Li Shi-Qun

Transparency and Strong Gain Without Population Inversion in Photonic Bandgap Crystals

Without using the weak-field approximation, we investigate the transient properties of a Λ-type atom with one transition near resonant with a photonic bandgap edge. Whatever the initial state of the atom is, the atom can become transparent to a probe field, and strong gain without population inversion is also possible if the atom has been pre-excited. The defect mode formed by atomic doping can have a strong effect on the absorption properties of the atom.

Incoherently Coupled Bright-Dark Soliton Pairs in Biased Centrosymmetric Photorefractive Media

It is shown theoretically that incoherently coupled bright-dark soliton pairs can exist in biased centrosymmetric photorefractive media under steady-state conditions. These soliton pairs can be established provided that the two optical beams have the same polarization, wavelength, and are mutually incoherent.

Manakov Soliton Pairs in Biased Photovoltaic Photorefractive Crystals

We study, theoretically, incoherently coupled screening-photovoltaic soliton pairs in biased photovoltaic photorefractive crystals. It is shown that when the total intensity of two coupled solitons is much lower than the effective dark irradiance, the coupled soliton equations reduce to the Manakov equations. The dark-dark, bright-bright and dark-bright soliton pair solutions of these Manakov equations are obtained under an appropriate external bias field and a photovoltaic field, and the characteristics of these Manakov soliton pairs are also discussed in detail.

Mode suppression, its elimination, and generation of small frequency differences in birefringence He-Ne lasers

The birefringence He-Ne dual-frequency laser (DFL) has so far provided only frequency differences (FDs) larger than 40 MHz, as a result of mode suppression (competition) between the two frequencies. This paper reports experiments on mode suppression in cavity tuning and demonstrates an approach to eliminate this suppression in order to generate small FD ranging from 3 MHz to hundreds of megahertz in birefringence He-Ne DFLs. This is realized with two intracavity elements: a calcite plate and a quartz crystal plate. The calcite plate is used to separate the paths of o light and e light spatially, in parallel, in the laser core of about 1-mm diam, so that light of each polarization acquires gain from its own group of active atoms without mode suppression. Also, the quartz plate is used to precisely tune the magnitude of the FD. This work has filled in the gap of FD between 3 and 40 MHz, which neither tradi- tional Zeeman nor birefringence He-Ne DFLs can supply.

Light-induced nonlinear effects on dispersion relation of ultracold Bose gas

We have investigated the optical properties of Λ-configuration ultracold dense Bose gas interacting with two laser pulses, which usually result in electromagnetically induced transparency. With the nonrelativistic quantum electrodynamics and taking into account the atomic dipole-dipole interaction and local field effect, we have derived the Maxwell-Bloch equations of the system. The dispersion relation of an ultracold Bose gas has been obtained and the light-induced nonlinear effects have been analysed. The light-induced nonlinear effects are different from the effects induced by two-body collision of Bose-Einstein condensation atoms which have a frequency shift of transparent window.

Control of Cold Atoms by Slow Waves in a Hollow Fibre

We propose that a slowly-moving standing wave can be used to manipulate the motion of an atomic wave packet in a hollow optical fibre. A cloud of cold atoms can follow the motion of such a slow wave. The evolution of an initial Gaussian atomic wave packet in the moving standing wave is calculated by numerically solving the Schrodinger equation. Our calculation shows that by using a sufficiently strong field and by adjusting the velocity of the standing wave pattern, the motion of the atomic cloud can be controlled in a hollow fibre.

Nonlinear Theory of Light Speed Reduction in a Three-Level Λ System

We present a nonlinear theory of light velocity reduction in a three-level Λ system based on electromagnetically induced transparency. Analysis shows that the probe field propagates with a velocity that is quite strongly dependent on its intensity instead of being merely approximately dependent on the coupling intensity. Moreover, the minimum group velocity of the probe field is analytically given for a given input power.

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.

Slow Light by Modified Reservoir Induced Transparency with a Non-Singular Density of Modes

We investigate the dispersive property of a ?-type atomic system embedded in photonic band gap structures. The atomic coherence is established by the structured reservoir which consists of a double-band photonic band gap (PBG) and defect modes. With an atomic density of 7?1015?atoms/cm3, the group velocity of a probe laser being detuned far away from the gap is shown to be reduced to 3?102?m/s near the transparency window associated with the PBG edges and 6?103?m/s at the transparency window associated with the defect modes, without using any deriving field. The influence of the smoothness of the density of modes and the decay rate of the second ground state of atoms are analysed.

Nonadiabatic effects on population transfer of two Bose-Einstein condensates induced by atomic interaction

We investigate the stimulated Raman adiabatic passage for Bose-Einstein condensate (BEC) states which are trapped in different potential wells or two ground states of BEC in the same trap. We consider that lasers are nearly resonant with the atomic transitions. The difference of population transfer processes between BEC atoms and usual atoms is that the atomic interaction of the BEC atoms can cause some nonadiabatic effects, which may degrade the process. But with suitable detunings of laser pulses, the effects can be remedied to some extent according to different atomic interactions.