Multiband diffraction and refraction control in binary arrays of periodically curved waveguides

The possibility of controlling discrete diffraction and refraction in a multiband waveguide array by periodic waveguide bending is theoretically demonstrated. Resonance effects, leading to the enhancement or inhibition of discrete diffraction, are found and related to the quantum analog of field-induced n-photon resonances in semiconductor superlattices. A very distinct behavior for light refraction is found for odd or even resonances. In particular, for even resonances, the two-band behavior of the straight binary array is quenched, resulting in the inhibition of double refraction.     

Nonadiabatic Landau-Zener tunneling in waveguide arrays with a step in the refractive index

Landau-Zener tunneling is discussed in connection with optical waveguide arrays. Light injected in a specific band of the Bloch spectrum in the propagation constant can be transmitted to another band, changing its physical properties. This can be achieved using two coupled waveguide arrays with different refractive indices. The step in the refractive index causes wave "acceleration" and thus induces strongly nonadiabatic Landau-Zener tunneling. Theoretically, the analysis is performed by considering a Schr?dinger equation in a periodic potential with a step. The region of physical parameters where this phenomenon can occur is analytically determined and a realistic experimental setup is suggested. Its application could allow the realization of light filters.     

Light emission properties in a double quantum dot molecule immersed in a cavity: Phonon-assisted tunneling

Two main mechanisms dictate the tunneling process in a double quantum dot: overlap of excited wave functions, effectively described as a tunneling rate, and phonon-assisted tunneling. In this paper, we study different regimes of tunneling that arise from the competition between these two mechanisms in a double quantum dot molecule immersed in a unimodal optical cavity. We show how such regimes affect the mean number of excitations in each quantum dot and in the cavity, the spectroscopic resolution and emission peaks of the photoluminescence spectrum, and the second-order coherence function which is an indicator of the quantumness of emitted light from the cavity.     

Visualization of coherent destruction of tunneling in an optical double well system

We report on a direct visualization of coherent destruction of tunneling (CDT) of light waves in a double well system which provides an optical analog of quantum CDT as originally proposed by Grossmann, Dittrich, Jung, and H?nggi [Phys. Rev. Lett. 67, 516 (1991)10.1103/PhysRevLett.67.516]. The driven double well, realized by two periodically curved waveguides in an Er:Yb-doped glass, is designed so that spatial light propagation exactly mimics the coherent space-time dynamics of matter waves in a driven double well potential governed by the Schr?dinger equation. The fluorescence of Er ions is exploited to image the spatial evolution of light in the two wells, clearly demonstrating suppression of light tunneling for special ratios between the frequency and amplitude of the driving field.     

Tunneling and nonhyperbolicity in quantum dots

We argue that many major features in electronic transport in realistic quantum dots are not explainable by the usual semiclassical approach, due to the contributions of the quantum-mechanical tunneling of the electrons through the Kolmogorov-Arnol'd-Moser islands. We show that dynamical tunneling gives rise to a set of resonances characterized by two quantum numbers, which leads to conductance oscillations and concentration of wave functions near stable and unstable periodic orbits. Experimental results agree very well with our theoretical predictions, indicating that tunneling has to be taken into account to understand the physics of transport in generic nanostructures.     

Magnetic-field-dependent excitation transfer in quantum wells of diluted magnetic semiconductor

We studied the excitation transfer in double quantum wells of a diluted magnetic semiconductor using a scanning near-field optical microscope at 7 K in external magnetic fields up to 9 T. In each quantum well, local energy minima are generated by local fluctuation of layer thickness and doping concentration of magnetic components. Excitons relax into the local energy minima and transfer between the minima via near-field optical interactions even across quantum wells toward stable sites at which to localize. We measured the intensity maps of near-field photoluminescence with spatial resolution estimated to be 30 nm under varying external magnetic fields. The measurement position reproducibility was confirmed by scanning tunneling microscope images. Analysis of the maps derived the magnetic-field dependence of the typical size of exciton-localization sites for each quantum well. Based on these results, we investigated the excitation transfer between the two quantum wells lying in different layers of the double quantum well system, and showed that the exciton transfer takes place at the two specific applied magnetic-field intensities that result in the crossing of Zeeman-split energy levels of the two different wells. We concluded that both the localization and the inter-quantum-well transfer of excitons are able to be controlled by an external magnetic field. This provides the basis for functional devices operating without any wiring.     

