线宽增长因子对光纤色散传输系统的影响

文章讨论了半导体激光器频率啁啾产生的物理机制 .利用波动方程 ,得到了线宽增长因子同光源线宽的关系 ;重点分析了激光器方程中的线宽增长因子和光纤通信系统中啁啾参数的一致性 ;并以线宽增长因子为切入点 ,将高斯脉冲作为信号源 ,数值解光纤传输的非线性薛定谔方程 ,考察了线宽增长因子和群速度色散共同对光纤通信系统传输性能的影响 .     

Scattering of screened excitons by free carriers in semiconductingquantum well structures

The theoretical treatment of the scattering of excitons by free electrons and holes in a two-dimensional semiconducting quantum-well system is extended to take into account screening by the free carriers. The scattering cross sections are calculated using the Born approximation for elastic scattering of the excitons by the free carriers. For the heavy-hole exciton, the screening by the free carriers reduces the cross section for free-carrier exciton scattering for all values of the energy of relative motion of the free carriers and the excitors. For the light-hole exciton, however, screening can actually lead to an enhancement of the scattering cross section for low values of the energy of relative motion when the density of free carriers is high. This is because screening not only reduces the interaction between the free carriers and the exciton, but also decreases the binding of the exciton, leading to a larger effective radius of the exciton. The results for the scattering cross sections are then applied to calculate the contribution of the exciton linewidth due to elastic scattering of the excitons by free carriers. It is found that this contribution to the exciton linewidth is decreased below its value in the absence of screening for both the heavy- and light-hole excitons     

Migration of an exciton in organic polymers driven by a nonuniform internal electric field

Migration of excitons is of vital importance for the photovoltaic process in polymer-based donor/acceptor (D/A) photovoltaic system. Due to the existence of some trapping charges as well as the mismatch of energy levels at the D/A heterointerface, there will exist a nonuniform internal electric field, which may induce the migration or dissociation of an exciton or charge transfer state generated near the interface. In this work, by choosing the nonuniform internal electric field as a linear gradient form, we theoretically simulate the migration dynamics of an exciton in a polymer. It is found that the exciton will be polarized under the electric field. Especially, the polarized positive and negative charges in the exciton lie in different electric fields, which will induce a net force driving the exciton to migrate. Effects of some factors, such as the electron-phonon (e-ph) coupling, on the exciton migration are analyzed. This research reveals a new exciton migration mechanism in polymer-based D/A photovoltaic system, which might be timely for further understanding their photovoltaic process, and thus provide a new strategy to optimize the photovoltaic efficiency of the devices.     

Electron correlation effects on exciton dissociation in the presence of an electric field in polyacetylene

Based on the Su–Schrieffer–Heeger and Pariser–Parr–Pople (SSH + PPP) model and the multiconfigurational time-dependent Hartree-Fock formalism, the dynamics of exciton dissociation in conjugated polymers systems has been simulated using a nonadiabatic molecular dynamics method. Within this approach, the appropriate spin symmetry of the electronic wavefunction is taken into account, which allows us to distinguish between singlet and triplet excited states. The different dynamic dissociation behaviors in the presence of an electric field of singlet excitons (SE) and triplet excitons (TE) due to electron correlation are emphasized. Using numerical simulations, it is found that the dissociation of TE under an applied electric field is more difficult than in the case for SE. The critical dissociation electric field strength for TE is about 3 times higher than that of SE. When the conjugated length of a polymer chain is short, SE and TE can each remain as one entity even under high electric field strengths owing to the confinement effect of the chain ends. In addition, the dependence of exciton dissociation on the Coulomb interaction strength, shielding factor and the conjugated length is discussed.     

Ba蒸气受激喇曼散射中的碰撞加宽效应

研究了缓冲气体分子所引起的Ba原子喇曼能级的碰撞加宽对于XeCl激光光泵Ba蒸气的喇曼增益系数及喇曼能量转换效率的影响。     

Measurement of linewidth broadening factor in gain-switched InGaAsP injection lasers by CHP method

First measurements of the semiconductor laser linewidth broadening factor ? using a recently proposed CHP (chirp-halfwidth product) method are reported. Gain-switched optical pulses have been generated by sinusoidal RF and comb generator drive. The simultaneous recording of streak-camera and monochromator traces provides the required data for determination of ?. The values of ? between 6 and 7.5 are obtained for a 1.3 ?m mesa laser. Computer simulations predict that the CHP formula should provide an estimate of ? within ±15% for either comb generator or hard sinusoidal drive.     

