Nonlinear Optical studies of the Transient Coherence in the Quantum Hall System

We review recent investigations of the femtosecond non-linear optical response of the two-dimensional electron gas (2DEG) in a strong magnetic field. We probe the Quantum Hall (QH) regime for filling factors $\nu \sim 1$. Our focus is on the transient coherence induced via optical excitation and on its time evolution during early femtosecond timescales. We simultaneously study the interband and intraband coherence in this system by using a nonlinear spectroscopic technique, transient three-pulse four wave mixing optical spectroscopy, and a many-body theory. We observe striking differences in the temporal and spectral profile of the nonlinear optical signal between a modulation doped quantum well system (with the 2DEG) and a similar undoped quantum well (without a 2DEG). We attribute these qualitative differences to Coulomb correlations between the photoexcited electron-hole pairs and the 2DEG. We show, in particular, that intraband many-particle coherences assisted by the inter-Landau-level magnetoplasmon excitations of the 2DEG dominate the femtosecond nonlinear optical responce. The most striking effect of these exciton-magnetoplasmon coherences is a large off-resonant four-wave-mixing signal in the case of very low photoexcited carrier densities, not observed in the undoped system, with strong temporal oscillations and unusually symmetric temporal profile.Comment: 22 pages, 9 figures; review article to be published in Solid State Communication

Photocarrier dynamics in the very early time regime

The authors present a study of carrier dynamics in the semiconductor GaAs, during and just after creation by ultrashort laser pulses. Using a pump-probe setup with independently adjustable characteristics for the pump and probe pulses they measure the differential absorption spectra and their time-delay derivative. They observe a quasi-instantaneous spread of the carrier population in momentum-energy space, and negative scattering rates just below the center of the pump spectra. Attempts to fit these two results, even qualitatively, with the Semiconductor Bloch Equations in the relaxation time approximation yield to unphysical results. They interpret this as a failure of Boltzmann Kinetics. The observations are, however, consistent with recent Quantum Kinetic theories of carrier-carrier and carrier-phonon scattering

Radiative corrections to the excitonic molecule state in GaAs microcavities

The optical properties of excitonic molecules (XXs) in GaAs-based quantum well microcavities (MCs) are studied, both theoretically and experimentally. We show that the radiative corrections to the XX state, the Lamb shift $\Delta^{\rm MC}_{\rm XX}$ and radiative width $\Gamma^{\rm MC}_{\rm XX}$, are large, about $10-30$ of the molecule binding energy $\epsilon_{\rm XX}$, and definitely cannot be neglected. The optics of excitonic molecules is dominated by the in-plane resonant dissociation of the molecules into outgoing 1$\lambda$-mode and 0$\lambda$-mode cavity polaritons. The later decay channel, ``excitonic molecule $\to$ 0$\lambda$-mode polariton + 0$\lambda$-mode polariton'', deals with the short-wavelength MC polaritons invisible in standard optical experiments, i.e., refers to ``hidden'' optics of microcavities. By using transient four-wave mixing and pump-probe spectroscopies, we infer that the radiative width, associated with excitonic molecules of the binding energy $\epsilon_{\rm XX} \simeq 0.9-1.1$ meV, is $\Gamma^{\rm MC}_{\rm XX} \simeq 0.2-0.3$ meV in the microcavities and $\Gamma^{\rm QW}_{\rm XX} \simeq 0.1$ meV in a reference GaAs single quantum well (QW). We show that for our high-quality quasi-two-dimensional nanostructures the $T_2 = 2 T_1$ limit, relevant to the XX states, holds at temperatures below 10 K, and that the bipolariton model of excitonic molecules explains quantitatively and self-consistently the measured XX radiative widths. We also find and characterize two critical points in the dependence of the radiative corrections against the microcavity detuning, and propose to use the critical points for high-precision measurements of the molecule bindingenergy and microcavity Rabi splitting.Comment: 16 pages, 11 figures, accepted for publication in Phys. Rev.

