomes the ratio of the two electric fields a t the interface between the dead layer and the bulk and does not depend only on r but also on the thickness of the dead layer which is a p r i o r i an unknown parameter. In this c a s e one cannot completely eliminate the surface effects.