The effect of the external lateral electric field on the luminescence intensity of InAs/GaAs quantum dots

We report on low-temperature microphotoluminescence (μ-PL) measurements of InAs/GaAs quantum dots (QDs) exposed to a lateral external electric field. It is demonstrated that the QDs’ PL signal could be increased severalfold by altering the external and/or the internal electric field, which could be changed by an additional infrared laser. A model which accounts for a substantially faster lateral transport of the photoexcited carriers achieved in an external electric field is employed to explain the observed effects. The results obtained suggest that the lateral electric fields play a major role for the dot luminescence intensity measured in our experiment—a finding which could be used to tailor the properties of QD-based optoelectronic applications. The text was submitted by the authors in English.     

Spin-polarized transport through a time-periodic non-magnetic semiconductor heterostructure

Spin-dependent Floquet scattering theory is developed to investigate the photon-assisted spin-polarized electron transport through a semiconductor heterostructure in the presence of an external electric field. Spin-dependent Fano resonances and spin-polarized electron transport through a laser irradiated time-periodic non-magnetic heterostructure in the presence of Dresselhaus spin-orbit interaction and a gate-controlled Rashba spin-orbit interaction are investigated. The electric field due to laser along with the spin-orbit interactions help to get spin-dependent Fano resonances in the conductance, whereas the external bias can be appropriately adjusted to get a near 80% spin-polarized electron transmission through heterostructures. The resultant nature of the Floquet scattering depends on the relative strength of these two electric fields.     

High-Field Effect of THz Radiation from Biased Large-Aperture Photoconducting Antennas

By introducing into the hot electron model, the authors have analyzed the THz radiation, which is excited by femtosecond laser from Large-Aperture InP photoconducting antennas biased electric field. Theoretic simulation shows that the departure of detected electric field from linear relationship with biased electric field comes from the high field effect of the transport process of free carriers.     

An electrohydrodynamics model for non-equilibrium electron and phonon transport in metal films after ultra-short pulse laser heating

The electrons and phonons in metal films after ultra-short pulse laser heating are in highly non-equilibrium states not only between the electrons and the phonons but also within the electrons. An electrohydrodynamics model consisting of the balance equations of electron density, energy density of electrons, and energy density of phonons is derived from the coupled non-equilibrium electron and phonon Boltzmann transport equations to study the nonlinear thermal transport by considering the electron density fluctuation and the transient electric current in metal films, after ultra-short pulse laser heating. The temperature evolution is calculated by the coupled electron and phonon Boltzmann transport equations, the electrohydrodynamics model derived in this work, and the two-temperature model. Different laser pulse durations, film thicknesses, and laser fluences are considered. We find that the two-temperature model overestimates the electron temperature at the front surface of the film and underestimates the damage threshold when the nonlinear thermal transport of electrons is important. The electrohydrodynamics model proposed in this work could be a more accurate prediction tool to study the non-equilibrium electron and phonon transport process than the two-temperature model and it is much easier to be solved than the Boltzmann transport equations.     

Recollision during the High Laser Intensity Ionization of C60

Through C60, we address the role of electron recollision in the nonresonant, femtosecond laser ionization of large, highly polarizable molecules. We show how the electron trajectories are influenced by the laser field, the laser induced dipole field, and the Coulomb field of the ion core. Working at long wavelengths we observe recollision in C60 through the ellipticity dependence of the fragmentation it produces. The ionizing electron emerges from C60z+ (z = 3, 4) with a lateral velocity of approximately 12 angstroms/fs, approximately half its Fermi velocity. Despite the large lateral velocity and competing forces on the electron, recollision remains relatively probable for this scale of molecule.     

Laser induced electronic acceleration process in nano dot

A physical model of the laser induced electron transport in dielectric with small size is discussed. The model assumes that free electrons are originally existent or produced by impact detrapping. The free electrons transporting in low dimensional dielectrics interact with phonons and surface. When the laser electric field strength is high enough, the inelastic electron scattering such as impact ionization and cascade processes will play important roles. A small size effect has been found in electron absorbing laser energy under the conditions that the laser wavelength is in near infrared zone and the material has size in nanometer level. This is a very significative effect to enlighten us on preparing ultrahigh threshold laser films with new nano-structure.     

飞秒激光固体靶相互作用中超热电子的输运特性

 实验研究了在100 TW掺钛宝石超短超强脉冲激光装置上完成的飞秒激光-固体靶相互作用中超热电子的输运特性,获得了超热电子的能谱、产额、注量及超热电子在靶内输运能量沉积范围。测量结果表明:超热电子的注量和总能量随靶厚度的增加而减少,超热电子约80％的能量主要沉积在靶内的前约一个激光脉冲宽度的范围内,且能量沉积范围随激光脉冲宽度的增加而增加,这主要是静电场的影响所致。     

Nonparabolic multivalley balance-equation approach to impact ionization: Application to wurtzite GaN

