Dephasing processes in a single semiconductor quantum dot

We discuss the decoherence dynamics in a single semiconductor quantum dot and analyze two dephasing mechanisms. In the first part of the review, we examine the intrinsic source of dephasing provided by the coupling to acoustic phonons. We show that the non-perturbative reaction of the lattice to the interband optical transition results in a composite optical spectrum with a central zero-phonon line and lateral side-bands. In fact, these acoustic phonon side-bands completely dominate the quantum dot optical response at room temperature. In the second part of the article, we focus on the extrinsic dephasing mechanism of spectral diffusion that determines the quantum dot decoherence at low temperatures. We interpret the variations of both width and shape of the zero-phonon line as due to the fluctuating electrostatic environment. In particular, we demonstrate the existence of a motional narrowing regime in the limit of low incident power or low temperature, thus revealing an unconventional phenomenology compared to nuclear magnetic resonance. To cite this article: G. Cassabois, R. Ferreira, C. R. Physique 9 (2008).     

Cavity QED effects with single quantum dots

A single quantum dot embedded in a photonic crystal defect cavity allows for the investigation of cavity quantum electrodynamics effects in a solid-state environment. We present experiments demonstrating the quantum nature of this fundamental system in the strong coupling regime. Photon correlation measurements are used to characterize the fundamental properties of this unique system: through these experiments, we identify an unexpected, efficient sustaining mechanism that ensures strong cavity emission and is quantum correlated with the exciton resonance, even when all the quantum dot resonances are far detuned from the cavity mode. To cite this article: A. Badolato et al., C. R. Physique 9 (2008).     

Intersublevel transitions in self-assembled quantum dots

Intersublevel transitions in semiconductor quantum dots are transitions of a charge carrier between quantum dot confined states. In InAs/GaAs self-assembled quantum dots, optically active intersublevel transitions occur in the mid-infrared spectral range. These transitions can provide a new insight on the physics of semiconductor quantum dots and offer new opportunities to develop mid-infrared devices. A key feature characterizing intersublevel transitions is the coupling of the confined carriers to phonons. We show that the effect of the strong coupling regime for the electron–optical phonon interaction and the formation of mixed electron–phonon quasi-particles called polarons drastically affect and control the dynamical properties of quantum dots. The engineering of quantum dot relaxation rates through phonon coupling opens the route to the realization of new devices like mid-infrared polaron lasers. We finally show that the measurement of intersublevel absorption is not limited to quantum dot ensembles and that the intersublevel ultrasmall absorption of a single quantum dot can be measured with a nanometer scale resolution by using phonon emission as a signature of the absorption. To cite this article: P. Boucaud et al., C. R. Physique 9 (2008).     

Optical nonlinearity enhanced by metal nanoparticle in CdTe quantum dots

The optical nonlinearity of a CdTe quantum dot enhanced by a gold nanoparticle has been theoretically studied by employing the multi-bands effective mass method. The energy levels have been computed using 6×6 k.p model for the valence band. The semiconductor quantum dot-size-dependent third-order susceptibility of third harmonic generation in a CdTe-Au nanocrystal complex has been analyzed. It is found that the metal nanoparticle enhances the optical nonlinearity of the semiconductor quantum dot due to the dipole/multipole interaction that will bring in the strong damping and the field enhancement. By choosing the radius of CdTe quantum dot, the third-order nonlinear susceptibility for third harmonic generation can be optimized for the one- and multi-photon resonance. Also, we can alter the optical nonlinearity by changing the ratio of semiconductor-metal nanoparticle distance to the metal nanoparticle radius.     

