Results 1 - 10 of 24576
Results 1 - 10 of 24576. Search took: 0.037 seconds
|Sort by: date | relevance|
[en] We discuss the optical properties of semiconductor microcavities. We introduce to both weak and strong coupling regimes of the light - matter interaction. We discuss the modifications of the radiative emission rate and pattern in the weak coupling regime and the cavity - polariton features in the strong coupling regime. (orig.)
[en] The electron-hole (e-h) exchange interaction leads to the splitting of the exciton into a pair of bright and a pair of dark states. This bright-dark-or singlet-triplet-exciton splitting was historically calculated as the sum of a long-range (LR) and a short-range (SR) component. Using a numerical atomistic approach, we are able to calculate the exchange integrals as a function of the e-h range of interaction S, revealing the 'internal' structure of the integrals. We apply this procedure to thickness-fluctuation GaAs/AlGaAs quantum dots (QDs), self-assembled InAs/GaAs QDs and colloidal InAs QDs. We find a heterogeneous situation, where the SR component contributes ∼10, ∼20-30 and ∼20-50% to the total e-h exchange splitting, which is in the range of 10, 100 and 10 000 μeV, for the three types of QDs, respectively. The balance between SR and LR is found to depend critically on the size, shape and type of structure. For all types of QDs we find, surprisingly, a range of interaction, close to the physical dimension of the structures, contributing to a reduction of the integral's magnitude. These results highlight the complexity of the exchange interaction, warning against simplified models, and establish the basic features of the nature and origin of dark-bright excitonic splitting in QDs.
[en] All six new arsenides were prepared by arc-melting of preheated mixtures of the monoarsenides MAs with the 3d metals Fe, Co, and Ni, respectively. The isostructural title compounds all form the Co2Si structure type, in contrast to the corresponding phosphides with ZrNiP occurring in the Ni2In type. The anomalous expansions of the unit cells from ZrCoAs to ZrNiAs (V = 178.5(3) A3 versus V = 182.5(1) A3) and from HfCoAs to HfNiAs (V = 175.03(5) A3 versus V 177.0(1) A3) can be understood based on extended Hueckel calculations of the electronic structure of HfCoAs. (orig.)
[en] We use a picosecond acoustics technique to modulate the laser output of electrically pumped GaAs/AlAs micropillar lasers with InGaAs quantum dots. The modulation of the emission wavelength takes place on the frequencies of the nanomechanical extensional and breathing (radial) modes of the micropillars. The amplitude of the modulation for various nanomechanical modes is different for every micropillar which is explained by a various elastic contact between the micropillar walls and polymer environment
[en] The results of a series of studies concerned with formation of epitaxially integrated InGaAs/AlGaAs and AlGaAs/AlGaAs heterostructures with several emitting regions and with investigation of properties of laser diodes based on the above structures operating in the spectral ranges λ = 800–810, 890–910, and 1040–1060 nm are summarized. It is shown that the suggested approach to integration of individual laser structures by the method of the MOVPE epitaxy operates efficiently in fabrication of laser diodes for a wide spectral range on the basis of various types of heterostructures. This approach made it possible to efficiently increase the output power of the laser diodes practically without variation in their mass-and-dimension characteristics. The main advantages of this approach and its limitations are outlined. Epitaxial integration of two laser heterostructures made it possible to increase the differential quantum efficiency by 1.7–2.0 times, while integration of three laser heterostructures increases the differential quantum efficiency by a factor of 2.5–3.0.
[en] Generation of a difference-frequency wave by two electromagnetic waves propagating in a heterolaser is analyzed theoretically. Calculations are carried out for InGaAs/GaAs/AlGaAs heterostructures of design optimized to attain maximum lasing power. It is shown that phase matching between the primary waves and the difference-frequency wave may persist over a distance of ∼1 mm, comparable to the cavity length (2-3 mm), and the conversion coefficient can be as large as several percent
[en] The photocurrent spectra induced by the anomalous Hall effect (AHE) of the (001)-oriented GaAs/AlGaAs quantum wells (QWs) with well width of 3 and 7 nm have been investigated at room temperature. Ultra-thin InAs layers with a thickness of 1 monolayer have been inserted at GaAs/AlGaAs interfaces to tune the asymmetry of the QWs. It is demonstrated that the AHE current can be effectively tuned by the inserted ultra-thin InAs layers and by the well width. A method has been proposed to separate the intrinsic and extrinsic mechanisms of the AHE, which can be also applied to spin Hall effect.
[en] The coupling between Bloch oscillations and longitudinal optical (LO) phonons is investigated in ternary In0.53Ga0.47As/In0.52Al0.48As superlattices of different well widths. In femtosecond time-resolved studies, a strong increase of the coherent LO phonon amplitudes is observed when the Bloch oscillations are subsequently tuned into resonance with the different optical phonon modes of the ternary semiconductor materials. In a narrow-well superlattice where electronic minibands are energetically shifted close to the confining barrier potential, the phonon amplitudes are asymmetrically enhanced on the high-frequency edge of the resonance. Here, at high electric fields, field induced tunneling into above-barrier continuum states leads to a rapid dephasing of Bloch oscillations. The associated polarization change provides an additional excitation process for coherent LO phonons. (copyright 2004 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
[en] 120 nm gate-length In0.7Ga0.3As/In0.52Al0.48As InP-based high electron mobility transitions (HEMTs) are fabricated by a new T-shaped gate electron beam lithograph (EBL) technology, which is achieved by the use of a PMMA/PMGI/ZEP520/PMGI four-layer photoresistor stack. These devices also demonstrate excellent DC and RF characteristics: the transconductance, maximum saturation drain-to-source current, threshold voltage, maximum current gain frequency, and maximum power-gain cutoff frequency of InGaAs/InAlAs HEMTs is 520 mS/mm, 446 mA/mm, -1.0 V, 141 GHz and 120 GHz, respectively. The material structure and all the device fabrication technology in this work were developed by our group.
[en] When InAs is grown above GaAs or AlAs, a lattice mismatch between the two layers causes strain fields to develop. Above a critical thickness, these strain fields cause the InAs to form islands or dots. The dots are typically 10nm in diameter, and 4nm high. As the dimensions of the dots are below the typical electron wavelength, zero dimensional states are formed. In this thesis quantum dots are grown within an AlAs tunnel barrier. When a bias is applied between the top and bottom contacts of the device, resonant tunnelling is observed of electrons from the two dimensional electron gas formed in front of the AlAs tunnel barrier, through individual InAs quantum dots, despite the presence of several million dots within a typical sample. By measuring the tunnel current through a dot as a function of the applied bias, information can be gained as to the properties of the dot itself, and the electron gas from which tunnelling occurs. Conventional magneto-transport measurements are sensitive only to effects at the Fermi energy of the electron emitter gas. Due to the zero-dimensionality of the quantum dots, a single dot can be used as a spectroscopic probe of the occupied density of states of an electron gas at all energies between the Fermi energy and the subband edge, in a manner not available to any other measurement technique. Further, many body effects cause an enhancement of the tunnel current when the dot ground state is resonant with the Fermi energy of the two dimensional electron gas. The ground state of the quantum dot can also therefore be used to study many body processes at the Fermi level. (author)