Abstract:The behavior of carrier-carrier screemng IS investigated in a GaAs-GaAIAs quantum well structure by measuring the band-to-band polarization dephasing with femtosecond photon echoes. The variation of the electron-hole polarization dephasing time with the carrier concentration reveals the two-dimensional character of the short range screening between the interacting carriers. 1The phase coherence time between an electron and the corresponding hole created during a band-to-band optical transition in a semiconductor decreases with increasing carrier density due to the effects of carrier-carrier scattering. At low carrier densities, ignoring the Coulomb interaction between carriers, the carrier-carrier scattering time is expected to scale inversely with the carrier density. At higher densities, screening due to the Coulomb potential of the charged particles will act to suppress the process of carrier-carrier scattering. Coulomb screening, which was originally studied in metals [1], depends on the electron and hole environment and therefore is a function of the dimensionality of the system under study.We report here measurements that show the influence of carrier-carrier interactions on the dephasing processes which occur in a GaAs-GaAlAs Quantum Well (QW) structure. Due to the confmement of the carriers in the well, they are expected to behave like a two-dimensional (2D) electron gas. Indeed, our measurements performed at different carrier densities indicate a screening behavior that reveals the bi-dimensionality of the system. The 2D screening that we deduce from the experiment is well understood if we assume that a given carrier interacts only with its nearest neighbors, indicating that the range of the screened Coulomb interaction is of the order of the average inter-carrier spacing. We discuss the validity of this nearest-neighbor interaction picture with respect to the experimental conditions in which a nonequilibrium, nondegenerate electron-hole population is created over a large range of energies and wave vectors.Femtosecond optical spectroscopy now allows the dynamics of energy relaxation and dephasing of excited carriers in bulk semiconductors or 2D QW structures to be studied with 10 fs resolution [2]. In bulk GaAs it is known that under high densities of excitation, carriercarrier scattering plays an important role in the thermalization of hot carriers (carriers with high excess energy with respect to the bottom of the conduction or valence band [3]), and it has been shown with both photon echo [4] and time-resolved polarization rotation measurements 2 [5,6] that it is also the dominant mechanism for momentum redistribution. In QW structures the carrier therrnalization has been extensively studied in undoped [7,8] and doped [9,10] structures, where the imponance of collisions between carriers was found to be the main process for the redistribution of energy through inelastic carrier-carrier scattering.Recent work [8,9,10] has also shown that phase space filling due to the Pauli exclusion principle...

The electronic dephasing of large molecules in solution is investigated with three-pulse photon echoes generated by 6-fs optical pulses. The observed dephasing behavior, which is analyzed with a model based on a stochastic coupling of the molecules with their heat bath, is clearly shown to be non-Markovian. Moreover, the correlation function of the fluctuations asymptotically approaches an exponential with characteristic times varying from 20 to 80 fs, depending on the solvent environment.PACS numbers: 42.50.Md Ultrafast spectroscopy performed with femtosecond optical pulses has provided important new information on the nature of the optical absorption of large dye molecules in solution. Spectral hole-burning and coherent transient measurements have been used to determine the degree of inhomogeneity of the optical-absorption spectrum and the appropriate statistical description of the molecule in its solvent environment. Hole-burning experiments 1 and two-pulse photon-echo experiments 2 have clearly shown the large molecules Nile Blue and Malachite Green to be inhomogeneously broadened. In this paper we present experimental results and analysis which go beyond the interpretation of the data in terms of a relaxation-time approximation to directly measure the frequency fluctuation correlation function with a threepulse photon-echo technique using 6-fs optical pulses.Weiner, Silvestri, and Ippen 3 have previously attempted to measure a three-pulse photon echo using pulses an order of magnitude longer than those used in this experiment, but were unable to resolve the polarization dephasing. Rosker, Wise, and Tang 4 and Chesnoy and Mokhtari 5 have observed what they term "quantum beats" using transient absorption spectroscopy. Although these experiments were originally interpreted in terms of coherence between electronic levels in the dye molecules, recent measurements 6 and theory 7 have shown that the observed oscillations are the result of wave-packet motion brought about by the coherent excitation of the manifold of vibrational levels. Most recently, Nibbering, Duppen, and Wiersma 8 studied optical dephasing in solution using line-shape analysis and resonance light-scattering measurements.We use femtosecond optical techniques to study the interactions of the dye molecule with its environment and the mechanisms responsible for the loss of electronic coherence. Such interactions may be explicitly analyzed by molecular-dynamic simulations, but usual tractable models collect all the degrees of freedom of the solute and solvent into a coupling with a thermal bath. With this type of description, the dephasing behavior of the molecules is determined by the statistical properties of the molecule-heat-bath coupling. The simplest model of a dephasing process assumes that the correlation time T C of the fluctuations of the molecule electronic gap is much shorter than Ti (the limit r c =0 corresponds to the particular Markovian case of a ^-correlation function). In general, the statistics are more complicated and may or may no...

We investigate the k-space scattering dynamics of carriers in GaAs by measuring the transient relaxation of an anisotropic population produced by a linearly polarized 9 femtosecond infrared pulse. The redistribution of carrier momentum is shown to occur in the first tens of femtoseconds after excitation, and the relaxation times are found to vary with carrier density, from 25 to 60 fs for a carrier density range from 8×1017 to 8×1016 cm−3. Our results indicate that carrier momentum is rapidly redistributed via carrier–carrier scattering, and the nature of the density dependence suggests that the scattering is mediated by a screened Coulomb interaction between the carriers.

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