R. Takayama

On the identification of intervalence-band coherences in semiconductor quantum wells

A theoretical analysis of intervalence-band coherences is presented. These optically created non-radiative coherences are generalizations of non-radiative coherences in atomic and molecular three-level systems. Whereas in three-level systems these coherences are the basis of important and well-established nonlinear effects, the interpretation of experimental evidence for such coherences in semiconductors needs to be supported by many-body theory. Based on the dynamics-controlled truncation formalism, the respective contributions of intervalence-band coherences and coherent biexcitonic correlations in time-integrated differential transmission spectroscopy is investigated. It is found that the contribution of the biexcitonic correlations to the observable heavy-hole–light-hole beats can be eliminated, either by reducing the light pulse duration or by choosing the central frequency of the light pulses at the heavy-hole exciton.

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.

Excitonic Correlations in the Nonlinear Optical Response of Two-dimensional Semiconductor Microstructures: A Nonequilibrium Green’s Function Approach

Important Coulomb correlations among excitons have been extensively demonstrated in weakly-nonlinear optical measurements on quasi-2D semiconductor structures in the coherent regime. The Dynamics Controlled Truncation (DCT) scheme, a perturbation (in the applied field amplitude) theory within the density matrix formalism, has scored much success in elucidating the correlation structures revealed by these experiments. Practically, however, DCT is ill-suited to treat the incoherent aspects of the electronic or excitonic dynamics, and therefore its range of efficient applications is limited to short times and/or low carrier densities. Traditionally, the most powerful approach to study incoherent effects and correlations in highly excited semiconductors is that of nonequilibrium Green’s functions (NGF). A combination of the insights and technical advantages of DCT and NGF can lead to more comprehensive theories for the nonlinear response of semiconductors. This contribution reviews some of our efforts in this direction.

Electromagnetically-induced transparency at the exciton resonance

We report the experimental demonstration of electromagnetically-induced transparency (EIT) in a GaAs quantum well, in which the absorption of an exciton resonance is reduced by more than twenty-fold. The destructive quantum interference for EIT is set up by a biexciton 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.

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.

Theoretical study of the polarization dependence of third-order optical nonlinearities in semiconductor microcavities

We present a microscopic theory of the coherent third order optical response of semiconductor quantum well micro cavities, specialized to the four-wave-mixing configuration in the spectral vicinity of the lowest exciton frequency. The theory is that of a quantum mechanical many-electron system dipole-coupled to a classical radiation field. The many-electron dynamics is treated within the dynamics- controlled-truncation formalism restricted to the 1s-exciton subspace. Within this limitation, al Coulomb correlation effects are included, resulting in an effective theory of exciton-polariton scattering. The theory is evaluated for various polarization configurations each of which depends differently on the underlying many-body effects, such as phase-space filing, Hartree-Fock exchange, and two-exciton correlations.