Sr离子自终止和复合激光交替振荡的动力学模型

建立了一个反映高重复率脉冲放电激励的Sr离子自终止激光和复合激光交替振荡的动力学模型，得到了与实测光电脉冲波形相一致的模拟结果.给出了He-Sr放电等离子中长寿命粒子、激光上下能级粒子数密度和电子温度随时间的演化过程.分析了两种激光交替振荡的发射过程、脉冲宽度特性和粒子数反转机理，认为在放电早期和余辉期电子温度的急剧升高和降低是Sr离子自终止和复合激光实现交替振荡的关键所在. 关键词： 自终止激光 复合激光 交替振荡 动力学模型     

硅衬底InGaN/GaN基蓝光发光二极管droop效应的研究

InGaN/GaN基阱垒结构LED当注入的电流密度较大时, LED的量子效率随注入电流密度增大而下降, 即droop效应.本文在Si (111)衬底上生长了 InGaN/GaN 基蓝光多量子阱结构的LED,通过将实验测量的光电性能曲线与利用ABC模型模拟的结果进行对比, 探讨了droop效应的成因.结果显示:温度下降会阻碍电流扩展和降低空穴浓度, 电子在阱中分布会越来越不平衡,阱中局部区域中因填充了势能越来越高的电子而溢出阱外, 从而使droop效应随着温度的降低在更小的电流密度下出现且更为严重, 不同温度下实验值与俄歇复合模型模拟的结果在高注入时趋势相反.这此结果表明,引起 droop效应的主因不是俄歇非辐射复合而是电子溢出,电子溢出的本质原因是载流子在阱中分布不均衡.     

Population inversion between the ground and first excited states in a recombining hydrogen plasma

Plasma conditions for generating a population inversion between the ground and first excited states in a recombining hydrogen plasma have been investigated on the basis of the CR model. Population inversion can be expected when three-body recombination plays a dominant role; the required regions of electron density and temperature are specified. It is shown that upper bounds exist for the ground-state population density at given electron density and temperature. Larger inversion densities can be obtained between the ground and first excited states than between excited levels. Numerical results are presented.     

Microscopic theory of Auger recombination in quantum wires

An analysis is made of mechanisms for Auger recombination of nonequilibrium carriers in cylindrical quantum wires. It is shown that two different Auger recombination mechanisms take place in these wires: a quasi-threshold and a nonthreshold mechanism. Both mechanisms are associated with the presence of heterobarriers but are of a different nature. The quasi-threshold mechanism is attributed to the spatial confinement of the carrier wave functions to the region of the quantum wire and in this case the quasi-momentum conservation law is violated and the Auger recombination process is intensified. As the radius of the wire increases, the quasi-threshold Auger recombination process goes over to a threshold process. The nonthreshold mechanism is caused by the scattering of an electron (hole) at the heterojunction; the rate of this nonthreshold Auger recombination tends to zero in the limit of an infinite-radius wire.     

A visible helium plasma recombination laser

A small (active volume ≈ 6 cm³), fast Blümlein type, pulsed, transverse-discharge driven laser device exhibited afterglow lasing on the 3s³S-2p³P He transition at 706.5 nm in mixtures of helium and hydrogen or deuterium over a pressure range of 200 mbar. The main features of the laser pulses and relevant time-resolved fluorescence investigations are presented. Population inversion is attributed to the enhanced recombination pumping of upper level population and to hampered lower level population in the presence of H₂ (D₂).     

A kinetic model for alternate oscillation of self-terminating and recombination lasers in strontium ions

A self-consistent model has been developed to simulate the kinetics of alternate oscillation of self-terminating and recombination lasers in univalent ions of strontium excited by the high-repetition-rate (HRR) pulsed discharge. Photoelectric pulse waveforms predicted by the model are in good agreement with the experimental measurement. The model also gives temporal behaviors of long-life particles, the population densities of upper and lower laser levels and the electron temperature (Te) in He-Sr discharge plasma. The emission process of the two laser alternate oscillations, the characteristic of the laser pulse width and the population inversion mechanics are also analyzed.     

