R. Binder

Optical limiting in Bragg-spaced semiconductor quantum wells

We propose a new mechanism for optical limiting in which nonlinear absorption and nonlinear reflection act in concert. The mechanism is based on the light-induced shift of the band gap in Bragg-spaced semiconductor quantum wells.

Material and light-pulse parameter dependence of the nonlinear optical susceptibilities in the coherent χ (3) regime in semiconductor quantum wells

A detailed numerical study of the third-order nonlinear optical susceptibilities (χ(3)) of semiconductor quantum wells is presented. The dependence of χ(3) on material parameters (electron-hole mass ratio and exciton linewidths), on the light polarization configuration (co- and countercircularly polarized) and on the spectral configuration is discussed. The goal of this study is to map out the nonlinear phase shift per quantum well and a related figure of merit caused by quasi-resonant excitonic and biexcitonic nonlinearities induced by picosecond light pulses. The study is based on the dynamics-controlled truncation formalism and evaluated under the assumption that only 1s-heavy-hole excitons contribute to the nonlinearities. It includes all correlation effects (exciton–exciton scattering in the singlet and triplet channels and coherent biexciton formation in the singlet channel) that contribute within the coherent excitonic χ(3) regime.

Spatiotemporal bright soliton Y junctions in nonlinear planar waveguides

The splitting and steering of ultrashort pulses propagating through a Kerr medium are investigated. This investigation extends earlier ones of cw spatial soliton Y junctions into the area of subpicosecond solitonlike pulse propagation. Besides that of the all-optical steering process, the influence of normal group-velocity dispersion and two-photon absorption is discussed. It is found that both processes modify the transmission characteristics of the system (i.e., output versus input beam intensity) in a similar way. However, two-photon absorption inhibits the steering property of Y junctions, whereas normal group-velocity dispersion enhances it. A model based on an adiabatic propagation approximation is used to explain this behavior qualitatively.

Polarization-dependence of ultrafast optical nonlinearities of Bragg-spaced quantum wells

We present a microscopic theory for the polarization dependence of the nonlinear reflection of Bragg-spaced quantum wells. Our theory includes polariton correlations beyond third order. Comparisons with experimental results show reasonably good agreement.

Theory of inducing electron spin coherence without spin precession in GaAs quantum well waveguides

We present a theory for a differential transmission experiment that generated electron spin coherence in quantum well waveguides without DC magnetic fields. We show how optical selection rules explain the spin coherences creation and detection.

Higher order correlations and semiconductor optical nonlinearities

A microscopic theory for optical semiconductor nonlinearities systematically including Coulomb correlation effects is able to explain AC Stark shifts measured with all possible pump/probe circular polarizations and gives evidence for a 3-level-atom-like intervalence band coherence.

Evidence of nonperturbative exciton-exciton scattering in four-wave-mixing signals in quantum well microcavities

Summary form only given. The role of exciton-exciton scattering in the nonlinear optical response of semiconductor quantum wells is well established. However, an interesting question arising from the systems dimensionality has so far not been addressed. In the quantum theory of scattering in two dimensions, the scattering amplitude behaves non-smoothly and non-perturbatively at low energies. While the exact scattering amplitude vanishes as 1/ln (energy) as energy /spl darr/0, the second Born approximation diverges logarithmically. These properties result from the systems dimensionality and hold for any short-range well-behaved potential. One may ask whether the second Born approximation also fails for exciton-exciton scattering in two-dimensional structures and how this breakdown may be experimentally detected. In this contribution, we address this question within a microscopic theory of frequency-degenerate four-wave-mixing signals from a quantum-well microcavity. Analyzing an experiment reported in with this theory, we argue that the data are already sufficiently sensitive to show the breakdown of the second Born approximation for exciton-exciton scattering.

