A lattice of electromagnetic solitons in a superlattice formed by a system of quantum dots

Considering the Maxwell equations for the electromagnetic-field propagation in a solid with a three-dimensional superlattice of quantum dots linked by strong tunneling along one axis, we obtained a phenomenological equation in the form of the classical 2+1-dimensional sine-Gordon equation. Electrons were considered classically in the formalism of the Boltzmann kinetic equation for the distribution function. Solutions were obtained as a soliton lattice for the vector potential of the electric field. These lattices emerge as a consequence of the coherent change of the classical distribution function and the electric field generated by tunneling electrons in a system of quantum wells.     

High-frequency response of two-well nanostructures

A consistent analytical theory is developed for coherent resonant electron tunnelling in a two-well nanostructure in the presence of a weak electromagnetic field. Simple expressions derived for the transmission coefficient and linear response of the two-well nanostructure make it possible to clarify the physics of processes and to express the gain as a function of the structure parameters. It is shown that the high-frequency response of the two-well nanostructure considerably exceeds the response of a one-well structure (resonance-tunnel diode) and that the application of a constant electric field makes it possible to tune the oscillation frequency and to increase the gain. It is concluded that two-well nanostructures can be used in designing terahertz oscillators. It is shown that, in contrast to a resonance-tunnel diode, interference of electrons between the wells and radiative “laser-type” transition play a decisive role in such structures.     

Stark ladder laser with a coherent electron subsystem

A theory of generation in a two-subband “Stark ladder” with a coherent electron subsystem is developed. In the proposed model, electrons reach the upper level of a quantum well due to resonant tunneling and pass to the lower level of the well (vertical transitions), emitting a photon ?ω, then tunnel resonantly to the upper level of a neighboring well, performing a radiative transition, and so on until electrons leave the lower level of the last well. A static electric field applied to the superlattice shifts the levels so that the lower level of the nth well coincides with the upper level of the (n+1)th well. Analytic expressions are derived for the wave functions and polarization currents of an N-well structure. The possibility of bulk oscillation of the N-well structure in the optimal mode with an efficiency close to unity, weak reflection, and a linear dependence of the power on the pumping current is demonstrated. The total generation power is proportional to the number of wells. For structures with an even number of wells, the energy of electrons from the emitter must simply coincide with the resonance energy for any laser fields; i.e., the energy tuning which is necessary in a single-well structure is not required. Universal relations are derived for parameters of the N-well structure, which ensure the simultaneous fulfillment of resonance conditions in all the wells. The possibility of coherent lasing in a one-subband Stark ladder with a lower gain is also indicated.     

Self—Trapping State and Atomic Tunnelling Current of an Atomic Bose—Einstein Condensate Interacting with a Laser Field in a Double—Well Potential

We present a theoretical treatment of dynamics of an atomic Bose-Einstein condensation interacting with a single-mode quantized travelling-wave laser field in a double-well potential.When the atom-field system is initially in a coherent state,expressions for the energy exchange between atoms and photons are derived.It is revealed that atoms in the two wells can be in a self-trapping state when the tunnelling frequency satisfies two specific conditions,in which the resonant and far off-resonant cases are included.It is found that there is an alternating current with two different sinusoidal oscillations between the two wells,but no dc characteristic of the atomic tunnelling current occurs.It should be emphasized that when without the laser field,both the population difference and the atomic tunnelling current are only a single oscillation.But they will respectively become a superposition of two oscillations with different oscillatory frequencies in the presence of the laser field.For the two oscillations of the population difference,one always has an increment in the oscillatory frequency,the other can have an increment or a decrease under different cases.These conclusions are also suitable to those of the atomic tunnelling current.As a possible application,by measurement of the atomic tunnelling current between the two wells,the number of Bose-condensed atoms can be evaluated.By poperly selecting the laser field,the expected atomic tunnelling current can be obtained too.     

Energy filtration effect and the possibility of the generation of terahertz radiation in resonant tunneling structures with several quantum wells

The properties of a high-frequency response in resonant tunneling double-well nanostructures have been considered for various energies of electrons arriving to a structure of electrons, various frequencies of the external electromagnetic field, and various features associated with the interaction of electronic states in neighboring quantum wells in double-well nanostructures. The energy filtration effect that is caused by the breaking of the symmetry of the high-frequency response in double-well nanostructures in a static electric field has been revealed. This effect leads to a sharp increase in the gain under conditions of the quantum amplification regime and opens real prospects of a significant increase in the efficiency of solid amplifying and generating devices based on resonant tunneling double-well nanostructure in the subterahertz and terahertz frequency ranges.     

Self-induced shapiro effect in semiconductor superlattices

We observe that the oscillatory motion of photoinjected electron-hole pairs in a biased semiconductor superlattice (Bloch oscillation) is accompanied by a coherent quasi-dc current that is generated by the interaction of the carriers with the self-induced oscillating field. It is shown that this novel macroscopic quantum effect, which is a coherent analog of the Shapiro effect observed in Josephson junctions, can be controlled by changing the spectral position of the exciting laser pulse, which in turn determines the amplitude and phase of the wave packet oscillations. It is thereby possible to coherently drive the electrons either downwards or upwards in the potential of the static field.     

