Search

The electronic and optical properties of several smedium to high qualityd 4H-SiC epitaxial sensors for particle detection have been studied. The samples are n-doped Schottky diodes with different nitrogen concentrations (6×1013 cm-3-5×1015 cm-3d and thicknesses (20-40 μm). A full electrical and optical characterization has been performed by capacitance versus voltage measurements and near-band-edge low-temperature photoluminescence. The effective doping along the epilayer and the depletion width have been determined and data are consistent with the charge collection efficiency characterization performed with a minimum ionizing μ-source. All the investigated samples exhibit a 100% collection efficiency. In particular, the best samples yield a highly reproducible signal, well separated from the pedestal. Photoluminescence results show a linear relationship between the effective doping and the ratio of nitrogen-bound excitonic emission (Q0) and free excitonic line (I76), in agreement with a previous work on 4H-SiC with a higher doping concentration fI. G. Ivanov, C. Hallin, A. Henry, O. Kordina, and E. Janzén, J. Appl. Phys. 80, 3504 s1996dg. Moreover we show that the dependence of the major spectral features as a function of the penetration depth of the exciting laser beam can quantitatively provide information on substrate contribution to the photoluminescence. In conclusion, we bring evidence that a detailed characterization of SiC-based detectors, by all optical techniques, yields an accurate value for the net doping and gives a qualitative information on the epilayer thickness prior to any electrical wafer tests. [ABSTRACT FROM AUTHOR]

We present a detailed investigation performed at low temperature (T < 50 K) concerning the exciton dynamics in GaN epilayers grown on c-plane sapphire substrates, focusing on the exciton formation and the transition from the nonthermal to the thermal regime. The time-resolved kinetics of longitudinal-optical-phonon replicas is used to address the energy relaxation in the excitonic band. From picosecond time-resolved spectra, we bring evidence for a long lasting nonthermal excitonic distribution, which accounts for the first 50 ps. Such a behavior is confirmed in different experimental conditions when both nonresonant and resonant excitations are used. At low excitation power density, the exciton formation and their subsequent thermalization are dominated by impurity scattering rather than by acoustic phonon scattering. The estimate of the average energy of the excitons as a function of delay after the excitation pulse provides information on the relaxation time, which describes the evolution of the exciton population to the thermal regime. [ABSTRACT FROM AUTHOR]

Organic functionalization of carbon nanotube sidewalls is a tool of primary importance in material science and nanotechnology, equally from a fundamental and an applicative point of view.1, 2 Here, an efficient and versatile approach for the organic/organometallic functionalization of single-walled carbon nanotubes (SWCNTs) capable of imparting multimodality to these fundamental nanostructures, is described. Our strategy takes advantage of well-established Cu-mediated acetylene-azide coupling (CuAAC) reactions applied to phenylazido-functionalized SWCNTs for their convenient homo-/heterodecoration with a number of organic/organometallic frameworks, or mixtures thereof, bearing terminal acetylene pendant arms. Phenylazido-decorated SWCNTs were prepared by chemoselective arylation of the CNT sidewalls with diazonium salts under mild conditions, and subsequently used for the copper-mediated cycloaddition protocol in the presence of terminal acetylenes. The latter reaction was performed in one step by using either single acetylene derivatives or equimolar mixtures of terminal alkynes bearing either similar functional groups (masked with orthogonally cleavable protecting groups) or easily distinguishable functionalities (on the basis of complementary analytical/spectroscopic techniques). All materials and intermediates were characterized with respect to their most relevant aspects/properties by TEM microscopy, thermogravimetric analysis coupled with MS analysis of volatiles (TG-MS), elemental analysis, cyclic voltammetry (CV), Raman and UV/Vis spectroscopy. The functional loading and related chemical grafting of both primary amino- and ferrocene-decorated SWCNTs were spectroscopically (UV/Vis, Kaiser test) and electrochemically (CV) determined, respectively. [ABSTRACT FROM AUTHOR]

Examines the photoluminescence properties of multiple stacked planes of GaN or AlN quantum dots studied by near-field optical microscopy. Development of quantum structure with lower dimensionality; Usability of quantum wires or quantum dots to achieve higher brightness light emitting diodes; Application of photoluminescence techniques.

