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A systematic study of nonsequential double ionization (NSDI) of alkaline-earth metal atoms with mid-infrared femtosecond pulses is reported. We find that the measured NSDI yield shows a strong target dependence and it is more suppressed for alkaline-earth metal with higher ionization potential. The observation is attributed to the differences in the recollision induced excitation and ionization cross sections of alkaline-earth metals. This work indicates that NSDI of alkaline-earth metals can be generally understood within recollision picture and sheds light on ultrafast control of electron correlation and dynamics of ionic excited states during NSDI of atoms with complex structures.

X-ray Thomson scattering technique for diagnosing dense plasma was demonstrated on Shenguang-II laser facility. Laser plasma x-ray source of titanium He-α lines (~4.75 keV), generated by laser beam (1.5 kJ/527 nm/2 ns) heated titanium thin foil, was used as x-ray probe beam. The x-ray probe was then scattered by cold CH foam column of 1 g/cm3 density. The scattered radiation at 90° was diffracted by polyethylene terephthalate (PET) crystal and recorded on x-ray charge-coupled device. Well-defined scattering spectra were obtained with good signal to noise ratio.

We investigate the behaviour of an atom–cavity system via an adiabatic passage technique using a three-level ladder atom both for one-mode and for two-mode cavity QED. For one-mode cavity QED, we find that a single-photon Fock state can be generated via the technique of stimulated Raman adiabatic passage in the microwave domain. For two-mode cavity QED, we find that a two-photon pair can be generated via the technique of Stark shift rapid adiabatic passage (SRAP). We also find, for the two-mode cavity QED system, that an entangled state of the two modes in a microwave cavity can be prepared via the SRAP technique, in which the number of steps required is significantly reduced due to the adiabatic passage of the ladder atom–cavity coupled system.

We show that it is possible to achieve population inversion and upconversion lasing when a strong coherent field induces an exchange in the relative position of one dynamical Stark sublevel and a neighbouring level. This prediction is based on the fact that the dynamical level crossing results in a change in the direction of spontaneous emission. This provides a potential way towards short-wavelength laser generation.

We present phase control of the fluctuation-induced bistability in the three-level Λ-type system where a weak, amplitude-fluctuating component and a strong coherent component are used as a control field. With the addition of the stochastic component, optical bistability can be produced even if detunings of the probe and control fields from corresponding atomic transition frequencies are equal; without the addition, the medium is transparent to the input light. Depending on the relative phase between the stochastic and coherent parts, bistability appears or disappears and the threshold intensity is raised or lowered.

The photon statistics of the two-photon fluorescence of a two-level atom in a squeezed vacuum is investigated by using the second-order correlation function g (2) ( t ). Based on its analytical sol...

The nondegenerate two-photon interaction of a two-level atom with a broadband multimode squeezed vacuum is investigated. We find that in the two-photon process the squeezed vacuum has a driving effect on the atom which can lead to an ac Stark effect when the average photon number of the squeezed vacuum is larger than a critical value.

The fact that electrons are accelerated by intense laser pulses in a vacuum is particularly apparent from recently published work, but up to now no high-beam-quality laser vacuum acceleration scheme has been presented. In this paper, numerical simulations demonstrate that free electrons can be accelerated axially (high beam quality) in a vacuum by using two intersecting ultrashort, high-intensity laser pulses. As the maximum intensities are on the laser axes, a hollow channel is formed between the two laser beams. The electrons will be expelled by the transverse ponderomotive force of each beam to the axis of the symmetry, and can be trapped inside the channel until they are accelerated to relativistic energies. This makes possible axial rather than radial acceleration.

It is shown that, in an open ladder-type atomic system with spontaneously generated coherence (SGC), regardless of the existence of an incoherent pumping, a lasing without inversion (LWI) gain is always remarkable larger than in the system without SGC, by adjusting the strength of SGC. Moreover, LWI gain in the system without incoherent pumping is much larger than that with incoherent pumping, within some strength of SGC; while in the corresponding closed system with SGC, we can't obtain LWI gain at any strength of SGC, if no incoherent pumping is applied.

