Exciton Polarization Research Articles

Coherent control of the exciton and exciton-biexciton transitions in the generation of nonlinear wave-mixing signals in a semiconductor quantum well

By use of an optical coherent-control technique we demonstrate and analyze the control of transitions involved in the generation of four-wave-mixing signals in a semiconductor quantum well for the two different four-wave-mixing directions and for different sequences of the excitation pulses. Results are presented for frequency- as well as for time-resolved signals. A doubling of the coherent switching frequency is found which occurs if the laser pulses performing the control process contribute quadratically to the wave-mixing signal. A direct comparison between experiment and microscopic theory reveals how the coherent-control process acts on the coherent polarization of the exciton-biexciton system in all different configurations. An additional interpretation of the experimental results based on a simplified few-level model is able to provide an intuitive understanding of the relevant processes which govern the coherent control of the excitonic and biexcitonic wave-mixing polarizations.

Coherent control of excitonic excitations in II–VI quantum wells

AbstractWe present results of experimental work in which the coherent‐control technique has been applied to study the coherent manipulation of excitonic polarization in ZnSe quantum‐well samples. We will introduce the linear control of excitonic polarization in pulse‐transmission experiments and afterwards focus on the question if and how the optical transition from the excitonic states to the bound‐biexciton state can be subject to a directed coherent control. The experimental results will be compared to simulations based on a microscopic theory and will also be explained on the base of a phenomenological few‐level model. The overall suitability of coherent control for the investigation of exciton systems in low‐dimensional wide‐gap materials will be discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic-field-induced reduction of the exciton polarization splitting in InAs quantum dots

By the application of an in-plane magnetic field, we demonstrate control of the fine structure polarisation splitting of the exciton emission lines in individual InAs quantum dots. The selection of quantum dots with certain barrier composition and confinement energies is found to determine the magnetic field dependent increase or decrease of the separation of the bright exciton emission lines, and has enabled the splitting to be tuned to zero within the resolution of our experiments. Observed behaviour allows us to determine g-factors and exchange splittings for different types of dots.

Microscopic theory of optical excitations, photoluminescence, and terahertz response in semiconductors

This article presents a comprehensive many-body theory for optically excited semiconductors. The coupled equations of motion for the correlation functions of the Coulomb-interacting electron-hole system are derived and solved for different excitation conditions. The generation of a coherent excitonic polarization and its conversion into incoherent populations is analyzed. The spontaneous emission properties of the excited system are evaluated using a fully quantized theory. Luminescence from excitonic and electron-hole plasma populations is computed, and significant hole burning in the exciton center of mass distributions is predicted. It is shown how different excitations states of the many-body system can be identified by their characteristic signatures in the absorption spectra of a terahertz probe field.

Theory of ultrafast exciton dynamics in the Quantum Hall system

We discuss a theory of the ultrafast non-linear optical response of excitons in the Quantum Hall system. Our theory focusses on the role of the low energy collective electronic excitations of the cold, strongly correlated, two-dimensional electron gas (2DEG) present in the ground state. It takes into account ground state electron correlations and Pauli exchange and interaction effects between the photoexcited excitons and the collective excitations. Our formulation addresses both the initial coherent regime, where the dynamics is determined by exciton and 2DEG polarizations, and the subsequent incoherent regime, dominated by population dynamics. We describe non-Markovian memory, dephasing, and correlation effects and a non-linear exciton hybridization due to the non-instantaneous interactions. We identify the signature of the coherent inter-Landau level magnetoplasmon (MP) in the temporal profile of the three-pulse time-integrated four-wave-mixing signal in the initial coherent regime.

Mean-field dynamics of a quantum dot-microcavity system

Mean-field evolution equations for the exciton and photon populations and polarizations (Bloch–Lamb equations) are written and numerically solved in order to describe the dynamics of electronic states in a quantum dot coupled to the photon field of a microcavity. The equations account for phase space filling effects and Coulomb interactions among carriers, and include also (in a phenomenological way) incoherent pumping of the quantum dot, photon losses through the microcavity mirrors, and electron–hole population decay due to spontaneous emission of the dot. When the dot may support more than one electron–hole pair, asymptotic oscillatory states, with periods between 0.5 and 1.5 ps, are found almost for any values of the system parameters.