We report on time-resolved photoluminescence measurements carried out along the thickness gradient of two types of GaN/AlGaN quantum wells with low Al content in the barriers (5% and 9%, respectively). A reduction of the biexciton binding energy with increasing well thickness is observed, as a result of the reduced quantum confinement and the increasing impact of the quantum confined Stark effect. In the sample with 5% Al content in the barriers, for thicknesses of the well region larger than ∼9 monolayers, the biexciton binding energy is found to be smaller than that measured for bulk GaN. [ABSTRACT FROM AUTHOR]

We discuss the presence of a polariton bottleneck in bulk GaN microcavities. Angle resolved photoluminescence measurements were performed using low excitation power densities for several negative detunings between the exciton and photon modes. At low temperatures, we observe an enhancement in the emission intensity at angles corresponding to the anticrossing of the lower and upper polariton modes, a clear demonstration of the polariton relaxation bottleneck. This feature becomes less prominent with increasing temperature, eventually disappearing at room temperature. We conclude that polariton-acoustic phonon scattering is the dominant polariton relaxation mechanism in bulk GaN microcavities, as supported by theoretical simulations. [ABSTRACT FROM AUTHOR]

We present an experimental study of the carrier recombination dynamics in high-quality (InGa)(AsN)/GaAs and Ga(AsN)/GaAs quantum-well structures after picosecond excitation. A comparison among samples with and without nitrogen and with different In concentration shows that nonradiative channels originated in the host matrix [i.e., (InGa) As and GaAs] play a dominant role in the recombination dynamics of these heterostructures. [ABSTRACT FROM AUTHOR]

Studies the controlled tuning of the radiative lifetime in indium arsenic (InAs) self-assembled quantum dots through vertical ordering. Spontaneous formation of ordered bidimensional and tridimensional arrays of semiconductor nanostructures; Determination of the photoilluminescence spectra using standard photon counting techniques.

Investigates the electron and hole capture times in double-barrier quantum well (QW) infrared detectors. Relaxation of photoexcited carriers from aluminum arsenide/aluminum gallium arsenide/aluminum arsenide barriers into gallium arsenide (GaAs) QW; Nonresonance of the electron capture process for GaAs; Occurrence of electron tunneling via T-minimum.

Investigates the dependence of exciton spin relaxation in quantum well structures on an external electric field along growth direction and on the well thickness. Variation of the exciton spin-relaxation rate; Dominance of the electron-hole exchange interaction; Effect of increased field along the growth direction.

Presents the optical investigation of interface roughness and defect incorporation in gallium arsenide/aluminum gallium arsenide quantum well (QW) structures. Growth of QW structures by molecular beam epitaxy; Dependence of temperature on nonradiative recombination intensity.

We present an experimental study of the radiative recombination dynamics in size-controlled anatase TiO2 nanoparticles in the range 20–130 nm. From time-integrated photoluminescence spectra and picosecond time-resolved experiments as a function of the nanoparticle size, excitation density, and temperature, we show that photoluminescence comes out from a bulk and a surface radiative recombination. The spectral shift and the different time dynamics provide a clear distinction between them. Moreover, the intrinsic nature of the emission is also proven, providing a quantitative evaluation of volume and surface contributions. [ABSTRACT FROM AUTHOR]

We report an experimental study on the recombination kinetics of single strain-free GaAs quantum dots (QDs) grown by modified droplet epitaxy. The different bands composing the single quantum dot emission line at high temperature show identical dynamics, proving the common origin of all contributions. Our results thus agree with the interpretation of the broad pedestal band appearing when increasing the temperature as originated from the phonon replica. Finally, the relative weight of the phonon replicas depends both on temperature and on the QD size, in agreement with the theoretical predictions. [ABSTRACT FROM AUTHOR]

