E. Runge

Ultrafast Manipulation of Strong Coupling in Metal−Molecular Aggregate Hybrid Nanostructures

We demonstrate an ultrafast manipulation of the Rabi splitting energy Ω(R) in a metal-molecular aggregate hybrid nanostructure. Femtosecond excitation drastically alters the optical properties of a model system formed by coating a gold nanoslit array with a thin J-aggregated dye layer. Controlled and reversible transient switching from strong (Ω(R) ≃ 55 meV) to weak (Ω(R) ≈ 0) coupling on a sub-ps time scale is directly evidenced by mapping the nonequilibrium dispersion relations of the coupled excitations. Such a strong, externally controllable coupling of excitons and surface plasmon polaritons is of considerable interest for ultrafast all-optical switching applications in nanoscale plasmonic circuits.

Excitons in Narrow Quantum Wells: Disorder Localization and Luminescence Kinetics

For narrow quantum wells, the energetic spread due to interface and alloy disorder is large enough to dominate the excitonic optical spectra. The resulting disorder potential enters the center-of-mass Schrodinger equation. We derive a kinetic equation for the exciton distribution over disorder eigenstates including acoustic phonon scattering and radiative recombination. The photoluminescence lineshape as function of temperature is studied. Both peak shift and width exhibit a nonmonotonous temperature dependence which is due to phonon-assisted thermal activation of localized excitons. A sharp low-energy drop in the photoluminescence excitation spectra can be understood in terms of a ‘relaxation mobility edge’.

Theory of Trion Spectra in Semiconductor Nanostructures

A consistent density-matrix approach for the absorption spectra of semiconductor nanostructures at finite carrier densities is presented that describes both bound and scattering three-particle states (trions). It automatically includes the two-particle bound states (excitons). In optical transitions, the initial electron or hole momentum is transferred to the final trion state, which - due to recoil energy - gives rise to low-energy tails at the trion and exciton line. A high-energy tail at the exciton is due to the exciton-electron scattering states (trion continuum). The incomplete transfer of exciton oscillator strength to the main trion line and the trion continuum is exemplified by a quantum wire calculation.

Spatially Resolved Spectra, Effective Mobility Edge, and Level Repulsion in Narrow Quantum Wells

Exciton spectra of narrow quantum wells with non-ideal interfaces are determined by the nature of the disorder-localized quantum mechanical eigenstates. We point out that the level statistics of spatially resolved spectra allows an unexpected approach to estimate the size of the quantum mechanical center-of-mass wavefunctions. In photoluminescence, the exciton relaxation has to be considered, too. We present large scale simulations for a model including static interface-related disorder and acoustic phonon scattering. Strong and systematic deviations between calculated photoluminescence excitation spectra and absorption are found. A sharp drop on the low-energy side of the former is discussed in terms of an effective mobility edge for exciton relaxation below which excitons cannot relax further within their radiative lifetime

Excitons in semiconductor nanostructures

Publisher Summary This chapter summarizes the comprehensive understanding of linear excitonic spectra from quantum nanostructures with disorder. The description of the relevant elementary processes has allowed quantitative predictions for new experiments and analysis. Exciton dispersions and wave functions for ideal quantum structures are fully accounting for the k . p valence band structure. The kinetic exciton mass is necessary for any modeling of disorder effects. Besides strong asymmetry with a tail toward higher energies, the explicit relation between the line width and the underlying disorder strength and the two stages of motional narrowing are important for quantitative understanding and description of interface disorder. The amount of level repulsion at a given spatial resolution permits derivation of a characteristic size of the wave functions. The distribution of wave function values changes dramatically within the inhomogeneous exciton line.

Trions in GaAs quantum wells: Photoluminescence lineshape analysis

Trion photoluminescence spectra were measured in undoped high-quality GaAs quantum wells. The lineshape is asymmetric with a temperature-dependent tail towards lower photon energies. We have solved numerically the trion Schrodinger equation in the quantum well and found good agreement for both the trion binding energy and the luminescence lineshape at different temperatures. To analyse the relative weight of trion and exciton emission, an equilibrium mass-action law for the respective densities is studied. Using calculated radiative lifetimes, the emission decay after pulsed excitation is found to deviate strongly from a single-exponential shape in case the trions are initially saturated.

Theory of resonant secondary emission: Rayleigh scattering versus luminescence

Publisher Summary This chapter discusses the theory of resonant secondary emission. The light emission after optical excitation in a semiconductor quantum structure is dominated by exciton effects. Excitons, that is, coulomb bound states of electron and hole, determine the optical properties of semiconductors near the fundamental band edge. They show up as distinct lines in absorption, reflection, and photoluminescence (PL). For understanding the interplay between homogeneous and inhomogeneous broadening of lines, excitons in quantum wells with interface roughness are investigated theoretically. Using the disorder eigenstates as basis, a density matrix theory is derived for excitons in interaction with the light field and with acoustic phonons. The secondary emission is decomposed into the coherent part (Rayleigh scattering) and the incoherent part (photoluminescence). This distinction is based on the speckle analysis. Quantum mechanical features such as level repulsion and enhanced resonant backscattering are discussed. The polarization dependence of the emission is related to the exchange (spin) splitting of anisotropic exciton states. The interplay of disorder and polaritonic effects is exemplified for the time-dependent emission.

Center-of-mass properties of the exciton in quantum wells

We present high-quality numerical calculations of the exciton center-of-mass dispersion for

Electron and Hole Trions in Wide GaAs Quantum Wells

{\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}

Spin relaxation without coherence loss: Fine-structure splitting of localized excitons

quantum wells of widths in the range 2--20 nm. The