双势阱产生正负电子对过程中的正电子波干涉与克莱因隧穿现象

通过将狄拉克场量子化并且数值求解狄拉克方程的方法研究在一维情况下双势阱激发正负电子对产生的过程.研究发现双势阱激发的正电子波在双势阱之间会出现干涉现象,过程中伴随着克莱因隧穿效应,并且在双势阱之间的距离取正电子波对应的驻波条件时,在双势阱之间会表现出驻波形式的正电子干涉波.并且驻波的出现对正负电子对的产生过程也存在相应的影响,驻波最终会通过克莱因隧穿效应衰减消失.

Hydrogen Concentration Dependence of Bragg Wavelength of UV-Written Fiber Grating

Hydrogen loading provides highly enhanced photosensitivity in Ge-doped fiber. But the Bragg wavelength of UV-written fiber grating will shift from its initial value as the remaining hydrogen diffuses out of such fiber. A simple model to calculate the index change of hydrogenated fibers is presented. The theoretical model is in good agreement with experimental results and it can well predict the final wavelength position of UV-written weak fiber grating.

Understanding Tunneling Ionization of Atoms in Laser Fields using the Principle of Multiphoton Absorption

The elaborate energy and momentum spectra of ionized electrons from atoms in laser fields suggest that the ionization dynamics described by tunneling theory should be modified. Although great efforts have been carried out within semiclassical models, there are few discussions describing the multiphoton absorption process within a quantum framework. Comparing the results obtained with the time-dependent Schr?dinger equation(TDSE)and the Keldysh–Faisal–Reiss(KFR) theory, we study the nonperturbative effects of ionization dynamics beyond the KFR theory. The difference in momentum spectra between multiphoton and tunneling regimes is understood in a unified picture with virtual multiphoton absorption processes. For the multiphoton regime, the momentum spectra can be obtained by coherent interference of each periodic contribution. However, the interference of multiphoton absorption peaks will result in a complex structure of virtual multiphoton bands in the tunneling regime. It is shown that the virtual spectra will be almost continuous in the tunneling regime instead of the discrete levels found in the multiphoton regime. Finally, with a model combining the TDSE and the KFR theory,we try to understand the different effects of virtual multiphoton processes on ionization dynamics.

Controllable optical bistability of Bose-Einstein condensate in an optical cavity with a Kerr medium

We study the optical bistability for a Bose-Einstein condensate of atoms in a driven optical cavity with a Kerr medium. We find that both the threshold point of optical bistability transition and the width of optical bistability hysteresis can be controlled by appropriately adjusting the Kerr interaction between the photons. In particular, we show that the optical bistability will disappear when the Kerr interaction exceeds a critical value.

Attosecond transient absorption spectroscopy:Comparative study based on three-level modeling

In the present paper, the time-resolved transient absorption spectroscopy of helium atoms is investigated based on the three-level modeling. The helium atoms are subjected to an extreme ultraviolet （XUV） attosecond pulse and a time-delayed infrared （IR） few-cycle laser field. The odd excited state are populated from the ground state by the XUV pulse due to the dipole selection rule, and probed by the time-delayed IR laser. The time-resolved transient absorption spectroscopy based on the different coupling mechanism demonstrate some different features, the photoabsorption spectrum based on three-level model with rotating wave approximation （RWA） cannot repeat the fast oscillation and the sideband structure which have been observed in the previous experimental investigation. The dressing effect of IR laser pulse on the ground state can contribute new interference structures in the photoabsorption spectrum.

Instability, adiabaticity, and controlling effects of external fields for the dark state in a homonuclear atom tetramer conversion system

In the present paper, we investigate the instability, adiabaticity, and controlling effects of external fields for a dark state in a homonuclear atom-tetramer conversion that is implemented by a generalized stimulated Raman adiabatic passage. We analytically obtain the regions for the appearance of dynamical instability and study the adiabatic evolution by a newly defined adiabatic fidelity. Moreover, the effects of the external field parameters and the spontaneous emissions on the conversion efficiency are also investigated.

