Coherent Ultrafast Optical Dynamics of the Fermi Edge Singularity

We develop a non-equilibrium many-body theory of the coherent femtosecond nonlinear optical response of the Fermi edge singularity. We study the role of the dynamical Fermi sea response in the time-evolution of the pump-probe spectra. The electron-hole correlations are treated nonperturbatively with the time-dependent coupled cluster cxpansion combined with the effective Hamiltonian approach. For short pulse durations, we find a non-exponential decay of the differential transmission during negative time delays, which is governed by the interactions. This is in contrast to the results obtained within the Hartree-Fock approximation, which predicts an exponential decay governed by the dephasing time. We discuss the role of the optically-induced dephasing effects in the coherent regime.Comment: 41 pages including 11 figs. Final version to appear in Phys. Rev.

Observation of inter-Landau-level quantum coherence in semiconductor quantum wells

Using three-pulse four-wave-mixing femtosecond spectroscopy, we excite a non-radiative coherence between the discrete Landau levels of an undoped quantum well and study its dynamics. We observe quantum beats that reflect the time evolution of the coherence between the two lowest Landau level magnetoexcitons. We interpret our observations using a many-body theory and find that the inter Landau level coherence decays with a new time constant, substantially longer than the corresponding interband magnetoexciton dephasing times. Our results indicate a new intraband excitation dynamics that cannot be described in terms of uncorrelated interband excitations.Comment: 5 pages, 5 figures, to appear in Phys. Rev. B Rapid Communication

Ultrafast dynamics of coherences in the quantum Hall system

Using three-pulse four-wave-mixing optical spectroscopy, we study the ultrafast dynamics of the quantum Hall system. We observe striking differences as compared to an undoped system, where the 2D electron gas is absent. In particular, we observe a large off-resonant signal with strong oscillations. Using a microscopic theory, we show that these are due to many-particle coherences created by interactions between photoexcited carriers and collective excitations of the 2D electron gas. We extract quantitative information about the dephasing and interference of these coherences.Comment: 4 pages, 4 figures, to be published in Phys. Rev. Let

Photon-energy dissipation caused by an external electric circuit in "virtual" photo-excitation processes

We consider generation of an electrical pulse by an optical pulse in the ``virtual excitation'' regime. The electronic system, which is any electro-optic material including a quantum well structure biased by a dc electric field, is assumed to be coupled to an external circuit. It is found that the photon frequency is subject to an extra red shift in addition to the usual self-phase modulation, whereas the photon number is conserved. The Joule energy consumed in the external circuit is supplied only from the extra red shift.Comment: 4 pages, 1 fugur

Constructing Modular and Universal Single Molecule Tension Sensor Using Protein G to Study Mechano-sensitive Receptors

Recently a variety of molecular force sensors have been developed to study cellular forces acting through single mechano-sensitive receptors. A common strategy adopted is to attach ligand molecules on a surface through engineered molecular tethers which report cell-exerted tension on receptor-ligand bonds. This approach generally requires chemical conjugation of the ligand to the force reporting tether which can be time-consuming and labor-intensive. Moreover, ligand-tether conjugation can severely reduce the activity of protein ligands. To address this problem, we developed a Protein G (ProG)-based force sensor in which force-reporting tethers are conjugated to ProG instead of ligands. A recombinant ligand fused with IgG-Fc is conveniently assembled with the force sensor through ProG:Fc binding, therefore avoiding ligand conjugation and purification processes. Using this approach, we determined that molecular tension on E-cadherin is lower than dsDNA unzipping force (nominal value: 12 pN) during initial cadherin-mediated cell adhesion, followed by an escalation to forces higher than 43 pN (nominal value). This approach is highly modular and potentially universal as we demonstrate using two additional receptor-ligand interactions, P-selectin & PSGL-1 and Notch & DLL1

Superfluidity of "dirty" indirect excitons and magnetoexcitons in two-dimensional trap

The superfluid phase transition of bosons in a two-dimensional (2D) system with disorder and an external parabolic potential is studied. The theory is applied to experiments on indirect excitons in coupled quantum wells. The random field is allowed to be large compared to the dipole-dipole repulsion between excitons. The slope of the external parabolic trap is assumed to change slowly enough to apply the local density approximation (LDA) for the superfluid density, which allows us to calculate the Kosterlitz-Thouless temperature $T_{c}(n(r))$ at each local point $r$ of the trap. The superfluid phase occurs around the center of the trap ($\mathbf{r}=0$) with the normal phase outside this area. As temperature increases, the superfluid area shrinks and disappears at temperature $T_{c}(n(r=0))$. Disorder acts to deplete the condensate; the minimal total number of excitons for which superfluidity exists increases with disorder at fixed temperature. If the disorder is large enough, it can destroy the superfluid entirely. The effect of magnetic field is also calculated for the case of indirect excitons. In a strong magnetic field $H$, the superfluid component decreases, primarily due to the change of the exciton effective mass.Comment: 13 pages, 3 figure

Parity forbidden excitations of Sr2CuO2Cl2 revealed by optical third-harmonic spectroscopy

We present the first study of nonlinear optical third harmonic generation in the strongly correlated charge-transfer insulator Sr2CuO2Cl2. For fundamental excitation in the near-infrared, the THG spectrum reveals a strongly resonant response for photon energies near 0.7 eV. Polarization analysis reveals this novel resonance to be only partially accounted for by three-photon excitation to the optical charge-transfer exciton, and indicates that an even-parity excitation at 2 eV, with a_1g symmetry, participates in the third harmonic susceptibility.Comment: Requires RevTeX v4.0beta

Theory of exciton-exciton correlation in nonlinear optical response

We present a systematic theory of Coulomb interaction effects in the nonlinear optical processes in semiconductors using a perturbation series in the exciting laser field. The third-order dynamical response consists of phase-space filling correction, mean-field exciton-exciton interaction, and two-exciton correlation effects expressed as a force-force correlation function. The theory provides a unified description of effects of bound and unbound biexcitons, including memory-effects beyond the Markovian approximation. Approximations for the correlation function are presented.Comment: RevTex, 35 pages, 10 PostScript figs, shorter version submitted to Physical Review