Self-trapping of Bose-Einstein condensates in optical lattices

The self-trapping phenomenon of Bose-Einstein condensates (BECs) in optical lattices is studied extensively by numerically solving the Gross-Pitaevskii equation. Our numerical results not only reproduce the phenomenon that was observed in a recent experiment [Anker {\it et al.}, Phys. Rev. Lett. {\bf 94} (2005)020403], but also find that the self-trapping breaks down at long evolution times, that is, the self-trapping in optical lattices is only temporary. The analysis of our numerical results shows that the self-trapping in optical lattices is related to the self-trapping of BECs in a double-well potential. A possible mechanism of the formation of steep edges in the wave packet evolution is explored in terms of the dynamics of relative phases between neighboring wells.Comment: 8 pages, 15 figure

Sensitive frequency-dependence of the carrier-envelope phase effect on bound-bound transition: an interference perspective

We investigate numerically with Hylleraas coordinates the frequency dependence of the carrier-envelope phase (CEP) effect on bound-bound transitions of helium induced by an ultrashort laser pulse of few cycles. We find that the CEP effect is very sensitive to the carrier frequency of the laser pulse, occurring regularly even at far-off resonance frequencies. By analyzing a two-level model, we find that the CEP effect can be attributed to the quantum interference between neighboring multi-photon transition pathways, which is made possible by the broadened spectrum of the ultrashort laser pulse. A general picture is developed along this line to understand the sensitivity of the CEP effect to laser's carrier frequency. Multi-level influence on the CEP effect is also discussed

Terrace-like structure in the above-threshold ionization spectrum of an atom in an IR+XUV two-color laser field

Based on the frequency-domain theory, we investigate the above-threshold ionization (ATI) process of an atom in a two-color laser field with infrared (IR) and extreme ultraviolet (XUV) frequencies, where the photon energy of the XUV laser is close to or larger than the atomic ionization threshold. By using the channel analysis, we find that the two laser fields play different roles in an ionization process, where the XUV laser determines the ionization probability by the photon number that the atom absorbs from it, while the IR laser accelerates the ionized electron and hence widens the electron kinetic energy spectrum. As a result, the ATI spectrum presents a terrace-like structure. By using the saddle-point approximation, we obtain a classical formula which can predict the cutoff of each plateau in the terrace-like ATI spectrum. Furthermore, we find that the difference of the heights between two neighboring plateaus in the terrace-like structure of the ATI spectrum increases as the frequency of the XUV laser increases

Pulse-duration dependence of high-order harmonic generation with coherent superposition state

We make a systematic study of high-order harmonic generation (HHG) in a He$^+$-like model ion when the initial states are prepared as a coherent superposition of the ground state and an excited state. It is found that, according to the degree of the ionization of the excited state, the laser intensity can be divided into three regimes in which HHG spectra exhibit different characteristics. The pulse-duration dependence of the HHG spectra in these regimes is studied. We also demonstrate evident advantages of using coherent superposition state to obtain high conversion efficiency. The conversion efficiency can be increased further if ultrashort laser pulses are employed

Interrelation between high-order harmonic generation and above-threshold ionization

We study the interrelation between high-order harmonic generation ͑HHG͒ and above-threshold ionization ͑ATI͒ in the frequency domain. HHG can be described simply as an ATI followed by laser assisted recombination ͑LAR͒. The plateau reflects mainly the characteristics of LAR. We also study the correspondence between frequency-domain and time-domain pictures of HHG

Laser-assisted collision effect on nonsequential double ionization of helium in a few-cycle laser pulse

Nonsequential double ionization (NSDI) of helium in an intense few-cycle laser pulse is investigated by applying the three-dimensional semi-classical re-scattering method. It is found that the momentum distribution of He$^{2+}$ shows a single-double-single peak structure as the pulse intensity increases. According to the different mechanisms dominating the NSDI process, the laser intensity can be classified into three regimes where the momentum distribution of He$^{2+}$ exhibits different characteristics. In the relatively high intensity regime, an NSDI mechanism named the "laser-assisted collision ionization" is found to be dominating the NSDI process and causing the single peak structure. This result can shed light on the study of non-sequential ionization of a highly charged ion in a relatively intense laser pulse

Fourth-order coherence-function theory of laser-induced molecular reorientational grating and population grating

We have employed fourth-order coherence-function theory to study the influence of the partial-coherence properties of pump beams on the laser-induced gratings. First, we examine the formation of molecular reorientational grating. The different roles of phase fluctuation and amplitude fluctuation have been pointed out. A time-delayed method has been proposed to distinguish molecular reorientational grating from thermal grating. We then apply the fourth-order theory to study the Bragg reflection from a population grating. We obtain an analytic solution which enables us to make an extensive investigation on the temporal behaviour of the Bragg reflection signal. This study is especially helpful for elucidating the generation mechanism of population grating.Nous avons utilisé la théorie des fonctions de corrélation au quatrième ordre pour étudier l'influence des propriétés de cohérence partielle des faisceaux pompe sur les réseaux induits par laser. Tout d'abord, nous examinons la formation du réseau de réorientation moléculaire. Les rôles respectifs des fluctuations de phase et des fluctuations d'amplitude sont dégagés. On propose une méthode de retard temporel pour distinguer le réseau de réorientation moléculaire du réseau de population. Nous appliquons ensuite la théorie au quatrième ordre pour étudier la réflexion de Bragg sur le réseau de population. Nous obtenons une solution analytique qui nous permet d'étudier en détail le comportement temporel du signal de réflexion de Bragg. Cette étude est tout particulièrement utile pour éclaircir le mécanisme de formation du réseau de population