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Ultrafast excited-state dynamics in chlorosomes isolated from the photosynthetic filamentous green bacterium <i>Chloroflexus aurantiacus</i>
摘要： Bacteriochlorophyll (BChl) c pigments in the aggregated state are responsible for efficient light harvesting in chlorosomes of the filamentous anoxygenic photosynthetic bacterium, Chloroflexus (Cfx.) aurantiacus. Absorption of light creates excited states in the BChl c aggregates. After sub-picosecond intrachlorosomal energy transfer, redistribution and relaxation, the excitation is transferred to the BChl a complexes and further to reaction centres on the picosecond time scale. In this work, the femtosecond excited state dynamics within BChl c oligomers of isolated Cfx. aurantiacus chlorosomes was studied by double difference pump-probe spectroscopy at room temperature. Difference (Alight ? Adark) spectra corresponding to excitation at 725 nm (blue side of the BChl c absorption band) were compared with those corresponding to excitation at 750 nm (red side of the BChl c absorption band). A very fast (time constant 70 ± 10 fs) rise kinetic component was found in the stimulated emission upon excitation at 725 nm. This component was absent at 750-nm excitation. These data were explained by the dynamical red shift of the stimulated emission due to excited state relaxation. The nature and mechanisms of the ultrafast excited state dynamics in chlorosomal BChl c aggregates are discussed.
关键词： excited-state dynamics,Chloroflexus aurantiacus,Bacteriochlorophyll,chlorosomes,pump-probe spectroscopy
更新于2025-09-09 09:28:46
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Nonequilibrium electron dynamics in pump-probe spectroscopy: Role of excited phonon populations
摘要： We study the role of excited phonon populations in the relaxation rates of nonequilibrium electrons using a nonequilibrium Green’s function formalism. The transient modifications in the phononic properties are accounted for by self-consistently solving the Dyson equation for the electron and phonon Green’s functions. The pump-induced changes manifest in both the electronic and phononic spectral functions. We find that the excited phonon populations suppress the decay rates of nonequilibrium electrons due to enhanced phonon absorption. The increased phonon occupation also sets the nonequilibrium decay rates and the equilibrium scattering rates apart. The decay rates are found to be time dependent, and this is illustrated in the experimentally observed population decay of photoexcited Bi1.5Sb0.5Te1.7Se1.3.
关键词： relaxation rates,nonequilibrium electron dynamics,excited phonon populations,nonequilibrium Green’s function formalism,pump-probe spectroscopy
更新于2025-09-09 09:28:46
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Fano resonance between coherent acoustic phonon oscillations and electronic states near the bandgap of photoexcited GaAs
摘要： Impulsive photo-excitation of solids results in a travelling strain pulse which manifests itself as coherent acoustic phonon oscillations. These oscillations have been extensively studied using time-resolved pump-probe spectroscopy. In the present work, we report the generation of extremely long-lived, coherent longitudinal acoustic phonon oscillations in intrinsic GaAs (100), with clear and unambiguous evidence of Fano interference between these oscillations and the continuum of electronic states close to the bandgap. Fano resonance is a widespread phenomenon observed in atomic systems and condensed media that arises from quantum interference between a continuum of quantum states and a discrete quantum state. Among other techniques, Fano resonance has been investigated with respect to optical phonons studied with Raman Spectroscopy. In the present work, we investigate Fano resonance in coherent phonon oscillations generated without the aid of any capping layer, dopants or substrate/interface effects. Since Fano resonance is sensitive to changes in electronic structure, doping and defects, these observations are important to the field of picosecond ultrasonics which is used for non-destructive depth profiling of solids and for carrier diffusion studies.
关键词： GaAs,coherent acoustic phonon oscillations,Fano resonance,quantum interference,pump-probe spectroscopy
更新于2025-09-04 15:30:14
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Measurement of ultrafast dynamics of photoexcited carriers in <i>β</i> -Ga <sub/>2</sub> O <sub/>3</sub> by two-color optical pump-probe spectroscopy
摘要： We report results from ultrafast two-color optical pump-probe spectroscopy on bulk b-Ga2O3. A two-photon absorption scheme is used to photoexcite carriers with the pump pulse and free-carrier absorption of the probe pulse is used to record the subsequent dynamics of the photoexcited carriers. Our results are consistent with carrier recombination via defect-assisted processes. We also observe transient polarization-selective optical absorption of the probe pulse by defect states under nonequilibrium conditions. A rate equation model for electron and hole capture by defects is proposed and used to explain the data. Whereas the rate constants for electron capture by defects are found to be temperature-independent, they are measured to be strongly temperature-dependent for hole capture and point to a lattice deformation/relaxation process accompanying hole capture. Our results shed light on the mechanisms and rates associated with carrier capture by defects in b-Ga2O3.
关键词： two-color optical pump-probe spectroscopy,ultrafast dynamics,photoexcited carriers,carrier recombination,defect-assisted processes,β-Ga2O3
更新于2025-09-04 15:30:14