arXiv subjects

S. W. Koch

Publications and source records attributed to S. W. Koch.

At least 37 records · Page 2Linked to original sources

arXiv2012

Observation of Coulomb-Assisted Dipole-Forbidden Intraexciton Transitions in Semiconductors

We use terahertz pulses to induce resonant transitions between the eigenstates of optically generated exciton populations in a high-quality semiconductor quantum-well sample. Monitoring the excitonic photoluminescence, we observe transient quenching of the $1s$ exciton emission, which we attribute to the terahertz-induced $1s$-to-$2p$ excitation. Simultaneously, a pronounced enhancement of the $2s$-exciton emission is observed, despite the $1s$-to-$2s$ transition being dipole forbidden. A microscopic many-body theory explains the experimental observations as a Coulomb-scattering mixing of the 2$s$ and 2$p$ states, yielding an effective terahertz transition between the 1$s$ and 2$s$ populations.

cond-mat.mes-hall↗

arXiv2011

Optical Response and Ground State of Graphene

The optical response and the ground state of graphene and graphene-like systems are determined self-consistently. Deriving equations of motion for the basic variables, graphene Bloch equations are introduced and combined with a variational Ansatz for the ground state. Within the Hartree--Fock approximation, this approach reproduces the gap equation for the ground state. The results show that the Coulomb interaction drastically influences the optical response of graphene and introduces an extremely sensitive dependency on the dielectric environment via screening. Regarding the effective fine-structure constant as control parameter, a transition from a semimetal to an excitonic insulator is predicted as soon as the effective graphene fine-structure constant exceeds a value of roughly 0.5. Above this critical value, the computed optical spectra exhibit a pseudogap and several bright $p$-like excitonic resonances.

cond-mat.mtrl-sci↗

arXiv2011

Excitons in Graphene and the Influence of the Dielectric Environment

The exciton Wannier equation for graphene is solved for different background dielectric constants. It is shown that freestanding graphene features strong Coulomb effects with a very large exciton binding energy exceeding $3\,$eV. A second-order transition to a weak Coulomb regime is found if the effective background dielectric constant exceeds a critical value. All bound-state solutions vanish for epitaxial graphene on a substrate with large background permittivity, such as SiC.

cond-mat.mtrl-sci↗

arXiv2010

Anisotropic ultrafast electron dynamics induced by high-field terahertz pulses in n-doped InGaAs

The anisotropic effective mass of electrons is directly measured using time-resolved THz- pump/THz-probe techniques in a n-doped InGaAs semiconductor thin film. A microscopic theory is used to attribute this anisotropy in the THz probe transmission to the nonparabolicity of the conduction band. Self-consistent light-matter coupling is shown to contribute significantly to the THz response.

cond-mat.mtrl-sci↗

arXiv2009

Extraction of many-body configurations from nonlinear absorption in semiconductor quantum wells

Detailed electronic many-body configurations are extracted from quantitatively measured timeresolved nonlinear absorption spectra of resonantly excited GaAs quantum wells. The microscopic theory assigns the observed spectral changes to a unique mixture of electron-hole plasma, exciton, and polarization effects. Strong transient gain is observed only under co-circular pump-probe conditions and is attributed to the transfer of pump-induced coherences to the probe.

cond-mat.mes-hall↗

arXiv2008

Characterization of strong light-matter coupling in semiconductor quantum-dot microcavities via photon-statistics spectroscopy

It is shown that spectrally resolved photon-statistics measurements of the resonance fluorescence from realistic semiconductor quantum-dot systems allow for high contrast identification of the two-photon strong-coupling states. Using a microscopic theory, the second-rung resonance is analyzed and optimum excitation conditions are determined. The computed photon-statistics spectrum displays gigantic, experimentally robust resonances at the energetic positions of the second-rung emission.

quant-ph↗

arXiv2007

Analytical Analysis of Single-Photon Correlations Emitted by Disordered Semiconductor Heterostructures

In a recent publication [Phys. Rev. Lett. 97, 227402 (2006), cond-mat/0611411], it has been demonstrated numerically that a long-range disorder potential in semiconductor quantum wells can be reconstructed reliably via single-photon interferometry of spontaneously emitted light. In the present paper, a simplified analytical model of independent two-level systems is presented in order to study the reconstruction procedure in more detail. With the help of this model, the measured photon correlations can be calculated analytically and the influence of parameters such as the disorder length scale, the wavelength of the used light, or the spotsize can be investigated systematically. Furthermore, the relation between the proposed angle-resolved single-photon correlations and the disorder potential can be understood and the measured signal is expected to be closely related to the characteristic strength and length scale of the disorder.

cond-mat.dis-nn↗

arXiv2006

Characterization of Disorder in Semiconductors via Single-Photon Interferometry

The method of angular photonic correlations of spontaneous emission is introduced as an experimental, purely optical scheme to characterize disorder in semiconductor nanostructures. The theoretical expression for the angular correlations is derived and numerically evaluated for a model system. The results demonstrate how the proposed experimental method yields direct information about the spatial distribution of the relevant states and thus on the disorder present in the system.

cond-mat.dis-nn↗

arXiv2006

Influence of bound and unbound electron-hole-pair populations on the excitonic luminescence in semiconductor quantum wells

