Showing papers on "Electric field published in 2001"

Quantum cascade laser

Abstract: The invention concerns a quantum cascade laser comprising in particular a gain region (14) consisting of several layers (20) each including: alternating strata of a first type (28) defining each an AllnAs quantum barrier and strata of a second type (28) defining each an InGaAs quantum barrier, and injection barriers (22), interposed between two of the layers (20). The layers of the gain region (14) form each an active zone extending from one to the other of the injection barriers (22) adjacent thereto. The strata (26, 28) are dimensioned such that: each of the wells comprises, in the presence of an electric field, at least a first upper subband, a second median subband, and a third lower subband, and the probability of an electron being present in the first subband is highest in the proximity of one of the adjacent injection barriers, in the second subband in the median part of the zone and in the third subband in the proximity of the other adjacent barriers. The laser is formed by a succession of active zones and injection barriers, without interposition of a relaxation zone.

Electrostatic field-assisted alignment of electrospun nanofibres

Abstract: This paper describes an electrostatic field-assisted assembly technique combined with an electrospinning process used to position and align individual nanofibres (NFs) on a tapered and grounded wheel-like bobbin. The bobbin is able to wind a continuously as-spun nanofibre at its tip-like edge. The alignment approach has resulted in polyethylene oxide-based NFs with diameters ranging from 100-300 nm and lengths of up to hundreds of microns. The results demonstrate the effectiveness of this new approach for assembling NFs in parallel arrays while being able to control the average separation between the fibres.

Aligning single-wall carbon nanotubes with an alternating-current electric field

Abstract: Single-wall carbon nanotubes (SWCNTs) were highly aligned by an external electric field. The results suggest that the alignment of SWCNTs shows significant dependencies on the frequency and the magnitude of the applied electric field. The electric field with 5 MHz straightened out the SWCNTs and created highly oriented samples with fewer large particles. We also discussed the mechanism and applications.

Dielectrophoretic liquid actuation and nanodroplet formation

Abstract: Water, like any polarizable medium, responds to a nonuniform electric field by collecting preferentially in regions of maximum field intensity. This manifestation of dielectrophoresis(DEP) makes possible a variety of microelectromechanicalliquid actuation schemes. In particular, we demonstrate a new class of high-speed DEP actuators, including “wall-less” flowstructures, siphons, and nanodroplet dispensers that operate with water. Liquid in these microfluidic devices rests on a thin, insulating, polyimide layer that covers the coplanar electrodes. Microliter volumes of water, deposited on these substrates from a micropipette, are manipulated, transported, and subdivided into droplets as small as ∼7 nl by sequences of voltage application and appropriate changes of electrode connections. The finite conductivity of the water and the capacitance of the dielectric layer covering the electrodes necessitate use of rf voltage above ∼60 kHz. A simple RC circuit model explains this frequency-dependent behavior. DEP actuation of small water volumes is very fast. We observe droplet formation in less than 0.1 s and transient, voltage-driven movement of water fingers at speeds exceeding 5 cm/s. Such speed suggests that actuation can be accomplished using preprogrammed, short applications of the rf voltage to minimize Joule heating.

Giant lateral electrostriction in ferroelectric liquid-crystalline elastomers

Abstract: Mechanisms for converting electrical energy into mechanical energy are essential for the design of nanoscale transducers, sensors, actuators, motors, pumps, artificial muscles, and medical microrobots. Nanometre-scale actuation has to date been mainly achieved by using the (linear) piezoelectric effect in certain classes of crystals (for example, quartz), and 'smart' ceramics such as lead zirconate titanate. But the strains achievable in these materials are small--less than 0.1 per cent--so several alternative materials and approaches have been considered. These include grafted polyglutamates (which have a performance comparable to quartz), silicone elastomers (passive material--the constriction results from the Coulomb attraction of the capacitor electrodes between which the material is sandwiched) and carbon nanotubes (which are slow). High and fast strains of up to 4 per cent within an electric field of 150 MV x m(-1) have been achieved by electrostriction (this means that the strain is proportional to the square of the applied electric field) in an electron-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymer. Here we report a material that shows a further increase in electrostriction by two orders of magnitude: ultrathin (less than 100 nanometres) ferroelectric liquid-crystalline elastomer films that exhibit 4 per cent strain at only 1.5 MV x m(-1). This giant electrostriction was obtained by combining the properties of ferroelectric liquid crystals with those of a polymer network. We expect that these results, which can be completely understood on a molecular level, will open new perspectives for applications.