The effects of hydrogen incorporation on carrier relaxation and recombination efficiencies in a large series of InAs self-assembled quantum dot structures deposited on InGaAs lower confining layers with different thicknesses and compositions have been addressed. With increasing H dose we observe an improvement in the radiative efficiency. By comparing steady state and time resolved photoluminescence measurements, it is established that the H passivation does not enhance the relaxation and capture efficiencies, but instead directly improves the emission yield from carriers in the dots. We therefore conclude that the H-passivated defects are located nearby, or even inside, the dots. [ABSTRACT FROM AUTHOR]

The role of structural and dynamical disorder in semiconductors is a topic of fundamental relevance because of its contribution to the spectral line shape of the photoluminescence, and it plays a major role in ruling the carrier transport properties at the band edge. In this regard, a class of semiconductors, i.e., halide perovskites, deeply investigated in the last decade, shows a peculiar degree of disorder, which has only been recently under investigation. The interest to study disorder in halide perovskites is related to the large set of innovative applications of this class of materials, spanning from energy harvesting to high brilliance incoherent and coherent light emitters. In this perspective, we show that quantitative information on the disorder in halide perovskites can be extracted by deep analysis of the photoluminescence in different experimental conditions. Our study, conducted on a large set of samples of a metal halide perovskite, CsPbBr 3 , prepared with various synthesis/deposition methods, clarifies the relative weight of the static and dynamic contributions. A comparison with theoretical predictions is provided, gaining insights into the exciton/carrier–phonon interaction in metal halide perovskites. [ABSTRACT FROM AUTHOR]

Focuses on the photoluminescence and photoluminescence excitation spectra performed on gallium arsenide/aluminum-gallium-arsenic quantum well structures. Thermal equilibrium between excitons, electrons and holes; Comparison of the emission from different wells grown in identical conditions; Establishment of the thermal equilibrium between excitons, electrons and holes.

Halide perovskites are extremely interesting semiconductors for innovation in optoelectronics and photovoltaics. In particular, they are efficient emitters of both incoherent and coherent light and, therefore, of interest as active materials in lasers, light amplification systems (resonators and waveguides), and other photonic devices. Here, we present a set of experimental results concerning nonlinear effects in the radiative emission of CsPbBr3 films deposited by spin-coating on a silicon substrate and on metasurfaces realized by microspheres having a core of SiO2 and a shell of TiO2 (T-Rex). We evidence the presence of amplified spontaneous emission which, depending on the sample structure, shows different behavior as a function of the excitation power. In particular, we distinguish between two processes: light amplification by randomly arranged emitters and amplification by a quite ordered distribution of nanocrystals around a resonator. [ABSTRACT FROM AUTHOR]

Alkenes or alkynes bearing a wide variety of functional groups including carboxylic acids and ammonium salts are subjected to optimized conditions. [ABSTRACT FROM AUTHOR]

Halide perovskites are a novel class of semiconductors that have attracted great interest in recent decades due to their peculiar properties of interest for optoelectronics. In fact, their use ranges from the field of sensors and light emitters to ionizing radiation detectors. Since 2015, ionizing radiation detectors exploiting perovskite films as active media have been developed. Recently, it has also been demonstrated that such devices can be suitable for medical and diagnostic applications. This review collects most of the recent and innovative publications regarding solid-state devices for the detection of X-rays, neutrons, and protons based on perovskite thin and thick films in order to show that this type of material can be used to design a new generation of devices and sensors. Thin and thick films of halide perovskites are indeed excellent candidates for low-cost and large-area device applications, where the film morphology allows the implementation on flexible devices, which is a cutting-edge topic in the sensor sector. [ABSTRACT FROM AUTHOR]

The response of resistive In2O3−x sensing devices was investigated as a function of the NO2 concentration in different operative conditions. Sensing layers are 150 nm thick films manufactured by oxygen-free room temperature magnetron sputtering deposition. This technique allows for a facile and fast manufacturing process, at same time providing advantages in terms of gas sensing performances. The oxygen deficiency during growth provides high densities of oxygen vacancies, both on the surface, where they are favoring NO2 absorption reactions, and in the bulk, where they act as donors. This n-type doping allows for conveniently lowering the thin film resistivity, thus avoiding the sophisticated electronic readout required in the case of very high resistance sensing layers. The semiconductor layer was characterized in terms of morphology, composition and electronic properties. The sensor baseline resistance is in the order of kilohms and exhibits remarkable performances with respect to gas sensitivity. The sensor response to NO2 was studied experimentally both in oxygen-rich and oxygen-free atmospheres for different NO2 concentrations and working temperatures. Experimental tests revealed a response of 32%/ppm at 10 ppm NO2 and response times of approximately 2 min at an optimal working temperature of 200 °C. The obtained performance is in line with the requirements of a realistic application scenario, such as in plant condition monitoring. [ABSTRACT FROM AUTHOR]