Room temperature intra-band lasing in quantum dot arrays placed in high photon density cavities—a theoretical study

Intersubband (intra-band) transitions are very attractive forlong wavelength lasers due to the high degree of tailoring possible in the emission spectra. In general, if intra-band population inversion is to be created in a conduction band quantum well by carrier injection at the barrier energy, it is necessary that the electron non-radiative intra-band energy relaxation times are long. Additionally, the extraction time for the electron from the lower state should be short. In a bipolar device studied here, this means the bandedge electron-hole recombination times should be short.The use of sub-two-dimensional (2D) structures (quantum dots) allows us to increase the intra-band energy relaxation time from about a picosecond for bulk or quasi-2D systems to several hundred picoseconds at room temperatures. Also, by placing these structures in a cavity with a high photon number, it is possible to decrease the bandedge electron-hole recombination times through stimulated emission. Our studies show that strong population inversion and lasing under d.c. conditions is possible at room temperature in such systems.     

Mechanisms of Auger recombination in semiconducting quantum dots

Microscopic calculation of the probability of Auger recombination of charge carriers localized in a semiconducting quantum dot (QD) is carried out. It is shown that two mechanism of Auger recombination (nonthreshold and quasi-threshold) operate in the QD. The nonthreshold Auger recombination mechanism is associated with scattering of a quasimomentum from a heterobarrier, while the quasi-threshold mechanism is connected with spatial confinement of the wave functions of charge carriers to the QD region; scattering of carriers occurs at the short-range Coulomb potential. Both mechanisms lead to a substantial enhancement of Auger recombination at the QD as compared to a homogeneous semiconductor. A detailed analysis of the dependence of Auger recombination coefficient on the temperature and QD parameters is carried out. It is shown that the nonthreshold Auger recombination process dominates at low temperatures, while the quasi-threshold mechanism prevails at high temperatures. The dependence of the Auger recombination coefficient on the QD radius experiences noticeable changes as compared to quantum wells and quantum filaments.     

Elastic and inelastic contributions to the auger electron appearance potential spectrum of titanium

The energy distribution of electrons contributing to the L-shell Auger electron appearance potential spectrum of a polycrystalline titanium surface has been measured. The Auger electron appearance potential spectrum is obtained by differentiating the total secondary electron yield of an electron bombarded sample as a function of incident electron energy. At the threshold for scattering from a core level the secondary yield increases. Most of the electrons contributing to this increase have energies below 30 eV, and result from secondary processes following Auger recombination of the core hole. The elastic yield decreases at the threshold, however, due to opening a new channel for inelastic scattering. A comparison of the elastic yield spectrum (DAPS), the total yield spectrum (AEAPS) and the soft X-ray yield spectrum (SXAPS), shows very similar line shapes, but differences in the relative strengths of the lines.     

Theoretical investigation of Auger recombination on internal quantum efficiency of blue light-emitting diodes

The Auger recombination is recently proposed as one of the possible origins for the deteriorated internal quantum efficiency of InGaN light-emitting diodes. The Auger recombination behavior is quite different under widely varied Auger coefficients. The effect of Auger coefficient on the efficiency and output power is investigated numerically. The simulation results indicate that the Auger recombination with large Auger coefficient greatly decreases the efficiency in the whole current range under study. It is found that the electron current leakage and nonuniform hole distribution are the possible mechanisms responsible for the efficiency droop at high injection current.     

Intersubband population inversion and induced transitions between the Landau levels in resonance-tunneling multiple quantum-well structures

A new mechanism of creating the population inversion in the system of Landau levels in resonance-tunneling multiple quantum-well structures has been proposed. It has been shown that when the distance between the lower subbands is smaller than the energy of an optical phonon (i.e., when the scattering by optical phonons is suppressed), the population of the ground (zeroth) Landau level of the upper subband can be substantially higher than the population of the first Landau level of the lowest subband, which opens the opportunity of obtaining the tunable-frequency generation of induced terahertz electromagnetic radiation at such a transition.     

