Binding energies of shallow impurities in asymmetric strained wurtzite Al_x Ga_(1-x)N/GaN/AIy Ga1-y N quantum wells

The ground state binding energies of hydrogenic impurities in strained wurtzite Al_xGa_1-xN/GaN/Al_yGa_1-yN quantum wells are calculated numerically by a variational method.The dependence of the binding energy on well width,impurity location and Al concentrations of the left and right barriers is discussed,including the effect of the built-in electric field induced by spontaneous and piezoelectric polarizations.The results show that the change in binding energy with well width is more sensitive to the impurity position and barrier heights than the barrier widths,especially in asymmetric well structures where the barrier widths and/or barrier heights differ.The binding energy as a function of the impurity position in symmetric and asymmetric structures behaves like a map of the spatial distribution of the ground state wave function of the electron.It is also found that the influence on the binding energy from the Al concentration of the left barrier is more obvious than that of the right barrier.     

Effect of a magnetic field on the energy levels of donor impurities in the ZnO parabolic quantum well

Energy levels of a donor impurity in the ZnO parabolic quantum well under the magnetic field are investigated using the variational method.The binding energy of the ground state,the energies of 2p±state and 1 s→2p±transition energies of a hydrogenic donor in the ZnO parabolic quantum well are numerically calculated as a function of the strength of magnetic field for different parabolic potential fields.The results show that the external magnetic field has an obvious influence on the binding energies and the 1 s→2p±transition energies of a hydrogenic donor.The Is to 2p±transition energy increases linearly with the strength of magnetic field,but the Is to 2p_ transition energy decreases when the strength of magnetic field increases for the small field strength. Compared to the GaAs parabolic well,the donors are more tightly bound to the ZnO parabolic well and the influence of external magnetic field on the binding energy of a donor is much stronger in the ZnO parabolic well.     

Effect of p-d exchange with an itinerant carrier in a GaMnAs quantum dot

Hydrogenic acceptor binding energy as a function of dot radius in a GaMnAs/Ga_0.6Al_0.4As quantum dot is calculated including the exchange interaction of Mn alloy content with an itinerant carrier.Calculations are performed by varying its dot radius,for various Mn alloy contents in GaMnAs quantum dot within a single band effective mass approximation using variational method.The spin polaronic energy of the acceptor impurity for different Mn²⁺ is evaluated for different dot radii using a mean field theory in the presence of magnetic field strength.The magnetization is computed in the influence of magnetic field and the Mn ions.The effective g-factor of the valence band electron with the inclusion of effects of Mn ion impurities is found in the influence of the magnetic field.The exchange coupling constant is calculated for various magnetic field strengths.The results show that the p-d exchange interaction in the GaMnAs/Ga_0.6Al_0.4As quantum dot has a strong dependence on spatial confinement,effect of magnetic field strength and the Mn alloy content.Our results are in good agreement with the other investigators.     

Magnetic field and temperature dependence of the groundstate energy of weak-coupling magnetopolaron in quantum rods with hydrogenic impurity

The dependence of the ground-state properties of weak-coupling bound magnetopolarons in quantum rods (QRs) with hydrogenic impurity on magnetic field and temperature is studied by means of the Lee-Low-Pines (LLP) transformation method and Huybrechts linear combination operator method. The expression for the ground-state energy of the magnetopolaron is derived. Results of the numerical calculations show that the ground-state energy of weak-coupling bound magnetopolarons in QRs with hydrogenic impurity increases with increasing the cyclotron frequency of the magnetic field, the confinement strength of QRs and the temperature, but decreases with increasing the electron-phonon coupling strength and the dielectric constant ratio. The stability of the ground state of magnetopolarons is closely related to the aspect ratio e′of the QR. The ground state of magnetopolarons is the most stable at e′=1. The stability of the ground state of magnetopolarons can remarkably decrease when the value of the aspect ratio increases or decreases from 1.     

Effects of hydrostatic pressure and external electric field on the impurity binding energy in strained GaN/AlxGa1?xN spherical quantum dots

The binding energy and Stark effect energy shifts of a shallow donor impurity state in a strained GaN/AlxGa1-xN spherical finite-potential quantum dot (QD) are calculated using a variational method based on the effective mass approximation. The binding energy is computed as a function of dot size and hydrostatic pressure. The numerical results show that the binding energy of the impurity state increases, attains a maximum value, and then decreases as the QD radius increases for any electric field. Moreover, the binding energy increases with the pressure for any size of dot. The Stark shift of the impurity energy for large dot size is much larger than that for the small dot size, and it is enhanced by the increase of electric field. We compare the binding energy of impurity state with and without strain effects, and the results show that the strain effects enhance the impurity binding energy considerably, especially for the small QD size. We also take the dielectric mismatch into account in our work.     