Due to their high optical efficiency, low-cost fabrication and wide variety in composition and bandgap, halide perovskites are recognized nowadays as real contenders for the development of the next generation of optoelectronic devices, which, among others, often require high quality over large areas which is readily attainable by vacuum deposition. Here, we report the amplified spontaneous emission (ASE) properties of two CsPbBr3 films obtained by single-step RF-magnetron sputtering from a target containing precursors with variable compositions. Both the samples show ASE over a broad range of temperatures from 10 K up to 270 K. The ASE threshold results strongly temperature dependent, with the best performance occurring at about 50 K (down to 100 µJ/cm2), whereas at higher temperatures, there is evidence of thermally induced optical quenching. The observed temperature dependence is consistent with exciton detrapping up to about 50 K. At higher temperatures, progressive free exciton dissociation favors higher carrier mobility and increases trapping at defect states with consequent emission reduction and increased thresholds. The reported results open the way for effective large-area, high quality, organic solution-free deposited perovskite thin films for optoelectronic applications, with a remarkable capability to finely tune their physical properties. [ABSTRACT FROM AUTHOR]

Abstract: We present an experimental study of the radiative recombination dynamics in size-controlled TiO2 nanoparticles in the range 20–130 nm. Time-integrated photoluminescence spectra clearly show a dominance of self-trapped exciton (STE) emission, with main features not dependent on the nanoparticle size and on its environment. From picosecond time-resolved experiments as a function of the excitation density and the nanoparticle size we address the STE recombination dynamics as the result of two main processes related to the direct STE formation and to the indirect STE formation mediated by non-radiative surface states. [ABSTRACT FROM AUTHOR]

Abstract: We discuss the thermalization of the polariton population from a bulk GaN microcavity at room temperature. Simultaneous optical measurements of reflectivity and photoluminescence (PL), as well as time resolved measurements, reveal strong light–matter coupling. Transfer matrix theory is used to calculate reflectivity, absorption, and transmission coefficients for the structure. The PL emission is found to be thermalized, despite its very short lifetime, suggesting the existence of very fast energy relaxation channels. [Copyright &y& Elsevier]

We present a detailed analysis of the emission of individual GaAs/AlGaAs complex nano-systems composed of two concentric and topologically distinct quantum nanostructures, namely, a quantum dot and a quantum ring. Time resolved, temperature and excitation power density dependence of the photoluminescence from single and ensemble dot/ring structures have been used in order to determine the carrier dynamics. Despite the small spatial separation between the dot and the ring, the exciton dynamics in the two nanostructures is completely decoupled at low temperatures. At higher temperatures, we observe a clear change in the carrier dynamics, which shows the onset of the coupling between the two nanostructures. We attribute such change in carrier dynamics to the breaking of topology induced selection rules which allows the transfer of the carriers between the dot and the ring via an electronic quantum state, common to the two nanostructures. [ABSTRACT FROM AUTHOR]

Hybrid systems based on carbon nanotubes emitting in the telecom wavelength range and Si-photonic platforms are promising candidates for developing integrated photonic circuits. Here, we consider semiconducting single walled carbon nanotubes (s-SWNTs) emitting around 1300 nm or 1550 nm wavelength. The nanotubes are deposited on quartz substrate for mapping their photoluminescence in hyperspectral near-field microscopy. This method allows for a sub-wavelength resolution in detecting the spatial distribution of the emission of single s-SWNTs at room temperature. Optical signature delocalized over several micrometers is observed, thus denoting the high quality of the produced carbon nanotubes on a wide range of tube diameters. Noteworthy, the presence of both nanotube bundles and distinct s-SWNT chiralities is uncovered. [ABSTRACT FROM AUTHOR]