Performance characteristics of deep violet InGaN DQW laser diodes with InGaN/GaN superlattice waveguide layers

The effect of the indium (In) composition of In_xGa_1−xN (GaN) waveguide layers on the performance of deep violet In_0.082Ga_0.918N/GaN double quantum well (DQW) laser diodes (LDs) emitting at 390 nm output emission wavelength has been numerically investigated. Simulation results indicated that by increasing In composition of the In_xGa_1−xN waveguide layers, the threshold current decreases, the slope efficiency, and differential quantum efficiency (DQE) increase, whereas the output power decreases. The increase in the In composition of the InGaN waveguide layers increases the refractive index and consequently increases the optical confinement factor (OCF) which result in the increase in the slope efficiency and DQE and the decrease in the threshold current. The decreasing movement of electron and hole carriers from the bulk waveguide layers to the active regions also causes to decrease the output power. A new LD structure with InGaN/GaN superlattice (SL) waveguide layers has been proposed to exploit the increased OCF of InGaN waveguide structures, and the enhanced electron and hole mobilities and the tunneling effect of the periodic structure of the SL structures. The results also showed that the use of InGaN/GaN SL waveguide structures effectively improves the output power, slope efficiency and DQE and decreases the threshold current of the LD compared with (In)GaN bulk waveguide structure.     

弱耦合GaAs/AlGaAs/InGaAs双势阱隧穿结构的磁隧穿特性研究

研究了低温(15 K)条件下弱耦合GaAs/AlGaAs/InGaAs双势阱结构的纵向磁隧穿特性. 研究表明,器件在零偏压下处于共振状态. 通过分析不同偏压下的磁电导振荡曲线,可以得到双量子阱中的基态束缚能级随偏压的变化规律,从而可以确定隧穿电流峰对应的隧穿机制. 所得结果可为弱耦合双量子点器件的制备提供基础. 关键词： 双量子阱 隧穿结构 磁电导振荡     

冷原子物理中的一维少体问题

作为构成量子多体系统的基本单元,一维少体系统的研究不仅可以在理论上为多体系统的量子关联及动力学等性质提供更为基本的理解,也可以为实验上制备多体系统提供更加方便和功能更加全面的方法.本文回顾了冷原子物理中一维少体系统最新的实验和理论进展.首先介绍了少体实验中实现的谐振子势阱中确定原子数的精确制备,亚稳态势阱和双阱系统中原子的隧穿,以及强相互作用下等效自旋链的实验结果.然后深度解析了理论研究方面,特别是基于精确可解模型的一些重要结果,包括亚稳态势阱中相互作用原子的隧穿概率,以及相应实验上常见势阱的能谱分析、密度分布、隧穿动力学以及强相互作用极限下的有效自旋链模型等.     

Effect of strain on hole tunneling dynamics in double barrier heterostructures

A study of the effects of strain due to external stress and lattice mismatch on hole tunneling times in double barrier heterostructures is presented. The band structure of holes is calculated based on a k·p method within the envelope function approximation, including band mixing effects. The phase delay time is obtained from the energy derivative of the total phase shift of the wave function upon tunneling and is used to estimate the hole tunneling time. The results demonstrate that strain can be utilized to tailor hole tunneling times by changing the energy separation and, consequently, the mixing between heavy hole and light hole states in the quantum well.     

Charge and energy transfer in double asymmetric quantum wells with quantum dots

The luminescence and luminescence excitation spectra of CdSe/ZnSe quantum dots are studied in a set of double quantum wells with the ZnSe barrier of width 14 nm, the same amount of a deposited CdSe layer forming a deep well and shallow wells with different depths. It is found that for a certain relation between the depths of shallow and deep wells in this set, conditions are realized under which the exciton channel in the luminescence excitation spectrum of a shallow well dominates in the region of kinetic exciton energies exceeding 10 longitudinal optical phonons above the bottom of the exciton band of the ZnSe barrier. A model is developed for the transfer of electrons, holes, and excitons between the electronic states of shallow and deep quantum wells separated by wide enough barriers. It is shown that the most probable process of electronic energy transfer between the states of shallow and deep quantum wells is indirect tunneling with the simultaneous excitation of a longitudinal optical phonon in the lattice. Because the probability of this process for single charge carriers considerably exceeds the exciton tunneling probability, a system of double quantum wells can be prepared in which, in the case of weak enough excitation, the states of quantum dots in shallow quantum wells will be mainly populated by excitons, which explains experimental results obtained.     

Inelastic tunneling in a double quantum dot coupled to a bosonic environment

Coupling a quantum system to a bosonic environment always give rise to inelastic processes, which reduce the coherency of the system. We measure energy-dependent rates for inelastic tunneling processes in a fully controllable two-level system of a double quantum dot. The emission and absorption rates are well reproduced by Einstein's coefficients, which relate to the spontaneous emission rate. The inelastic tunneling rate can be comparable to the elastic tunneling rate if the boson occupation number becomes large. In the specific semiconductor double dot, the energy dependence of the inelastic rate suggests that acoustic phonons are coupled to the double dot piezoelectrically.     