Shallow impurity states in Alx Gam_xAs cylindrical quantum wire

Polarons bound to a shallow Coulomb impurity center in cylindrical quantum wire is studied by a vari- ational approach. The binding energies of the shallow impurity states in AlxGal-xAs cylindrical quantum wire are calculated as functions of the composition x and the impurity position. It is confirmed that the binding energies are reduced obviously by the influence of the electron-phonon interaction and the binding energies are increased with increasing the composition x.     

Influence of strain on hydrogenic impurity states in a GaN/ Al_xGa_(1-x)N quantum dot

Within the effective-mass approximation, we calculated the influence of strain on the binding energy of a hydrogenic donor impurity by a variational approach in a cylindrical wurtzite GaN/AlxGa1-xN strained quantum dot, including the strong built- in electric field effect due to the spontaneous and piezoelectric polarization. The results show that the binding energy of impurity decreases when the strain is considered. Then the built-in electric field becomes bigger with the Al content increasing and the bin...     

CHARACTERISTIC STUDY OF A FREE ELECTRON LASER WITH LINEARLY POLARIZED WIGGLER AND RECTANGULAR WAVEGUIDE

The characteristics of coherent radiation produced by a cylindrical electron beam passing through a rectangular waveguide and linearly polarized wiggler are studied. The instability analysis is based on the linearized Vlasov-Maxwell equations for the perturbations about a self-consistent beam equilibrium. The dispersion equation of TMmn mode is deduced and by making use of numerical calculation the radiation frequency and growth rate as a function of electron beam energy and radius, axial magnetic field, wiggler field and wave length are presented and discussed.     

Double balanced differential configuration for high speed InGaAs/InP single photon detector at telecommunication wavelengths

Recently,some kinds of structures have been found to show the property of extraordinary optical transmission(EOT).In this paper,we present a novel composite structure based on array of annular apertures(AAA)with compound lattice.The lattice includes two kinds of annular apertures with the same outer radius and different inner radii.The transmission spectrum of this compound periodic AAA can be achieved by adding up the spectra of two corresponding simple periodic AAAs,and the transmission shows EOT property.The transmission peaks of this kind of structure can be adjusted to desire wavelengths by changing the inner radius of aperture or the index of the dielectric material in the aperture.This structure can be used as a filter with dual pass bands when the difference between inner radii or indices of dielectric inside is large enough for two kinds of apertures.     

Tailoring electron vortex beams with customizable intensity patterns by electron diffraction holography

An electron vortex beam(EVB) carrying orbital angular momentum(OAM) plays a key role in a series of fundamental scientific researches, such as chiral energy-loss spectroscopy and magnetic dichroism spectroscopy. So far, almost all the experimentally created EVBs manifest isotropic doughnut intensity patterns. Here, based on the correlation between local divergence angle of electron beam and phase gradient along azimuthal direction, we show that free electrons can be tailored to EVBs with customi...     

Energy spectrum in a concentric double quantum ring of GaAs–(Ga,Al)As under applied magnetic fields

The energy spectrum of an electron in a two-dimensional concentric double quantum ring is exactly calculated in the presence of an axial magnetic field using linear combinations of confluent hypergeometric functions. The influence of the geometric confinement on the energy spectrum is analyzed as a function of the inner and outer ring radii. The applied magnetic field modifies the electron energy spectrum and we discuss the appearance of the well-known Aharonov–Bohm oscillations. Interesting behavior of the probability amplitude of the electron state was found reflecting the competition between geometric and magnetic confinements. Electronic tunneling between the coupled rings may be reinforced or suppressed by conveniently modulating the barrier size separating the two rings, leading to drastic change in the electronic charge distribution through the nanostructure.     

磁场对GaAs/Al_xGa_(1-x)As异质结系统中束缚极化子的影响

对GaAs/AlxGa1-xAs单异质结系统引入三角势近似异质结势,同时考虑体纵光学(LO)声子和有效近似下两支界面光学(IO)声子的影响,采用变分法讨论了外界恒定磁场对束缚于近界面杂质的光学极化子结合能的影响.利用改进的Lee-Low-Pines(LLP)中间耦合方法处理电子-声子和杂质-声子的相互作用,计算了杂质态结合能随杂质位置、磁场强度、电子面密度的变化关系.结果表明,极化子结合能随磁场呈现增加的趋势,其中LO声子对结合能的负贡献受磁场影响显著,而IO声子的负贡献受磁场的影响并不明显,但当杂质靠近界面时,杂质-IO声子相互作用对磁场的影响很敏感.结果还表明,     