Gate control of quantum dot-based electron spin–orbit qubits

We investigate theoretically the coherent spin dynamics of gate control of quantum dot-based electron spin–orbit qubits subjected to a tilted magnetic field under electric-dipole spin resonance (EDSR). Our results reveal that Rabi oscillation of qubit states can be manipulated electrically based on rapid gate control of SOC strength. The Rabi frequency is strongly dependent on the gate-induced electric field, the strength and orientation of the applied magnetic field. There are two major EDSR mechanisms. One arises from electric field-induced spin–orbit hybridization, and the other arises from magnetic field-induced energy-level crossing. The SOC introduced by the gate-induced electric field allows AC electric fields to drive coherent Rabi oscillations between spin-up and -down states. After the crossing of the energy-levels with the magnetic field, the spin-transfer crossing results in Rabi oscillation irrespective of whether or not the external electric field is present. The spin–orbit qubit is transferred into the orbit qubit. Rabi oscillation is anisotropic and periodic with respect to the tilted and in-plane orientation of the magnetic field originating from the interplay of the SOC, orbital, and Zeeman effects. The strong electrically-controlled SOC strength suggests the possibility for scalable applications of gate-controllable spin–orbit qubits.     

Anharmonic Bloch Oscillation of Electrons in Biased Superlattices

The oscillatory motion of electrons in a periodic potential under a constant applied electric field, known as Bloch oscillations (BO), is one of the most striking and intriguing quantum effects and was predicted more than eighty years ago. Oscillating electrons emit electromagnetic radiation and here we consider this BO effect for emission in the THz region. To date, it has been assumed that the Bloch oscillation of an electron is anharmonic oscillation, therefore with radiation emitted at the single Bloch frequency. We analyze scenarios when Bloch oscillations can be accompanied by the emission of radiation not only at the Bloch frequency but also with double and triple Bloch frequencies. The first scenario means that electrons could jump over neighboring Stark states. The second scenario of anharmonic emission is coupled to an opening of the minigap in the miniband.     

Interaction of counterpropagating electromagnetic waves in an absorbing plate inside a waveguide

The interaction of two counterpropagating coherent transverse electric and transverse magnetic electromagnetic waves in an absorbing plate that is placed in a waveguide with an arbitrary transverse cross section is analyzed. It is assumed that the waves with different initial phases are incident on the plate boundaries from two sides. An analytical expression for the interference transmission coefficient with respect to power is derived. Several physical features of the tunneling interference in the plate are revealed. It is demonstrated that a scenario in which an electromagnetic energy flux exists on the left-hand side of the plate and vanishes on the right-hand side or vice versa is possible at certain relations of the initial phases and amplitudes of the counterpropagating waves.     

双模非经典光场辐照下介观约瑟夫森结中超流的动力学行为

研究了直流偏置电压下介观约瑟夫森结在SU（1，1）双模相干态光场作用下超流的动力学行为、流压台阶和直流分量等特性．研究发现超流能够呈现明显的崩塌与复苏现象，对于经典电磁场和非耦合的双模相干态光场作用下的情况，超流不出现此现象，且其流压台阶结构与后两种情况也不相同． 关键词：     

Zener tunneling between Landau levels in a double quantum well at high filling factors

Differential resistance r _xx in a double GaAs quantum well with two occupied size-quantization subbands has been studied at a temperature of 4.2 K in magnetic fields B < 2 T. The oscillations of r _xx with a period in the inverse magnetic field determined by the value of a dc bias current I _dc have been discovered in the electron system under investigation at high filling factors in the presence of I _dc. The amplitude of magneto-intersubband oscillations has been shown to increase in the r _xx oscillation maxima, while the oscillation reversal has been observed in the minima. The discovered oscillations have been shown to be due to Zener tunneling of electrons between Landau levels tilted by a Hall electric field. The experimental data are qualitatively explained by the effect of intersubband transitions on the I _dc-dependent component of the electron distribution function.     

Coherent oscillations in a superconducting multilevel quantum system

We have observed coherent oscillations in a multilevel quantum system, formed by a current-biased dc SQUID. These oscillations have been induced by applying resonant microwave pulses of flux. Quantum measurement is performed by a nanosecond flux pulse that projects the final state onto one of two different voltage states of the dc SQUID, which can be read out. The number of quantum states involved in the coherent oscillations increases with increasing microwave power. The dependence of the oscillation frequency on microwave power deviates strongly from the linear regime expected for a two-level system and can be very well explained by a theoretical model taking into account the anharmonicity of the multilevel system.     

