E. J. Mayer

Pump-probe investigations of biexcitons in GaAs quantum wells

Abstract The observation of biexcitonic contributions to time-resolved pump-probe measurements of a 25 nm multiple quantum well are reported. Results from wavelength and polarization dependent studies of the differential transmission in the vicinity of the heavy hole exciton transition demonstrate the strong influence of the biexciton state on the nonlinear response. Intensity dependent measurements are consistent with biexciton formation. The previously predicted angle dependence of the beating between light and heavy hole excitons is also observed.

Dephasing of interacting heavy-hole and light-hole excitons in GaAs quantum wells

We describe a novel three-pulse degenerate four-wave mixing configuration that permits the direct observation of the coherence between the σ+ and the σ− exciton states in a GaAs quantum well excited in a two-photon process. It is found that the phase coherence between the two single heavy-hole exciton states decays with a time constant that is considerably longer than the dephasing time of the coherence between these states and the ground state (interband coherence). All the experimental data are well described by numerical solutions of the optical Bloch equations for a phenomenological multilevel model.

Polarization selection rules for quantum beating between light- and heavy-hole excitons in GaAs quantum wells

Abstract We report results on selection rules and polarization conditions for quantum beating between light hole and heavy hole excitons in GaAs quantum wells. The measurements were conducted using time-integrated degenerate-four-wave-mixing in a three-pulse configuration. By choosing particular polarization configurations of the ultrashort optical pulses exciting heavy and light hole excitons simultaneously, degenerate four-wave-mixing signals show either quantum beating or no modulation at all. The experiments are analyzed using a model that takes into account exciton/exciton interaction.

Optical Dephasing of Excitons in III-V Semiconductors

Rapid progress in the development of mode-locked laser systems during the past decade has boosted the time resolution attainable in nonlinear optical spectroscopy well below 100 fs. Ti: sapphire lasers which can directly generate pulses as short as 12 fs [1] mark the most recent milestone of this evolution. This new generation of fs solid state lasers surpasses the older colliding-pulse mode-locked (CPM) dye laser [2] by far with respect to output power and stability, and most importantly, by the broad tunability of the fs output between 680–1000 nm. Combining these lasers with frequency converters like harmonic generators [3] or optical parametric oscillators [4] extends the tunability range to the ultraviolet, visible, near and middle infrared regime.

Polarization dependence of degenerate four-wave mixing on 2D excitons in GaAs quantum wells

The polarization dependence of the coherent optical response of GaAs QWS reveals that modelling of 2D excitons as non-interacting two-level systems is inadequate. Extensions of the model taking into account many-body Coulomb effects and disorder-induced coupling of the valence band states results in a fairly good description of all experimental data.

Influence of Many-Body Effects on the Quantum Coherence of 2D Excitons in GaAs Quantum Wells

The polarization dependence of time-integrated (TI) and time-resolved (TR) degenerate four-wave-mixing (DFWM) on the lowest heavy hole (hh) and light hole (lh) exciton transitions in GaAs quantum wells (QW) demonstrates that heavy and light hole excitons cannot be described in the framework of the two non-interacting three-level systems with opposite circular polarization selection rules suggested by Schmitt-Rink et al.1 In several recent papers, 2–5 we have shown that the discrepancies between experiment and theory can be resolved if many-body Coulomb interactions and disorder-induced coupling of excitonic states are taken into account. Exciton/exciton Coulomb interaction which involves a variation of the local electric fields (LFE), excitation-induced dephasing (EID) and energy shifts of excitonic states, especially the formation of biexcitons (BIF), is phenomenologically implemented into the theory by solving the optical Bloch equations for multi-level model systems which represent single-exciton and two-exciton contributions to the nonlinear optical response by discrete transitions with adjustable oscillator strengths, dephasing rates, and frequencies. Comparison of a large variety of experimental data with theoretical curves calculated with this model demonstrates, in particular, the important contribution of the heavy hole biexciton to the third-order nonlinear optical response of GaAs QW’s. Whereas modulation of the signal with a frequency corresponding to the biexciton binding energy are the unique “fingerprint” for the formation of biexcitons, their importance is further proved by the polarization dependence of the signal strength.

Polarization selection rules for quantum beating between light and heavy holes in GaAs quantum wells

QThB3 Fig. 1. (a) Time integrated cross-correlation function of the femtosecond pump and probe pulses in the mid-infrared, giving time zero and the pulse durations of 300 fs. (b) to (e) Transient increase of HH to SO intervalence band absorption of p-type germanium after femtosecond excitation at E, = 430 meV (X = 2.9 pm, T, = 10 K). The change of absorption AA = -In (T/T,) is plotted versus the delay between pump and probe pulses for probe energies of (b) 445 meV, (c) 430 meV, (d) 415 meV, and (e) 400 meV (To, T transmission before and after excitation). The solid lines represent a numerical calculation with a rise time of 700 fs.