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S W Koch

Publications and source records attributed to S W Koch.

17 recordsLinked to original sources
PubMed2006

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.

Journal Article↗
PubMed2006

Semiconductor excitons in new light.

Excitons are quasi-particles that form when Coulomb-interacting electrons and holes in semiconductors are bound into pair states. They have many features analogous to those of atomic hydrogen. Because of this, researchers are interested in exploring excitonic phenomena, from optical, quantum-optical and thermodynamic transitions to the possible condensation of excitons into a quantum-degenerate state. Excitonic signatures commonly appear in the optical absorption and emission of direct-gap semiconductor systems. However, the precise properties of incoherent exciton populations in such systems are difficult to determine and are the subject of intense debate. We review recent contributions to this discussion, and argue that to obtain detailed information about exciton populations, conventional experimental techniques should be supplemented by direct quasi-particle spectroscopy using the relatively newly available terahertz light sources. Finally, we propose a scheme of quantum-optical excitation to generate quantum-degenerate exciton states directly.

Journal Article↗
PubMed2005

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, many-body Coulomb correlations, and the carrier LO-phonon coupling on the second-order Born-Markov level, as well as nonperturbative light-field-induced intraband and interband excitations. A nonmonotonic 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 Coulomb scattering.

Journal Article↗
PubMed2005

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.

Journal Article↗
PubMed2004

Exciton-population inversion and terahertz gain in semiconductors excited to resonance.

The buildup of exciton populations in resonantly laser excited semiconductors is studied microscopically. For excitation around 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.

Journal Article↗
PubMed2004

Entanglement between a photon and a quantum well.

The lack of translational invariance perpendicular to the plane of a single quantum well causes equal probability for spontaneous emission to the left or right. Combining one emission path from the left and one from the right into a common detector leads to interference fringes for fundamentally indistinguishable paths corresponding to geometries where the same in-plane momentum is transferred to the quantum well. For all other paths, no interference is observed because of the entanglement between the photon and extended Bloch states of the many-body system. In multiple-quantum-well structures the interference can be controlled via the spacing between the wells.

Journal Article↗
PubMed2004

Five-wave-mixing spectroscopy of ultrafast electron dynamics at a si(001) surface.

The optically induced electron dynamics at a Si(001) surface is studied using a five-wave-mixing setup which measures the diffracted second-harmonic intensity induced by three ultrashort (13 fs) laser pulses. Depending on the time ordering of the pulses, this technique is capable of monitoring the temporal evolution of photoexcited one- or two-photon coherences, or populations. For a particular pulse sequence, the experiments show a delayed rise and a decay of the diffracted signal intensity on time scales of 50 and 250 fs, respectively. This response can be described by optical Bloch equations by including rapid scattering of the photoexcited carriers in the D(down) band of Si(001).

Journal Article↗
PubMed2004

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 exciton and electron-hole plasma contributions. For low temperatures and low densities, the excitonic emission is extremely sensitive to details of the electron-hole-pair population making it possible to identify even minute fractions of optically active excitons.

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PubMed2001

Exciton formation in semiconductors and the influence of a photonic environment.

A fully microscopic theory is presented for interacting electrons, holes, photons, and phonons in semiconductor heterostructures. The formation dynamics and statistics of incoherent excitons are analyzed for different densities, lattice temperatures, and photonic environments. Luminescence experiments are shown to depend strongly on the photonic environment in contrast to suggested terahertz absorption measurements. Whereas luminescence in free space is dominated by plasma contributions, terahertz absorption should be able to directly measure excitonic populations.

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PubMed2001

Nonlinear pulse propagation in semiconductors: hole burning within a homogeneous line.

Features reminiscent of spectral hole burning in a homogeneous line are predicted to result from the interaction of small area pulses with the semiconductor exciton resonance. The small area pulses may be designed through pulse shaping or evolve naturally in bulk semiconductors via polaritonic effects. The spectral features exhibit signatures that are characteristic for the underlying material nonlinearity and should occur in any system with isolated spectral resonances and coherent nonlinearities.

