Strong generation of coherent acoustic phonons in semiconductor quantum dots

We present experimental results obtained in two-color pump-probe experiments performed in semiconductor self-assembled quantum dot (QD) layers. The sample reflectivities present several acoustic contributions, among which are strong acoustic phonon wave packets. A comparison between one- and two-color experiments and a fine analysis of the echo shape attest that a high magnitude phonon pulse emerges from each single QD layer. This conclusion is supported by a numerical modeling which perfectly reproduces our experimental signals only if we introduce a strong generation in each QD layer. We explain such a strong emission thanks to an efficient capture of the carriers by the QDs.     

Light absorption involving longitudinal optical phonons in semiconductor quantum dots

A theory of single-photon interband transitions involving optical phonons in semiconductor quantum dots (QDs) has been developed. This theory assumes that the electron subsystem of QDs with infinite potential walls is in strong confinement, and its energy spectrum can be described according to the two-band semiconductor model. Longitudinal optical phonons are considered to be related to the QD electron subsystem via polar (Fröhlich) electronphonon interaction. It is shown that, in these approximations, only the off-diagonal part of electron-phonon interaction leads to the generation of electron-hole pairs with the participation of phonons; the selection rules for these transitions differ from those for zero-phonon transitions. Analytical expressions for the light-absorption coefficients of ensembles of identical and size-distributed QDs have been obtained.     

The role of acoustic phonons for Rabi oscillations in semiconductor quantum dots

The damping of Rabi oscillations in quantum dots as well as the renormalization of the carrier-light coupling, due to the interaction with longitudinal acoustic phonons are studied as a function of temperature and laser pulse parameters. Numerical results are obtained by using a correlation expansion within the density matrix theory. The observed features like a non-monotonous dependence of the damping on the pulse duration are characteristic for the strongly non-Markovian nature of the phonon coupling in these systems. The results can be well interpreted on the level of a perturbation expansion in the carrier-phonon interaction. PACS 78.67.Hc; 63.20.Kr; 03.65.Yz     

Electrodeposited quantum dots IV. Epitaxial short-range order in amorphous semiconductor nanostructures

The structure of diffraction-amorphous CdSe (a-CdSe) quantum dots (QDs) electrodeposited on evaporated Pd substrate was studied by high resolution transmission electron microscopy (HRTEM), and compared with epitaxial (crystalline) QDs obtained by the same procedure on Au, as well as with a simulated image of random a-CdSe. Digital analysis of HRTEM images established the existence of repeating ordered motifs in a-CdSe QDs on Pd substrates, in the form of epitaxial sub-nanometre to nanometre size clusters. The QDs are shown to be intermediate between crystalline and random amorphous material. Digital Fourier analysis indicated epitaxial relationship with the 111inPd substrate, rotated 30° relative to the orientational relationship on 111_Au.     

Optical phonons in semiconductor quantum rods

Surface polar-optical phonons are analyzed for semiconductor quantum rods (QR). The cylindrical symmetry is modelled in the form of a prolate spheroid where the aspect-ratio is identified with the corresponding ratio of the ellipsoid semiaxes. Using a theory based on the dielectric continuum approach, we consider a single CdSe QR inserted in a host material, assumed as an infinite dielectric, and viewed as an inactive rigid medium to the oscillations. Our results for CdSe are discussed, together with the differences from spherical and quasi-spherical quantum dots. We believe these are important steps to help the understanding of future experiments in this new class of materials.     

Infrared photodetection with semiconductor self-assembled quantum dots

Semiconductor self-assembled quantum dots are potential candidates to develop a new class of midinfrared quantum photodetectors and focal plane arrays. In this article, we present the specific midinfrared properties of InAs/GaAs quantum dots associated with the intersublevel transitions. The electronic structure, which accounts for the strain field in the islands, is obtained within the framework of a three-dimensional 8 band k.p formalism. The midinfrared intersublevel absorption in n-doped quantum dots is described. We show that the carrier dynamics can be understood in terms of polarons which result from the strong coupling regime for the electron–phonon interaction in the dots. The principle of operation of vertical and lateral quantum dot infrared photodetectors is described and discussed by comparison with quantum well infrared photodetectors. We review the performances of different type of detectors developed to date and finally give some orientation to realize high performance quantum dot infrared photodetectors. To cite this article: P. Boucaud, S. Sauvage, C. R. Physique 4 (2003).     

Pseudospin quantum computation in semiconductor nanostructures

We theoretically show that spontaneously interlayer-coherent bilayer quantum Hall droplets should allow robust and fault-tolerant pseudospin quantum computation in semiconductor nanostructures with voltage-tuned external gates providing qubit control and a quantum Ising Hamiltonian providing qubit entanglement. Using a spin-boson model, we estimate decoherence to be small (approximately 10(-5)).     

Few-electron semiconductor quantum dots with Gaussian confinement

We have performed Hartree-Fock calculations of the electronic structure of N ≤ 10 electrons in a quantum dot modeled with a confining Gaussian potential well. We discuss the conditions for the stability of N bound electrons in the system. We show that the most relevant parameter determining the number of bound electrons is V ₀ R ². Such a feature arises from widely valid scaling properties of the confining potential. Gaussian Quantum dots having N = 2, 5, and 8 electrons are particularly stable in agreement with the Hund rule. The shell structure becomes less and less noticeable as the well radius increases.      