Dynamical instability and adiabatic evolution of the atom-homonuclear-trimer dark state in a condensate system

This paper investigates the dynamical instability and adiabatic evolution of the atom homonuclear-trimer dark state of a condensate system in a stimulated Raman adiabatic passage aided by Feshbach resonance. It obtains analytically the regions for the appearance of dynamical instability caused by the interparticle interactions. Moreover, the adiabatic property of the dark state is also studied in terms of a newly defined adiabatic fidelity. It shows that the nonlinear collisions have a negative effect on the adiabaticity of the dark state and hence reduce the conversion efficiency.

Majorana stellar representation for mixed-spin (s, 1/2) systems

By describing the evolution of a quantum state with the trajectories of the Majorana stars on a Bloch sphere,Majorana’s stellar representation provides an intuitive geometric perspective to comprehend the quantum system with highdimensional Hilbert space.However,the representation of a two-spin coupling system on a Bloch sphere has not been solved satisfactorily yet.Here,a practical method is presented to resolve the problem for the mixed-spin(s,1/2)system and describe the entanglement of the system.The system can be decomposed into two spins:spin-(s+1/2)and spin-(s−1/2)at the coupling bases,which can be regarded as independent spins.Besides,any pure state may be written as a superposition of two orthonormal states with one spin-(s+1/2)state and the other spin-(s−1/2)state.Thus,the whole initial state can be regarded as a state of a pseudo spin-1/2.In this way,the mixed spin decomposes into three spins.Therefore,the state can be represented by(2s+1)+(2s−1)+1=4s+1 sets of stars on a Bloch sphere.Finally,some examples are given to show symmetric patterns on the Bloch sphere and unveil the properties of the high-spin system by analyzing the trajectories of the Majorana stars on the Bloch sphere.

Linear instability and adiabaticity of a dark state during conversion of two species of fermionic atoms to stable molecules

In the present paper, we investigate the linear instability and adiabaticity of a dark state during conversion of two species of fermionic atoms to stable molecules through the stimulated Raman adiabatic passage aided by Feshbach resonance. We analytically obtain the regions for the appearance of linear instability. Moreover, taking 40K and 6Li atom molecule conversion systems as examples, we give the unstable regions numerically. We also attempt to obtain the adiabatic criterion for this nonlinear system with classical adiabatic dynamics and study the adibaticity of the dark state with the adiabatic condition.

Dynamical instability of the dark state in the conversion of Bose-Fermi mixtures into stable molecules

In this paper, we investigate the dynamical instability of the dark state in the conversion of Bose-Fermi mixtures into stable molecules through a stimulated Raman adiabatic passage aided by Feshbach resonance. We analytically obtain the regions where the dynamical instability appears and find that such instability in the Bose-Fermi mixture system is caused not only by bosonic interparticle interactions but also by Pauli blocking terms, which is different from the scenario of a pure bosonic system where instability is induced by nonlinear interparticle collisions. Taking a 40K-87Rb mixture as an example, we give the unstable regions numerically.

Double Barrier Resonant Tunneling in Spin-Orbit Coupled Bose–Einstein Condensates

We study the double barrier tunneling properties of Dirac particles in spin-orbit coupled Bose–Einstein Condensates.The analytic expression of the transmission coefficient of Dirac particles penetrating into a double barrier is obtained.An interesting resonance tunneling phenomenon is discovered in the Klein block region which has been ignored before.

Quantum Cyclotron Orbits of a Neutral Atom Trapped in a Triple Well with a Synthetic Gauge Field

The strong effective magnetic fields with flux to the order of one flux quantum per plaquette has been realized for ultracold atoms,and the quantum cyclotron orbit of a single atom in a single plaquette exposed to the magnetic field was directly revealed recently [Phys.Rev.Lett.107 (2011) 255301].We study the quantum cyclotron orbits of a bosonic atom in a triple well with a synthetic gauge field,and find that the dynamics of the atom in real space is similar to a classical dynamic billiard.It is interesting that the billiard-like motion is a signature of the quantum evolution of the three-level system,and its behaviors are determined by the ratio of the two energy gaps of the three energy levels.