Optical and electronic properties of quantum dots with magnetic impurities

The article discusses some of the recent results on semiconductor quantum dots with magnetic impurities. A single Mn impurity incorporated in a quantum dot strongly changes the optical response of a quantum-dot system. A character of Mn-carrier interaction is very different for II-VI and III-V quantum dots (QDs). In the II-VI QDs, a Mn impurity influences mostly the spin-structure of an exciton. In the III-V dots, a spatial localization of hole by a Mn impurity can be very important, and ultimately yields a totally different spin structure. A Mn-doped QD with a variable number of mobile carriers represents an artificial magnetic atom. Due to the Mn-carrier interaction, the order of filling of electronic shells in the magnetic QDs can be very different to the case of the real atoms. The “periodic” table of the artificial magnetic atoms can be realized in voltage-tunable transistor structures. For the electron numbers corresponding to the regime of Hund's rule, the magnetic Mn-carrier coupling is especially strong and the magnetic-polaron states are very robust. Magnetic QD molecules are also very different to the real molecules. QD molecules can demonstrate spontaneous breaking of symmetry and phase transitions. Single QDs and QD molecules can be viewed as voltage-tunable nanoscale memory cells where information is stored in the form of robust magnetic-polaron states. To cite this article: A.O. Govorov, C. R. Physique 9 (2008).     

Can multiband coupling effects due to remote bands cause significant normal-incidence absorption inn-type direct-gap semiconductor quantum wells?

Surprisingly, several experiments have reported that normal-incidence light absorption due to inter-conduction-subband transitions in direct-gap semiconductor quantum wells is as strong as in-plane-incidence absorption. In contrast to other models, a recent theoretical study claimed that a 14-bandk  pmodel including multiband coupling terms due toremote-conduction bandsis able to explain the experimental results. In this work, a concise formulation extends the model beyond 14 bands. Nevertheless, after rederiving the optical transition matrix elements, this analysis clearly shows that the oscillator strength for the in-plane polarized optical intersubband transition due to the multiband coupling effects is much smaller than the oscillator strength for the normal-to-plane polarized optical intersubband transition. These results indicate that the multiband coupling effects due to remote-conduction bands cannot cause a sufficient in-plane polarized optical intersubband transition to produce the observed normal-incidence absorption in the desirablen-type III–V compound semiconductor quantum wells.     

A study of nonlinear optical properties of a negative donor quantum dot

In this paper, we studied the nonlinear optical properties of a negative donor center (D⁻) in a disk-like quantum dot (QD) with a Gaussian confining potential. Calculations are carried out by using the method of numerical diagonalization of Hamiltonian matrix within the effective-mass approximation. A detailed investigation of the linear, third-order nonlinear, total optical absorptions and refractive index changes has been carried out for the D⁻ QD and the D⁰ QD. The linear, third-order nonlinear, total optical absorptions and refractive indices have been examined for a double-electron QD with and without impurity. Our results show that the optical absorption coefficients and refractive indices in a disk-like QD are much larger than their values for quantum wells and spherical QDs and the nonlinear optical properties of QDs are strongly affected not only with the confinement barrier height, dot radius, the number of electrons but also the electron-impurity interaction.     

The infrared spin-galvanic effect in semiconductor quantum wells

The spin-galvanic effect generated by homogeneous optical excitation with infrared circularly polarized radiation in quantum wells (QWs) is reviewed. The spin-galvanic current flow is driven by an asymmetric distribution of spin-polarized carriers in k-space of systems with lifted spin degeneracy due to k-linear terms in the Hamiltonian. Spin photocurrents provide methods to investigate the spin-splitting of the band structure and to make conclusion on the in-plane symmetry of QWs.     

Nuclear spin diffusion effects in optically pumped quantum wells

We studied the influence of the nuclear spin diffusion on the dynamical nuclear polarization of low dimensional nanostructures subject to optical pumping. Our analysis shows that the induced nuclear spin polarization in semiconductor nanostructures will develop both a time and position dependence due to a nonuniform hyperfine interaction as a result of the geometrical confinement provided by the system. In particular, for the case of semiconductor quantum wells, nuclear spin diffusion is responsible for a nonzero nuclear spin polarization in the quantum well barriers. As an example we considered a 57 Å GaAs square quantum well and a 1000 Å Al _x Ga_1?x As parabolic quantum well both within 500 Å Al_0.4Ga_0.6As barriers. We found that the average nuclear spin polarization in the quantum well barriers depends on the strength of the geometrical confinement provided by the structure and is characterized by a saturation time of the order of few hundred seconds. Depending on the value of the nuclear spin diffusion constant, the average nuclear spin polarization in the quantum well barriers can get as high as 70% for the square quantum well and 40% for the parabolic quantum well. These results should be relevant for both time resolved Faraday rotation and optical nuclear magnetic resonance experimental techniques.     