弱湍流大气中部分相干涡旋光束的轨道角动量特性

根据轨道角动量谱理论,推导了部分相干拉盖尔-高斯光束轨道角动量态的功率表达式。分析了相干长度、束宽对轨道角动量的影响,讨论了弱湍流大气中部分相干-拉盖尔高斯光束轨道角动量特性。结果表明:部分相干拉盖尔-高斯光束在相干长度与束腰半径比值固定的情况下,其初始轨道角动量态相对功率不会随着束腰半径的改变而变化。在部分相干拉盖尔-高斯光束初始相干长度与束腰半径取值大小相同的情况下,随着束腰半径的增大,光束在弱湍流大气中传输1 km处的初始轨道角动量态相对功率减小。     

非对称应变纤锌矿AlxGa1-xN/GaN/AlyGa1-yN量子阱中浅杂质态的结合能

本文采用变分法数值计算应变纤锌矿AlxGa1−xN/GaN/AlyGa1−yN量子阱中类氢杂质的基态结合能. 计及由自发极化和压电极化引起的内建电场, 讨论阱宽、杂质位置以及左右垒中Al组分对结合能的影响. 结果表明, 尤其在非对称量子阱即势垒宽度或（和）高度不一样的情形下, 杂质位置和垒高对结合能随阱宽变化关系的影响比垒宽更为明显. 对称或非对称结构中, 结合能随杂质位置的变化形如电子基态波函数的空间分布. 此外, 左垒中Al组分对结合能的影响较右垒更甚.     

Energy levels of a quantum ring in a lateral electric field

The electronic states of a semiconductor quantum ring (QR) under an applied lateral electric field are theoretically investigated and compared with those of a quantum disk of the same size. The eigenstates and eigenvalues of the Hamiltonian are obtained from a direct matrix diagonalization scheme. Numerical calculations are performed for a hard-wall confinement potential and the electronic states are obtained as a function of the electric field and the ratio r₂/r₁, where r₂ (r₁) is the outer (inner) radius of the ring. The effects of decreasing symmetry and mixing on the energy levels and wave functions due to the applied electric field are also studied. The direct optical absorption are reported as a function of the electric field.     

Binding energies of shallow impurities in asymmetric strained wurtzite AlxGa1-xN/ GaN/Aly Ga1-yN quantum wells

The ground state binding energies of hydrogenic impurities in strained wurtzite AlxGa1-xN/GaN/AlyGa1-y>N quantum wells are calculated numerically by a variational method. The de-pendence of the binding energy on well width, impurity location and Al concentrations of the left and right barriers is discussed, including the effect of the built-in electric field induced by spontaneous and piezoelectric polarizations. The results show that the change in binding energy with well width is more sensitive to the impurity position and barrier heights than the barrier widths, especially in asymmetric well structures where the barrier widths and/or barrier heights differ. The binding energy as a function of the impurity position in symmetric and asymmetric structures behaves like a map of the spatial distribution of the ground state wave function of the electron. It is also found that the influence on the binding energy from the Al concentration of the left barrier is more obvious than that of the right barrier.     

Experimental aspects of hot electron distribution functions

A review is presented on the experimental efforts to obtain hot electron distribution functions in bulk materials in high d.c. electric fields. Three optical methods will be discussed in detail: (i) inter- and intraband absorption measurements, (ii) emission due to radiative transitions between band states, and band- and impurity states, (iii) inelastic light scattering experiments. A distinction is made between methods which yield the energy distribution functions and those which give information on the anisotropic distribution of hot carriers in the momentum space. The latter experiments involve a determination of the electric field induced dichroism in absorption or emission and the dependence of the scattering cross section on scattering geometry in inelastic light scattering experiments. The influence of the nonequilibrium phonon distribution on the interpretation of the experimental results is discussed, too. In addition, current work on energy distribution functions of hot carriers in quantizing magnetic fields as obtained from absorption or emission of infrared radiation due to transitions of carriers between Landau states or magnetic field split impurity states will be presented.     

量子环中电子的激发能量和跃迁谱线频率

在量子环中电子与体纵光学声子强耦合的情况下,通过求解能量本征方程,得出了电子的基态和第一激发态的本征能量及其波函数.数值计算结果表明:电子的激发能量和跃迁谱线频率随着电子-声子耦合强度的增大而增大,并且随着量子环内径(或外径)的增大而减小。     

电磁波泵浦的自由电子激光器

本文研究了电磁波泵浦的自由电子激光器。从跃变磁场结构出来的大半径迴旋环形电子束通过圆柱形波导与圆柱形波导的TE11模入射电磁波相作用。利用电子束的弗拉索夫分布函数理论和三维波导模的波动方程求得在康普顿区域中的散射波色散关系。通过数值分析,讨论了轴向引导磁场,电子束能量,电子迴旋比和电子束环的径向位置等与散射波频率和增长率的关系。