Geospace Environment Modeling magnetic reconnection challenge: Simulations with a full particle electromagnetic code

Abstract: The objective of the Geospace Environment Modeling (GEM) magnetic reconnection challenge is to understand the collisionless physics that controls the rate of magnetic reconnection in a two-dimensional configuration. The challenge involves investigating a standard model problem based on a simple Harris sheet configuration by means of a variety of physical models in order to isolate the essential physics. In the present work the challenge problem is modeled using an electromagnetic particle-in-cell code in which full particle dynamics are retained for both electrons and ions and Maxwell's equations are solved without approximation. The timescale for reconnection is of the order of 10 Ωi-1 (where Ωi is the ion cyclotron frequency based on the asymptotic field B0), and the corresponding reconnection electric field is (c/vA)Ey/B0 ∼ 0.24. The diffusion region near the neutral line is observed to develop a multiscale structure based on the electron and ion inertial lengths c/ωpe and c/ωpi. The difference between the ion and electron dynamics in the diffusion region gives rise to in-plane (Hall) currents which produce an out-of-plane By field with a quadrupolar structure. In the diffusion region the magnetic field is no longer frozen-in to the electrons; the inductive Ey field is supported primarily by the off-diagonal electron pressure terms in the generalized Ohm's law. The reconnection rate is found to be insensitive to electron inertia effects and to the presence of a moderate out-of-plane initial field component B0y ≲ B0. The results are consistent with the theory that the reconnection rate is independent of the mechanism which breaks the frozen-in condition and is controlled by dynamics at length scales much greater than the electron dissipation region.

Geotail observations of the Hall current system: Evidence of magnetic reconnection in the magnetotail

Abstract: In a two-fluid picture of magnetic reconnection, inflow electrons flow with the magnetic field line to the diffusion region, whereas inflow ions cannot reach the diffusion region and rest around a distance of the ion inertial length. The relative motion of electrons and ions results in electric currents, that is, the Hall currents. The Hall current system produces a quadrupole structure in the cross-tail component of the magnetic field near the magnetic reconnection region. Furthermore, this relative motion forms the electric field, whose direction is toward the equatorial plane (midplane). We have investigated the plasma and magnetic field structure near the magnetic reconnection region in the magnetotail with the Geotail spacecraft. We commonly observed inflowing low-energy (less than 5 keV) electrons in the outermost layer of the plasma sheet in magnetic reconnection events, where accelerated ions and electrons flow away from the magnetic reconnection region. These electrons can carry currents to form part of the Hall current system. The observed east-west variations in the magnetic field are consistent with the quadrupole structure produced by the Hall current system. We also noted that inflowing ions have consistently a dawnward motion, almost perpendicular to the magnetic field. These ions indicate the presence of the electric field toward the equatorial plane. The present observations demonstrate the ion-electron decoupling processes for magnetic reconnection in the magnetotail.

On the heating of linear conductive structures as guide wires and catheters in interventional MRI.

Abstract: The interest in performing vascular interventions under magnetic resonance (MR) guidance has initiated the evaluation of the potential hazard of long conductive wires and catheters. The objective of this work is to present a simple analytical approach to address this concern and to demonstrate the agreement with experimental results. The first hypothesis is that a long conductive structure couples with the electric field of the radio frequency (RF) transmit coil. The second hypothesis is that this coupling induces high voltages near the wire ends. These voltages can cause tissue heating due to induced currents. The experimental results show an increase in coupling when moving a guide wire toward the wall of an RF transmit coil, documented with a temperature increase of a saline solution in close proximity to the tip of the guide wire. The coupling of the wire not only presents a potential hazard to the patient, but also interferes with the visualization of the wire. A safe alternative would be the use of nonconducting guide wires. J. Magn. Reson. Imaging 2001;13:105-114.

Plasmon resonant coupling in metallic nanowires

Abstract: We investigate the plasmon resonances of interacting silver nanowires with a 50 nm diameter. Both non–touching and intersecting configurations are investigated. While individual cylinders exhibit a single plasmon resonance, we observe much more complex spectra of resonances for interacting structures. The number and magnitude of the different resonances depend on the illumination direction and on the distance between the particles. For very small separations, we observe a dramatic field enhancement between the particles, where the electric field amplitude reaches a hundredfold of the illumination. A similar enhancement is observed in the grooves created in slightly intersecting particles. The topology of these different resonances is related to the induced polarization charges. The implication of these results to surface enhanced Raman scattering (SERS) are discussed.