Evidence for intervalence band coherences in semiconductors via coherently coupled optical Stark shifts

Summary form only given. The optical Stark shift in semiconductors transient nonlinearity which helps to understand fundamental ultrafast microscopic processes. Studies of similarities and differences between optical Stark shifts in atomic two-level systems and those observed in semiconductors led to the understanding of semiconductor-specific many-body effects such as biexcitons and Coulomb memory effects. While the two-level analogies of Stark shifts in semiconductors have been successfully exploited, the question of what one can learn from a comparison of coherent atomic three-level effects with optical Stark shifts in semiconductors using the coherent coupling between heavy-hole (hh) and light-hole (lh) excitons has not been addressed so far. In atomic three-level systems, nonradiative or Raman coherences are the foundation of important nonlinear optical effects such as electromagnetically-induced transparency and lasing without inversion. We investigate these coherences in an InGaAs multiple quantum well utilizing coherently-coupled Stark shift measurements of the hh- and lh-exciton resonances.

Excitonic correlations in nonlinear phase shift in semiconductor quantum wells

Summary form only given. The nonlinear optics near the fundamental absorption edge is commonly formulated in terms of a limited number of material parameters characterizing the strength of specific nonlinear processes. Once the nonlinear response function underlying these material parameters is known, one can optimize the relevant nonlinear optical effect for specific device applications. This study is focused on optical nonlinearities at quasi-resonant conditions in the picosecond-pulse regime, where excitonic correlations, such as biexcitonic resonances, contribute prominently to the physical processes. Using a microscopic theory, we have calculated systematically the parameter dependencies of some representative figures of merit, nonlinear phase shift /spl Delta//spl phi/ and differential transmissivity AT, which provide complementary information on the nonlinear susceptibility-/spl chi//sup (3)/:/spl Delta//spl phi//spl sim/Re/spl chi//sup (3)/, /spl Delta/T/spl sim/Im/spl chi//sup (3)/, approximately. We will show in detail how the various aspects of excitonic dynamics affect these quantities around the exciton resonance.

Coherently coupled optical stark shifts provide evidence for intervalence band coherences in semiconductor quantum wells

The optical Stark shift in semiconductors is an important transient nonlinearity that helps researchers to understand fundamental ultrafast microscopic processes. Studies of similarities and differences between optical Stark shifts in atomic two-level systems and those observed in semiconductors led to the understanding of semiconductor-specific many-body effects. We recently investigated the semiconductor counterparts of coherent atomic three-level effects utilizing the coherent coupling between heavy-hole (hh) and light-hole (lh) excitons. By analyzing the coherently coupled optical Stark effect on hh and lh exciton resonances we showed1 that our data provide evidence for the existence of hh-lh Raman coherences in semiconductor quantum wells. In atomic three-level systems, Raman coherences are the foundation of important nonlinear optical effects such as electromagnetically induced transparency and lasing without inversion. Figures 1(a) and 1(b) show the transient absorption changes measured at both hh and lh exciton resonances in an InGaAs quantum-well structure. The two degenerate three-band systems are effectively reduced to a single three-band system by driving a single hh transition with a circularly polarized pump and probing with either coor counter-circular polarization. For co-circular polarization we observed the well-known transient blueshift of the hh exciton resonance. We did not observe any nonlinear response at the lh exciton resonance indicating negligible direct coupling between the pump field and the lh exciton. With a countercircularly polarized probe pulse a definite blueshift appears at the coherently coupled lh exciton resonance. Our theoretical analysis is based on a full many-body approach within the third-order nonlinear optical regime.2 We considered a semiconductor quantumwell analog for the atomic three-level system, that is, the three-band system that consists of a conduction band as well as hh and lh valence bands. While in the band picture the analogy holds for each in-plane momentum state, the Coulomb interaction prevents the analogy from being completely accurate. Exciton scattering and biexcitonic effects have no counterparts in three-level systems. Nevertheless, as long as those processes do not dominate the Stark shift, an approximate threelevel versus semiconductor Stark shift analogy is possible, and the definition of an excitonic Raman coherence (excitonic intervalence band coherence) is appropriate. Figure 1(c) shows theoretical results for the ratio of hh-lh Stark shifts with and without intervalence band coherences. When these coherences are included, the lh shift is much larger than without coherences and the hh-lh shift ratio is approximately two. Deviations by a factor of 2 come from the hh and lh mass differences, especially at small detunings, from correlation contributions. At large detunings the ratio behaves essentially like a three-level system: it is two (infinity) for the case with (without) Raman coherences. Clearly, the experimental observations (squares) provide direct evidence for the presence of an intervalence band Raman coherence, because the observed hh-lh shift ratio is close to the ideal value of two at large detunings.