Decoherence and Programmable Quantum Computation

An examination of the concept of using classical degrees of freedom to drive the evolution of quantum computers is given. Specifically, when externally generated, coherent states of the electromagnetic field are used to drive transitions within the qubit system, a decoherence results due to the back reaction from the qubits onto the quantum field. We derive an expression for the decoherence rate for two cases, that of the single-qubit Walsh-Hadamard transform, and for an implementation of the controlled-NOT gate. We examine the impact of this decoherence mechanism on Grover's search algorithm, and on the proposals for use of error-correcting codes in quantum computation.Comment: submitted to Phys. Rev. A 35 double-spaced pages, 2 figures, in LaTe

Photocarrier relaxation in the quantum kinetics regime

We present a study of carrier relaxation in GaAs in the quantum- kinetics regime. The results cannot be explained by Bolzmann kinetics. They are in satisfactory agreement with quantum-kinetic theory calculation of the occupation number in k-space

Ultrafast Nonlinear Optical Response of Strongly Correlated Systems: Dynamics in the Quantum Hall Effect Regime

We present a theoretical formulation of the coherent ultrafast nonlinear optical response of a strongly correlated system and discuss an example where the Coulomb correlations dominate. We separate out the correlated contributions to the third-order nonlinear polarization, and identify non-Markovian dephasing effects coming from the non-instantaneous interactions and propagation in time of the collective excitations of the many-body system. We discuss the signatures, in the time and frequency dependence of the four-wave-mixing (FWM) spectrum, of the inter-Landau level magnetoplasmon (MP) excitations of the two-dimensional electron gas (2DEG) in a perpendicular magnetic field. We predict a resonant enhancement of the lowest Landau level (LL) FWM signal, a strong non-Markovian dephasing of the next LL magnetoexciton (X), a symmetric FWM temporal profile, and strong oscillations as function of time delay, of quantum kinetic origin. We show that the correlation effects can be controlled experimentally by tuning the central frequency of the optical excitation between the two lowest LLs.Comment: 21 pages, 10 figure

ABSORPTION MODULATION IN FIELD EFFECT QUANTUM WELL STRUCTURES

Nous présentons des études de la modulation d'absorption dans des puits quantiques InGaAs/InAlAs à modulation de dopage, utilisés comme canal conducteur de transistors à effet de champs. Par application d'un potentiel aux électrodes, la concentration électronique peut être variée continuement entre N ≈ 0 et N ≈ 6.5x1011cm-2. Cet effet donne d'importantes informations in situ sur le gas bidimensionnel d'électrons. De plus l'élimination complète de l'absorption autour des résonances nz = 1 a des applications intéressantes en interconnection optique de l'électronique à base de semiconducteurs III-V et en optoélectronique.We present investigations of modulation of the absorption in InGaAs/InlAs modulation doped single quantum well structures used as conducting channel in field effect transistors. The electron concentration can be tuned continuously from N≈ 0 to N≈ 6.5x1011cm-2 using the gate voltage. The effects give in situ information on the 2D-electron gas. Furthermore the total quenching of the absorption seen at the nz = 1 edge has potential applications to optical interconnects for III-V electronics and to lightwave optoelectronic devices

Optical Nonlinearities in Semiconductors

Nonlinear optical processes in semiconductors presents some very specific aspects which originate from the nature and properties of elementary electronic excitations in these materials. In this talk we discuss the origins and characteristics of nonlinear optical effects in semiconductors and we illustrate our presentation with selected experimental results.</jats:p

Femtosecond Spectroscopy of Magneto-Excitons

Recently, quasi-2D excitonic (X) effects have been investigated extensively in semiconductor quantum well (QW), resulting in fundamental advances in our understanding of the effects of dimensionality reduction from 3D to 2D on the optical response of semiconductors, and have lead to numerous new applications in optoelectronic [1]. Efforts to make new semiconductor structures where electronic states are further confined in quasi-0 or quasi-1 dimensions, have had only very limited success because of the difficulty of obtaining narrow size-distribution of defect free samples. However, electronic states strongly confined in all dimensions, in material of excellent uniformity and optical quality, can be obtained by immersing a high quality QW well in a perpendicular magnetic field [2,3]. In such a situation, the crystal band discontinuities confine the electronic states in the direction perpendicular to the QW. In the QW plane, in addition to the Coulomb potential, -e2/εor, the e-h pairs experience the confinement of the quadratic-potential, (eHr)2/8mc2, imposed by the magnetic field, H. The relative strength of two confining potentials is measured by the dimensionless parameter, λ = (ao/lc)2, where ao is the Bohr radius and lc = (c/eH)1/2 is the cyclotron radius. The e-h pairs then form magneto-excitons (MX), which extrapolate continuously between quasi-2D excitons (λ = 0) and quasi-0D Landau levels (λ → ∞).</jats:p