Inorganic metal halide perovskites are relevant semiconductors for optoelectronic devices. The successful deposition of thin films of CsPbBr3 and CsPbCl3 has recently been obtained by Radio-Frequency magnetron sputtering. In this work we compare the morphological, structural and optical characteristics of the two materials obtained with this deposition technique. A detailed photoluminescence (PL) spectroscopy study of the as-grown samples was conducted at the macro and micro scale in a wide temperature range (10-300 K) to fully characterize the PL on sample areas of square centimeters, to assess the origin of the inhomogeneous broadening and to quantify the PL quantum yield quenching. Our results prove that this technique allows for the realization of high quality nanometric films with controlled thickness of relevance for optoelectronic applications. [ABSTRACT FROM AUTHOR]

We show that the epitaxial growth of height-controlled GaAs quantum dots, leading to the reduction of the inhomogeneous emission bandwidth, produces individual nanostructures of peculiar morphology. Besides the height controlled quantum dots, we observe nanodisks formation. Exploiting time resolved and spatially resolved photoluminescence we establish the decoupling between quantum dots and nanodisks and demonstrate the high optical properties of the individual quantum dots, despite the processing steps needed for height control. [ABSTRACT FROM AUTHOR]

We investigated the origin of radiative recombination in In[sub x]Ga[sub 1-x]As[sub 1-y]N[sub y]/GaAs quantum wells by photoluminescence (PL) after picosecond excitation and under a magnetic field, B. Continuous wave and time-resolved PL show that at low temperature T localized states are mainly involved in the radiative recombination processes. Most importantly, the shift of the PL peak position induced by B depends dramatically on temperature, being higher at lower T. This result indicates that the PL emission at low temperature is determined by the recombination of loosely bound electron-hole pairs in which one carrier is localized by N-induced potential fluctuations, and the other carrier is delocalized. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Significant optical nonlinearity has been found in InGaAs (311)A sidewall quantum wires by means of time resolved photoluminescence measurements. A strong reverse quantum confined Stark effect has been observed and attributed to the dynamical screening of both the internal piezoelectric field and the Coulomb interaction between carriers. The time evolution of the quantum wire emission has been reproduced by means of self-consistent calculations that take into account excitonic effects, strain, and induced piezoelectric charges. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report a detailed experimental investigation of the power dependence of excitonic complexes (neutral exciton, neutral biexciton, and charged exciton) confined in single self-assembled GaAs/AlGaAs strain-free quantum dots grown by droplet epitaxy. By using the random population theory we show that, under stationary excitation, the power dependence of the excitonic complexes precisely follows the Poissonian statistics. This result allows us to determine with great accuracy the state filling condition of the quantum dots (QDs) and therefore to estimate the capture volume of the QDs. [ABSTRACT FROM AUTHOR]

We present a combined theoretical and experimental analysis to describe the interplay between polarization field, charge screening, and radiative and nonradiative recombinations in AlGaN/ GaN-based nanostructures. We perform the study of photoluminescence (PL) in both stationary and time-resolved PL (TR-PL) conditions, considering also the effect of temperature on the different recombination mechanisms, and especially on nonradiative recombinations. The theoretical analysis is obtained by coupling a complete self-consistent solution of Schrödinger and Poisson equations to determine the electronic states in the nanostructure with a rate equation model to account for time-dependent effects of charge rearrangement. We review the topic of polarization field screening taking into account the most recent results on the evaluation of the very strong coefficients for spontaneous and piezoelectric polarizations that have recently been predicted for III-V nitride semiconductors with natural wurtzite symmetry, and we show how the screening influences both static and dynamic recombination mechanisms. Experimental results are obtained for high-quality GaN/AlGaN multiple quantum wells by means of both continuous-wave (cw) and TR-PL techniques. The case of single quantum well is also considered. The PL measured decays show a time dependence that is not only controlled by radiative lifetimes, which depend on the fields inside the GaN wells, but also on the carrier recombinations through nonradiative channels. We demonstrate that PL emission is influenced by charge accumulation in the well, and a loss of carriers from the ground level induced by an interplay between radiative and nonradiative recombination processes. Moreover, from the analysis of the temperature dependence of the TR-PL decays, we deduce important confirmations of a thermally activated detrapping mechanism that strongly affects the nonradiative recombination processes. [ABSTRACT FROM AUTHOR]

