Excited Semiconductor Quantum Research Articles

Cell Imaging Using Two-Photon Excited CdS Fluorescent Quantum Dots Working within the Biological Window.

In recent years, two-photon excited semiconductor quantum dots (QDs) have been the subject of intense investigation due to their long excitation wavelength which helps to achieve deeper penetration and higher image resolution in optical bioimaging. In this paper, water-soluble CdS QDs were synthesized using a hydrothermal method and applied to human liver hepatocellular carcinoma (HepG2) cells. The first-principles calculation suggested that the S-rich defected structure contributes to a narrower band gap compared to the pristine structure. The resulting fluorescence wavelength was significantly red shifted, which was attributed to the deep defect states emission. The large Stokes shifts (> 200 nm) of the QDs can eliminate the possible cross-talk between the excitation light and the emission light. Two-photon induced red fluorescence emission can avoid overlapping with the autofluorescence emission of biological samples. The uptake and cell viability measurements of the HepG2 cells showed a good biocompatibility and a low toxicity of CdS QDs. Two-photon excited scanning microscopy images revealed that the HepG2 cells incubated with CdS QDs emitted bright red upconversion fluorescence and the fluorescence brightness was 38.2 times of that of the control group. These results support CdS QDs as a good candidate for application in cellular imaging.

Generation of superfluorescent bursts from a fully tunable semiconductor magneto‐plasma

AbstractQuantum particles sometimes cooperate to develop a macroscopically ordered state with extraordinary properties. Superconductivity and Bose‐Einstein condensation are examples of such cooperative phenomena where macroscopic order appears spontaneously. Here, we demonstrate that such an ordered state can also be obtained in an optically excited semiconductor quantum well in a high magnetic field. When we create a dense electron‐hole plasma with an intense laser pulse, after a certain delay, an ultrashort burst of coherent radiation emerges. We interpret this striking phenomenon as a manifestation of superfluorescence (SF), in which a macroscopic polarization spontaneously builds up from an initially incoherent ensemble of excited quantum oscillators and then decays abruptly producing giant pulses of coherent radiation. SF has been observed in atomic gases, but the present work represents the first observation of SF in a solid‐state setting. While there is an analogy between the recombination of electron‐hole pairs and radiative transitions in atoms, there is no a priori reason for SF in semiconductors to be similar to atomic SF. This is a complex many‐body system with a variety of ultrafast interactions, where the decoherence rates are at least 1,000 times faster than the radiative decay rate, an unusual situation totally unexplored in previous atomic SF studies. We show, nonetheless, that collective many‐body coupling via a common radiation field does develop under certain conditions and leads to SF bursts. The solid‐state realization of SF resulted in an unprecedented degree of controllability in the generation of SF, opening up opportunities for both fundamental many‐body studies and device applications. We demonstrate that the intensity and delay time of SF bursts are fully tunable through an external magnetic field, temperature, and pump laser power.

Microscopic equation-of-motion approach to the multiphonon assisted quantum emission of a semiconductor quantum dot

We present a microscopical model for the multiphonon assisted quantum emission from an externally excited semiconductor quantum dot. In the high driving field limit, our model predicts several strong coupling features, in particular, a phonon-assisted Mollow-triplet splitting and anticrossings between the Mollow sidebands and the phonon satellite peaks. The observed anticrossing scales with the effective LO-phonon coupling and can be used as a measure for the phonon coupling strength. Further, we investigate the non-Markovian temporal emission dynamics of the main and multiphonon assisted triplet structures.

Interaction of terahertz radiation with semiconductors

AbstractThe microscopic theory of the interaction of terahertz radiation with semiconductors is reviewed.The theory is applied to analyze the intersubband terahertz response of optically excited semiconductor quantum wells (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

CdSe Quantum Dot Sensitized Solar Cells. Shuttling Electrons Through Stacked Carbon Nanocups

The charge separation between excited CdSe semiconductor quantum dots and stacked-cup carbon nanotubes (SCCNTs) has been successfully tapped to generate photocurrent in a quantum dot sensitized solar cell (QDSC). By employing an electrophoretic deposition technique we have cast SCCNT-CdSe composite films on optically transparent electrodes (OTEs). The quenching of CdSe emission, as well as transient absorption measurements, confirms ultrafast electron transfer to SCCNTs. The rate constant for electron transfer increases from 9.51 x 10(9) s(-1) to 7.04 x 10(10) s(-1) as we decrease the size of CdSe nanoparticles from 4.5 to 3 nm. The ability of SCCNTs to collect and transport electrons from excited CdSe has been established from photocurrent measurements. The morphological and excited state properties of SCCNT-CdSe composites demonstrate their usefulness in energy conversion devices.

Fano Signatures in the Intersubband Terahertz Response of Optically Excited Semiconductor Quantum Wells

Absorption and transmission spectra of broadband terahertz pulses are measured to probe the intersubband response of an optically excited quantum-well heterostructure. While the terahertz absorption shows the single peak of the resonant intersubband transition, the transmission spectra display strong Fano signatures due to the phase sensitive superposition of ponderomotive and terahertz currents as predicted by our microscopic theory.