First results of electric field and density observations by Cluster EFW based on initial months of operation

Abstract: Highlights are presented from studies of the electric field data from various regions along the CLUS- TER orbit. They all point towards a very high coherence for phenomena recorded on four spacecraft that are sepa- rated by a few hundred kilometers for structures over the whole range of apparent frequencies from 1 mHz to 9 kHz. This presents completely new opportunities to study spatial- temporal plasma phenomena from the magnetosphere out to the solar wind. A new probe environment was con- structed for the CLUSTER electric field experiment that now produces data of unprecedented quality. Determination of plasma flow in the solar wind is an example of the capability of the instrument.

Bioelectrics-new applications for pulsed power technology

Abstract: Electric phenomena play an important role in biophysics. Bioelectric processes control the ion transport processes across membranes, and are the basis for information transfer along neurons. These electrical effects are generally triggered by chemical processes. However, it is also possible to control such cell functions and transport processes by applying pulsed electric fields. This area of bioengineering, bioelectrics, offers new applications for pulsed power technology. One such application is prevention of biofouling, an effect that is based on reversible electroporation of cell membranes. Pulsed electric fields of several kV/cm amplitude and submicrosecond duration have been found effective in preventing the growth of aquatic nuisance species on surfaces. Reversible electroporation is also used for medical applications, e.g. for delivery of chemotherapeutic drugs into tumor cells, for gene therapy, and for transdermal drug delivery. Higher electric fields cause irreversible membrane damage. Pulses in the microsecond range with electric field intensities in the tens of kV/cm are being used for bacterial decontamination of water and liquid food. A new type of field-cell interaction, "Intracellular Electromanipulation", by means of nanosecond pulses at electric fields exceeding 50 kV/cm has been recently added to known bioelectric effects. It is based on capacitive coupling to cell substructures, has therefore the potential to affect transport processes across subcellular membranes, and may be used for gene transfer into cell nuclei. There are also indications that it triggers intracellular processes, such as programmed cell death, an effect, which can be used for cancer treatment. In order to generate the required electric fields for these processes, high voltage, high current sources are required. The pulse duration needs to be short to prevent thermal effects. Pulse power technology is the enabling technology for bioelectrics. The field of bioelectrics, therefore opens up a new research area for pulse power engineers, with fascinating applications in biology and medicine.

Runaway breakdown and electric discharges in thunderstorms

Abstract: This review concerns the theory of the avalanche multiplication of high-energy (0.1 – 10 MeV) electrons in a neutral material, a newly discovered phenomenon known as runaway breakdown (RB). In atmospheric conditions RB takes place at electric fields an order of magnitude weaker than those needed for normal breakdown in air. Experimental work of the past few years has shown that RB determines the maximum electric field strength in thunderclouds and is behind a variety of phenomena newly observed in thunderstorm atmosphere, such as giant high-altitude discharges between thunderclouds and the ionosphere, anomalous amplifications of X-ray emission, intense bursts of gamma radiation, etc. These phenomena are becoming increasingly active areas of study. A necessary condition for the occurrence of runaway avalanche is the presence of high energy seed electrons. In the atmosphere, these are cosmic ray secondary electrons. Therefore, the observed effects reflect the close relationship between cosmic rays and electrodynamic processes in the thunderstorm atmosphere. The first laboratory results on RB are also presented. Further studies in this area may be of interest for high-current electronics.

The Generation of Nonthermal Particles in the Relativistic Magnetic Reconnection of Pair Plasmas

Abstract: Particle acceleration in the magnetic reconnection of electron-positron plasmas is studied by using a particle-in-cell simulation. It is found that a significantly large number of nonthermal particles are generated by the inductive electric fields around an X-type neutral line when the reconnection outflow velocity, which is known to be an Alfven velocity, is on the order of the speed of light. In such a relativistic reconnection regime, we also find that electrons and positrons form a power-law-like energy distribution through their drift along the reconnection electric field under the relativistic Speiser motion. A brief discussion of the relevance of these results to the current sheet structure, which has an antiparallel magnetic field in astrophysical sources of synchrotron radiation, is presented.

