G.R. Hayes

Is Luminescence from Quantum Wells Due to Excitons

Femtosecond luminescence measurements combined with pump-probe experiments using selective excitation of either free carriers or excitons have been performed. The time-resolved luminescence spectra show a striking disparity compared with recently published theoretical models. Large differences in the pump-probe spectra are observed for resonant and non-resonant (continuum) pumping. The origin of these differences is discussed.

Ultrafast Coherent Control of Excitons and Exciton-Polaritons in Quantum Nanostructure

We have demonstrated ultrafast coherent control of excitons and exciton-polaritons in semiconductor quantum nanostructures. An InGaAs/AlGaAs multi-quantum-well Bragg structure and an AlGaAs/GaAs crescent-shaped quantum wire structure were used as samples for the control of the exciton-polariton and excitons in the nanostructure. In the multi-quantum-well Bragg sample, a two-stage decay due to the super-radiant exciton-polariton mode is clearly observed under the resonance conditions. Highly sensitive coherent control measurement enables us to observe the dephasing characteristics in the crescent-shaped quantum wire sample, and a very fast dephasing time T-2 of between 200 fs and 300 fs with powder dependence was observed.

Relevance of Dephasing Processes for the Ultrafast Rise of Emission from Resonantly Created Excitons in Quantum Wells

We present a comparative study of time-integrated four-wave-mixing and femtosecond emission under resonant excitation on excitons in weakly disordered GaAs quantum wells. At highest exciton densities when dephasing dominates the spectral width (homogeneous broadening), we find that the rise time of the incoherent luminescence signal is given by T-2/2. At lowest densities, optical coherence times approach the exciton radiative lifetime (15 to 20 ps). This confirms our previous result that coherent resonant Rayleigh scattering is responsible for the short rise time of the excitonic emission. We also show clear evidence for dephasing due to exciton-phonon interaction, as the rise time of the emission decreases dramatically when the sample temperature is increased.

1.5-µm Intraband Transitions in PbSe Quantum Dots

Intraband transitions around 1.5 mum are studied in PbSe quantum dots at room temperature. Femtosecond pump-probe measurements reveal induced absorption by photoexcited carriers up to higher energy levels and its relaxation. A dominant decay component has a decay time of 10-40 ps, which is determined by the relaxation of carriers at the lowest absorption peak. The induced absorption is observed in a broad spectral range between 0.77 and 2.5 eV.

Speckle-averaged resonant Rayleigh scattering from quantum well excitons

Summary form only given. We have measured the ultrafast resonant Rayleigh scattering from quantum well excitons using Fourier transform spectral interferometry. Our novel approach enables us to simultaneously measure a statistically significant number of speckles with one hundred femtosecond temporal resolution. We show that theories based on ensemble-averaging can be compared directly with the experimental resonant Rayleigh scattering dynamics. We also deduce the importance of other contributions to the total secondary emission.

Direct observation of the decay of a coherent polarization into an incoherent exciton population in quantum wells

Summary form only given. We experimentally demonstrate the validity of the concept that separates coherent polarization and incoherent exciton population. We observed directly the transfer of polarization into population. The decay of the microscopic polarization builds up the population, but the macroscopic polarization in form of Rayleigh scattering decays faster when inhomogeneous broadening dominates.

Ultrafast secondary radiation of excitons in quantum wells: The transition from the coherent to the incoherent regime

Coherent Rayleigh scattering and incoherent luminescence are studied by femtosecond upconversion under resonant excitation as a function of exciton density in GaAs quantum wells. Comparing samples with different well widths allows to establish the relation between inhomogeneous broadening and the shape of resonant Rayleigh scattering. We find an excellent agreement between experiment and a classical model. In samples with moderate inhomogeneous broadening we provide evidence for the transition from coherent to incoherent emission. The coinparison with excitonic dephasing times T2 obtained from four-wave-mixing experiments demonstrates that the incoherent exciton population rises within T2/2, at any exciton density when changed by more than an orders of magnitude. The experimental results confirm very clearly the concept predicting that the coherent polarization made up by localized excitons decays into an incoherent exciton population under conservation of the number of excited quanta.