K.-H. Pantke

Exciton dynamics in CdSe

Abstract The time evolution of the adsorption due to exciton-biexciton transitions and the subsequent relaxation, diffusion and recombination of excitons and exciton complexes are investigated in CdSe by differential transmission experiments and by laser-induced grating experiments. Transitions from free excitons and impurity bound excitons as well as direct two-photon absorption (TPA) to the biexciton state are identified. For a quasi-equilibrium situation, a few hundred picoseconds after the excitation, a diffusion coefficient D x = 3.6 cm 2 /s and a recombination lifetime T x = 1500 ps is determined for free excitons at a lattice temperature of 30 K.

NONLINEAR QUANTUM BEAT SPECTROSCOPY IN SEMICONDUCTORS

We review quantum beats from extended electronic states in bulk semiconductors and multiple quantum well structures obtained by nonlinear spectroscopy. The nonlinear methods are degenerate four-wave mixing, photon echo and nonlinear transmission. The role of the spectral resolution of the nonlinear signal emitted from a four-wave mixing process is discussed in detail. Results obtained by linear methods like fluorescence or transmission are briefly discussed.

Bound biexcitons in II-VI semiconductors

Abstract Biexcitons localized at neutral acceptor sites in the direct-gap II–VI semiconductors CdS and CdSe are identified with different spectroscopic techniques such as photoluminescence, two-photon absorption and nonlinear quantum beat spectroscopy (NQBS). The NQBS offers the possibility to distinguish between quantum beats from a three-level system and polarization interference from independent two-level systems. The localized biexciton states are discussed in analogy with excited states of holes in neutral donor complexes.

Energy transfer from excitonic polaritons to deep impurities in CdS

Abstract Energy transfer processes from excitons to deep impurities are investigated by luminescence excitation and absorption spectroscopy. The measured dips and peaks in the excitation spectra indicate a strong competition between different deep centre transitions and are connected with the annihilation mechanism of excitonic polaritons. They can be captured directly by deep impurities from the photonic or excitonic part of the polariton branches, respectively. In addition excitonic polaritons can be absorbed by deep centres after having undergone a multistep process: a polariton can be scattered to another branch and thereby converted from a photon-like to an exciton-like polariton via acoustic phonons. This multistep process is verified by comparing the calculated excitations spectra using the dielectric function and the damping dispersion of excitonic polaritons with our experiments.

Temperature dependent damping dispersion of excitonic polaritons in CdS

Abstract The damping dispersion Γ(ω) is determined by analysing transmission spectra of CdS at temperatures between 2 and 30 K. It is shown that the low energy side of the spectrum is governed by temperature independent impurity scattering of lower branch polaritons. For the high energy side phonon scattering is the dominant process, especially at higher temperatures.

Experimental determination of additional boundary conditions for excitonic polaritons in CdS

Abstract By analyzing simultaneously measured transmission and reflection spectra of thin CdS crystals in the spectral region of excitonic polaritons, we succeded in determining the energy dependence of γ to be proportional to the group velocity dω/dk of the polaritons. Such a dependence can be explained by polariton scattering at charged impurities. With the help of this known energy dependence of the abcs it could be unambiguously shown that Pekars assumption is the only one to explain all the experimental results.