Optical properties of arrays of quantum dots with internal disorder

Optical properties of large arrays of isolated quantum dots are discussed in order to interpret the existent photoluminescence data. The presented theory explains the large observed shift between the lowest emission and absorption energies as the average distance between the ground and first excited states of the dots. The lineshape of the spectra is calculated for the case when the fluctuations of the energy levels in quantum dots are due to the alloy composition fluctuations. The calculated lineshape is in good agreement with the experimental data. The influence of fluctuations of the shape of quantum dots on the photoluminescence spectra is also discussed.Comment: 7 pages (twocolumn) LATEX, 6 Postscript figure

Exciton lifetime in InAs/GaAs quantum dot molecules

The exciton lifetimes $T_1$ in arrays of InAs/GaAs vertically coupled quantum dot pairs have been measured by time-resolved photoluminescence. A considerable reduction of $T_1$ by up to a factor of $\sim$ 2 has been observed as compared to a quantum dots reference, reflecting the inter-dot coherence. Increase of the molecular coupling strength leads to a systematic decrease of $T_1$ with decreasing barrier width, as for wide barriers a fraction of structures shows reduced coupling while for narrow barriers all molecules appear to be well coupled. The coherent excitons in the molecules gain the oscillator strength of the excitons in the two separate quantum dots halving the exciton lifetime. This superradiance effect contributes to the previously observed increase of the homogeneous exciton linewidth, but is weaker than the reduction of $T_2$. This shows that as compared to the quantum dots reference pure dephasing becomes increasingly important for the molecules

Probing energy barriers and quantum confined states of buried semiconductor heterostructures with ballistic carrier injection: An experimental study

A three-terminal spectroscopy that probes both subsurface energy barriers and interband optical transitions in a semiconductor heterostructure is demonstrated. A metal-base transistor with a unipolar p-type semiconductor collector embedding InAs/GaAs quantum dots (QDs) is studied. Using minority/majority carrier injection, ballistic electron emission spectroscopy and its related hot-carrier scattering spectroscopy measures barrier heights of a buried AlxGa1-xAs layer in conduction band and valence band respectively, the band gap of Al0.4Ga0.6As is therefore determined as 2.037 +/- 0.009 eV at 9 K. Under forward collector bias, interband electroluminescence is induced by the injection of minority carriers with sub-bandgap kinetic energies. Three emission peaks from InAs QDs, InAs wetting layer, and GaAs are observed in concert with minority carrier injection.Comment: 11 pages, 4 figures, submitted to Physical Review

Multi-Exciton Spectroscopy of a Single Self Assembled Quantum Dot

We apply low temperature confocal optical microscopy to spatially resolve, and spectroscopically study a single self assembled quantum dot. By comparing the emission spectra obtained at various excitation levels to a theoretical many body model, we show that: Single exciton radiative recombination is very weak. Sharp spectral lines are due to optical transitions between confined multiexcitonic states among which excitons thermalize within their lifetime. Once these few states are fully occupied, broad bands appear due to transitions between states which contain continuum electrons.Comment: 12 pages, 4 figures, submitted for publication on Jan,28 199

Optimization of heat treatment in cold-drawn 6063 aluminium tubes

The effect of heat treatment condition on mechanical properties and bendability of 6063 aluminium cold-drawn tubes was investigated. The standardized heat treatment presently used in manufacturing of cold-drawn tubes increases the cost and time of the process which ultimately reduces plant productivity. The effects of time, temperature, and furnace heating rate were studied in order to identify an optimized heat treatment for tubes with different cold work levels. Drawn from the as-extruded state, tubes were heat treated to under-aged, peak-aged, and over-aged conditions with time and temperature ranging from 1 min to 24 h and 130\u2013200 \u25e6C, respectively. Mechanical properties were determined with full section tensile tests whereas tube bendability was evaluated on an industrial draw bending machine. These characteristics were evaluated in each condition in order to identify the heat treatment which allows conforming to 6063-T832 temper requirements and gives sufficient bendability. Moreover, bendability was successfully correlated to fracture strain measured during a uniaxial tensile test and a threshold value over which problem-free bending operation was determined.On a \ue9tudi\ue9 l\u2019effet des conditions de traitement thermique sur les propri\ue9t\ue9s m\ue9caniques et la flexibilit\ue9 de tubes en aluminium 6063 \ue9tir\ue9s \ue0 froid. Le traitement thermique standardis\ue9 pr\ue9sentement utilis\ue9 pour la production de tubes \ue9tir\ue9s \ue0 froid fait cro\ueetre le co\ufbt et la dur\ue9e du proc\ue9d\ue9, ce qui finalement r\ue9duit la productivit\ue9 de l\u2019usine. Les effets du temps, de la temp\ue9rature et de la vitesse de chauffage du four ont \ue9t\ue9 \ue9tudi\ue9s afin d\u2019identifier un traitement thermique optimis\ue9 pour les tubes avec diff\ue9rents niveaux de travail \ue0 froid. \uc9tir\ue9s \ue0 partir de l\u2019\ue9tat extrud\ue9 tel quel, les tubes ont \ue9t\ue9 chauff\ue9s dans des conditions de sous-vieillissement, vieillissement pic et sur-vieillissement avec des temps et des temp\ue9ratures allant de 1 min \ue0 24 h et de 130 \ue0 200 \ub0C. On a d\ue9termin\ue9 les propri\ue9t\ue9s m\ue9caniques au moyen d\u2019essais de tension pleine section, alors que la flexibilit\ue9 a \ue9t\ue9 \ue9valu\ue9e sur une machine industrielle de cintrage par enroulement. Ces caract\ue9ristiques ont \ue9t\ue9 \ue9valu\ue9es pour chaque condition afin d\u2019identifier le traitement thermique qui permet de se conformer aux exigences de duret\ue9 du 6063 T832 et donne une flexibilit\ue9 suffisante. En plus de corr\ue9ler avec succ\ue8s la flexibilit\ue9 \ue0 la d\ue9formation de rupture mesur\ue9e au cours d\u2019un essai de tension dans un axe, une valeur seuil au-dessus de laquelle un cintrage sans probl\ue8me est possible a \ue9t\ue9 d\ue9termin\ue9e.Peer reviewed: YesNRC publication: Ye