A new fluctuation phenomenon is observed through Langmuir probe measurements at the edge plasma in the KT-5C tokamak by applying a -90 degrees phase shift feedback. Using a two point correlation technique, it is found that this fluctuation mode has a longer poloidal wavelength and a definite frequency when compared with the usual edge turbulence. It is also found through bispectral analysis that this mode is a spontaneously excited quasi-coherent mode, which has almost no contribution to the cross-field particle flux

We investigate, using a nonlinear theory of laser physics, for a simple three-level atomic system the steady-state intensity behavior due to the effects of quantum coherence and decoherence. We find that a change from a noninversion laser to an inversion laser action can occur as the Rabi frequency of the driving field increases. The steady-state intensity of the laser field can arrive at a maximal value for a certain Rabi frequency of the driving field. We also find that the linear gain and the laser intensity tend to decrease for the linewidth of the driving field.

It is found on KT-5C tokamak that an edge fluctuation mode can be excited by using a pair of floating Langmuir probes. This mode is propagating along the magnetic field with its amplitude decreasing drastically when it is off the magnetic field line. Preliminary feedback experiments show that the broadband edge turbulence can be reduced or enhanced depending upon the phase shift of the feedback network.

For atoms of three-state ladder or V configuration, a large population may be 'locked' into a single high-lying excited state via atomic lower-state coherence even when it exceeds the total population in the two lower states. The conditions are analysed and the two systems are compared.

The interaction of a pair of coupling two-level atoms with a radiation field is studied. The effects of a dispersion force (dipole-dipole interaction) on the emission spectrum are analysed. It is found that the dipole-dipole interaction induces an asymmetry of the emission spectrum, which is most obvious in a vacuum field.

The ionization behavior of linear polyatomic molecular ions in ultrashort, intense laser fields was investigated with the time-dependent Schrodinger equation solved by the standard split-operator algorithm. The enhanced ionization effect was found as a general feature in the ions when the internuclear separation was in a critical distance range and can be explained in terms of the field-induced over-barrier ionization mechanism. In addition, the ionization energy does not influence the ionization probability significantly.

R6G was incubated in three kinds of silver colloid with different low concentrations, and its surface-enhanced resonance Raman scattering (SERRS) was studied with a confocal microscopy system. Each sample exhibited different spectrum character. At the single-molecule level the SERRS-spectra were recorded in 10(-13) mol x L(-1) dye colloidal solution. Some spectral inhomogeneous behavior was recorded such as spectral diffusion, intensity fluctuation of vibrational lines and spectral polarization, and even on/off blinking and splitting of some lines within the spectrum of one molecule. Polarization of excitation light and background subtraction of signal have an important influence on the SERRS spectra detection of single-molecule, which was analyzed in the present paper.

The combination of the confocal scanning microscopy and the defocusing imaging in the plane of imaging system was proposed. A switch lens and a pinhole with fine focusing aid are used to switch between the confocal scanning imaging mode and defocusing imaging mode. Objectives of different magnification can be used to get different field of vision. The complementary images can offer more information than any single one. Image blur or distortion caused by scanning, such as the displacement of the target and the excursion of scanning stage, can be corrected according to the comparison of two images. Location and adjustment of the target can be achieved easily and the accuracy of scanning image will be improved obviously. Some samples were tested on a home-built system.

Soft X-ray microscopy is a microimaging technique using soft X-ray as illuminative source. It fills the gap between optical and electron microscopy. Soft X-ray microscopes have better resolution than visible microscopes. In comparison with electron microscopes, it can examine thick (up to 10 micrometers for biological samples) and wet specimens in their natural states without being dehydrated, sectioned and stained. In addition, soft X-ray microscopy can map elements and analyze the biological macromolecules such as protein and DNA in the examined samples. In this paper, the advantages of soft X-ray microscopy for biology are briefly described. The applications of soft X-ray microscopy to the analysis of mammal and human sperm are illustrated.

In recent years it has been shown that atomic coherence or interference of pathways can be established through incoherent processes. We have investigated the generation of optical bistability via interference of incoherent pump processes, which is a new incoherent method leading to atomic coherence. The standard model of optical bistability with an atomic sample is considered.

We propose a scheme to generate the entangled state of two Lambda-type three-level atoms trapped in distant cavities by using interference of polarized photons. Two possible spontaneous emission channels of each excited atom result in a coherent superposition of the states of two atoms. The subsequent detection of the different polarized photons reveals that both atoms are in different ground states, but an interference effect prevents us from distinguishing which atom is in which ground state; the atoms are thus entangled. In comparison with the original proposal of interference-induced entanglement [C. Cabrillo, J. Cirac, P. Garcia-Fernandez, and P. Zoller, Phys. Rev. A 59, 1025 (1999)]], in our scheme the weakly driven condition is not required, and the influence of atomic excitement and atomic recoil on the entanglement fidelity can be eliminated.

