R. Zimmermann

Dephasing in Quantum Dots: Quadratic Coupling to Acoustic Phonons

A microscopic theory of optical transitions in quantum dots with a carrier-phonon interaction is developed. Virtual transitions into higher confined states with acoustic phonon assistance add a quadratic phonon coupling to the standard linear one, thus extending the independent boson model. Summing infinitely many diagrams in the cumulant, a numerically exact solution for the interband polarization is found. Its full time dependence and the absorption line shape of the quantum dot are calculated. It is the quadratic interaction which gives rise to a temperature-dependent broadening of the zero-phonon line, calculated here for the first time in a consistent scheme.

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.

Optical exciton Aharonov-Bohm effect, persistent current, and magnetization in semiconductor nanorings of type I and II

The optical exciton Aharonov-Bohm effect—i.e., an oscillatory component in the energy of optically active bright states—is investigated in nanorings. It is shown that a small effective electron mass, strong confinement of the electron, and high barrier for the hole, achieved, e.g., by an InAs nanoring embedded in an AlGaSb quantum well, are favorable for observing the optical exciton Aharonov-Bohm effect. The second derivative of the exciton energy with respect to the magnetic field is utilized to extract Aharonov-Bohm oscillations even for the lowest bright state unambiguously. A connection between the theories for infinitesimal narrow and finite width rings is established. Furthermore, the magnetization is compared to the persistent current, which oscillates periodically with the magnetic field and confirms thus the nontrivial connected topology of the wave function in the nanoring.

Brillouin-Wigner perturbation theory in open electromagnetic systems

A Brillouin-Wigner perturbation theory is developed for open electromagnetic systems which are characterised by discrete resonant states with complex eigenenergies. Since these states are exponentially growing at large distances, a modified normalisation is introduced that allows a simple spectral representation of the Greens function. The perturbed modes are found by solving a linear eigenvalue problem in matrix form. The method is illustrated on exactly solvable one- and three-dimensional examples being, respectively, a dielectric slab and a microsphere.

Exciton dephasing in quantum dots due to LO-phonon coupling: An exactly solvable model

It is widely believed that, due to its discrete nature, excitonic states in a quantum dot coupled to dispersionless LO phonons form everlasting mixed states (exciton polarons) showing no line broadening in the spectrum. This is indeed true if the model is restricted to a limited number of excitonic states in a quantum dot. We show, however, that extending the model to a large number of states results in LO phonon-induced spectral broadening and complete decoherence of the optical response.

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.

Optical absorption in quantum dots: Coupling to longitudinal optical phonons treated exactly

Optical transitions in a semiconductor quantum dot are theoretically investigated, with emphasis on the coupling to longitudinal optical phonons, and including excitonic effects. When limiting to a finite number of

Observation of memory effects in electron-phonon quantum kinetics

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Electron and Hole Trions in Wide GaAs Quantum Wells

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

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