Escape Rate Of Electrons Research Articles

The Effect of Electron Escape Rate on the Nonlinear Dynamics of Quantum Dot Lasers under Optical Feedback

When theoretically investigating the nonlinear dynamics of quantum dot lasers (QDLs), the parameter value of the electron escape rate (Ce) is sometimes approximated to zero to simplify the calculation. However, the value of Ce is dependent on the energy interval between the ground state (GS) and the excited state (ES) in the conduction band and is affected by the operation temperature. As a result, such simplified approximation treatments may lead to inaccurate results. In this study, after considering the effect of Ce, we investigate the nonlinear dynamics of QDLs with and without optical feedback based on the asymmetric electron-hole carrier rate equation model. The simulation results show that without optical feedback, the lasing conditions for ES and GS in free-running QDLs are dependent on the value of Ce. A larger Ce is more helpful for the ES emission, and the GS emission will stop lasing if Ce is large enough. Through analyzing the dynamical characteristics of GS and ES in QDLs with optical feedback under different Ce values, it can be found that the dynamical characteristics are strongly correlative with Ce.

Photoelectric and deep level study of metamorphic InAs/InGaAs quantum dots with GaAs confining barriers for photoluminescence enhancement

Metamorphic InAs/InGaAs quantum dots (QDs) have been proposed as active elements for optoelectronic light-emitting devices operating in the infrared range. However, advanced structure design to allow efficient and stable enhancement of quantum yield at room temperature are still needed. Here, we compare a metamorphic InAs/In0.15Ga0.85As QD heterostructure with and without GaAs confining barriers, to investigate the effect of introducing GaAs barriers on the photo- and thermo-electrical properties. GaAs confining barriers allow to enhance the QD photoluminescence intensity at 1.3 µm, i.e. second telecommunication window, by more than two orders of magnitude at room temperature and at 80 K. We also discuss the effect of GaAs barriers on the carrier transport and on defect-related levels detected by means of photocurrent and deep level thermally stimulated current spectroscopies. GaAs confining barriers decrease the thermal escape rate of electrons confined in QD and wetting layer, thereby highly increasing the radiative recombination and also quenching the photocurrent. At low temperatures, the barriers also reduce the capture of electrons generated in InGaAs by the QD layer and, on the other hand, prevent the trapping of electrons outside the QD layer, decreasing carrier lifetimes. The deep levels identified as point and extended defects have been detected in the InGaAs layers. There are no new types of defects introduced in the structure by the addition of the barriers, but this causes a weakly increased density of traps near QDs. Our results show that InAs/InGaAs QDs with GaAs confining barriers can be efficient light emitters with only a slight increase of defects in the structure. Hence, such advanced design for metamorphic QDs can be of relevant interest for applications in energy-efficient QD lasers, optical amplifiers and single-photon emitters operating at 1.3–1.55 µm.

Power-induced lasing state switching and bistability in a two-state quantum dot laser subject to optical injection

We theoretically investigate power-induced lasing state switching and bistability in a two-state quantum dot laser subject to optical injection. The simulated results show that, for a free-running two-state quantum dot laser operating at the ground state under low current, a power-induced lasing state switching between the ground state and the excited state can be achieved through introducing optical injection with a frequency (winj) close to the lasing frequency of excited state (wES). The injection power required for the state switching depends on the scanning route of injection power, i.e. there may exist state bistability for the injection power within a certain region. For forward scanning injection power, with the increase of frequency detuning (ΔΩ = winj – wES), the injection power required for the state switching shows a decreasing trend accompanied by slight fluctuations. However, for backward scanning injection power, the injection power required for the state switching exhibits obvious fluctuations with the increase of ΔΩ. The width of the hysteresis loop fluctuates with ΔΩ, and the fluctuation amplitude is increased with the increase of the injection current. Additionally, the influences of the inhomogeneous broadening factor and the electron escape rate on the bistability performances are analyzed.

Theory of strain-induced confinement in transition metal dichalcogenide monolayers

Recent experimental studies of out-of-plane straining geometries of transition metal dichalchogenide (TMD) monolayers have demonstrated sufficient band gap renormalisation for device application such as single photon emitters. Here, a simple continuum-mechanical plate-theory approach is used to estimate the topography of TMD monolayers layered atop nanopillar arrays. From such geometries, the induced conduction band potential and band gap renormalisation is given, demonstrating a curvature of the potential that is independent of the height of the deforming nanopillar. Additionally, with a semi-classical WKB approximation, the expected escape rate of electrons in the strain potential may be calculated as a function of the height of the deforming nanopillar. This approach is in accordance with experiment, supporting recent findings suggesting that increasing nanopillar height decreases the linewidth of the single photon emitters observed at the tip of the pillar, and predicting the shift in photon energy with nanopillar height for systems with consistent topography.