Spontaneous emission, elastic scattering and localization of exciton‐polaritons in disordered quantum wells

Steady-state resonant effects due to multiple scattering of exciton-polaritons are studied for quantum wells in disordered semiconductor multilayers. Single quantum wells with laterally fluctuating width and random arrays of quantum wells are considered as the basic objects with intra- and inter-well disorder. Discussed are the following issues: a model of fluctuating quasi-2D exciton polarization, steady-state resonant Rayleigh scattering and resonant localization of exciton-polaritons. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Polariton propagation in shallow-confinement heterostructures: Microscopic theory and experiment showing the breakdown of the dead-layer concept

Polariton effects due to the interplay of the excitonic polarization of a semiconductor and a propagating light field are studied for transmission spectra of $\mathrm{Zn}\mathrm{Se}∕\mathrm{Zn}{\mathrm{S}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x}$ heterostructures. Calculations in terms of microscopic boundary conditions for the exciton motion within a finite-height confinement potential can explain the measured transmission spectra. These calculations also show the absence of polarization-free regions near the sample interfaces. Macroscopic models based on Pekar's additional boundary conditions can only reproduce the spectra if the band alignment at the $\mathrm{Zn}\mathrm{Se}∕\mathrm{Zn}{\mathrm{S}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x}$ interfaces is modified in comparison to the microscopic calculation and if a sample thickness is used that exceeds the independently determined experimental value. Our findings demonstrate the breakdown of the dead-layer concept for shallow confinement potentials.

Exciton spin relaxation in quasiresonantly excitedCdTe∕ZnTeself-assembled quantum dots

We study the exciton spin relaxation in CdTe self-assembled quantum dots by using polarized photoluminescence spectroscopy in magnetic field. The experiments on single CdTe quantum dots and on large quantum dot ensembles show that by combining phonon-assisted absorption with circularly polarized resonant excitation the spin-polarized excitons are photo-excited directly into the ground states of quantum dots. We find that for single symmetric quantum dots at B=0 T, where the exciton levels are degenerate, the spins randomize very rapidly, so that no net spin polarization is observed. In contrast, when this degeneracy is lifted by applying external magnetic field, optically created spin-polarized excitons maintain their polarization on a time scale much longer than the exciton recombination time. We also observe that the exciton spin polarization is conserved when the splitting between exciton states is caused by quantum dot shape asymmetry. Similar behavior is found in a large ensemble of CdTe quantum dots. These results show that while exciton spins scatter rapidly between degenerate states, the spin relaxation time increases by orders of magnitude as the exciton spin states in a quantum dot become non-degenerate. Finally, due to strong electronic confinement in CdTe quantum dots, the large spin polarization of excitons shows no dependence on the number of phonons emitted between the virtual state and the exciton ground state during the excitation.

Excitonic intraband relaxation and polarization anisotropies in PTCDA on femtosecond and picosecond timescales

We report on investigations of optical excitations in polycrystalline organic molecular crystals with quasi-1D-stacked crystal structure and negative exciton dispersion. As model system, we choose thin films of the perylene derivative 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). Using pump–probe spectroscopy, we show how the relaxation from the absorbing state towards the border of the Brillouin zone occurs on a 120 fs timescale. Time-resolved luminescence anisotropy gives evidence that as a result of the coherent coupling between adjacent stacks, populations of the Davydov-split states that are prepared during photo-excitation relax into the emitting states in less than 5 ps. The behavior of the luminescence anisotropy can be explained by the orientation of the two PTCDA molecules in the unit cell. However, a full understanding of the ultrafast pump–probe anisotropy requires novel explanations beyond current models.

Excitons in small dielectric crystals: spectral distribution of oscillator strength

Excitonic states with small wave vectors, agreed with light by spatial phase and responsible for the formation of optical and luminescence properties, are considered for a dielectric crystal with an allowed excitonic transition. Spectroscopic manifestations of such excitons in a cubic-symmetry crystal are traced depending on the ratio of crystal size L to light wavelength λ. In a large crystal, the exciton energy spectrum is known to consist of two degenerate levels related to exciton polarization parallel and perpendicular to its wave vector. The inward part of this longitudinal–transverse splitting interval, making no contribution to optical characteristics at L / λ → ∞ , begins to play a growing role with a decrease of L / λ . The energy distribution of oscillator strength inside the longitudinal–transverse splitting interval was obtained vs. L / λ ; it was found that at L / λ ▪ 1 excitonic states inside this interval intercept a significant part of oscillator strength and participate essentially in the formation of optical properties.

The Green function of cavity polariton with radiative correction: a new description ofresonant second-order optical processes

A previously proposed Green function of cavity polariton associated with quantum wellexcitons in a distributed Bragg reflector cavity has been generalized to an arbitrarythree-dimensional cavity containing multilevel excitons. This Green function describes theelectromagnetic field, both inside and outside the cavity, caused by the cavitydielectrics and the linear polarization of excitons. The electromagnetic field inducedby any other polarizations can be calculated as a convolution with the Greenfunction. In this way, we can include in the Green function approach all the problemswhich deal with boundary conditions and the radiative shift/width of excitons.