Mechanisms of Auger recombination in quantum wells

The main mechanisms for the Auger recombination of nonequilibrium carriers in semiconductor quantum-well heterostructures are investigated. It is shown for the first time that there are three fundamentally different Auger recombination mechanisms in quantum wells: 1) a threshold-free mechanism, 2) a quasithreshold mechanism, and 3) a threshold mechanism. The rate of the threshold-free process has a weak temperature dependence. The rate of the quasithreshold Auger process exhibits an exponential temperature dependence. However, the threshold energy depends significantly on the quantum-well width and is close to zero for narrow quantum wells. It is shown that the threshold-free and quasithreshold processes are dominant in fairly narrow quantum wells, while the quasithreshold and threshold Auger processes are dominant in wide quantum wells. The limiting transition to a three-dimensional Auger process is accomplished for a quantum-well width tending to infinity. The value of the critical quantum-well width, at which the quasithreshold and threshold Auger processes combine to form a single three-dimensional Auger recombination process, is found. Zh. éksp. Teor. Fiz. 113, 1491–1521 (April 1998)     

Soft X-ray gain in hydrogenic ions: Line pumping,ionization pumping and transient excitation

In a plasma containing hydrogenic ions which is irradiated by appropriate X-ray radiation, gain on the n=3n=2 (Balmer-) transition can be generated. Depending on the spectrum of the pump radiation, there are two different ways of obtaining the inversion: direct excitation of the upper laser level by line radiation, or ionization of the hydrogenic ions by broad band radiation with subsequent population of the upper level by recombination.In the first part of this paper, we present numerical calculations which compare the two pumping methods. The gain is evaluated under idealized conditions, i.e., with a pump spectrum containing only radiation useful for generating an inversion and under more realistic conditions, with a pump spectrum containing radiation at other frequencies as well.In the second part of the paper the possibility of obtaining Balmer- gain in hydrogenic ions under highly transient conditions is investigated, assuming a pump pulse with a duration comparable to the decay time of the lower laser level. Considerable gain is predicted even for a black-body pump spectrum.     

电子碰撞激发机制中自电离与双电子俘获

 以Ge为例，研究了双电子复合代替自电离与双电子俘获对离子布居的影响；通过解包括双激发态和自电离与双电子俘获过程的速率方程组，研究了类F离子与类Ne离子基态对19.6nm与23.6nm激光线上、下能级的布居贡献因子及类Na离子与类Ne离子的电离速率，并讨论了这两条激光线的反转与增益。     

Number of electrons active in slow collisions of and with

Resonant excitation or resonant electron scattering is a two step process in which Auger rates are involved in both steps. First an electron is captured into a bound state and a bound electron is excited (inverse Auger effect). Then an Auger transition leads to the emission of the electron from the ion. The corresponding cross-sections are very sensitive to the Auger rates and allow a detailed study of the Breit interaction which is a current-current contribution to the static electron-electron interaction. The contribution of the Breit interaction to the cross-section of resonant excitation on hydrogen-like uranium ions is discussed and shown that it is roughly twice as large as in the case of dielectronic recombination. Received 4 August 1999 and Received in final form 29 September 1999     

Carrier kinetics and population inversion in Landau level system in cascade GaAs/AlGaAs quantum well structures

The carrier distribution over Landau levels was studied in resonant tunneling GaAs/AlGaAs quantum well structures under tunneling pumping of the upper subband. The numerical calculations of the Landau levels population for various values of pumping intensity (tunneling time), magnetic field and the structure doping were carried out. The effect of various scattering mechanisms, as two-electron (electron–electron scattering) as single-electron (acoustic phonon and interface roughness scattering) ones on level population was studied. The population inversion between the zeroth Landau level of the upper subband and the first Landau level of the lowest subband was shown to exist in wide range of the magnetic field strength thus providing the possibility of wide range tunable stimulated terahertz emission.     

Laser without inversion driven by short pulse

Lasing without population inversion in a three-level atomic system driven by a short pulse is considered. In the case of adiabatic interaction, the gain at a laser transition is shown to be caused by a simple mechanism associated with the fact that the driving field creates an unoccupied lower laser level. As a result, the system operates as a standard two-level laser. To control the population of the upper laser level, various schemes of incoherent pumping of this state are analyzed. The example of sodium vapors is considered. An important application of the method under study to metal vapor lasers is discussed.     

Pseudopotential theory of Auger processes in CdSe quantum dots

Auger rates are calculated for CdSe colloidal quantum dots using atomistic empirical pseudopotential wave functions. We predict the dependence of Auger electron cooling on size, on correlation effects (included via configuration interaction), and on the presence of a spectator exciton. Auger multiexciton recombination rates are predicted for biexcitons as well as for triexcitons. The results agree quantitatively with recent measurements and offer new predictions.