Imaging Localized Energy States in Silicon-Doped InGaN Nanowires Using 4D Electron Microscopy

Introducing dopants into InGaN NWs is known to significantly improve their device performances through a variety of mechanisms. However, to further optimize device operation under the influence of large specific surfaces, thorough knowledge of ultrafast dynamical processes at the surface and interface of these NWs is imperative. Here, we describe the development of four-dimensional scanning ultrafast electron microscopy (4D S-UEM) as an extremely surface-sensitive method to directly visualize in space and time the enormous impact of silicon doping on the surface-carrier dynamics of InGaN NWs. Two time regimes of surface dynamics are identified for the first time in a 4D S-UEM experiment: an early time behavior (within 200 ps) associated with the deferred evolution of secondary electrons due to the presence of localized trap states that decrease the electron escape rate and a longer time scale behavior (several ns) marked by accelerated charge carrier recombination. The results are further corroborated by ...

ANALYSIS OF KINETIC PROCESS OF ENERGETIC ELECTRON DURING A FLARE ON 2004 DECEMBER 1

We analyze the M1.1 flare event peaked at 07:20:00 UT on 2004 December 1 mainly from radio and hard X-ray (HXR) observations. By eliminating the thermal component from the observed total radio emission flux, the non-thermal part of the radio and HXR burst process is investigated in a self-consistent way. The spectral index of energetic electrons deduced from the radio burst evolves as a Soft-Hard-Hard pattern and that from HXR as a Soft-Hard-Soft pattern corresponding to an initial-main-decay phase. The trap-plus-precipitation model is applied in the kinetic process of energetic electrons for this flare event. The radio fluxes at six frequencies selected from the 2.6-7.6 GHz range are fitted with a gyrosynchrotron radiation mechanism. It is found that a linearly increasing electron escape rate can best fit to the observed radio fluxes from 07:00:00 UT to 07:40:00 UT and the slope of the electron escape rate for the six selected frequencies decreases with increasing frequency. During the decay phase from 07:15:00 UT to 07:20:00 UT, the hardened spectrum of the radio burst may be due to the increasing amount of trapped electrons with higher energy by a lower escape rate. Meanwhile, the increasing amount of precipitating electrons with a lower energy band may soften the HXR spectrum. In the decay phase after 07:20:00 UT, thermal emission is the dominating component for the radio burst.

Estimation of the escape of photoelectrons from Mars in 2004 liberated by the ionization of carbon dioxide and atomic oxygen

Photoelectron peaks in the atmosphere of Mars caused by the ionization of carbon dioxide and atomic oxygen by solar 30.4 nm photons have been observed by the Electron Spectrometer (ELS), a component of the Mars Express (MEx) Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3) experiment. Ionization mostly occurs at the Mars exobase with the majority of the photoionized electron flux trapped in the remanent and induced magnetic field, with a portion of that flux escaping the planet down its tail. Since Mars is overall charge neutral, the number of electrons must be identical to the number of ion charges which escape the planet. An estimate of the fraction of the total number of escaping electrons is obtained for the year 2004, specifically those produced by the ionization of carbon dioxide and atomic oxygen by solar 30.4 nm photons. In achieving this process, an illustrative example pass is used to show how the electron spectrum is adjusted for the potential on the spacecraft; then the region of the electron spectrum which shows photoelectron peaks is integrated over energy, yielding a flux of 5.74 × 10 6 electrons/(cm 2 s sr). This technique is then applied to a subset of 22 sample averaged spectra from the 2004 data (5 January 2004 through 25 January 2005), yielding an average result of 4.15 × 10 6 electrons/(cm 2 s sr) for the 22 cases. The observation cone of 33.75° is used to integrate over solid angle (assuming the flux is constant), giving 4.39 × 10 6 electrons/(cm 2 s). This average value was taken as representative of the full data interval. Frequency of occurrence statistics showing about a 6.2% occurrence rate for the 2004 data is applied to give an average escape flux from Mars of 2.72 × 10 5 electrons/(cm 2 s) during 2004. By estimating the outflow area as 1.16 × 10 18 cm 2 at X = −1.5 R Mars the electron escape rate of 3.14 × 10 23 electrons/s is obtained. Thus about 9.92 × 10 30 electrons or 16.5 Mmole of electrons escaped Mars during 2004 due to the ionization of carbon dioxide and atomic oxygen by the He 30.4 nm line. Due to the caveats of the analysis, these derived escape rates should be considered lower limits on the total electron escape rate from Mars.

