We report measurements of ultralong coherence in self-assembled quantum dots. Transient four-wave mixing experiments at 5 K show an average dephasing time of 372 ps, corresponding to a homogeneous linewidth of 3.5 microeV, which is significantly smaller than the linewidth observed in single-dot luminescence. Time-resolved luminescence measurements show a lifetime of the dot ground state of 800 ps, demonstrating the presence of pure dephasing at finite temperature. The homogeneous width is lifetime limited only at temperatures approaching 0 K.

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We report the first investigation of resonant secondary emission from quantum well excitons using ultrafast spectral interferometry. Observation of high-contrast spectral fringes demonstrates a significant phase correlation of the spectral components of the Rayleigh scattered light and allows us to determine the temporal dynamics of the coherent emission. The results contradict the present theories for resonant Rayleigh scattering based on ensemble averaging, and show the nonergodic nature of resonance Rayleigh scattering from semiconductor quantum wells. [S0031-9007(98)07109-9] PACS numbers: 78.35. + c, 78.47. + p, 78.66. -w The nature of scattered light has been a topic of interest for almost a century. Rayleigh scattering has been a wellestablished concept since the pioneering work of Lord Rayleigh on the scattering of sunlight by atmospheric index fluctuations on a subwavelength scale [1]. A central issue in the theoretical treatment of scattered light has been the problem of how to describe scattering from a large number of scatterers. One usually resorts to statistical methods, where ensemble averaging is often the only way to get meaningful experimental predictions [2]. Interference between fields from different scatterers is ignored in the ensemble averaging process and only the incoherent sum of intensities from each scatterer is considered. All effects of spatial coherence-the ability of the ensemble of scatterers to form speckles-are thus lost [3]. Theories based on ensemble averaging postulate therefore a zero expectation value for the scattered field amplitude in order to calculate a well-defined expectation value for the scattered intensity. The motivation for using ensemble averages lies in the "ergodic hypothesis," which states that a time average for a single realization of the ensemble is identical to an ensemble average. Whether the ergodic theorem can be used depends on the specific experimental situation. In particular, the typical fluctuations of the field must be faster than the time resolution of the actual experiment [4,5].The properties of secondary emission (i.e., emission in nonspecular directions) from quantum well excitons following ultrafast resonant excitation have recently been the focus of an intense scientific debate [6][7][8][9]. It is now commonly accepted that the emission is composed of exciton luminescence and resonance Rayleigh scattering (RRS). The origin of the RRS is the spatially fluctuating exciton resonance frequency causing the inhomogeneous broadening of the absorption line [10]. The experimental distinction between luminescence and RRS is difficult due to their spectral similarities. However, it was inferred that the rise of the secondary emission is dominated by RRS at low exciton densities by comparing experimental data with a theoretical model based on ensemble averaging [7,9].Whether models based on ensemble averages can provide meaningful experimental predictions for the femtosecond dynamics of RRS is not clear. An understanding of the dynamics of RRS is o...

Using a model structure for a two-dimensional ͑2D͒ biexciton confined in a quantum well, it is shown that the form of the Hamiltonian of the 2D biexciton reduces into that of an exciton. The binding energies and Bohr radii of a 2D biexciton in its various internal energy states are derived analytically using the fractional dimension approach. The ratio of the binding energy of a 2D biexciton to that of a 2D exciton is found to be 0.228, which agrees very well with the recent experimental value. The results of our approach are compared with those of earlier theories. ͓S0163-1829͑96͒02623-9͔

Abstract-A novel design strategy for the epitaxial structure of monolithic mode-locked semiconductor lasers is presented. Using an all-active design, we fabricate 40-GHz lasers generating 2.8-ps almost chirp-free pulses with record low high-frequency jitter and more than 7-mW fiber coupled output power.