Quantum control of polaron states in semiconductor quantum dots

We study phonon-assisted dephasing in optically excited semiconductor quantum dots within the frameworks of the independent-boson model and optimal control. Using a realistic description for the quantum-dot states and the phonon coupling, we demonstrate that such dephasing has a drastic impact on the coherent optical response. We employ optimal-control theory to search for control strategies that allow us to fight decoherence, and show that appropriate tailoring of laser pulses allows a complete control of the optical excitation despite the phonon dephasing.

Amplification and quantum statistics of microcavity polaritons under nonresonant excitation

A model describing amplification and quantum statistics of the exciton polaritons with k=0 in a non-resonantly excited semiconductor quantum well embedded in a microcavity is presented. Exploiting the bottleneck effect for exciton polaritons, it is assumed that the polaritons with k different from 0 act as a reservoir. The time evolution of the k=0 polaritons is described by a master equation, in which two- and one-polariton transitions between the mode with k=0 and the reservoir are accounted for. The k=0 mode exhibits a threshold depending on the material parameters and on the injected exciton density. Below threshold the quantum statistics of the emission is characteristic of an incoherent process, while above threshold it approaches that of a laser.

Absorption and gain spectra of optically excited semiconductor quantum wires: Effects of Coulomb correlation and screening

We present a systematic study on optical spectra of one-dimensional semiconductor quantum wires by solving the semiconductor Bloch equations. Many-body Coulomb interactions are substantially enhanced by strong geometrical confinement in quantum wire systems. Our theoretical calculation reveals that phase-space filling has strong influence on the absorption spectra. Increasing carrier density bleaches the exciton peak in low-density regime, and induces considerable optical gain in very high density. Band-gap renormalization is partially cancelled by reduction of exciton binding energy, which increases as the cross-section of quantum wire decreases.

Angular correlation of photons emitted from an excited quantum well

We study the angular correlation of single photons emitted from excited semiconductor quantum wells. The considered physical system is described in terms of two subsystems, the electronic part constituting the bath and the photonic part constituting the bathed subsystem, both being coupled by the light-matter interaction. From the master equations describing the coarse-grained Markovian evolution of the photonic subsystem, we derive the corresponding equations of motion for the photonic angular correlation functions. These equations are solved in the stationary, low-density limit. Experimentally, the angular correlations can be assessed by studying the interference of light emitted in different directions. In agreement with recent experimental results, we find that for ordered quantum wells angular correlations exist only in emission directions for which the projections of the photon momenta onto the plane of the quantum well are equal. This feature is a direct consequence of the Bloch character of the electronic states in an ordered quantum well. Thus the experimental study of the angular correlations of emitted photons may provide an interesting diagnostic tool to reveal the presence of disorder in semiconductor heterostructures and to characterize its influence on the electronic states near the band edges.

Optimal quantum control of electron–phonon scatterings in artificial atoms

We study the phonon-induced dephasing dynamics in optically excited semiconductor quantum dots within the frameworks of the independent Boson model and optimal control. It is shown that appropriate tailoring of laser pulses allows a complete control of the optical excitation despite the phonon dephasing, a finding in marked contrast to other environment couplings.

Direct Observation of the Mott Transition in an Optically Excited Semiconductor Quantum Well

We have studied density-dependent time-resolved photoluminescence from a 80 A InGaAs/GaAs single quantum well excited by picosecond pulses. We succeed in giving evidence for the transition from an exciton-dominated population to an unbound electron-hole pair population as the pair density increases. For pair densities below this excitonic Mott transition we observe a spectrally separate emission from free electron-hole pairs in addition to excitonic luminescence, thereby proving the coexistence of both species. Exciton binding energy and band gap remain unchanged even near the upper bound of this coexistence region. Above the Mott density we observe a purely exponential high energy tail of the photoluminescence and a redshift of the band gap with pair density. The transition occurs gradually between 1 x 10(10) and 1 x 10(11) cm(-2) at the carrier temperatures of our experiment.

The origin of the transverse relaxation time in optically excited semiconductor quantum wells

The origin of the transverse relaxation time in optically excited semiconductor quantum wells is investigated based on the vector property of the interband transition matrix elements. The dephasing rate due to carrier–carrier (CC) scattering is found to be equal to half of the common momentum relaxation rate. The analytical expression of the polarization dephasing due to CC scattering in two-dimension is established and the dependence of the dephasing rate Γcc on the carrier density N is determined to be Γcc=Constant·N1/2, which is used to explain the experimental results and provides a promising physical picture.

Quantum control of electron-phonon scatterings in artificial atoms.