Energy-Dependent Tunneling from Few-Electron Dynamic Quantum Dots

We measure the electron escape rate from surface-acoustic-wave dynamic quantum dots (QDs) through a tunnel barrier. Rate equations are used to extract the tunneling rates, which change by an order of magnitude with tunnel-barrier-gate voltage. We find that the tunneling rates depend on the number of electrons in each dynamic QD because of Coulomb energy. By comparing this dependence to a saddle-point-potential model, the addition energies of the second and third electron in each dynamic QD are estimated. The scale ( approximately a few meV) is comparable to those in static QDs as expected.

Measurement of Coulomb-energy-dependent tunnelling rates in surface-acoustic-wave-defined dynamic quantum dots

We present measurements of the escape rate of electrons from dynamic quantum dots (QDs), generated in surface-acoustic-wave minima, as the dynamic QD passes a tunnel barrier. The tunnelling time for this interaction is ∼ 350 ps . A rate-equation formulation is used to extract the tunnelling rates, which are strongly dependent on the number of electrons in the dynamic QD, and can be changed by over an order of magnitude by varying the tunnel-barrier gate voltage. We compare this dependence to a saddle-point-potential model for non-interacting electrons, to obtain the addition energies of the dynamic QD, which are found to be of a comparable order of magnitude to those measured in static few-electron QDs.

Propagation of cosmic-ray electrons in the Galaxy

We formulate the global propagation model of cosmic-ray electrons including the source region, which is currently considered to be supernova remnants (SNRs). The model is characterized by the escape rate of electrons from SNRs into the interstellar space. It becomes clear that the energy index of the escape rate influences the high energy side of the interstellar spectrum and makes it possible to explain the observed data up to 2 TeV in the case of source spectral index smaller than 2.2 that is expected from the radio spectrum in SNRs. The escape lifetime of electrons in SNRs is also discussed by using the ratio of the radio flux in two regions: SNRs and the Galaxy. The result shows the mean lifetime in SNRs of ∼10 4 yr around 1 GeV, which corresponds to the SNR age in the Sedov phase.

Efficient Computational Method for Ballistic Currents and Application to Single Quantum Dots

We present an efficient method for the calculation of the ballistic transmission function and current through an arbitrarily shaped, multi-terminal two- or three-dimensional open device. The method is applicable to cases where a ballistic current model is meaningful and charge self-consistency is not relevant, such as quantum dot devices, quantum wires, or interferometer type of structures. As a concrete example, we study the electron escape rate in self-assembled GaAs/InGaAs single quantum dots as a function of applied bias, as measured by photocurrent experiments.

Conductance fluctuations in a quantum dot under almost periodic ac pumping

It is shown that the variance of the linear dc-conductance fluctuations in an open quantum dot under a high-frequency ac pumping depends significantly on the spectral content of the ac field. For a sufficiently strong ac field $\ensuremath{\gamma}{\ensuremath{\tau}}_{\ensuremath{\varphi}}\ensuremath{\ll}1,$ where $1/{\ensuremath{\tau}}_{\ensuremath{\varphi}}$ is the dephasing rate induced by ac noise and $\ensuremath{\gamma}$ is the electron escape rate, the dc-conductance fluctuations are much stronger for the harmonic pumping than in the case of the noise ac field of the same intensity. The reduction factor r in a static magnetic field takes the universal value of 2 only for the white-noise pumping. For the strictly harmonic pumping ${A(t)=A}_{0}\mathrm{cos}\ensuremath{\omega}t$ of sufficiently large intensity the variance is almost insensitive to the static magnetic field $r\ensuremath{-}1=2\sqrt{{\ensuremath{\tau}}_{\ensuremath{\varphi}}\ensuremath{\gamma}}\ensuremath{\ll}1.$ For the quasiperiodic ac field of the form ${A(t)=A}_{0}[\mathrm{cos}({\ensuremath{\omega}}_{1}t)+\mathrm{cos}({\ensuremath{\omega}}_{2}t)]$ with ${\ensuremath{\omega}}_{1,2}\ensuremath{\gg}\ensuremath{\gamma}$ and $\ensuremath{\gamma}{\ensuremath{\tau}}_{\ensuremath{\varphi}}\ensuremath{\ll}1$ we predict effect of enchancement of conductance fluctuations at commensurate frequencies ${\ensuremath{\omega}}_{2}/{\ensuremath{\omega}}_{1}=P/Q.$

Direct observation of single-electron decay from an artificial nucleus

We describe the behaviour of an isolated quantum dot charged up with an excess of 30 electrons over its equilibrium value. A non-invasive voltage probe with a charge resolution of e/10 is fabricated next to the dot and used to measure the electron decay rate. The system behaves like an artificial nucleus with the electron escape rate following a Poisson distribution. We observe times for tunnelling of electrons out of the dot in excess of 1000 s.