Ensemble interactions in strained semiconductor quantum dots

Large variations in InxGa1-xAs quantum dot concentrations were obtained with simultaneous growths on vicinal GaAs [001] substrates with different surface step densities. It was found that decreasing dot-dot separation blueshifts all levels, narrows intersublevel transition energies, shortens luminescence decay times for excited states, and increases inhomogeneous photoluminescence broadening. These changes in optical properties are attributed to a progressive strain deformation of the confining potentials and to the increasing effects of positional disorder in denser dot ensembles

Multiple Wavelength InGaAs Quantum Dot Lasers Using Ion Implantation Induced Intermixing

We demonstrate multiple wavelength InGaAs quantum dot lasers using ion implantation induced intermixing. Proton implantation, followed by annealing is used to create differential interdiffusion in the active region of the devices. The characteristics (lasing-spectra, threshold currents and slope efficiencies) of the multi-wavelength devices are compared to those of as-grown devices and the differences are explained in terms of altered energy level spacing in the annealed quantum dots

What do evidence-based secondary journals tell us about the publication of clinically important articles in primary healthcare journals?

BACKGROUND: We conducted this analysis to determine i) which journals publish high-quality, clinically relevant studies in internal medicine, general/family practice, general practice nursing, and mental health; and ii) the proportion of clinically relevant articles in each journal. METHODS: We performed an analytic survey of a hand search of 170 general medicine, general healthcare, and specialty journals for 2000. Research staff assessed individual articles by using explicit criteria for scientific merit for healthcare application. Practitioners assessed the clinical importance of these articles. Outcome measures were the number of high-quality, clinically relevant studies published in the 170 journal titles and how many of these were published in each of four discipline-specific, secondary "evidence-based" journals (ACP Journal Club for internal medicine and its subspecialties; Evidence-Based Medicine for general/family practice; Evidence-Based Nursing for general practice nursing; and Evidence-Based Mental Health for all aspects of mental health). Original studies and review articles were classified for purpose: therapy and prevention, screening and diagnosis, prognosis, etiology and harm, economics and cost, clinical prediction guides, and qualitative studies. RESULTS: We evaluated 60,352 articles from 170 journal titles. The pass criteria of high-quality methods and clinically relevant material were met by 3059 original articles and 1073 review articles. For ACP Journal Club (internal medicine), four titles supplied 56.5% of the articles and 27 titles supplied the other 43.5%. For Evidence-Based Medicine (general/family practice), five titles supplied 50.7% of the articles and 40 titles supplied the remaining 49.3%. For Evidence-Based Nursing (general practice nursing), seven titles supplied 51.0% of the articles and 34 additional titles supplied 49.0%. For Evidence-Based Mental Health (mental health), nine titles supplied 53.2% of the articles and 34 additional titles supplied 46.8%. For the disciplines of internal medicine, general/family practice, and mental health (but not general practice nursing), the number of clinically important articles was correlated withScience Citation Index (SCI) Impact Factors. CONCLUSIONS: Although many clinical journals publish high-quality, clinically relevant and important original studies and systematic reviews, the articles for each discipline studied were concentrated in a small subset of journals. This subset varied according to healthcare discipline; however, many of the important articles for all disciplines in this study were published in broad-based healthcare journals rather than subspecialty or discipline-specific journals

Spin interactions and switching in vertically tunnel-coupled quantum dots

We determine the spin exchange coupling J between two electrons located in two vertically tunnel-coupled quantum dots, and its variation when magnetic (B) and electric (E) fields (both in-plane and perpendicular) are applied. We predict a strong decrease of J as the in-plane B field is increased, mainly due to orbital compression. Combined with the Zeeman splitting, this leads to a singlet-triplet crossing, which can be observed as a pronounced jump in the magnetization at in-plane fields of a few Tesla, and perpendicular fields of the order of 10 Tesla for typical self-assembled dots. We use harmonic potentials to model the confining of electrons, and calculate the exchange J using the Heitler-London and Hund-Mulliken technique, including the long-range Coulomb interaction. With our results we provide experimental criteria for the distinction of singlet and triplet states and therefore for microscopic spin measurements. In the case where dots of different sizes are coupled, we present a simple method to switch on and off the spin coupling with exponential sensitivity using an in-plane electric field. Switching the spin coupling is essential for quantum computation using electronic spins as qubits.Comment: 13 pages, 9 figure

Electronic States and Light Absorption in a Cylindrical Quantum Dot Having Thin Falciform Cross Section

Energy level structure and direct light absorption in a cylindrical quantum dot (CQD), having thin falciform cross section, are studied within the framework of the adiabatic approximation. An analytical expression for the energy spectrum of the particle is obtained. For the one-dimensional “fast” subsystem, an oscillatory dependence of the wave function amplitude on the cross section parameters is revealed. For treatment of the “slow” subsystem, parabolic and modified Pöschl-Teller effective potentials are used. It is shown that the low-energy levels of the spectrum are equidistant. In the strong quantization regime, the absorption coefficient and edge frequencies are calculated. Selection rules for the corresponding quantum transitions are obtained