Dependence of electron spin g-factor on magnetic field in quantum wells

The dependence of electron spin g-factor on magnetic field has been investigated in GaAs/AlGaAs quantum wells. We have estimated the electron g-factor from spin precession frequency in time-resolved photoluminescence measurements under a magnetic field in different configurations; the magnetic field perpendicular (g_⊥) and parallel (g_∥) to the quantum confinement direction. When the angle between the magnetic field and the confinement direction is 45°, we have found that g-factor varies depending on the direction of magnetic field and the circular polarization type of excitation light (σ⁺ or σ^?). These dependences of g-factor exhibit main features of Overhauser effect that nuclear spins react back on electron spin precession. The value of g_⊥ and g_∥ corrected for the nuclear effects agree well with the results of four-band k·p perturbation calculations.     

Deformation potentials and photo-response of strained PbSe quantum wells and quantum dots

Deformation potentials (D_u and D_d) for PbSe where analyzed using transmission data of PbSe/PbEuSeTe multi-quantum wells (MQWs). We use calculations based on a k·p model to obtain the strain induced intervalley splitting in the quantum wells. For the reduction of the Fabri–Pérot interference fringes of the multilayer structures we design a PbEuSeTe/BaF₂ anti-reflex coating which is deposited on top of the MQWs. At low temperature we found PbSe deformation potentials D_u=-2.36 and 5.88. The results of the transmission measurements are compared with photo-current spectra measured with self-assembled PbSe/PbEuTe quantum dot superlattices.     

Spin interaction and magnetic field strength effects on the system of two interacting electrons in a 2D quartic confinement potential

In this study we review concepts of double quantum dot, quantum chaos with shifted 1/N expansion method associated with semiquantum nonlinear system. We present a numerical study of two interacting particle motions in a time dependent magnetic field in quartic geometry. It is evident that, the area where the interaction particle motions are stochastic decreases as the spin interaction strength decreases, as well as, the magnetic field strength decreases. Moreover, we describe their possible connections with other aspects of quantum information. Furthermore, we pay attention to a system of two interacting electrons in a two-dimensional quartic confinement potential and hypothesis leading to analytical energy expression. The dynamics of double quantum dot gallium arsenide are of great importance and are also emphasized.     

Magnetooptics of (Zn,Cd,Mn)Te/ZnTe heterostructures with a small discontinuity in the valence band potential

Magnetooptics in (Zn,Cd,Mn)Te/ZnTe semimagnetic heterostructures with quantum wells has been thoroughly studied. For the Mn-containing quantum wells, an ordinary giant spin splitting effect is observed: the complete circular polarization in magnetic fields B> 0.5 T, a great increase in the intensity associated with the suppression of Auger recombination in the Mn ions, and a strong red shift of the σ⁺ polarized component are observed. On the contrary, the behavior of structures with nonmagnetic quantum wells and distant semimagnetic ZnMnTe layers is opposite. This unusual behavior is attributed to a high sensitivity of the exciton bond energy to the variation of the valence band potential in the structures with a small band discontinuity; this is confirmed by the computations and allows us to estimate the band chemical discontinuity as ?80 ± 20 meV in the ZnTe/CdTe system close to the photoemission data (T. M. Due, Phys. Rev. Lett. 58, 1127 (1987)).     

