Energy spectrum and effective mass of carriers in the InSe/GaSe superlattice

Within an effective mass approximation the energy spectrum and mass of carriers in the InSe/GaSe superlattice have been calculated. The superlattice belongs to type II: electrons are primarily confined to the InSe layers whereas the holes are mosfly confined to the GaSe layers. The characteristic feature of electronic structure of the superlattice is the existence of minibands of light carriers at the θ point of the Brillouin zone and minibands of heavy carriers at theM point. The dependence of the miniband structure on thickness of layers has been computed. It is shown that the minibands of light and heavy carriers compete with one another in energy. A general conclusion is made concerning the influence of the competition between the minibands on optic and kinetic properties of the superlattice.     

Optical spectra of one-dimensional defect photonic crystals

The effect exerted by defects that represent a local disturbance of the structural periodicity on the band spectrum of a one-dimensional photonic crystal has been investigated. A classification of single defects has been proposed. It has been demonstrated that the position of defect minibands in the band gap of the spectrum of the crystal can be controlled by varying the type of a defect and its location in the crystal structure. The presence of two defects in the structure leads to the formation of two minibands, so that the spacing between the minibands and their intensity depend on the type of defects and on the distance between them.     

Energy spectrum and effective mass of carriers in the InSe/GaSe superlattice

Within an effective mass approximation the energy spectrum and mass of carriers in the InSe/GaSe superlattice have been calculated. The superlattice belongs to type II: electrons are primarily confined to the InSe layers whereas the holes are mosfly confined to the GaSe layers. The characteristic feature of electronic structure of the superlattice is the existence of minibands of light carriers at the θ point of the Brillouin zone and minibands of heavy carriers at theM point. The dependence of the miniband structure on thickness of layers has been computed. It is shown that the minibands of light and heavy carriers compete with one another in energy. A general conclusion is made concerning the influence of the competition between the minibands on optic and kinetic properties of the superlattice.     

Interaction of wide-spectrum electromagnetic pulses with materials containing a superstructure

The interaction of wide-spectrum electromagnetic pulses with materials containing a superstructure is considered. The effective equation is obtained, which has a form of classical one-dimensional sine-Gordon equation, perturbed by quantum transitions between minibands. Numerical solutions are presented, and a region is established, where transitions between minibands play a significant role in the electromagnetic pulse dynamics.     

Tamm minibands in graphene-based planar superlattices

The Tamm minibands in planar superlattices composed of graphenes with different Fermi velocities and energy gaps have been investigated. It is shown that Tamm minibands arise only in the case of intersection of the dispersion curves of charge carriers in different superlattice regions.     

Application of optimal control theory to ultrafast nonresonant multiphoton transitions in polyatomic molecules

Optimal control theory (OCT) including nonresonant multiphoton transitions (NMT) in the femtosecond time regime has been implemented. The NMT process is modelled using an effective time-dependent Schrödinger equation, which considers a reduced space of the two levels which are involved in the transition, coupled to a density of high-lying off-resonant states. The specification of OCT to two-photon transitions is done within the rotating wave approximation. As an application, the transfer and displacement of a wavepacket in the organometallic system CpMn(CO)₃, from the ground to an electronically excited state using two nonresonant photons, is simulated.     

Band structures of symmetrical graphene superlattice with cells of three regions

We study the electronic band structures of massless Dirac fermions in symmetrical graphene superlattice with cells of three regions. opening gaps and additional Dirac points. Finally, we inspect the potential effect on minibands, the anisotropy of group velocity and the energy bands contours near Dirac points. We also discuss the evolution of gap edges and cutoff region near the vertical Dirac points.     

含各向异性左手材料一维光子晶体微腔的Wannier-Stark态

通过传输矩阵方法,研究了含各向异性左手材料的一维光子晶体耦合微腔结构。结果表明,该结构中存在两类微带,一类位于常规的Bragg带隙,另一类位于零均值折射率带隙。当腔的厚度发生调制时,两类微带变成两类Wannier-Stark态。耦合微腔厚度梯度因子增大时,两类带隙中Bragg振荡周期都减小,而且透过率降低。     

由宽波段多光子作用产生的锂分子高位态受激辐射

本文报道了以６２０．６￣６６５．０ｎｍ范围内任意波长激光作用下产生的锂分子高位态紫外扩散带受激辐射的实验结果,在锂分子－原子系统中,由激光双光子激发、并通过碰撞能量转移过程以及多光子电离复合过程使锂分子高位态获得布居,从而得到紫外区扩散受激辐射,该辐射来自较高的某个三重态向低排斥态ａ＾３Σｕ＾＋的跃迁的结果。文中对有关的辐射特性及激发机制进行了详细的分析和讨论。     

Diffraction properties of binary graphene sheet arrays

We theoretically and numerically investigate the diffraction properties of surface plasmon polariton(SPP) in binary graphene sheet arrays. The single SPP band splits into two minibands by alternatively arranging the graphene waveguides with two different chemical potentials. Numerical simulations show that SPP beams in the array split into two different paths due to the different diffraction relation.     