Distribution of the electric field in high-resistivity MSM structures illuminated by nonmonochromatic light

The sensitivity of the electric field distribution to the spectrum of the incident radiation in highly biased, high-resistivity, semiconductor structures of the MSM (metal-semiconductor-metal) type illuminated by nonmonochromatic light is investigated theoretically. It is shown that in the presence of deep impurity levels the field distribution depends strongly on the spectral composition of the incident light. The frequency interval corresponding to optical thicknesses of the order of unity is found to significantly influence the space charge in the bulk of the structure and the electric field distribution E(x), even when the fraction of energy in this region of the spectrum relative to the total flux is extremely minimal. The trapping of holes by a deep impurity level in the bulk of the structure forms a positive space charge and produces qualitatively new field distributions, which increase near the dark electrode with a positive curvature of the function E(x). The impurity trapping of electrons near the illuminated anode imparts a negative space charge to the impurity levels. This phenomenon induces a substantial increase of the field in the electrode sheath and forms in the vicinity of the anode a region wherein the field varies only slightly. All the prominent features disclosed by the calculations in the electric field distributions are observed in experiment. Fiz. Tekh. Poluprovodn. 33, 815–823 (July 1999)     

Effect of quantum dot shape on dynamical dephasing suppression in exciton qubits under applied electric field

We investigate the efficiency of periodic dynamical decoupling of an exciton qubit confined in a self-assembled quantum dot in the presence of an applied electric field. The shape of the quantum dot is found to have a large effect on the excitonic dephasing. It is shown that dynamical suppression of dephasing, through a simple series of equally spaced bit flips, is most efficient for quantum dots that are close to spherical. In addition, compared to the no field case, the presence of an electric field increases the efficiency of the decoupling technique as the quantum dot becomes more oblate. Our calculations show that dephasing can be significantly suppressed in GaAs/AlAs quantum dots suitable for quantum information processing.     

An analytical expression for the electric field in MSM structures

In this work the conformal mapping is used to determine an analytical expression for the electric field in MSM structures. The expression simplifies the analysis of optically generated carriers transport, i.e. the determination of the response of the MSN photodetectors. It can also be useful in the analysis of MSM electrooptical modulators operation.     

Photocarrier escape time in quantum-well light-absorbing devices: effects of electric field and well parameters

We analyze the dependence of the carrier escape time from a single-quantum-well optoelectronic device on the applied electric field and well width and depth. For this purpose, a new simple and computationally efficient theory is developed. This theory is accurate in the case of electrons, and the assessment of the applicability for holes is given. Semi-analytical expressions for the escape times are derived. Calculations are compared to experimental results and previous numerical simulations. Significant correlations between the position of quantum-well energy levels and the value of the escape time are found. The main escape mechanism at room temperature is established to be thermally assisted tunneling/emission through near-barrier-edge states. The formation of a new eigenstate in the near-barrier-edge energy region is found to reduce the electron escape time significantly, which can be used for practical device optimization.     

Determination of the direction of piezoelectric field in InGaN/GaN multiple quantum-well structures grown by metal-organic chemical vapor deposition

The direction of the piezoelectric field in InGaN/GaN multiple quantum-well (MQW) structures grown by metal-organic vapor deposition (MOCVD) was determined using excitation-power-density variable photoluminescence (PL). By comparing the excitation-power-density dependence of the shift of the PL peak and the change of the full-width at half-maximum (FWHM) of the peak from an InGaN/GaN MQW structure and an InGaN MQW-based light-emitting diode (LED), the piezoelectric field in the InGaN/GaN MQW structures was unambiguously determined to be pointing toward the substrate. This result helps to identify the surface polarity of the LED wafer as Ga-faced.     