Extended nonparabolic multivalley balance equations including impact ionization (II) process are presented and are applied to study electron transport and impact ionization in wurtzite-phase GaN with a , L-M, and conduction band structure at high electric field up to 1000kV/cm. Hot-electron transport properties and impact ionization coefficient are calculated taking account of the scatterings from ionized impurity, polar optical, deformation potential, and intervalley interactions. It is shown that, for wurtzite GaN when the electric field approximately equals 530kV/cm, the II process begins to contribute to electron transport and results in an increase of the electron velocity and a decrease of the electron temperature, in comparison with the case without the II process. Similar calculations for GaAs are also carried out and quantitative agreement is obtained between the calculated II coefficients by this present approach and the experimental data. Relative to GaAs, GaN has a higher threshold electric field for II and a smaller II coefficient. Received: 27 April 1998 / Revised: 17 July 1998 / Accepted: 13 August 1998     

Above threshold ionization in tightly focused,strongly relativistic laser fields

The dynamics of electrons ionized from high charge states by lasers with intensity >10(20) W/cm(2) have been studied. At these intensities vxB forces drive the electrons subsequent to ionization in a trajectory nearly parallel to the laser propagation direction. This gives rise to large energy gains as the electron rides in phase with the laser field over a long distance. Monte Carlo simulations illustrate that, unlike in case of ionization in sub- and near-relativistic intensity fields (<10(19) W/cm(2)), the electron dynamics in the ultrarelativistic case are strongly influenced by the longitudinal electric fields found near the focus of a tightly focused laser.     

Self-consistent modelling of X-ray preionized XeCl-laser discharges

In this theoretical work a 0-D model for a self-sustained X-ray preionized XeCl-laser discharge is presented. The model is self-consistent in the sense that it simultaneously solves, contrarily to the usual decoupling procedure, the Boltzmann equation for electrons, the kinetic equations for excited and ionic species, the equations for the electrical circuit and the laser photon density. It includes a rather complete kinetics of HCl(v) vibrational excitation, dissociation and dissociative attachment. The influence of electron collisions with excited species and of e-e Coulomb collisions on the plasma parameters and transport coefficients is discussed. Some evidence of the non-stationary equilibrium between the electron distribution and the reduced electric field E/N is given. Results of the model are compared with experimental ones corresponding to a XeCl-laser discharge driven by a L-C inversion circuit. The model predicts well the main trends for the variation of the laser energy in a large range of experimental conditions. The discrepancy between experiment and model for absolute values of the laser energy is discussed.     

Mechanism of electron violent acceleration by extra-intense lasers in vacuum

The mechanism underlying the electron capture acceleration (Phys. Rev. E 58 (1998) 6575) is studied. It is found that the longitudinal electric field, resulting from the transverse spatial gradient of the laser fields, is responsible for the violent electron acceleration while the transverse fields play the role of confining the electron inside the laser beam.     

Numerical method for a 2D drift diffusion model arising in strained n-type MOSFET device

This paper reports the calculation of electron transport in metal oxide semiconductor field effects transistors (MOSFETs) with biaxially tensile strained silicon channel. The calculation is formulated based on two-dimensional drift diffusion model (DDM) including strain effects. The carrier mobility dependence on the lateral and vertical electric field model is especially considered in the formulation. By using the model presented here, numerical method based on finite difference approach is performed. The obtained results show that the presence of biaxially tensile strain enhances the current in the devices.     

Electron localization of linear symmetric molecular ion H_3~(2+)

Electron localization in the dissociation of the symmetric linear molecular ion H_3~(2+) is investigated. The numerical simulation shows that the electron localization distribution is dependent on the central frequency and peak electric field amplitude of the external ultrashort ultraviolet laser pulse. When the electrons of the ground state are excited onto the 2pσ~2Σ_u~+ by a one-photon process, most electrons of the dissociation states are localized at the protons on both sides symmetrically. Almost no electron is stabilized at the middle proton due to the odd symmetry of the wave function. With the increase of the frequency of the external ultraviolet laser pulse, the electron localization ratio of the middle proton increases, for more electrons of the ground state are excited onto the higher 3pσ~2Σ_u~+ ustate. 50.9% electrons of all the dissociation events can be captured by the middle Coulomb potential well through optimizing the central frequency and peak electric field amplitude of the ultraviolet laser pulse. Besides, a direct current(DC) electric field can be utilized to control the electron motions of the dissociation states after the excitation of an ultraviolet laser pulse, and 68.8% electrons of the dissociation states can be controlled into the middle proton.     

LaAlO_3/SrTiO_3界面增强光伏效应

探索LaAlO_3/SrTiO_3(LAO/STO)界面产生的新奇物理特性对理解关联电子系统中多自由度耦合和设计功能材料器件具有重要的价值.本文通过脉冲激光沉积方法在SrTiO_3基底上制备了LAO/STO薄膜,研究了正面照射LAO/STO膜面和侧面照射LAO/STO界面时的光伏效应,探讨了LAO/STO界面对光伏效应的影响.研究结果表明,在同样光照能量下侧面照射LAO/STO界面产生的光电压远高于正面照射LAO/STO膜面产生的光电压,说明LAO/STO界面对光伏效应有明显的增强作用.通过偏压调控可以进一步增强照射LAO/STO界面产生的光电压,当偏压为60 V时, LAO/STO样品的位置探测灵敏度达到了36.8 mV/mm.这些研究结果为设计场调控位置敏感探测器等新型光电子器件提供了新的思路.     