Two-point phase correlations of a one-dimensional bosonic Josephson junction

We realize a one-dimensional Josephson junction using quantum degenerate Bose gases in a tunable double well potential on an atom chip. Matter wave interferometry gives direct access to the relative phase field, which reflects the interplay of thermally driven fluctuations and phase locking due to tunneling. The thermal equilibrium state is characterized by probing the full statistical distribution function of the two-point phase correlation. Comparison to a stochastic model allows us to measure the coupling strength and temperature and hence a full characterization of the system.     

Photocurrent response in a double barrier structure with quantum dots–quantum well inserted in central well

We report the photocurrent response in a double barrier structure with quantum dots–quantum well inserted in central well. When this quantum dots–quantum well hybrid heterostructure is biased beyond +1 or −1 V, the photocurrent response manifests itself as a step-like enhancement, increasing linearly with the light intensity. Most probably, at proper bias condition, the pulling down of the X minimum of GaAs at the outgoing interface of the emitter barrier by the photovoltaic effect in GaAs QW will initiate the Γ–X–X tunneling at much lower bias as compared with that in the dark. That gives rise to the observed photocurrent response.     

Control of photon tunneling in optical waveguides

Engineered optical waveguide structures with a controlled photon tunneling rate are shown to provide an experimentally accessible optical system to test universal features of dynamic control of quantum mechanical decay [Phys. Rev. Lett. 87, 270405 (2001)]. Examples of deceleration and acceleration of photon tunneling, as well as of suppression of fractional decay, are presented for a periodically curved waveguide coupled to a semi-infinite waveguide array.     

Atomic Landau-Zener tunneling in Fourier-synthesized optical lattices

We report on an experimental study of quantum transport of atoms in variable periodic optical potentials. The band structure of both ratchet-type asymmetric and symmetric lattice potentials is explored. The variable atom potential is realized by superimposing a conventional standing wave potential of lambda/2 spatial periodicity with a fourth-order multiphoton potential of lambda/4 periodicity. We find that the Landau-Zener tunneling rate between the first and the second excited Bloch band depends critically on the relative phase between the two spatial lattice harmonics.     

石墨烯覆盖微纳光纤复合波导全光优先吸收特性

利用二氧化碳激光器加热法,将普通单模光纤拉制成微纳光纤,用湿法转移石墨烯覆盖在微纳光纤上构成复合波导,不同波长的光通过耦合器进入复合波导,以倏逝波的形式与石墨烯相互作用,开展石墨烯优先吸收特性的研究。当短波作为泵浦光时,随着入射强度的增长,测得输出端长波信号光光谱的变化,获得了约3.5 dB的调制深度,0.62 dB·mW-1的调制效率。当长波作为泵浦光并改变入射光强时,在输出端测得作为信号光的短波透过率变化约1.9%。实验结果表明,随着任意波长泵浦光入射光强的增长,复合波导对其表现出优先吸收的特性。实验还测试了长波和短波分别经过复合波导后透过率随输入功率的变化,得出长波的透过率增加速度比短波更快,并从能带和倏逝波两方面作出了对应的理论分析。     

Tunneling-induced enhancement of self-Kerr nonlinearity in asymmetric quantum wells

We propose an asymmetric AlGaAs/GaAs double quantum wells (QWs) structure for realizing the enhancement of self-Kerr nonlinearity. It is found, with resonant tunneling, that the self-Kerr nonlinearity can be clearly enhanced, while the absorption of probe field is very small and can be safely neglected. We attribute the enhancement of self-Kerr nonlinearity mainly to the constructive interference induced by resonant tunneling.     

Self-consistent calculations of coherent tunneling oscillations in asymmetric double quantum wells

Coherent carrier tunneling in asymmetric double quantum well heterostructures is analytically investigated through a self-consistent method of calculation. Longitudinal electric fields are applied to reach the interwell resonance conditions. Exact wave functions in asymmetric double quantum well systems (modified Airy functions) are used to include the electron-hole interaction in the coherent tunneling process. Results are in good agreement with the available previous works.     

Fluctuations and decoherence in chaos-assisted tunneling

We study quantum dynamical tunneling between two symmetry-related islands of stability in the phase space of a classically chaotic system. The setting for these experiments is the motion of carefully prepared samples of cesium atoms in an amplitude-modulated standing wave of light. We examine the dependence of the tunneling dynamics on the system parameters and indicate how the observed features provide evidence for chaos-assisted (three-state) tunneling. We also observe the influence of a noisy perturbation of the standing-wave intensity, which destroys the tunneling oscillations, and we show that the tunneling is more sensitive to the noise for a smaller value of the effective Planck constant.