M Lindberg’s research while affiliated with Åbo Akademi University and other places

We evaluate finite-temperature equilibrium correlators $\langle T_\tau \hat \psi ({\text{r}}_{\text{1}} )\hat \psi ^\dag ({\text{r}}_{\text{2}} )\rangle$ for thermal time τ ordered Bose fields $\hat \psi ,{\text{ }}\hat \psi ^\dag$ to good approximations by new methods of functional integration in d=1, 2, 3 dimensions and with the trap potentials V(r)≢0. As in the translationally invariant cases, asymptotic behaviors fall as $R^{ - 1} \equiv |{\text{r}}_1 - {\text{r}}_2 |^{ - 1}$ to longer-range condensate values for and only for d = 3 in agreement with experimental observations; but there are generally significant corrections also depending on $S \equiv ({\text{r}}_1 {\text{ + r}}_2 )/2$ due to the presence of the traps. For d = 1, we regain the exact translationally invariant results as the trap frequencies Ω → 0. In analyzing the attractive cases, we investigate the time-dependent c-number Gross–Pitaevskii (GP) equation with the trap potential for a generalized nonlinearity −2cψ|ψ|2n and c < 0. For n = 1, the stationary form of the GP equation appears in the steepest-descent approximation of the functional integrals. We show that collapse in the sense of Zakharov can occur for c = 0 and nd ≥ 2 and a functional E NLS[ψ] ≤ 0 even when V(r)≢0. The singularities typically arise as δ-functions centered on the trap origin r=0.

The interaction of strong low-area pulses with two-level systems shows absorption line narrowing and hole burning within the homogeneous linewidth as a result of nonlinear wave mixing. The wave mixing results from the two-level electronic saturation nonlinearity and occurs, depending on the sign of the pulse area, as a strong absorption enhancement or gain at the transition frequency of the two-level system for resonant excitation.

A scheme to generate an experimentally measurable current echo is proposed where current pulses are induced by a sequence of phase coherent optical pulses. Based on microscopic calculations for a one-dimensional tight-bindig model of a disordered two-band semiconductor it is predicted that excitation sequences can be taylored such that after two induced time-delayed current pulses a current echo pulse results.

The semiconductors show, when excited optically in the coherent regime, remarkable many similar phenomena as the atoms. We (in collaboration with R. Binder, Optical Sciences Center, University of Arizona, Tucson, AZ) have studied theoretically the possibility to observe so called dark states in coherent semiconductor spectroscopy. Dark states, well known from three-level spectroscopy of atoms, are a field dressed states which do not interact with the light field. In a semiconductor, like GaAs, the heavy and the light hole bands together with the conduction band form a V-type three band structure. Near the band gap this structure provides a semiconductor analogy of a three-level system. Our calculations show that the pump probe absorption of this system has a dip at equal detunings, which is the signature of the dark state. We also study the dynamics of the system under pulsed excitation and show that the Coulomb effects can strongly modify phenomena compared with those appearing in atoms due to the dark states.

Rigorous dipole selection rules are derived for an interacting electron-hole system in a multiband semiconductor. The electronic system is described by the Coulomb many-body Hamiltonian and the valence-band structure is modeled using the Luttinger Hamiltonian in the axial approximation. For the example of a third-order analysis of polarization dependent two- and three-beam four-wave-mixing experiments the polarizations of the mixing signals are computed. Besides situations with well-defined four-wave-mixing polarizations configurations are identified where the polarization state of the outgoing signal depends on the dynamic and coherent properties of the semiconductor.

Exciton coupling is studied for double quantum wells, which are separated by thick barriers. Results are presented for the linear optical spectrum, stationary and transient luminescence as well as four-wave-mixing signals. Additionally, coherent and incoherent transport through the barrier is considered.

Femtosecond-pulse propagation in resonantly excited semiconductors is investigated by numerically solving the semiconductor Maxwell-Bloch equations for plane waves. For excitation at the exciton resonance, it is shown that the pulse absorption exhibits a strongly nonlinear dependence on the input pulse area. Very long propagation distances for strong pulses are observed, but even when all dephasing processes have been neglected, no lossless propagation (self-induced transparency) was found. The influence of the electron-hole many-body effects, nonequilibrium carrier relaxation, and optical dephasing on the pulse-propagation dynamics is studied. The exchange interaction in the electron-hole plasma is shown to support large propagation distances. For excitation of the continuum states, the dependence of the absorption on the intensity of the input pulse is reduced due to the rapid carrier relaxation into quasiequilibrium distributions.

The phenomena of Rabi oscillations, resonant pulse propagation, and photon echo in semiconductors are investigated by numerically solving the full semiconductor Bloch equations for the example of bulk CdSe. The calculations are done for resonant femtosecond excitation at the exciton resonance. The many-body modifications of Rabi flopping and semiconductor photon echo are discussed for a thin medium, whereas the propagation of weak and strong pulses is analyzed for an extended material. Rabi flopping of the density can lead to field reamplification and thus to long propagation distances of sufficiently strong pulses and to pulse break-up. The area theorem known from self-induced transparency (SIT) in atomic systems is generalized for the case of phase modulated fields and shown to be modified significantly in semiconductors. Even though no ideal SIT occurs, the Coulomb interacting electron–hole system nevertheless seems to support extremely long propagation distances of so-called π-pulses.