At carrier densities above the Mott density Coulomb screening destroys the exciton resonance.This, together with band-gap renormalization and band filling, severely affects the optical spectra.We have experimentally studied these effects by ultrafast pump-probe reflectivity measurements on a ZnO single crystal at various wavelengths around the exciton resonance and in a broad carrierdensity range. Theoretically we determined the Mott density in ZnO to be 1.5 × 10 24 m −3 at 300 K. Taking a field-theoretical approach, we derived and solved the Bethe-Salpeter ladder equation and we computed the density-dependent reflectivity and absorption spectra. A carrier dynamics model has been developed, containing three-photon absorption, carrier cooling, and carrier trapping near the surface. The agreement between the theoretical reflectivity based on our model and the experimental data is excellent.

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Are excitons involved in lasing in ZnO nanowires or not? Our recently developed and experimentally tested quantum many-body theory sheds new light on this question. We measured the laser thresholds and Fabry-Pérot laser modes for three radically different excitation schemes. The thresholds, photon energies, and mode spacings can all be explained by our theory, without invoking enhanced light-matter interaction, as is needed in an earlier excitonic model. Our conclusion is that lasing in ZnO nanowires at room temperature is not of excitonic nature, as is often thought, but instead is electron-hole plasma lasing.

We demonstrate the development of high-amplitude picosecond strain pulses in a sapphire single crystal into an ultrafast compressional soliton train. For this purpose, large-intensity light pulses were used to excite a metal film, yielding a 2 orders of magnitude higher strain than that achieved in earlier studies. Propagation of the packets is monitored over a distance of several millimeters by means of Brillouin light scattering. A one-parameter model, based on the Korteweg-de Vries-Burgers equation, simultaneously explains the observed behavior at all strains and temperatures under study. We predict up to 11 solitons in the train, reaching pressures as high as 40 kbar and 0.5 ps temporal widths.

Acoustic solitons formed during the propagation of a picosecond strain pulse in a GaAs crystal with a ZnSe/ZnMgSSe quantum well on top lead to exciton resonance energy shifts of up to 10 meV, and ultrafast frequency modulation, i.e., chirping, of the exciton transition. The effects are well described by a theoretical analysis based on the Korteweg-de Vries equation and accounting for the properties of the excitons in the quantum well.