Formation and ordering of epitaxial quantum dots

Single quantum dots (QDs) have great potential as building blocks for quantum information processing devices. However, one of the major difficulties in the fabrication of such devices is the placement of a single dot at a pre-determined position in the device structure, for example, in the centre of a photonic cavity. In this article we review some recent investigations in the site-controlled growth of InAs QDs on GaAs by molecular beam epitaxy. The method we use is ex-situ patterning of the GaAs substrate by electron beam lithography and conventional wet or dry etching techniques to form shallow pits in the surface which then determine the nucleation site of an InAs dot. This method is easily scalable and can be incorporated with marker structures to enable simple post-growth lithographic alignment of devices to each site-controlled dot. We demonstrate good site-control for arrays with up to 10 micron spacing between patterned sites, with no dots nucleating between the sites. We discuss the mechanism and the effect of pattern size, InAs deposition amount and growth conditions on this site-control method. Finally we discuss the photoluminescence from these dots and highlight the remaining challenges for this technique. To cite this article: P. Atkinson et al., C. R. Physique 9 (2008).     

Nonlinear optical properties of a D system in a spherical quantum dot

The nonlinear optical properties of a D⁻ system confined in a spherical quantum dot represented by a Gaussian confining potential are studied. The great advantage of our methodology is that the model potential possesses the finite height and range. Calculations are carried out by using the method of numerical diagonalization of Hamiltonian matrix within the effective-mass approximation. We calculate the linear, third-order nonlinear and total optical absorption coefficients under the density matrix formalism. Numerical results for GaAs − Ga_1 − xAl_xAs QDs are presented. Our results show that the optical absorption coefficients in a spherical QD are much larger than their values for GaAs quantum wells. It is found that optical absorptions are strongly affected not only the confinement barrier height, dot radius, the electron-impurity interaction but also the position of the impurity.     

Coherent spin dynamics of electrons and excitons in nanostructures (a review)

The studies of spin phenomena in semiconductor low-dimensional systems have grown into the rapidly developing area of the condensed matter physics: spintronics. The most urgent problems in this area, both fundamental and applied, are the creation of charge carrier spin polarization and its detection, as well as electron spin control by nonmagnetic methods. Here, we present a review of recent achievements in the studies of spin dynamics of electrons, holes, and their complexes in the pump-probe method. The microscopic mechanisms of spin orientation of charge carriers and their complexes by short circularly polarized optical pulses and the formation processes of the spin signals of Faraday and Kerr rotation of the probe pulse polarization plane as well as induced ellipticity are discussed. A special attention is paid to the comparison of theoretical concepts with experimental data obtained on the n-type quantum well and quantum dot array samples.     

Accurate modelling of InGaN quantum wells

The internal field, the band structure and the oscillator strengths of the optical transitions of wurtzite strained InGaN quantum wells are accurately computed by a self-consistent solution of the Poisson equation and an eight-band k · p Schrödinger equation taking into account charges due to polarisation fields, doping and free carriers. The results are used to compare luminescence and gain spectra for single and triple quantum well structures and to elucidate the effect of the polarisation fields.     

From quantum optics to quantum communications

We review the possible roles of quantum optics and quantum information methods for future developments of optical telecommunications. To cite this article: I. Abram, P. Grangier, C. R. Physique 4 (2003).     

Thermal tripartite quantum correlations: quantum discord and entanglement perspectives

We investigate thermal tripartite quantum correlations for a spin star network and for a new extended version of it. In a spin star network, three peripheral spins interact with the central spin identically while in extended spin star network, three peripheral spins interact with two central spatially separated spins in the same way. We exploit the method of [C.C. Rulli, M.S. Sarandy, Phys. Rev. A 84, 042109 (2011)] to evaluate the tripartite quantum discord (TQD) and the method of [M. Li, S. Fei, Z. Wang, Rep. Math. Phys 65, 289 (2010)] called as lower bound of tripartite concurrence (LBTC) to evaluate the tripartite entanglement (TE) of the the peripheral parties in both systems. It is found that thermal TQD is much more robust than thermal TE as a function of temperature T. Also, the peripheral parties of the extended spin star network, in comparison with those of the spin star one, can exhibit higher values of TQD at T > 0. This, indeed, motivates us to realise improved quantum information and quantum computation tasks at finite temperatures.