Abstract

The emission dynamics of excitons in semiconductor quantum wells (QW) after a short-pulse excitation receives continued interest. A delayed secondary emission (SE) was noticed early on [1], unlike of what is observed in atomic vapors. We show that this difference is due to a, uniform distribution of the excitonic oscillator strength in the QW plane. When exciting only a part of the resonance, the in-plane uniformity of the initially excited polarization is broken, and an instantaneous rise of the SE is observed. This is experimentally achieved using a sample with large growth islands, which lead to a splitting of the exciton resonance into separated peaks corresponding to effective QW thicknesses differing by integer monolayers. We compare it to a sample without the large growth islands. While the spectral widths of the excited excitonic distributions are similar in both samples, the SE rise is instantaneous for the monolayer sample, and delayed for the other. The initial dynamics of the SE intensity for the monolayer sample is shown in the figure together with the response function of the streak-camera. The dynamics according to a model for a inhomogeneously broadened exciton resonance [2], convoluted with the streak response is given by the solid line (α=1). The SE data show a much faster rise, and thus differ from the typical quadratic rise determined by the inhomogeneous broadening σ. To include the influence of growth islands into the model we assign to each exciton a monolayer thickness with a random spatial distribution on a length scale of the light wavelength Only excitons belonging to one monolayer thickness are optically excited, and contribute to the SE. The calculated initial dynamics using the probability a of an exciton to belong to the excited monolayer is shown in the inset. The SE acquires an instantaneous contribution of 1-α of the total signal. For a small concentration of excited resonances, like in atomic vapors, the predicted RRS rise is thus fully instantaneous. For the present samples typically 2-3 different monolayer thicknesses exist in parallel, compatible with α ≈ 0.4, for which both an instantaneous and a delayed component is expected. The calculation convoluted with the streak response (dashed line) agrees with the measured data. We conclude that the observed instantaneous RRS contribution is due to the spatial nonuniformity of the excitons belonging to a specific monolayer thickness.

© 2000 IEEE