Ab initio study of BaTiO 3 and PbTiO 3 surfaces in external electric fields

Abstract: For the ferroelectric perovskite compounds ${\mathrm{BaTiO}}_{3}$ and ${\mathrm{PbTiO}}_{3},$ we have studied the effects of external electric fields on the structural properties of the (001) surfaces. The field-induced changes in the surface interlayer spacings and bucklings have been calculated using a first-principles ultrasoft-pseudopotential approach, and the change of the polarization and the ferroelectric distortions in the surface layers have been obtained. The surfaces are represented by periodically repeated slabs, and an external dipole layer is included in the vacuum region of the supercells to control the electric field normal to the surfaces. The influence of the electrical boundary conditions on the ferroelectric properties of the slabs is discussed. In the case of a vanishing internal electric field, our study indicates that even very thin slabs can show a ferroelectric instability.

Transient and steady-state behavior of space charges in multilayer organic light-emitting diodes

Abstract: A numerical study of space charge effects in multilayer organic light-emitting diodes (OLEDs) is presented. The method of solving the coupled Poisson and continuity equations, previously established for single-layer polymer LEDs, has been extended to treat internal organic interfaces. In addition, we consider the transient current and electroluminescence response. We discuss the accumulation of charges at internal interfaces and their signature in the transient response as well as the electric field distribution. Comparison to experimental transient data of a typical bilayer LED based on tris(8-hydroxyquinolinato)aluminum (Alq3) is provided and good agreement is found. Our results are consistent with commonly assumed operating principles of bilayer LEDs. In particular, the assumptions that the electric field is predominantly dropped across the Alq3 layer and that the electroluminescence delay time is determined by electrons passing through Alq3 to the internal interface are self-consistently supported by ...

Electrohydrodynamic instabilities in polymer films

Abstract: We have studied the influence of electric fields on highly viscous polymer films An electrohydrodynamic (EHD) instability causes a wave pattern with a characteristic wavelength λ, leading to an array of polymer columns which span the gap of a capacitor device When represented as a master curve, the data is quantitatively described by an EHD model, without any adjustable parameters Our results suggest that EHD experiments using polymer films are well suited to study non-equilibrium pattern formation in quasi-two-dimensional systems

Comprehensive computational model of Earth's ring current

Abstract: A comprehensive ring current model (CRCM) has been developed that couples the Rice Convection Model (RCM) and the kinetic model of Fok and coworkers. The coupled model is able to simulate, for the first time using a self-consistently calculated electric field, the evolution of an inner magnetosphere plasma distribution that conserves the first two adiabatic invariants. The traditional RCM calculates the ionospheric electric fields and currents consistent with a magnetospheric ion distribution that is assumed to be isotropic in pitch angle. The Fok model calculates the plasma distribution by solving the Boltzmann equation with specified electric and magnetic fields. To combine the RCM and the Fok model, the RCM Birkeland current algorithm has been generalized to arbitrary pitch angle distributions. Given a specification of height-integrated ionospheric conductance, the RCM component of the CRCM computes the ionospheric electric field and currents. The Fok model then advances the ring current plasma distribution using the electric field computed by the RCM and at the same time calculates losses along particle drift paths. We present the logic of CRCM and the first validation results following the H+ distribution during the previously studied magnetic storm of May 2, 1986. The H+ fluxes calculated by the coupled model agree very well with observations by AMPTE/CCE. In particular, the coupled model is able to reproduce the high H+ flux seen on the dayside at L ∼ 2.3 that the previous simulation, which employed a Stern-Volland convection model with shielding factor 2, failed to produce. Though the Stern-Volland and CRCM electric fields differ in several respects, the most notable difference is that the CRCM predicts strong electric fields near Earth in the storm main phase, particularly in the dusk-midnight quadrant. Thus the CRCM injects particles more deeply and more quickly.

Three-dimensional simulations of ion acceleration from a foil irradiated by a short-pulse laser.

Abstract: Using 3D particle-in-cell simulations we study ion acceleration from a foil irradiated by a laser pulse at 10(19) W/cm(2) intensity. At the front side, the laser ponderomotive force pushes electrons inwards, thus creating the electric field by charge separation, which drags the ions. At the back side of the foil, the ions are accelerated by space charge of the hot electrons exiting into vacuum, as suggested by Hatchett et al. [Phys. Plasmas 7, 2076 (2000)]. The transport of hot electrons through the overdense plasma and their exit into vacuum are strongly affected by self-generated magnetic fields. The fast ions emerge from the rear surface in cones similar to those detected by Clark et al. [Phys. Rev. Lett. 84, 670 (2000)].