电场对含空位缺陷硅结构影响的第一性原理研究

用分子动力学模拟和第一性原理计算分析方法,研究电场对含空位缺陷硅结构的影响。分子动力学结果表明,硅(100)晶面处的空位数量,总体随着电场强度的增强而增加。当电场在8.90×10~6 V/cm和1.35×10~7 V/cm之间时,空位数达到最大饱和数值10,并保持稳定。第一性原理计算结果表明,能带结构中的缺陷能级等效于降低价带和导带之间的带隙值,导致空位结构中隧穿电流变大。此外,缺陷能级也会导致空位结构中的静态介电常数变大,从而引起电容-电压特性值变大。     

A model for the electric field in lightly doped drain structures

A semi-quantitative model for the lateral channel electric field in LDD MOSFET's has been developed. This model is derived from a quasi-two-dimensional analysis under the assumption of a uniform doping profile. A field reduction factor and voltage improvement, indicating the effectiveness of an LDD design in reducing the peak channel field, are used to compare LDD structures with, without, and with partial gate/drain overlap. Approximate equations have been derived that show the dependencies of the field reduction factor on bias conditions and process parameters. Plots showing the trade-off between, and the process-dependencies of, the field reduction factor/voltage improvement and the series resistance are presented for the three cases. Structures with gate-drain overlap are found to provide greater field reduction than those without the overlap for the same series resistance introduced. This should be considered when comparing the double-diffused and spacer LDD structures. It is shown that gate-drain offset can cause the rise of channel field and substrate current at large gate voltages. This offset is also found to be responsible for nonsaturation of drain current. The model has also been compared with two-dimensional simulation results.     

Specific features of the field generation of minority charge carriers in Si-SiO2 structures

The specific features of generation processes occurring in Si-SiO₂ structures in the presence of a strong electric field in the oxide layer (E _ox ≈ 10 MV cm^?1) are considered. It is shown that, after a certain threshold electric-field strength is reached, an additional mechanism of minority charge-carrier generation, associated with the development of electroluminescence processes in the oxide layer, becomes operative.     

Studies of the mobility of charge carriers in low-dimensional systems in a transverse DC electric field

The mobility of charge carriers μ in a parabolic quantum well in an electric field E directed along the size-confinement axis is calculated. With consideration for scattering of charge carriers at a rough surface, the mobility μ is shown to decrease with increasing E. A physical interpretation of this effect is proposed.     

On the permanent dipole-induced two-level Raman scattering insemiconductor quantum wells in electric field

The permanent dipole-induced two-level Raman scattering in semiconductor quantum wells in a static electric field is analyzed. Calculations performed for GaAs/Al_xGa_1-xAs multiple quantum wells (MQWs) indicate that reasonable, but not high, Stokes wave amplification could be obtained, and eventually used for IR generation. The gain is limited primarily by reststrahlen absorption and a rather large transition linewidth     

沟道电场分布对AlN/GaN HFETs极化库仑场散射的影响

Based on the measured capacitance–voltage(C–V) curves and current–voltage(I–V) curves for the prepared differently-sized Al N/Ga N heterostructure field-effect transistors(HFETs), the I–V characteristics of the Al N/Ga N HFETs were simulated using the quasi-two-dimensional(quasi-2D) model. By analyzing the variation in the electron mobility for the two-dimensional electron gas(2DEG) with the channel electric field, it is found that the different polarization charge distribution generated by the different channel electric field distribution can result in different polarization Coulomb field(PCF) scattering. The 2DEG electron mobility difference is mostly caused by the PCF scattering which can reach up to 899.6 cm2/(V s)(sample a), 1307.4 cm2/(V s)(sample b),1561.7 cm2/(V s)(sample c) and 678.1 cm2/(V s)(sample d), respectively. When the 2DEG sheet density is modulated by the drain–source bias, the electron mobility for samples a, b and c appear to peak with the variation of the 2DEG sheet density, but for sample d, no peak appears and the electron mobility rises with the increase in the2 DEG sheet density.     

Simulation of a microprofile and electric field distribution in MIM structures

Simulation of etching of a microtip’s lower electrode and deposition of a dielectric film has shown that the film undergoes a strong thinning at lateral surfaces and at the microtip’s bases. It is demonstrated that, at a microtip density of 5 × 10⁸ cm⁻², the electric field’s strength at the base of the tip is 3.5 times that at its apex, which gives rise to additional emission centers.     

Investigation of electronic transitions in coupled-quantum-well structures with a built-in electric field by photoreflectance spectroscopy

Room-temperature photoreflectance spectra of coupled-quantum-well heterostructures with a built-in electric field are measured. The optical transition energies are determined and their dependences on the well width and the barrier thickness are examined. The experimental results are compared with the calculated energies of electron-hole transitions. Good agreement between calculations and experiment is found for narrow wells; in the case of wide wells, optical transitions are associated with groups of several closely spaced electron-hole transitions.