The excited-state structure and energy-transfer dynamics, including their dependence on temperature and redox conditions, were studied in chlorosomes of the green sulfur bacterium Chlorobium tepidum at low temperatures by two independent methods:  spectral hole burning in absorption and fluorescence spectra and isotropic one-color pump−probe spectroscopy with ∼100 fs resolution. Hole-burning experiments show that the lowest excited state (LES) of BChl c aggregates is distributed within approximately 760−800 nm, while higher excitonic states of BChl c (with absorption maximum at 750 nm) possess the main oscillator strength. The excited-state lifetime determined from hole-burning experiments at anaerobic conditions was 5.75 ps and most likely reflects energy transfer between BChl c clusters. Isotropic one-color absorption difference signals were measured from 720 to 790 nm at temperatures ranging from 5 to 65 K, revealing BChl c photobleaching and stimulated emission kinetics with four major components, with lifetimes of 200−300 fs, 1.7−1.8 ps, 5.4−5.9 ps, and 30−40 ps at anaerobic conditions. The lifetimes are attributed to different relaxation processes of BChl c, taking into account their different spectral distributions as well as limitations arising from results of hole burning. Evidence for at least two spectral forms of BChl c in chlorosome is reported. There is a striking similarity between the spectrum and kinetics of the 5.4−5.9 ps component with those of the LES determined from hole burning. A pronounced change of isotropic decays was observed at around 50 K. The temperature dependence of the isotropic decays is correlated with temperature-dependent changes of BChl c fluorescence emission. Further, the temperature decrease leads to an increase in the relative amplitude of the 200−300 fs component. At aerobic conditions, both hole burning and pump−probe spectroscopy show that the lifetime of the LES shortens to ∼2.6 ps, as a result of excitation quenching by a mechanism presumably protecting the cells against superoxide-induced damage. This mechanism operates on at least two levels, the second one being characterized by a 14−16 ps lifetime.

To investigate the properties of a two-dimensional carrier gas at intermediate densities, we perform picosecond transient four-wave mixing experiments on trions ͑charged excitons͒ and neutral excitons in modulation p-doped CdTe/Cd 1ϪxϪy Mg x Zn y Te quantum wells. The determination of trion and exciton dephasing rates reveals a localization of both trions and holes in potential fluctuations induced by the ionized remote acceptors. We demonstrate that trions can be efficiently used as a charged optical probe sensitive to electrostatic potential fluctuations which are imperceptible for neutral excitons. ͓S0163-1829͑99͒15731-X͔ Modifications of optical spectra due to the introduction of carriers into a semiconductor quantum well ͑QW͒ recently have been of considerable interest, in particular as they provide information on the electronic properties in the vicinity of the metal-insulator transition, and on the role of disorder. 1 It has been known for a long time that excitons characterize the optical spectra of undoped semiconductors while bandto-band transitions are observed on strongly doped samples. It has been recognized more recently that spectra of moderately doped QW's feature also charged excitons, either positively or negatively according to the type of doping-the so-called trions. 1-5 Extensive investigations have been reported on the binding energy of trions, 2-5 on their polarization in a magnetic field, 2,3 and on their behavior in an electric field 6 and at different carrier densities. 1,3,4,7 Up to now, however, only very few papers have been devoted to the dynamics of trions, including transport and relaxation. [7][8][9] Actually, there has been some controversy in the literature about the possible localization of trions in modulation doped QW's ͑MDQW͒, in which carrier gases are formed in the QW's by introducing doping layers in the barriers. It was argued that free carriers would screen the Coulomb interaction thus forbidding the formation of excitons and trions. 1,2 Hence the charge carriers should be localized by electrostatic potential fluctuations caused by the ionized remote donors or acceptors. Recent near-field photoluminescence experiments 10 directly evidence the localization of trions in n-doped GaAs QW's and conclude indirectly to the localization of the carriers themselves. Other authors 3,11 have reported the observation of free trions in high-quality MDQW's with relatively thick spacer layers, which decreases the fluctuation depth.In this paper we demonstrate that the study of the coherence properties of trions and excitons by degenerate fourwave mixing ͑FWM͒ is a very efficient method to investigate a possible localization of both trions and carriers in a MDQW. The idea is as follows: First, localized quasiparticles have a strongly reduced scattering efficiency as compared to free ones. Second, this scattering plays an important role in the dephasing of the quasiparticles. Consequently, the determination of exciton and trion dephasing rates under different conditions ͑exciton, trion a...

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