Exciton formation and relaxation in GaAs epilayers

Exciton formation and relaxation in GaAs bulk epilayers have been studied by means of time-resolved photoluminescence techniques. It is found that the time evolution of the free exciton luminescence, nonresonantly excited at low temperature and low intensity, is extremely slow, with a rise time of the order of 1 ns and a decay time of several ns. Simulations based on Monte Carlo solution of the set of coupled Boltzmann-like equations for free carriers and excitons show a nice agreement with the experimental data, and suggest a dominant role played by acoustic phonons in the exciton relaxation

Ultrafast terahertz probes of transient conducting and insulating phases in an electron–hole gas

Many-body systems in nature exhibit complexity and self-organization arising from seemingly simple laws. The long-range Coulomb interaction between electrical charges generates a plethora of bound states in matter, ranging from the hydrogen atom to complex biochemical structures. Semiconductors form an ideal laboratory for studying many-body interactions of quasi-particles among themselves and with lattice vibrations and light. Oppositely charged electron and hole quasi-particles can coexist in an ionized but correlated plasma, or form bound hydrogen-like pairs called excitons which strongly affect physical properties. The pathways between such states however remain elusive in near-visible optical experiments that detect a subset of excitons with vanishing center-of-mass momenta. In contrast, transitions between internal exciton levels which occur in the far-infrared at terahertz (10 s) frequencies are in dependent of this restriction suggesting their use as a novel pro be of pair dynamics. Here, we employ an ultrafast terahertz probe to directly investigate the dynamical interplay of optically-generated excitons and unbound electron-hole pairs in GaAs quantum wells. Our observations witness an unexpected quasi-instantaneous excitonic enhancement, reveal formation of insulating excitons on a hundred picosecond timescale and manifest conditions under which excitonic populations prevail

Ultrafast dynamics and modelling of Semiconductor Optical Amplifiers for WDM applications

We have developped a research program aimed at the study of the ultrafast dynamical properties of Semiconductor Optical Amplifiers (SOAs). This program is run in collaboration with the group of H. Melchior, and of H. Jaeckel in Zurich and the group of F. Devaux at Alcatel. Amongst the properties of these devices, we specifically study their possibilities in terms of switching and wavelength conversion. To this aim, we have developped a femtosecond setup which allows to probe the cross gain dynamics of the amplifiers. A cw beam is fed into the amplifier and is modified by a strong cross-polarized pump beam. The changes of the cw beam are then time resolved with 150 fs resolution in a non linear gate. Such a set-up corresponds to the configuration used for wavelength conversion, and has the great interest to provide at the same time the time resolution and the relative tunability of the two laser beams over a wide range.</jats:p

Phonon-assisted exciton formation and relaxation in GaAs/AlxGa1-xAs quantum wells

A microscopic analysis of exciton formation and relaxation in photoexcited quantum wells is presented. The theoretical approach is based on a Monte Carlo simulation of the coupled free-carrier and exciton dynamics, and includes various mechanisms contributing to exciton formation and relaxation. Our investigation clarifies the ori,ain of excitonic luminescence in time-resolved experiments. In particular, we address the problem of the relative efficiencies of exciton formation assisted by either LO phonons or acoustic phonons, respectively

Ultrafast probes of electron–hole transitions between two atomic layers

The investigation into the dynamical transitions of charged quasiparticles on interfaces remains technically challenging. Here, the authors use ultrafast, mid-infrared micro-spectroscopy to unveil the formation of tightly bound interlayer excitons between conducting graphene and semiconducting MoSe2

The plasticity of fungal interactions

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