Noncontact friction and force fluctuations between closely spaced bodies.

Abstract: Noncontact friction between a Au(111) surface and an ultrasensitive gold-coated cantilever was measured as a function of tip-sample spacing, temperature, and bias voltage using observations of cantilever damping and Brownian motion. The importance of the inhomogeneous contact potential is discussed and comparison is made to measurements over dielectric surfaces. Using the fluctuation-dissipation theorem, the force fluctuations are interpreted in terms of near-surface fluctuating electric fields interacting with static surface charge.

Multi-axial electrical switching of a ferroelectric: theory versus experiment

Abstract: Samples of the polycrystalline ferroelectric ceramic PZT-5H were poled by applying an electric field at room temperature. Subsequently, an electric field was applied to the samples at a range of angles to the poling direction. The measured non-linear responses in electric displacement are used to construct “yield surfaces” in electric field space corresponding to the onset of ferroelectric switching. The results are compared with predictions from three models: (i) a previous self-consistent polycrystal calculation with rate-independent, non-hardening crystal plasticity; (ii) a simplified crystal plasticity model with viscoplastic (rate-dependent) behaviour and a sufficient number of transformation systems to reproduce the polycrystalline behaviour; (iii) a phenomenological model based on rate-independent flow theory, using kinematic hardening and a quadratic yield surface in electric field and stress space. The experiments suggest that the self-consistent crystal plasticity formulation is most able to reproduce the multi-axial electrical response and yield surface of the polycrystal. The phenomenological model is able to reproduce the uniaxial response accurately, but gives relatively poor performance for multi-axial loading paths, in its present form. A tolerable compromise in multi-axial modelling is the simplified crystal plasticity approach. This is able to reproduce multi-axial constitutive behaviour with reasonable accuracy, whilst offering computational simplicity and speed similar to that of the phenomenological model.

Low-macroscopic-field electron emission from carbon films and other electrically nanostructured heterogeneous materials: hypotheses about emission mechanism ☆

Abstract: Thin flat dielectric films can be low-macroscopic-field (LMF) electron emitters, able to generate electrons when subject to a macroscopic electric field in the range 1–50 V μm −1 . This phenomenon is a known cause of pre-breakdown currents in high-voltage vacuum breakdown, and is now the basis of a broad-area electron-source technology, using carbon-based thin films and other materials. The phenomenon occurs because the dielectric film is, or becomes, an electrically nanostructured heterogeneous (ENH) material, with quasi-filamentary conducting channels between its surfaces. These channels connect to emitting features near or on the film/vacuum surface, or act as electron emitters themselves. The film may contain conducting or semiconducting particles that assist with conductivity and/or act as emitting features. Several forms of thin-film LMF emitter exist: in each case the situation geometry ensures that sufficient field enhancement occurs at the ‘tip’ of the emitting feature for the emission process to be some form of tunnelling field electron emission (probably ‘cold’ in some cases, ‘hot’ in others). This paper explores aspects of the theory of thin-film LMF emission, starting from the need to understand the behaviour of emitters based on amorphous carbon films. A summary review, with extensive references, is given of relevant past work outside the immediate ‘carbon field emission’ context. Relevant aspects of semiconductor field emission theory are noted. Comment is made on the original experiments on diamond field emission, and on theoretical misconceptions in the carbon field emission literature. Analysis of carbon-film emitter behaviour suggests that emission must primarily be due to geometrical field enhancement, that in at least some cases arises from conducting nanostructure inside the film. In one case, published film characteristics can be used to show that sufficient field enhancement should be available. Some problems with an ‘internal field enhancement’ hypothesis are considered and disposed of. Difficulties with Latham’s theory of field-induced emission from ENH materials are pointed out: a new theory, largely qualitative at this stage, can explain longstanding problems: this assumes that dielectric films must be treated as ‘hopping conductors’ not semiconductors. Electron emission takes place via localised surface states: transition to a channel-limited current regime takes place when the surface states no longer have high enough occupation probability to screen the external field, and is accompanied by anomalous band bending at the channel tip. Mathematical theories of band bending and field emission for hopping conductors are required. Some consequences for the design of LMF emitters are noted.