Transport properties of a lateral semiconductor quantum dot defined by a single connected metallic front-gate

We present studies on the electric transport in a lateral GaAs/AlGaAs quantum dot defined by a patterned single connected metallic front-gate. This gate design allows to easily couple a large number of quantum dots and therefore holds high potential in the design of new materials with tailor-made band structures based on quantum dot superlattices of controlled shape. Clear Coulomb diamond structures and well pronounced tunneling peaks observed in experiment indicate that single-electron control has been achieved. However, the dependence on electron density in the heterostructure embedding the dot, which is controlled by an additional back-gate, reveals that transport characteristics are strongly influenced supposedly by potential fluctuations in the dot and lead regions.     

金激光等离子体的空间分辨X射线发射谱及其应用

在星光II激光装置上,采用PET平面晶体谱仪与宽20μm的狭缝构成一维空间分辨光谱测量系统,对金平面靶激光等离子体进行观测,获得了沿靶面法向一维空间分辨的金M带发射谱。在实验谱中观察到了Au元素类Ni离子的电四极跃迁线3p63d10(1S0) 3p53d104d(3/2,5/2)J=1。利用电四极跃迁线对电子密度的敏感特性,开展了金激光等离子体电子密度诊断的尝试,确定出利用该谱线进行电子密度诊断的有效范围大致在1019～4.5×1021cm-3之间。     

基于异质双栅电极结构提高碳纳米管场效应晶体管电子输运效率

为改善碳纳米管场效应晶体管(CNTFET)器件性能, 提高电子输运效率, 提出了一种异质双金属栅(HDMG)电极结构CNTFET器件. 通过对单金属栅(SMG)-CNTFET器件输运模型的适当修改, 实现了对HDMG-CNTFET器件电子输运特性的研究.研究结果表明, 对于所提出的HDMG结构器件, 如果固定源端金属栅S-gate的功函数W_GS使其等于本征CNT 的功函数, 而选取漏端金属栅D-gate的功函数W_Gd, 使其在一定范围内小于W_GS, 可优化器件沟道中的电场分布, 提高器件沟道电子平均输运速率; 同时由于HDMG-CNTFET的D-gate对沟道电势具有调制作用, 使该器件阈值电压降低, 导致在相同的工作电压下, HDMG-CNTFET器件具有更大的通态电流; 而D-gate对漏电压的屏蔽作用又使HDMG-CNTFET与SMG-CNTFET相比具有更好的栅控能力 及减小 漏极感应势垒降低效应、热电子效应和双极导电性等优点. 本研究通过合理选取HDMG-CNTFET双栅电极的功函数, 有效克服了现有研究中存在的改善CNTFET性能需要以减小通态电流为代价的不足, 重要的是提高了器件的电子输运效率, 进而可提高特征频率、减小延迟时间, 有利于将CNTFET器件应用于高速/高频电路. 关键词： CNTFET 异质双栅 电子输运效率 双极导电性     

Flux-limited nonequilibrium electron energy transport in warm dense gold

An abrupt change in energy transport has been observed in femtosecond laser heated gold when the absorbed laser flux exceeds ~7×10(12) W/cm(2). Below this value, the absorbed flux is carried by ballistic motion of nonthermal electrons produced in interband excitation. Above this value energy transport appears to include ballistic transport by nonthermal electrons and heat diffusion by thermalized hot electrons. The ballistic component is limited to a flux of ~7×10(12) W/cm(2). This offers a unique benchmark for comparison with theory on nonequilibrium electron transport.     

Coherent single-cycle pulses with MV/cm field strengths from a relativistic transition radiation light source

Terahertz (THz) pulses with energies up to 100 μJ and corresponding electric fields up to 1 MV/cm were generated by coherent transition radiation from 500 MeV electron bunches at the free-electron laser Freie-Elektronen-Laser in Hamburg (FLASH). The pulses were characterized in the time domain by electro-optical sampling by a synchronized femtosecond laser with jitter of less than 100 fs. High THz field strengths and quality of synchronization with an optical laser will enable observation of nonlinear THz phenomena.     

Electric field driven plasmon dispersion in AlGaN/GaN high electron mobility transistors

We present a theoretical study on the electric field driven plasmon dispersion of the two-dimensional electron gas(2DEG)in AlGaN/GaN high electron mobility transistors(HEMTs).By introducing a drifted Fermi–Dirac distribution,we calculate the transport properties of the 2DEG in the AlGaN/GaN interface by employing the balance-equation approach based on the Boltzmann equation.Then,the nonequilibrium Fermi–Dirac function is obtained by applying the calculated electron drift velocity and electron temperature.Under random phase approximation(RPA),the electric field driven plasmon dispersion is investigated.The calculated results indicate that the plasmon frequency is dominated by both the electric field and the angle between wavevector and electric field.Importantly,the plasmon frequency could be tuned by the applied source–drain bias voltage besides the gate voltage(change of the electron density).