M. A. Bouchene

Temporal coherent control in the photoionization of Cs2: Theory and experiment

Two identical femtosecond pulses are used to create a coherent superposition of two vibrational wave packets in a bound electronic state of cesium dimers. The oscillations of these two wave packets are further detected after photoionization of the system. Quantum interferences between the two wave packets result in a temporal coherent control of the ionization probability. The interferogram exhibits the following features as a function of the time delay between the two laser pulses: high-frequency oscillation corresponding to Ramsey fringes (at the Bohr frequency of the transition) modulated by a slow envelope corresponding to the oscillations of vibrational wave packets (vibrational recurrences). Here the control parameter is the time delay between the two laser pulses which can be used to control the preparation of a wave packet in a quantum system and monitor its evolution. The detailed theory of this experiment is presented and compared with the pump-probe experiment. The temporal coherent control exp...

One-color coherent control in Cs2. Observation of 2.7 fs beats in the ionization signal

Abstract We present the theory of one-color coherent control with two identical time-delayed laser pulses and the experimental observation of the resulting wave packet interferences in the B 1 Π u state of Cs 2 . The B state population is detected by two-photon ionization. The wave packet interference produces beats in the Cs 2 + ion signal at the optical frequency, i.e. with a period of 2.7 fs which are resolved for the first time. These beats are modulated by the vibrational recurrences and allow a determination of the vibrational period. Furthermore, we show that such interferences can be observed even when the probe step involves an electronic state parallel to the excited state in which the wave packet oscillates.

Competition of different ionization pathways in K2 studied by ultrafast pump–probe spectroscopy: A comparison between theory and experiment

We present a comparison of experiment and theory of ultrafast one-color pump–probe multiphoton ionization spectrocopy of K2. The wave packet propagation in the A 1Σu+ state and in the (2) 1Πg Rydberg state is monitored in detail by changing systematically the pump and probe wavelength from 779 nm to 837 nm. The measured total ionization rates as a function of the delay time between pump and probe are shown to depend sensitively on the pump and probe wavelengths used and exhibit drastic changes and a variety of fascinating structures as the direct observation of inward and outward wave packet detection and frequency doubling of the detected wave packet oscillation. The time dependent quantum mechanical wave packet calculations are in excellent agreement with the experimental results and allow a clear interpretation of different ionization pathways and mechanisms observed in the femtosecond ion signal.

On the importance of damping phenomena in clusters irradiated by intense laser fields

We study the dynamics of large clusters irradiated by intense and short laser pulses, within the framework of the nanoplasma model. Particular attention is paid to the influence of electron–surface collisions, which have not been considered in previous versions of the model. We show that they dominate inverse bremsstrahlung collisions when plasmon resonance occurs. The dynamics of the cluster changes considerably and the predictions of the model are significantly modified. Moreover, there is no evidence for the presence of highly charged ions and the hydrodynamic pressure is found to be smaller than the Coulomb one.

Polarization-dependent pump-probe studies in atomic fine-structure levels: towards the production of spin-polarized electrons

The precession of orbital and spin angular momentum vectors has been observed in a pump-probe study of the 4P fine-structure states of atomic potassium. A femtosecond pump pulse prepared a coherent superposition of the two fine-structure components. A time-delayed probe pulse then ionized the system after it had been allowed to evolve freely. Oscillations recorded in the ion signal reflect the evolution of the orientation of the orbital and spin angular momentum due to spin-orbit coupling. This interpretation gives physical insight into the cause of the half-period phase shift observed when the relative polarizations of the laser pulses were changed from parallel to perpendicular. Finally, it is shown that these changes in the orientation of the spin momentum vector of the system can be utilized to produce highly spin-polarized free electrons on the femtosecond scale

Spin-polarized electrons produced by a sequence of two femtosecond pulses. Calculation of differential and global polarization rates

This paper presents a theoretical analysis of the possibilities of producing spin-polarized electrons with a sequence of two ultra-short time-delayed laser pulses. The first pulse, which is right circularly polarized, excites resonantly the fine structure np level of potassium atoms leading to a spin-flip and thus polarizing the atom. The second pulse ionizes the system thereby resulting in spin-polarized electron release. We examine and compare several situations corresponding to different polarizations of the ionizing pulse (σ+,σ- and π). For each case we derive analytical expressions for the angular and global electron spin-polarization rates as well as for the differential cross sections. The obtained rates can be very high (up to 100%) and can be controlled accurately on the femtosecond time scale by varying the time delay between the pulses.

Slowing light through Zeeman coherence oscillations in a duplicated two-level system

We present the theory of a method to slow a linearly polarized probe pulse as it propagates through a duplicated two-level system driven by an orthogonally polarized control field. The method makes use of Zeeman coherence oscillations that arise in the atomic system because of the spatial and temporal modulation of the total polarization. This method exhibits properties similar to those that rely on electromagnetically induced transparency but without the existence of any trapping dark state. We demonstrate also the propagation of a polariton in the medium.

Compensation of electron wavepacket spreading with linearly chirped pulses; theoretical analysis

We investigate in this paper the properties of electron wavepackets created in the continuum by ultrashort pulses, linearly chirped. We consider alkali atoms, where complete quantum calculations can be carried out. The spreading of the wavepacket is radial and mainly due to free propagation, except for very low kinetic energy where Coulomb interaction has to be considered. We show that the wavepacket can be focused into a position located at macroscopic distances far from the ion core. Distances as large as several centimetres are obtained for standard values of laser parameters. Moreover, we derive the conditions where the wavepacket is temporally optimally focused. Finally, we study several characteristics of the wavepacket, such as the temporal divergence (in ps cm-1) and the temporal profile, when the focusing is not maximum.

One-photon wave packet interacting with two separated atoms in a one-dimensional waveguide: Influence of virtual photons

We present a theoretical study of a one-photon wave packet scattered by two atoms in a one-dimensional waveguide. We investigate the role of terms beyond the rotating-wave approximation to correctly take into account the effects of the virtual photons that are exchanged between the atoms. These terms are shown to drastically influence the reflected and the transmitted fields, imposing strict constraints on their temporal envelopes.

Entanglement rebirth of multi-trapped ions with trap phonon modes: entanglement sudden death with recovery

We consider a multi-qubit system consisting of two trapped ions coupled in a laser field. The ions are identical three-level electronic systems which interact with one another through the phonon modes of their relative or center of mass motions, and the system is tuned so that two-phonon processes dominate the electronic transitions. The resulting evolution of the system is studied theoretically with a focus on the entanglement properties of the system. A method of quantifying the entanglement is discussed, and the time dependence of these quantifications is determined. The cases of the two ions coupled to the same phonon field and to two different isolated phonon fields are compared for Fock cavity modes. Instances of the entanglement sudden death recovery are identified in these various systems.