The effects of GaAs substrate misorientation from the (001) plane on the photoluminescence spectra of GaAs/AlGaAs quantum well structures grown by molecular beam epitaxy have been investigated using time-resolved spectroscopy. It is shown, by comparison with a (001) heterostructure having a high impurity content, that the broadening of photoluminescence spectra, observed when the substrate misorientation is towards the (111)As plane, is very unlikely due to a preferential impurity incorporation but is rather originated from the growth mechanism on this type of surface. [ABSTRACT FROM AUTHOR]

The multiexciton properties of extrinsic centers from AlGaAs layers on Ge and Si substrates are addressed. The two photon cascade is found both in steady state and in time resolved experiments. Polarization analysis of the photoluminescence provides clearcut attribution to neutral biexciton complexes. Our findings demonstrate the prospect of exploiting extrinsic centers for generating entangled photon pairs on a Si based device. [ABSTRACT FROM AUTHOR]

The electron transport layer (ETL) plays a fundamental role in perovskite solar cells. Recently, graphene-based ETLs have been proved to be good candidate for scalable fabrication processes and to achieve higher carrier injection with respect to most commonly used ETLs. Here, the effects of different graphene-based ETLs in sensitized methylammonium lead iodide (MAPI) solar cells are experimentally studied. By means of time-integrated and picosecond time-resolved photoluminescence techniques, the carrier recombination dynamics in MAPI films embedded in different ETLs is investigated. Using graphene doped mesoporous TiO2 (G+mTiO2) with the addition of a lithium-neutralized graphene oxide (GO-Li) interlayer as ETL, it is found find that the carrier collection efficiency is increased by about a factor two with respect to standard mTiO2. Taking advantage of the absorption coefficient dispersion, the MAPI layer morphology is probed, along the thickness, finding that the MAPI embedded in the ETL composed by G+mTiO2 plus GO-Li brings to a very good crystalline quality of the MAPI layer with a trap density about one order of magnitude lower than that found with the other ETLs. In addition, this ETL freezes MAPI at the tetragonal phase, regardless of the temperature. Graphene-based ETLs can open the way to significant improvement of perovskite solar cells. [ABSTRACT FROM AUTHOR]

Silicon photonic devices need to be complemented with other materials for the realization of optically active devices. The unique emission properties of single-walled carbon nanotubes are especially promising for efficient light emitters realization in the telecommunication wavelength ranges and for their integration in silicon photonic devices. However one of the problem of carbon nanotubes is their low quantum efficiency. One of the approach to address this problem is to couple the carbon nanotube emission with micro-resonators in order to the enhance their radiative emission. Here we report on the fabrication and optical characterization of a silicon micro-slot-ring resonator coupled with selected single walled semiconducting carbon nanotubes for high photoluminescence emission in the O band telecommunication wavelength. Hyperspectral photoluminescence maps show spectra with sharp resonances superimposed to the nanotube emission bands which demonstrate the coupling of the nanotubes with the evanescent tails in air of the electric field localized in the photonic modes of the micro-resonator. The enhancement of the photoluminescence is achieved over three different CNTs chirality for more than 30 sharp resonances in a single microring resonator. This work paves the way for the development of integrated light sources based on carbon nanotubes in silicon photonic devices. [ABSTRACT FROM AUTHOR]

We report on a novel lithographic approach for the fabrication of integrated quantum dot (QD)-photonic crystal (PhC) nanocavity systems. We exploit unique hydrogen's ability to tailor the band gap energy of dilute nitride semiconductors to fabricate isolated site-controlled QDs via a spatially selective hydrogenation at the nanometer-scale. A deterministic integration of the realized site-controlled QDs with PhC nanocavities is provided by the inherent realignment precision (~ 20 nm) of the electron beam lithography system used for the fabrication of both QDs and PhC cavities. A detailed description of the fabrication steps leading to the realization of integrated QD-PhC cavity systems is provided, together with the experimental evidence of a weak coupling effect between the single-photon emitter and the PhC cavity. [ABSTRACT FROM AUTHOR]