Theoretical Studies of Dark Current of Quantum Dots

ABSTRACTA time evolution method is used to compute the electron escape time and dark current of a quantum dot under electric field. We found that for quantum dots with just one bound state, the ground state wave function can be well described by the product of a 2D confined wave function in the x, y plane and a 1D confined wave function in the z direction with an effective quantum well potential. A comparison of phase-shift analysis and the full time-dependent calculation is presented. Good agreement between the two in the large time scale is found, but discrepancy exists in the small time scale. Our study shows that the electron escape rate (which determines dark current) of quantum dots is much lower than that of quantum wells with the same bound-to-continuum transition energy.

Geometrical effect on the electron escape rate in a mesoscopic ring with an Aharonov-Bohm magnetic flux.

An electron confined to a two-dimensional (2D) mesoscopic ring with a finite square-well potential threaded by an Aharonov-Bohm (AB) flux can escape from the ring due to an increase in the curvature of the ring or in the AB flux. Here, we remark mainly on the electron's escape rate due to the AB flux and show that the escape rate is strongly influenced by the curvature and potential width of the ring as compared to the case of a quasi-1D ring with a very small potential width and very high confining walls. Furthermore, we point out that in the case of a uniform magnetic field, no electron can escape from the ring irrespective of the curvature and potential width. \textcopyright{} 1996 The American Physical Society.

Transfer-energy-dependent escape rate of electrons influenced by dynamical flux fields.

A path-integral approach is used to study the dynamics of a tunneling electron coupled to a flux that is governed by a free field Lagrangian. The transfer-energy-dependent escape rate of electrons is obtained from an analytic bounce trajectory for a rectangular barrier. Results are compared with those previously obtained for the charge model.

Resonant tunneling in the presence of a two-level fluctuator: Low-frequency noise.

We study current noise in a double-barrier resonant-tunneling structure due to dynamic defects that switch states because of their interaction with a thermal bath. The time fluctuations of the resonant level result in low-frequency noise, the characteristics of which depend on the relative strengths of the electron escape rate, the coupling to the defects, and the defect's switching rate. If the number of defects is sufficiently large, the noise is of the 1/f type. It is shown that the temperature dependence of the noise intensity becomes more pronounced as the spacer gets thicker.

Electron escape rate from coupled multiple quantum wells: Effects of coherence length and hopping.

Electron escape rates from coupled quantum wells with a small number of periods have been determined by measuring the photoluminescence decay time. In the absence of an electric field, the escape time depends strongly on the coherence length of the wave function which is estimated to extend over six well layers. The escape time of electrons localized by an electric field reflects the hopping probability which is affected by the wave-function overlap between adjacent well layers. This probability is enhanced when LO-phonon-assisted hopping is possible.

Transfer-energy-dependent escape rate of electrons through a small-capacitance tunnel junction

The dynamics of a tunneling electron through a small-capacitance tunnel junction are studied by developing a method for self-consistently determining escape rate and barrier traversal time for those electrons that transfer a given energy to the electromagnetic environment. The transfer-energy-dependent escape rate lets us examine dissipative and nondissipative tunneling rates separately and its integral over the transfer energy gives the total escape rate that is obtained by the instanton technique. It is found that the escape rate increases exponentially with increasing the ratio of the elementary charging energy to the quantized energy of the environment, but that the traversal time peaks at one particular ratio. As the frequency of the electromagnetic mode increases, an effective potential is shown to change from linear (dominated by the zero-point fluctuations of charge on the junction) to parabolic (dominated by the static polarization of the charge). Possible experimental situations for observing the predicted effects are discussed.

Electron escape rate and barrier traversal time influenced by the electromagnetic environment.

Self-consistent determination of electron escape rate and barrier traversal time under the influence of the electromagnetic environment reveals that the escape rate increases monotonically with increasing ratio of the elementary charging energy to the barrier height but that the traversal time peaks at one certain ratio. As the freuqency of the electromagnetic mode increases, an effective potential for a tunneling electron changes from linear to parabolic, dominated, respectively, by the zero-point fluctuations of the charge on the junction and its static polarization. Possible experimental situations for observing these predictions are discussed.