Excitation-induced dephasing and biexcitonic effects in the coherent control of excitonic polarization in pulse-transmission experiments

Excitation-induced dephasing and biexcitonic effects in the coherent control of excitonic polarization in pulse-transmission experiments

Spin polarization of excitons in nonmagnetic quantum dots induced by a neighboring magnetic semiconductor quantum well

Magneto-photoluminescence (PL) experiments were carried out on double layer structures consisting of self-assembled nonmagnetic CdSe quantum dots (QDs) and a ZnCdMnSe diluted magnetic semiconductors quantum well (QW), separated by a thin ZnSe barrier layer. We observe two clearly defined PL peaks in all samples, corresponding to CdSe QD and ZnCdMnSe QW layers, respectively. The PL signal was detected using circular polarization. The degree of PL polarization from CdSe QDs in the double layer structures was much larger than that observed in single layers of nonmagnetic CdSe QD system. The effect was systematically studied by examining intensities, energy shifts, and polarizations of the PL peaks. We discuss the observed behavior in terms of antiparallel spin interaction between excitons localized in the nonmagnetic QDs and in the magnetic QWs.

Ultrafast near-field spectroscopy of single semiconductor quantum dots.

Excitonic and spin excitations of single semiconductor quantum dots (QDs) currently attract attention as possible candidates for solid-state-based implementations of quantum logic devices. Due to their rather short decoherence times in the picosecond to nanosecond range, such implementations rely on using ultrafast optical pulses to probe and control coherent polarizations. We combine ultrafast spectroscopy and near-field microscopy to probe the nonlinear optical response of a single QD on a femtosecond time-scale. Transient reflectivity spectra show pronounced oscillations around the QD exciton line. These oscillations reflect phase-disturbing Coulomb interactions between the excitonic QD polarization and continuum excitations. The results show that although semiconductor QDs resemble in many respects atomic systems, Coulomb many-body interactions can contribute significantly to their optical nonlinearities on ultrashort time-scales.

Real-time observation of the coherent control of linear- and four-wave-mixing polarization of excitons in a ZnSe single quantum well

The coherent control of excitonic and biexcitonic polarization in pulse-transmission and four-wave-mixing (FWM) experiments is analysed with real time resolution. As a model system a ZnSe single quantum well is exclusively excited at the heavy-hole exciton and exciton–biexciton resonance. In pulse-transmission experiments the coherent control of excitonic polarization is shown for low excitation intensities. At elevated excitation intensities the dephasing time of the polarization is found to be strongly dependent on the relative phase of two phase-locked laser pulses which perform the coherent manipulation of the polarization. In FWM experiments the separate coherent control of the excitonic and biexcitonic FWM polarization is demonstrated which enables a coherent manipulation of the amplitude and phase of exciton–biexciton beats which are observed on the FWM polarization transients.

Dephasing of excitonic and biexcitonic polarization in a ZnSe single‐quantum well

The decay of excitonic and biexcitonic polarization in a ZnSe single-quantum well is studied in transient, time-integrated four-wave-mixing measurements. The acoustic- and optical-phonon-scattering coefficients are obtained from the temperature dependence of the decay of the four-wave-mixing signal at the exciton and exciton-to-biexciton transition. The temperature dependence of the scattering coefficients is found to be comparable for both transitions. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

On the origin of time-resolved pump–probe differential signal from excitons in quantum wells

We investigate the origin and nature of femtosecond time-resolved (TR) pump–probe differential (PPD) signal from the exciton resonance in semiconductor quantum wells (QWs) in the coherent regime. The excitonic PPD signal is expressed as a sum of interferences of the excitonic nonlinear polarization with the incident probe field and the linear polarization induced by the probe. A nonzero excitonic TR-PPD signal from QWs can occur for time t larger than the pulse width ( t p) when the exciton dephasing time is longer than t p, and this signal can originate only from the second interference term for both the negative and positive pump–probe delays, the contribution from the first interference term being zero for t> t p. This is in contrast with the case of PPD signal in the spectral domain where the first interference term plays a major role. We discuss the effects of many body interactions on the TR-PPD signal. It is shown that excitation-induced dephasing can cause a significant increase in the rise time of the TR-PPD signal.

Counter polarized photoluminescence of trions in n‐doped selfassembled InAs/GaAs quantumdots

We report on negative polarization of charged excitons (or trions) photoluminescence in n-doped InAs/GaAs quantum dots at low temperature (T = 10 K) under optical orientation conditions. The dynamics is studied by time-resolved spectroscopy. A pump-probe experiment evidences the optical orientation of doping electrons, with a spin relaxation time larger than 10 ns. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Time-resolved excitonic coherent polarization dynamics in GaAs multiple quantum wells

We investigate pump induced modulation in the temporal evolution of the excitonic coherent polarization generated by a probe pulse in GaAs multiple quantum wells. For this, we perform fs time resolved pump-probe reflectivity measurements using frequency upconversion at both positive and negative pump-probe delays. A contribution arising from interference of excitonic linear and nonlinear polarization, which is usually not considered in describing pump-probe experiments, is required to understand the results.