Interface phonons in cylindrical quantum dot heterostructure

The top interface optical (TIO) and side interface optical (SIO) phonon modes of a cylindrical GaAs/ Al_xGa_1−xAs quantum dot are derived within the framework of dielectric continuum approximation. Results reveal that, in the case of taking the “two-mode” behavior of the Al_xGa_1−xAs material into account, there exist eight branches of TIO phonon modes and four branches of SIO phonon modes. The dispersion frequencies of TIO or SIO phonon modes sensitively depend on the Al mole fraction x$x$ in the Al_xGa_1−xAs material. With increasing wavevector q$q$ (κ$\kappa$), the frequency of each TIO (SIO) mode approaches one of the two frequency values of the single Al_xGa_1−xAs heterostructure.     

Interface phonons in asymmetric quantum well structures

In semiconductor microstructures with many layers, the phonon modes change from their bulk form and split into ‘confined LO phonons’ (LC) and ‘interface phonons’ (IF), the number and variety of which depends on both the number of layers and the number of different materials in the structure. This affects the electron–phonon scattering rates. Because of the current interest in inter-subband THz emitters, we use these LC and IF modes to evaluate the inter-subband electron–phonon scattering rate in THz emitter prototypes that are based on four-subband stepped quantum wells. These scattering rates in turn affect the population inversion predicted for these devices, so we compare the predicted population inversions for the most promising prototypes against those obtained using bulk phonon scattering rates.     

Excitonic polarons in semiconductor quantum dots

The discretization of the electronic spectrum in semiconductor quantum dots implies a strong coupling behavior between the optical phonons and the electron-hole pairs, despite the fact that a pair is electrically neutral. The excitonic polarons strongly modify the optical spectra. In particular, the ground excitonic polaron contains one or two phonon components, which leads to the existence of phonon replicas in the luminescence. The population and coherence decay times of the optical transition associated with the ground excitonic polaron are calculated.     

Coulomb effects in semiconductor quantum dots

This paper presents a new model for the Internet graph (AS graph) based on the concept of heuristic trade-off optimization, introduced by Fabrikant, Koutsoupias and Papadimitriou in [5] to grow a random tree with a heavily tailed degree distribution. We propose here a generalization of this approach to generate a general graph, as a candidate for modeling the Internet. We present the results of our simulations and an analysis of the standard parameters measured in our model, compared with measurements from the physical Internet graph.Received: 9 February 2004, Published online: 14 May 2004PACS: 89.75.-k Complex systems - 89.75.Hc Networks and genealogical trees - 89.75.Da Systems obeying scaling laws - 89.75.Fb Structures and organization in complex systems - 89.65.Gh Economics; econophysics, financial markets, business and managementLRI: http: //www.lri.fr/~ihameli; CNRS, LIP, ENS Lyon : http: //www.ens-lyon.fr/~nschaban     

半导体量子点中的杂化量子比特

文章介绍了半导体新型量子比特——杂化量子比特。通过与半导体量子点中自旋量子比特和电荷量子比特进行比较,阐述了杂化量子比特兼具长相干与快操控的优点。在总结了杂化量子比特发展与现状的基础上,进一步简单介绍了中国科学技术大学中国科学院量子信息重点实验室在改进型杂化量子比特方面的工作成果。     

Vector solitons in semiconductor quantum dots

A theory of an optical vector soliton of self-induced transparency in an ensemble of semiconductor quantum dots is considered. By using the perturbative reduction method, the system of the Maxwell–Liouville equations is reduced to the two-component coupled nonlinear Schrödinger equations. It is shown that a distribution of transition dipole moments of the quantum dots and phase modulation changes significantly the pulse parameters. The shape of the optical two-component vector soliton with the sum and difference of the frequencies in the region of the carrier frequency is presented. The vector soliton can be reduced to the breather solution of self-induced transparency with a different profile. Explicit analytical expressions in the presence of single-excitonic and biexcitonic transitions for the optical vector soliton are obtained with realistic parameters which can be reached in current experiments.     

Optical breathers in semiconductor quantum dots

A theory of resonant optical breathers in the presence of single and biexciton transitions in an ensemble of inhomogeneously broadened semiconductor quantum dots is constructed. Explicit analytical expressions for the breather shape and parameters for experimental investigations are proposed.     

Control of spin state in CdSe quantum dots by coupling with magnetic semiconductor quantum dots

We studied spin states of CdSe quantum dots (QDs) coupled with CdMnSe QDs by probing circular polarization of photoluminescence spectrum under external magnetic fields. The bandgap energies of CdSe and CdMnSe QDs are close to each other and photoluminescence mainly originates from CdSe QDs due to relatively low radiation efficiency of CdMnSe QDs. The photoluminescence lifetime as well as its intensity was decreased with increasing magnetic field, which was ascribed to the increase in the ground state wavefunctions in CdMnSe QDs. The decrease was more pronounced for spin down electrons, which was explained by the difference in spin up and down wave functions under magnetic fields. Our results show that the spin state of CdSe QDs can be manipulated by coupling with CdMnSe QDs.     

Influence of phonons on the electronic energy spectrum of small semiconductor quantum dots in a dielectric matrix

A diagrammatic technique developed for Green’s functions with inclusion of multiphonon processes is used to investigate the electronic energy levels and the phonon replicas corresponding to them in a semiconductor quantum dot (QD) embedded in a dielectric matrix. It is shown, with reference to GaAs, CdSe, and CuCl quantum dots embedded in glass, that in the case of QD potential wells of a finite depth the shifts of the electronic energy levels decrease with decreasing QD size, irrespective of the strength of electron-phonon coupling in the nanoheterostructure. Theoretically calculated positions of the phonon replicas for CdSe in glass agree with the experimental data on Raman scattering.