Compact Single-Mode Distributed Bragg Reflector Fiber Laser

A compact single-mode distributed Bragg reflector (DBR) fiber laser with narrow spectral linewidth is investigated. Firstly, based on our theoretical analysis the single longitudinal mode operation domain is obtained. Then, a single-mode DBR fiber laser of 7.9cm long with master oscillator power amplifier (MOPA) configuration is designed and constructed to operate in the single-mode domain. The fiber laser is pumped by a semiconductor laser at 975.5nm. The master oscillator operates at 1556.91 nm with a cw output power of 1.43mW for a pump power of 55.35 mW. Its slope efficiency is 2.7% and the spectral linewidth is less than 1.2MHz (instrument resolution limited). With the MOPA configuration the laser output power and slope efficiency are increased to 7.8mW and 16.9%, respectively.

New Method to Obtain Strain Gradient Chirped Fiber Bragg Gratings

We describe a new method for applying a strain gradient with different profiles to convert uniform fiber Bragg gratings into linear or non-linear chirped ones. The chirping profile and the chirp rate of the fiber gratings can be controlled by simply varying the tension applied. Both the theory and experimental results are presented.

Electrically Tunable Optical Fiber Mach- Zehnder Interferometer

An electrically tunable optical fibre Macli-Zehnder interferometer is reported,based on the magnetostrictive effect.The period and peak wavelength of the interference spectrum can be electrically tuned to match the International Telecommunication Union standard wavelength grid and so it is promising to be used in dense wavelength division multiplexing technology.

Photosensitivity and Hydroxyl in Hydrogenated Silica Fibers

Optical loss spectra and refractive index changes have been measured in the hydrogenated standard telecommu-nication fiber and photosensitive fiber.The index changes due to ultraviolet exposure were found to be closely related with the OH formation in both types of fibers.A direct photolyticd model is proposed to explain the effect of hydrogen loading on the enhancement of the photosensitivity in silica fibers.

Dynamical effects of switching a super-critical well potential on pair creation from a vacuum

The dynamical effects on electron-positron pair creation from a vacuum caused by the switching processes of a super- critical well potential are investigated in detail. The results show that only when the switching on and switching off time both increase will the final pair yield converge to the integer of embedded bound states nearly exponentially. But a single adiabatic switching on or switching off cannot lead to an integer pair yield. If the potential is turned on abruptly, associated with the discrete and embedded bound states, there is multi-frequency oscillation around the pair number＇s saturation. The slowly switching on can suppress the amplitude of this oscillation and reduce the final pair yield. The switching off can also reduce the final pair number in the same order of magnitude. The evolution of a single-pair number shows a robust long range correlation between particle and antiparticle. For an adiabatic switching case, the single-pair dominates the early pair creation, their upper limit value is equal to the integer, and these single-pairs will totally disentangle during the switching off.

Enhanced Correlation of Electron-Positron Pair in Two and Three Dimensions

Early time electron-positron correlation in vacuum pair-production in an external field is investigated. The entangled electron and positron wave functions are obtained analytically in the configuration and momentum spaces. It is shown that, relative to that of the one-dimensional theory, two- and three-dimensional calculations yield enhanced spatial correlation and broadened momentum spectra. In fact, at early times the electron and positron almost coincide spatially. The correlation also depends on the direction of the applied field. For the spatial correlation, the transverse correlation is stronger than the longitudinal correlation.

Reconstructing coherent dynamics of bound states induced by strong attosecond XUV pulses

We propose a scheme that utilizes weak-field-induced quantum beats to investigate the electronic coherences of atoms driven by a strong attosecond extreme ultraviolet(XUV)pulse.The technique involves using a strong XUV pump pulse to excite and ionize atoms and a time-delayed weak short pulse to probe the photoelectron signal.Our theoretical analysis demonstrates that the information regarding the bound states,initiated by the strong pump pulse,can be precisely reconstructed from the weak-field-induced quantum beat spectrum.To examine this scheme,we apply it to the attosecond XUV laser-induced ionization of hydrogen atoms by solving a three-dimensional time-dependent Schr?dinger equation.This work provides an essential reference for reconstructing the ultrafast dynamics of bound states induced by strong XUV attosecond pulses.