The phonon-induced dephasing dynamics in optically excited semiconductor quantum dots is studied within the frameworks of the independent boson model and optimal control. We show that appropriate tailoring of laser pulses allow complete control of the optical excitation despite the phonon dephasing, a finding in marked contrast to other environment couplings.

Spontaneous emission in highly excited semiconductors: Saturation of the radiative recombination rate

The spontaneous emission rate of semiconductors displays a saturation at high carrier injection densities due to the filling of the conduction and valence bands. We have carried out time-resolved experiments, whereby a semiconductor quantum well at room temperature is excited optically by a short laser pulse and the time decay of its luminescence is measured for different injected carrier densities. The luminescence decay rate, plotted as a function of injected carrier density, deviates strongly from the well-known quadratic recombination law, valid at low carrier densities, displaying a saturation. We have developed a simple analytic theory that accounts for this saturation and can describe adequately the recombination kinetics of highly excited semiconductor quantum wells.

New concept for organic LEDs: non-radiative electronic energy transfer from semiconductor quantum well to organic overlayer

The resonant electronic energy transfer from an excited semiconductor quantum well to a nearby strongly absorbing organic medium may occur on time scales of several tens of picoseconds for II–VI semiconductors and of several hundreds picoseconds for III–V semiconductors, which is in both cases significantly less than the semiconductor excitation life-time in the absence of such a transfer. Thus, a large fraction of the semiconductor excitation energy can be transfered to the organic medium. Due to energy transfer the non-radiative processes in the semiconductor quantum well with characteristic times larger than that of the energy transfer, will be suppressed. The advantage of the hybrid structures would be also the combination of comparatively good transport properties of semiconductors (allowing for electrical pumping) and good light-emitting properties of organic substances (high quantum yield, colour tuning). Such a favorable combination may be exploited in constructing new type of LEDs.

Cascade evolution and radiative recombination of quantum dot multiexcitons studied by time-resolved spectroscopy

We resolve spatially, spectroscopically, and temporally the photoluminescence emission from single self-assembled InAs/GaAs quantum dots. The rich photoluminescence spectrum and its evolution with time after pulse excitation and with the density of excitation is experimentally measured and analyzed using a theoretical multiexciton model. The model accounts quantitatively for the dynamics of a small number of interacting electrons and holes confined in optically excited semiconductor quantum dots.

Quantum-kinetic dephasing in resonantly excited semiconductor quantum wells

We investigate resonant time-resolved four-wave mixing on GaAs quantum wells at $T=300\mathrm{K}$ and at elevated carrier densities using excitation with 11 fs optical pulses. The experiments are compared with non-Markovian quantum-kinetic calculations including carrier-carrier and carrier-LO-phonon scattering. A self-consistently determined two-time-dependent screened random-phase approximation Coulomb potential is used for the carrier-carrier scattering. In experiment and theory we find an unusual real-time response, which is far from an ideal photon echo. Surprisingly, we also find that the dependence of the decay time \ensuremath{\tau} of the four-wave-mixing signal versus the optically excited nonequilibrium electron-hole pair density ${n}_{\mathrm{eh}}$ is quantitatively similar to the corresponding behavior of bulk GaAs. Thus both data sets can be fitted by the same form ${\ensuremath{\tau}}^{\ensuremath{-}1}={\ensuremath{\tau}}_{0}^{\ensuremath{-}1}{+cn}_{\mathrm{eh}}^{1/3}.$ This is not consistent with previous experimental work in a more limited range of carrier densities, which determined different behavior for quasi-two-dimensional (2D) and 3D, i.e., exponents of 0.3 and $0.55\ifmmode\pm\else\textpm\fi{}0.04,$ respectively.

Coulomb effects in the optical spectra of highly excited semiconductor quantum dots

We present a density functional calculation of the energy level structure and luminescence spectra of many electron–hole pairs confined in a semiconductor quantum dot. As the number of carriers in the dot is increased, the energies of the optical emission lines gradually shift to the lower energy side, similar to the band-gap renormalisation of bulk semiconductors. Further, Coulomb scattering leads to a substantial density-dependent broadening of the spectral lines, mainly by elastic electron–hole scattering processes that involve an electron transfering sufficient energy to a hole to excite the latter into a continuum state. The same scattering process also provides an efficient channel for relaxation of an electron captured into an excited state of the QD, without any assistance of phonons.

Electron–hole scattering in a highly excited semiconductor quantum well amplifier with terahertz-field-drifted carrier distributions: applications to all-optical switching

Anisotropic carrier–carrier scattering is theoretically investigated for high-density field-drifted electron–hole plasma distributions in semiconductor quantum wells. The drifted distributions, driven for example by a free-electron laser, are found to march microscopically and rapidly back to an isotropic, non-drifted plasma at the center of the Brillouin zone. This is in contrast to the single-species plasma case (p- or n-doped). We demonstrate that the presence of holes provides an efficient transport channel for electron relaxation and vice versa. Applications to all-optical switching and terahertz-optical control of direct-gap semiconductors are discussed.