Journal Article↗
PubMed2000

Quantum correlations in the nonperturbative regime of semiconductor microcavities.

The nonlinear optical response of semiconductor microcavities in the nonpertubative regime is studied in resonant single-beam-transmission and pump-probe experiments. In both cases a pronounced third transmission peak lying spectrally between the two normal modes is observed. A fully quantized theory is essential for the agreement with the experimental observations, demonstrating that quantum fluctuations leading to intraband polarizations are responsible for this effect.

Journal Article↗
PubMed2000

Dominance of radiative coupling over disorder in resonance Rayleigh scattering in semiconductor multiple quantum-well structures.

Resonance Rayleigh scattering by periodic semiconductor multiple quantum-well structures is studied experimentally and theoretically. Polaritonic effects are found to dominate disorder in the secondary emission dynamics. The coexistence of several radiant polaritonic modes with different radiative decay times leads to polarization beating between modes, strongly influences the rise times, and determines the fast decay times of the resonance Rayleigh scattered signals.

Journal Article↗
PubMed1987

Sertraline, a selective inhibitor of serotonin uptake, induces subsensitivity of beta-adrenoceptor system of rat brain.

Subacute administration (b.i.d. for 4 days) of sertraline, a potent and selective inhibitor of serotonin uptake, was found to reduce cyclic AMP generation by the norepinephrine receptor-coupled adenylate cyclase in rat limbic forebrain slices and decrease the number of beta-adrenoceptors in rat cerebral cortex without affecting the affinity of [3H]dihydroalprenolol binding. Co-administration of sertraline and the serotonin agonist, quipazine, at doses at which neither agent had an effect, resulted in desensitization of norepinephrine receptor-coupled adenylate cyclase and down-regulation of beta-adrenoceptors. These findings suggest that increased serotonergic activity may be involved in the induction of subsensitivity of the beta-adrenoceptor system of rat brain by sertraline.

1-Naphthylamine↗
PubMed1983

Differential effects of the enantiomers of 3-(3-hydroxyphenyl)-N-n-propylpiperidine (3-PPP) at dopamine receptor sites.

The agonist actions of 3-PPP at central dopamine (DA) autoreceptors were found to reside mostly in its (+) enantiomer, (+)-3-PPP also reduced striatal content of DOPAC and HVA, whereas (-)-3-PPP elevated HVA levels. Only (-)-3-PPP antagonized DA stimulation of DA-receptor linked adenylate cyclase. It was more effective than (+)-3-PPP at inhibiting [3H]DA binding to striatal membranes. The results suggest that (+)-3-PPP may act predominantly at DA autoreceptors, while (-)-3-PPP exhibits weak affinity for presynaptic and postsynaptic DA receptors.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1980

Dopamine receptor blockade by imidoline and its proposed active conformation.

Imidoline, 1-[2-(N,N-dimethylamino)ethyl]-3-m-chlorophenyl-2-imidazolidinone, has been found to be as potent as chlorpromazine in increasing striatal DOPA accumulation and prolactin secretion in vivo. In contrast, imidoline exhibited only weak inhibitory activity towards dopamine-sensitive adenylate cyclase and 3H-spiroperidol binding to striatal membranes in vitro. These neuroleptic effects in vivo are probably caused by blockade of dopamine receptors since imidoline did not deplete the striatum of dopamine. Imidoline is of interest because its structure is distinct from those of other neuroleptics. A proposed active conformation involves intramolecular hydrogen bonding between the protonated dimethylamino group and the oxygen of the imidazolidinone ring. The spatial relationship between the amine nitrogen and phenyl ring in this conformation allows proper fit of imidoline with key dimensions described for the dopamine receptor.

3,4-Dihydroxyphenylacetic Acid↗