We study the carbon-dope aluminum clusters by using time-of-flight mass spectrum experiments and {\em ab initio} calculations. Mass abundance distributions are obtained for anionic aluminum and aluminum-carbon mixed clusters. Besides the well-known magic aluminum clusters such as Al$_{13}^-$ and Al$_{23}^-$, Al$_7$C$^-$ cluster is found to be particularly stable among those Al$_n$C$^-$ clusters. Density functional calculations are performed to determine the ground state structures of Al$_n$C$^-$ clusters. Our results show that the Al$_7$C$^-$ is a magic cluster with extremely high stability, which might serve as building block of the cluster-assembled materials., 4 pages, 6 figures

A noninversion laser system which is closed lifetime broadened and free of the restriction on atomic decay rates is proposed. For the realization of lasing without inversion, the Rabi frequency of the applied field must exceed a lower threshold. Moreover, the coherence term in dressed states must enhance the gain. These results are obtained beyond the specific approximations

In this paper, the spectral, temporal and spatial properties of few cycle optical pulsed beam on propagation in free space has been studied. We analyze the spectral shifts of the ultrashort pulse in the far zone, and then discuss the scaling law of the pulsed sources. It is found that by the truncation of the pulsed beam, the scaling law may be destroyed. In this case the spectral shifts on propagation of the few cycle pulses may be observed in experiment. On the other hand, we reveal an analytical form of the pulsed Gaussian beam in free space and analyze the space-time coupling on propagation.

Two possible designs are considered in this paper for increasing the GL value of recombining X‐ray lasers. A ‘‘fibers target’’ structure was proposed to get a rapid 3‐dimensional adiabatic expanding at the initial state followed by a slower 2‐dimensional plasma expansion. A ‘‘hybrid model’’ was used to calculate the plasma behavior of the target and the results are presented. Theoretical results about X‐ray pumped recombination X‐ray laser are also reported.

Spectral diagnostic technique was used to study the properties of line‐shaped laser plasmas. Electron temperature and electron density profiles were deduced from the spectra obtained by a pinhole crystal spectrograph. The Stark‐broadened line‐wings of hydrogenic ions were used to deduce the electron density beyond the critical surface, and the results were compared with the theoretical model of planar laser‐driven ablation. Time integrated properties of line‐shaped laser plasma were also discussed and compared with those of spot‐shaped laser plasma and the laser absorption theoretical estimations.

A numerical method based on techniques of inverse Fourier convolution and nonlinear least square algorithm, etc., is presented for obtaining high‐resolved X‐ray spectra of laser plasmas, which can eliminate the line‐broadening induced by radiation sources and spectrographs, and can improve the spectral resolving power of spectrographs. The code ESDAP has now been successfully applied to the analysis of spectra from line‐shaped Mg and CaF2 laser plasmas.

A program was developed using the Marquardt algorithm method to fit the Stark‐broadened line wings of hydrogenic ions, which can be conveniently used to measure electron density in laser‐produced plasma. Applications of the method are illustrated using space‐resolved 2p‐1s resonance lines in line‐shaped laser plasmas of Mg slab targets irradiated with different laser intensity. Space‐resolved electron densities higher than the critical density are obtained by fitting the Stark‐broadened line wings with the program.

Summary form only given, as follows. The experiments were carried out at the LF12 Laser Facility of SIOFM. A 1.05 /spl mu/m laser beam with 200-600J energy and about 900 ps duration was focused onto magnesium slab targets by means of a novel cylindrical lens array combined with an asperical lens. A spectral diagnostic technique was used to study the properties of fine-shaped laser-produced plasmas. With the help of a new pinhole flat-crystal spectrograph, the spatial distributions of electron temperature and density along the normal of target surface in line-shaped plasmas were obtained. The Stark-broadened Ly-/spl alpha/ line wings of hydrogenic ions were used to deduce the electron density beyond the critical surface. A fitting program has been developed. The results were compared with a theoretical model of distributed absorption of laser light by plasma. The data of the electron density and width of conduction region are in good agreement with the theoretical predictions of a planar laser-driven ablation model of nonlocalized absorption. The results also show that the plasma fluid dynamic behavior of line- and spot-shaped laser-produced plasmas are quite different due to their different expansion dimensions.