Single and two-layer InAs/GaAs(001) samples were grown in a Molecular Beam Epitaxy chamber under critical conditions, leading to the selective growth of self-assembled InAs Quantum Dot chains over mounded GaAs surfaces. Changing the thickness of the spacer layer and the InAs deposition made it possible to tune the nucleation of 2-fold or single chains in the second layer. Finite Element Method simulations evidenced the major role of the strain field in favoring the formation of single stacked chains. On the other hand, tuning properly the As 4 /In flux ratio contributed to improving the QD ordering along the chains. Microphotoluminescence experiments demonstrated single photon emission properties of the observed QDs. Our growth approach did not degrade the optical quality of the InAs QDs, allowing a significant spatial correlation between the QDs aligned along the chain. [ABSTRACT FROM AUTHOR]

This paper describes a detailed analysis of the effects of high temperatures on the optical performance and structural characteristics of GaN-based LED structures with a high threading dislocation density. Results show that, as a consequence of storage at 900 °C in N2 atmosphere, the samples exhibit: (i) an increase in the efficiency of GaN and quantum-well luminescence, well correlated to an increase in carrier lifetime; (ii) a decrease in the parasitic luminescence peaks related to Mg acceptors, which is correlated to the reduction in the concentration of Mg in the p-type region, detected by Secondary Ion Mass Spectroscopy (SIMS); (iii) a diffusion of acceptor (Mg) atoms to the quantum well region; (iv) a reduction in the yield of Rutherford Backscattering Spectrometry (RBS)-channeling measurements, possibly due to a partial re-arrangement of the dislocations, which is supposed to be correlated to the increase in radiative efficiency (see (i)). [ABSTRACT FROM AUTHOR]

Abstract: We propose to develop a fast, thin silicon sensor with gain capable to concurrently measure with high precision the space (∼10μm) and time (∼10ps) coordinates of a particle. This will open up new application of silicon detector systems in many fields. Our analysis of detector properties indicates that it is possible to improve the timing characteristics of silicon-based tracking sensors, which already have sufficient position resolution, to achieve four-dimensional high-precision measurements. The basic sensor characteristics and the expected performance are listed, the wide field of applications are mentioned and the required R&D topics are discussed. [Copyright &y& Elsevier]

Abstract: In this work we show the possibility to increase the optical quality of quantum dots grown by droplet epitaxy on Ge-on-Si substrates in terms of single dot emission linewidth, decay time and efficiency by operating on the As pressure during the crystallization step without increasing the thermal budget. [Copyright &y& Elsevier]

We study the multiexciton properties of GaAs quantum dots self aggregated on Si substrates. Sequential emission of two photons radiative cascade is observed both in continuous wave and in time resolved measurements. Polarization resolved measurements, with high spectral resolution, allow us to attribute the observed photon cascade to positively charged biexciton. Our results highlight the possibility of obtaining systems showing quantum correlations on a Si based device. [ABSTRACT FROM AUTHOR]

We show that the epitaxial growth of thin layers of AlGaAs on Ge and Si substrates allows to obtain single photon sources by exploiting the sparse and unintentional contamination with acceptors of the AlGaAs. Very bright and sharp single photoluminescence lines are observed in confocal microscopy. These lines behave very much as single excitons in quantum dots, but their implementation is by far much easier, since it does not require 3D nucleation. The photon antibunching is demonstrated by time resolved Hanbury Brown and Twiss measurements. [ABSTRACT FROM AUTHOR]

We employ a far-field analysis of the angular emission pattern to experimentally assess the symmetry of localized modes in coupled photonic-crystal cavities. We demonstrate that the spatial distribution of localized modes in photonic-crystal nanocavities may change from a bonding to an antibonding orbital, a feature that is unusual in quantum mechanical coupled systems. Experimental data are confirmed by numerical calculations and interpreted in terms of the peculiar oscillatory behavior of the evanescent waves in photonic-crystal band gaps. [ABSTRACT FROM AUTHOR]