A fully microscopic theory for the spontaneous emission from semiconductors is discussed. The theory is evaluated for a quantum-well system and the role of excitonic and unbound electron-hole-pair contributions to the emission is analyzed. Simplifying aproximations to the full theory and their range of validity are discussed. Numerical solutions are presented for experimentally relevant situations and it is shown that a detailed analysis of measured spectra requires the knowledge of both absorption and emission under identical conditions.

cond-mat.mes-hall↗

arXiv2006

Microscopic modeling of photoluminescence of strongly disordered semiconductors

A microscopic theory for the luminescence of ordered semiconductors is modified to describe photoluminescence of strongly disordered semiconductors. The approach includes both diagonal disorder and the many-body Coulomb interaction. As a case study, the light emission of a correlated plasma is investigated numerically for a one-dimensional two-band tight-binding model. The band structure of the underlying ordered system is assumed to correspond to either a direct or an indirect semiconductor. In particular, luminescence and absorption spectra are computed for various levels of disorder and sample temperature to determine thermodynamic relations, the Stokes shift, and the radiative lifetime distribution.

cond-mat.dis-nn↗

arXiv2005

Many-Body Dynamics and Exciton Formation Studied by Time-Resolved Photoluminescence

The dynamics of exciton and electron-hole plasma populations is studied via time-resolved photoluminescence after nonresonant excitation. By comparing the peak emission at the exciton resonance with the emission of the continuum, it is possible to experimentally identify regimes where the emission originates predominantly from exciton and/or plasma populations. The results are supported by a microscopic theory which allows one to extract the fraction of bright excitons as a function of time.

cond-mat.mes-hall↗

arXiv2005

Microscopic analysis of the coherent optical generation and the decay of charge and spin currents in semiconductor heterostructures

The coherent optical injection and temporal decay of spin and charge currents in semiconductor heterostructures is described microscopically, including excitonic effects, carrier LO-phonon and carrier-carrier scattering, as well as nonperturbative light-field-induced intraband and interband excitations. A nonmonotonous dependence of the currents on the intensities of the laser beams is predicted. Enhanced damping of the spin current relative to the charge current is obtained as a consequence of spin-dependent Coulomb scattering.

cond-mat.other↗

arXiv2005

Polarization Conversion and "Focusing" of Light Propagating through a Small Chiral Hole in a Metallic Screen

Propagation of light through a thin flat metallic screen containing a hole of twisted shape is sensitive to whether the incident wave is left or right circularly polarized. The transmitted light accrues a component with handedness opposite to the incident wave. The efficiency of polarization conversion depends on the mutual direction of the hole's twist and the incident light's wave polarization handedness and peaks at a wavelength close to the hole overall size. We also observed a strong transmitted field concentration at the center of the chiral hole when the handedness of the chiral hole and the wave's polarization state are the same.

cond-mat.mtrl-sci↗

arXiv2004

Photoluminescence and Terahertz Emission from Femtosecond Laser-Induced Plasma Channels

Luminescence as a mechanism for terahertz emission from femtosecond laser-induced plasmas is studied. By using a fully microscopic theory, Coulomb scattering between electrons and ions is shown to lead to luminescence even for a spatially homogeneous plasma. The spectral features introduced by the rod geometry of laser-induced plasma channels in air are discussed on the basis of a generalized mode-function analysis.

physics.plasm-ph↗

arXiv2004

Direct Optical Excitation of Quantum-Degenerate Exciton States in Semiconductors

Quantum electrodynamic calculations predict that truly incoherent light can be used to efficiently generate quantum-degenerate exciton population states. Resonant incoherent excitation directly converts photons into excitons with vanishing center of mass momentum. The populated exciton state possesses long-range order, is very stable against perturbations, and should be observable via its unusual directional and density dependence in luminescence measurements.

cond-mat.mes-hall↗

arXiv2004

Excitation Induced Dephasing in Semiconductor Quantum Dots

A quantum kinetic theory is used to compute excitation induced dephasing in semiconductor quantum dots due to the Coulomb interaction with a continuum of states, such as a quantum well or a wetting layer. It is shown that a frequency dependent broadening together with nonlinear resonance shifts are needed for a microscopic explanation of the excitation induced dephasing in such a system, and that excitation induced dephasing for a quantum-dot excitonic resonance is different from quantum-well and bulk excitons.

cond-mat.mtrl-sci↗

arXiv2004

Exciton-Population Inversion and Terahertz Gain in Resonantly Excited Semiconductors

The build-up of exciton populations in resonantly laser excited semiconductors is studied microscopically. For excitation at the $2s$-exciton resonance, it is shown that polarization with a strict $s$-type radial symmetry can be efficiently converted into an incoherent $p$-type population. As a consequence, inversion between the $2p$ and $1s$ exciton states can be obtained leading to the appearance of significant terahertz gain.

cond-mat.mtrl-sci↗

arXiv2003

Excitonic Photoluminescence in Semiconductor Quantum Wells: Plasma versus Excitons

Time-resolved photoluminescence spectra after nonresonant excitation show a distinct 1s resonance, independent of the existence of bound excitons. A microscopic analysis identifies excitonic and electron-hole plasma contributions. For low temperatures and low densities the excitonic emission is extremely sensitive to even minute optically active exciton populations making it possible to extract a phase diagram for incoherent excitonic populations.

cond-mat↗