Interwell excitons in a lateral potential well in an inhomogeneous electric field

The luminescence of interwell excitons in double quantum wells based on GaAs/AlGaAs semiconductor heterostructures (n-i-n structures) in a lateral trap prepared with the use of an inhomogeneous electric field was studied at helium temperatures. A rather strong and inhomogeneous electric field occurred in the depth of the heterostructure when a current passed through the contact between the conducting tip of a tunneling microscope and the heterostructure surface to the bulk region containing a built-in gate. Because of the Stark shift of energy bands in the electric field, the photoexcited electrons and holes are spatially separated in neighboring quantum wells by a tunnel-transparent barrier and are bound into interwell quasi-two-dimensional excitons. These excitons have a dipole moment even in the ground state. Therefore, electrostatic forces in the inhomogeneous electric field cause the excitons to move in the plane of quantum wells toward the maximum field region and eventually accumulate in the lateral trap artificially prepared in such a way. The maximum trap depth achieved through the inhomogeneous electric field was 13.5 meV, and its lateral size was about 10 μm. It is shown that, in the traps prepared in this way, photoexcited interwell excitons behave with increasing concentration at sufficiently low temperatures (T=2K) in the same fashion as in the lateral traps caused by large-scale fluctuations of the random potential. At concentrations exceeding the percolation threshold, the interwell excitons condense into the lowest energy state in the trap.     

Ambipolar tunneling in near-surface quantum wells

We study the photoluminescence from a near-surface quantum well in the regime of ambipolar tunneling to the surface states. Under steady-state excitation an electric field develops self-consistently due to the condition of equal tunneling currents for electrons and holes. The field induces a Stark shift of the photoluminescence signal which compares well with experimental data from near-surface GaAs/AlGaAs single quantum wells.     

Nonlinear Electronic Transport Equations for Tunneling Superlattices

We present a nonlinear electronic transport theory for a tunneling superlattice which is composed of interacting electrons with impurity and phonon scatterings. The theory is based on the Lei-Ting balance-equation method and a newly developed matrix separation technique for the density-correlation-function. Taking account of the overlap of the wave-functions between adjacent wells, the nonlinear dc resistivity in a transverse strong electric field is explicitly expressed in terms of the matrixdensity- correlation-function, which can be straightforwardly calculated in RPA as long as the single well wave function is given.     

Spin current through a quantum dot in the presence of an oscillating magnetic field

Nonequilibrium spin transport through an interacting quantum dot is analyzed. The coherent spin oscillations in the dot provide a generating source for spin current. In the interacting regime, the Kondo effect is influenced in a significant way by the presence of the processing magnetic field. In particular, when the precession frequency is tuned to resonance between spin-up and spin-down states of the dot, Kondo singularity for each spin splits into a superposition of two resonance peaks. The Kondo-type cotunneling contribution is manifested by a large enhancement of the pumped spin current in the strong coupling low temperature regime.     

Forced charge oscillations in a double quantum dot

The two-electron wave function and charge distribution are obtained in a symmetric double quantum dot in a weak variable electric field. It is shown that the action of a variable field under resonance conditions when the perturbation frequency is close to the frequency of the quantum transition leads to the appearance of electron density oscillations between the dots having the characteristic form of beats. However, the Coulomb repulsion between the electrons strongly “quenches” the amplitude of the beats even in a resonant variable field.     

相互作用量子点系统中的场致自旋电流

从理论上研究了相互作用量子点在外部旋转磁场下的非平衡自旋输运性质，研究结果表明，量子点中的相干自旋振荡可以导致自旋电流的产生，当计入库仑关联相互作用后，近藤共振效应受外部进动磁场的影响很强，特别是当磁场的进动频率与塞曼能移满足共振条件时，每个自旋近藤峰就会劈裂为两个自旋共振峰的叠加，在低温强耦合区，这种近藤型共隧穿过程对自旋电流带来重要贡献。     

Radiative coupling of intersubband transitions in GaAs/AlGaAs multiple quantum wells

Radiative coupling of resonantly excited intersubband transitions in GaAs/AlGaAs multiple quantum wells can have a strong impact on the coherent nonlinear optical response, as is shown by phase and amplitude resolved propagation studies of ultrashort electric field transients. Upon increasing the driving field amplitude, strong radiative coupling leads to a pronounced self-induced absorption, followed by a bleaching due to the onset of delayed Rabi oscillations. A many-particle theory including light propagation effects accounts fully for the experimental results.     

CdSe/ZnSe复合结构非对称量子阱的发光特性

利用MOCVD技术在GaAs衬底上外延生长了非对称量子阱结构CdSe/ZnSe材料,通过对其稳态变温光谱及变激发功率光谱,研究了其发光特性。稳态光谱表明:在82～141K时,观测到的两个发光峰来源于不同阱层厚度的量子阱激子发光,用对比实验验证了高能侧发光的来源。宽阱发光强度先增加后减小,将其归结为激子隧穿与激子热离化相互竞争的结果。通过Arrhenius拟合,对宽阱激子热激活能进行了计算。82K时变激发功率PL光谱表明:由于激子隧穿的存在,使得窄阱发光峰位不随激发功率变化而变化,宽阱发光峰位随激发功率增加发生了蓝移,并对激子隧穿进行了实验验证。