High-Order Harmonic Generation in Aligned Molecules

Abstract: We demonstrate alignment control in high density vapors of $\mathrm{CS}{}_{2}$, hexane, and $\mathrm{N}{}_{2}$ $(\ensuremath{\sim}{10}^{17}$ molecules $\mathrm{cm}{}^{\ensuremath{-}3})$ using the electric field of a 300 ps duration laser pulse. This was sufficient to study for the first time the molecular orientation dependence of high-order harmonic generation using a second, 70 fs duration laser pulse of intensity $\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0ex}{0ex}}\mathrm{W}\mathrm{cm}{}^{\ensuremath{-}2}$. We were able to modulate and significantly enhance the harmonic intensity in aligned molecules compared to the randomly oriented case. Our results are consistent with the existence of an anisotropic dipole phase.

Strength of the electric field in apertureless near-field optical microscopy

Abstract: Enhancement γ of the electrical field at the end of a tip relative to the incident field in a focused radiation beam is calculated by the finite-element time-domain (FETD) method. First, the reliability of the FETD method is established by calculating the electric field on simple structures like thin cylinders, spheres, and ellipsoids, and comparing the results with analytical solutions. The calculations on these test structures also reveal that phase retardation effects substantially modify γ when the size of the structure is larger than approximately λ/4, λ being the radiation wavelength. For plasmon resonance, in particular, phase retardation severely reduces the resonance and the expected field enhancement for a gold tip. The small value of γ=4 calculated by FETD is about an order of magnitude smaller than the value found in recent published work. Resonance effects can be recovered for special tips, which have a discontinuity or a different material composition at the end of the tip. Some tuning of the discontinuity dimension is needed to maximize the resonance. Under optimal conditions for plasmon resonance, an enhancement in the electric field of about 50 is calculated at the end of a small gold protrusion mounted on a wider silicon or glass tip.

The dielectrophoretic and travelling wave forces generated by interdigitated electrode arrays: analytical solution using Fourier series

Abstract: In alternating current electrokinetics, electric fields are used to generate forces on particles. Techniques have been applied for the manipulation of particles and the measurement of their dielectric properties. The fields are typically generated by microelectrode structures fabricated on planar surfaces. One particular design, using interdigitated bar electrodes, is used both in dielectrophoretic field flow fractionation and travelling wave dielectrophoresis. This paper presents a Fourier series analysis of the dielectrophoretic force on a particle generated by this type of electrode array, for both dielectrophoresis and travelling wave dielectrophoresis. Simple expressions are derived for the force at a distance of the order of the electrode spacing from the electrodes. A full analytical expression is given for the dielectrophoretic force in two dimensions. Comparisons are made with previously published experimental observations.

Electrostatic surface plasmon resonance: direct electric field-induced hybridization and denaturation in monolayer nucleic acid films and label-free discrimination of base mismatches.

Abstract: We demonstrate that in situ optical surface plasmon resonance spectroscopy can be used to monitor hybridization kinetics for unlabeled DNA in tethered monolayer nucleic acid films on gold in the presence of an applied electrostatic field. The dc field can enhance or retard hybridization and can also denature surface-immobilized DNA duplexes. Discrimination between matched and mismatched hybrids is achieved by simple adjustment of the electrode potential. Although the electric field at the interface is extremely large, the tethered single-stranded DNA thiol probes remain bound and can be reused for subsequent hybridization reactions without loss of efficiency. Only capacitive charging currents are drawn; redox reactions are avoided by maintaining the gold electrode potential within the ideally polarizable region. Because of potential-induced changes in the shape of the surface plasmon resonance curve, we account for the full curve rather than simply the shift in the resonance minimum.

High internal electric field in a graded-width InGaN/GaN quantum well: Accurate determination by time-resolved photoluminescence spectroscopy

Abstract: Time-resolvedphotoluminescence (PL), at T=8 K, is used to study a graded-width InGaN/GaN quantum well. Across the sample, the well width continuously varies from ∼5.5 to 2.0 nm corresponding to PL peak energies varying between 2.0 and 2.9 eV and to PL decay rates covering four orders of magnitude. The plot of decay times versus PL energies is very well fitted by a calculation of the electron–hole recombination probability versus well width. The only fitting parameter is the electric field in the well, which we find equal to 2.45±0.25 MV/cm, in excellent agreement with experimental Stokes shifts for this type of samples.