Neuromyelitis optica spectrum disorder (NMOsd) is a group of demyelinating disorders recently redefined and associated with NMO-IgG/anti-aquaporin 4 antibodies. Because NMOsd is of unknown prevalence worldwide, we conducted a retrospective, cross-sectional study of 850 patients with demyelinating disorders hospitalized in North East Tuscany from 1998 to 2006 to examine the prevalence of NMO and related disorders among unselected consecutive neurological patients with inflammatory CNS diseases and to evaluate the clinical phenotype spectrum of identified cases. Clinical data were updated after at least 2 years of follow-up. An immunofluorescence technique was used to detect NMO-IgG on rat brain tissue. Sera from other 828 neurological patients, 65 non-neurological patients and 50 healthy donors served as controls. The prevalence of NMOsd was 1.5%, with a MS:NMOsd ratio of 42.7. Among 13 NMOsd patients, 77% had long spinal cord lesions, 38% had severe optic neuritis and 23% had brain or brainstem lesions. Only 56% had clinically definite NMO at follow-up. The final EDSS score ranged from 1 to 10, mainly depending on brainstem involvement occurrence. Our findings confirm a low prevalence of NMO and related disorders among demyelinating inflammatory diseases in a Caucasian population. Moreover, this study demonstrates an unexpectedly high prevalence of limited and atypical variants of this disease, not previously documented. [ABSTRACT FROM AUTHOR]

Abstract: We present a detailed experimental study aimed at demonstrating the polariton nature of the photoluminescence emission in a bulk GaN microcavity grown on (111) silicon. The comparison of the photoluminescence with coincident reflectivity measurements at different temperatures shows an anticrossing behaviour with a Rabi splitting of the order of 35 meV up to room temperature. Relevant confirmations of the mixing between excitons and photons are found in the analysis of the spectral linewidth and of the time resolved kinetics. [Copyright &y& Elsevier]

Abstract: The interplay of polarization fields and free carrier screening in InxGa1−xN/GaN () multiple quantum wells is studied by combining photoluminescence (time-integrated and time-resolved) and cathodoluminescence studies, in an excitation density range from 108 to 1012 cm−2 of generated e–h pairs. For such low In content, the quantum-confined Stark effect is verified to rule the recombination dynamics, while effects of carrier localization in potential fluctuations have a minor role. Efficient field screening is demonstrated in CL steady-state high-injection conditions and in PL time-resolved experiments at the maximum excitation density. Under recovered nearly flat band conditions, quantum confinement effects are revealed and a high and possibly composition-dependent bowing parameter is extrapolated. Information on radiative and non-radiative rates for carrier recombination in the wells is obtained, both from steady-state and from time-resolved experiments, modelling the carrier dynamics in the framework of a theoretical rate equation model, which calculates electronic states and recombination rates in the nanostructure by coupling complete self-consistent solutions of Schrödinger and Poisson equations. [Copyright &y& Elsevier]

We present a study of the optical properties and carrier dynamics in strained InGaAs sidewall quantum wires (QWR) on patterned GaAs (3 1 1)A substrates by means of picosecond time-resolved photoluminescence (PL). A pronounced dynamical red shift of the QWR-PL band when increasing the delay time after the pulse excitation is observed. In addition, time-resolved data show a significant shortening of the PL decay time from the wire at short delay and when high excitation power is used. The data are compared with theoretical predictions. The results, i.e. the dynamical red shift observed in the wire emission and the shortening of the PL decay with increasing the excitation density, are interpreted in terms of a dynamical screening effect of the piezoelectric field. [Copyright &y& Elsevier]

GaAs/InxGa1−xP quantum wells, with x=0.541 and 0.427, have been investigated by continuous wave and time resolved photoluminescence. Spatial localization of excitons due to alloy compositional defects seem to be responsible of the observed phenomenology: non-exponential decay behavior at long times and slow decrease of the effective recombination time by increasing temperature. A two-class exciton kinetic model, based on a mobility edge varying with the temperature explain the experimental results. [Copyright &y& Elsevier]