Ultrafast spectroscopy of high-lying excited states via eight-wave mixing

We demonstrate the use of eight-wave mixing (8WM) with femtosecond laser pulses for the detection of the high-lying D state of ethylene (C₂H₄) gas. The D state is reached through three-photon absorption of 427 nm light followed by a delayed single-photon probe interaction with the ionization continuum, resulting in an overall 8WM process. This scheme allows the detection of high-lying states with both spatial and temporal resolution before a thermal grating can form. Alternatively, by using a nanosecond delay between pulses, the induced thermal grating may be studied after the population grating has decayed. We verify the precise 8WM nature of this process and show how to use it to study the excited-state dynamics of ethylene and to measure the profile of an ethylene flow.     

Density of states of superlattices with multiple layers per period

Density-of-states (DOS) investigations of polytype superlattices (SLs), whose period consists of more than two different layers, are presented. Local DOS (LDOS) is computed as a function of both the electron energy and the space coordinate, which illustrates — in a direct way — the spatial localization, over the SL period, of states forming the energy minibands. A few examples of AlGaAs-based polytype SLs with a coupled-well, step-well and δ-doped basis are analysed, indicating that various LDOS distributions within SL minibands can be arranged. By changing the basis geometry, the electronic DOS can either be selectively confined to a particular SL layer or extend over the whole SL period, which provides a means of modifying the spatial overlap of states from different SL minibands and thus, tuning the respective interband transitions. In particular, polytype SLs with similar miniband structures may exhibit essentially different LDOS features and, consequently, the different transport and optical characteristics. Therefore, it is emphasized that — for a particular device application of a complex-basis SL — not only the electronic level structure should be appropriately designed, but also the corresponding space-charge distributions should be carefully considered.     

Acoustic analogue of electronic BLOCH oscillations and resonant Zener tunneling in ultrasonic superlattices

We demonstrate the existence of Bloch oscillations of acoustic fields in sound propagation through a superlattice of water cavities and layers of methyl methacrylate. To obtain the acoustic equivalent of a Wannier-Stark ladder, we employ a set of cavities with different thicknesses. Bloch oscillations are observed as time-resolved oscillations of transmission in a direct analogy to electronic Bloch oscillations in biased semiconductor superlattices. Moreover, for a particular gradient of cavity thicknesses, an overlap of two acoustic minibands occurs, which results in resonant Zener-like transmission enhancement.     

Zener tunneling of light waves in an optical superlattice

We report on the observation of Zener tunneling of light waves in spectral and time-resolved transmission measurements, performed on an optical superlattice made of porous silicon. The structure was designed to have two photonic minibands, spaced by a narrow frequency gap. A gradient in the refractive index was introduced to create two optical Wannier-Stark ladders and, at a critical value of the optical gradient, tunneling between energy bands was observed in the form of an enhanced transmission peak and a characteristic time dependence of the transmission.     

Coupling effects on Rabi oscillations in quantum dots chains

In this work, we calculate the temporal evolution of electronic states under AC electric fields. We do that by obtaining the eigenenergies and wave functions of states of the first two minibands for arrays of 10 cylindrical quantum dots, both vertically and laterally coupled, which exhibit notable difference in their symmetry levels and coupling magnitudes. We observe an important dependence of the Rabi amplitude on the wave function symmetry and the coupling magnitude, and conclude that Rabi oscillations at the terahertz range are best suited for the higher coupling between dots.     

Dipolar Excitations of a Trapped Bose-Fermi Mixture

We study the dipolar excitation of a trapped Bose-Fermi mixture at zero temperature, by using a scaling ansatz formalism and Thomas-Fermi approximation at mean-field level. We show that both frequencies of the low-lying and high-lying modes are strongly affected by the Bose-Fermi interaction. Possible implication of our results to the recent experiment has been commented.     

Spin-polarized transport in graphene nanoribbon superlattices

By the Green’s function method,we investigate spin transport properties of a zigzag graphene nanoribbon superlattice(ZGNS) under a ferromagnetic insulator and edge effect.The exchange splitting induced by the ferromagnetic insulator eliminates the spin degeneracy,which leads to spin-polarized transport in structure.Spin-dependent minibands and minigaps are exhibited in the conductance profile near the Fermi energy.The location and width of the miniband are associated with the geometry of the ZGNS.In the optimal structure,the spin-up and spin-down minibands can be separated completely near the Fermi energy.Therefore,a wide,perfect spin polarization with clear stepwise pattern is observed,i.e.,the perfect spin-polarized transport can be tuned from spin up to spin down by varying the electron energy.     

Dipolar Excitations of a Trapped Bose-Fermi Mixture

We study the dipolar excitation of a trapped Bose-Fermi mixture at zero temperature, by using a scalingansatz formalism and Thomas-Fermi approximation at mean-field level. We show that both frequencies of the low-lying and high-lying modes are strongly affected by the Bose-Fermi interaction. Possible implication of our results to the recent experiment has been commented.     

Stark localization and mixing phenomena between different Stark-ladders in coupled quantum wells

We have studied Stark localization of the minibands in coupled quantum wells by considering the effect of an external electric field, applied along the growth axis, on a multiple-quantum well system which is identical to the effect of the ladder-like increment in the potential energy. The mixing feature of the Stark ladders is explained by using an analysis of the spatial probability distribution of the carrier. We have found that at intermediate ladder values a certain degree of delocalization of the carrier wave functions exists and finally at large ladder values the Stark-ladder states become localized within individual wells and a combined band structure occurs.