High-Order Phase-Dependent Asymmetry in the Above-Threshold Ionization Plateau.

Above-threshold ionization spectra from cesium are measured as a function of the carrier-envelope phase (CEP) using laser pulses centered at 3.1 µm wavelength. The directional asymmetry in the energy spectra of backscattered electrons oscillates three times, rather than once, as the CEP is changed from 0 to 2π. Using the improved strong-field approximation, we show that the unusual behavior arises from the interference of few quantum orbits. We discuss the conditions for observing the high-order CEP dependence, and draw an analogy with time-domain holography with electron wave packets.

Helicity asymmetry in strong-field ionization of atoms by a bicircular laser field.

Ionization of atoms by an intense bicircular laser field is considered, which consists of two coplanar corotating or counterrotating circularly polarized field components with frequencies that are integer multiples of a fundamental frequency. Emphasis is on the effect of a reversal of the helicities of the two field components on the photoelectron spectra. The velocity maps of the liberated electrons are calculated using the direct strong-field approximation (SFA) and its improved version (ISFA), which takes into account rescattering off the parent ion. Under the SFA all symmetries of the driving field are preserved in the velocity map while the ISFA violates certain reflection symmetries. This allows one to assess the significance of rescattering in actual data obtained from an experiment or a numerical simulation.

Numerical solution of the time-dependent Schrödinger equation for H_{2}

Time evolution of the bound state of a molecular hydrogen cation in an intense, linearly polarized laser field is investigated by solving the full three-dimensional time-dependent Schrödinger equation. Our method is based on the Born-Oppenheimer and dipole approximations, and the wave function is expanded in finite series using B-spline functions and spherical harmonics in prolate spheroidal coordinates. After solving the stationary Schrödinger equation, the initial state is propagated under the influence of the laser field employing the Crank-Nicolson propagator. Using this method we calculate and present high-harmonic photon spectra and above-threshold ionization angle-resolved electron spectra.

Electron rescattering in a bicircular laser field.

Above-threshold ionization of rare-gas atoms by a bicircular field with its two components counterrotating is theoretically investigated by means of the improved strong-field approximation. Both direct and rescattered electrons are considered and the quantum orbits that lead into a specific final state are calculated and depicted. The angle-dependent spectrum reflects the discrete rotational symmetry of the bicircular field. The backward-scattering contributions are very similar to those generated by a linearly polarized field; several such contributions are rotated one versus the other by the symmetry angle of the discrete rotational symmetry. The forward-scattering contributions dramatically affect the velocity map at comparatively low momenta. The direct-electron spectrum observes reflection symmetry about several symmetry axes determined by the field symmetry. This is broken by rescattering.

Accurate determination of absolute carrier-envelope phase dependence using photo-ionization.

The carrier-envelope phase (CEP) dependence of few-cycle above-threshold ionization (ATI) of Xe is calibrated for use as a reference measurement for determining and controlling the absolute CEP in other interactions. This is achieved by referencing the CEP-dependent ATI measurements of Xe to measurements of atomic H, which are in turn referenced to ab initio calculations for atomic H. This allows for the accurate determination of the absolute CEP dependence of Xe ATI, which enables relatively easy determination of the offset between the relative CEP measured and/or controlled by typical devices and the absolute CEP in the interaction.

Application of the dressed-bound-state molecular strong-field approximation to above-threshold ionization of heteronuclear molecules: NO vs. CO.

We theoretically investigate high-order above-threshold ionization (HATI) of heteronuclear diatomic molecules applying the molecular strong-field approximation which includes dressing of the molecular bound state. We consider HATI of nitrogen monoxide molecules, which are characterized by the π symmetry of their highest occupied molecular orbital. We show that the HATI spectra of NO exhibit characteristic interference structures. We analyze the differences and similarities of the HATI spectra of NO molecules and the spectra of CO (σ symmetry) and O(2) (π(g) symmetry) molecules. The symmetry properties of the molecular HATI spectra governed by linearly and elliptically polarized fields are considered in detail. The yields of high-energy electrons, contributing to the plateau region of the photoelectron spectra, strongly depend on the employed ellipticity.

Electron rescattering in above-threshold photodetachment of negative ions.

We present experimental and theoretical results on photodetachment of Br(-) and F(-) in a strong infrared laser field. The observed photoelectron spectra of Br(-) exhibit a high-energy plateau along the laser polarization direction, which is identified as being due to the rescattering effect. The shape and the extension of the plateau is found to be influenced by the depletion of negative ions during the interaction with the laser pulse. Our findings represent the first observation of electron rescattering in above-threshold photodetachment of an atomic system with a short-range potential.

Two-source double-slit interference in angle-resolved high-energy above-threshold ionization spectra of diatoms.

When an electron from a diatomic molecule undergoes tunneling-rescattering ionization, a novel form of destructive interference can be realized that involves all four geometric orbits that are available to the electron when it is freed, because both ionization and rescattering may take place at the same or at different centers. We find experimentally and confirm theoretically that in orientation-averaged angle-resolved high-order above-threshold ionization spectra the corresponding destructive interference is visible for O_{2} but not for N_{2}. This effect is different from the suppression of ionization that is well known to occur for O_{2}.

Angle-resolved high-order above-threshold ionization of a molecule: sensitive tool for molecular characterization.

The strong-field approximation for ionization of diatomic molecules by an intense laser field is generalized to include rescattering of the ionized electron off the various centers of its molecular parent ion. The resulting spectrum and its interference structure strongly depend on the symmetry of the ground state molecular orbital. For N2, if the laser polarization is perpendicular to the molecular axis, we observe a distinct minimum in the emission spectrum, which survives focal averaging and allows determination of, e.g., the internuclear separation. In contrast, for O2, rescattering is absent in the same situation.

Attosecond double-slit experiment.

A new scheme for a double-slit experiment in the time domain is presented. Phase-stabilized few-cycle laser pulses open one to two windows (slits) of attosecond duration for photoionization. Fringes in the angle-resolved energy spectrum of varying visibility depending on the degree of which-way information are measured. A situation in which one and the same electron encounters a single and a double slit at the same time is observed. The investigation of the fringes makes possible interferometry on the attosecond time scale. From the number of visible fringes, for example, one derives that the slits are extended over about 500 as.

Accomplishments and perspectives of immunological interventions in lymphoproliferative disorders.

The purpose of this review is to consider evidences accumulated in the last few years which might lead to a new therapeutic strategy for lymphoproliferative disorders. Tumor-targeted and immunologically designed therapy has already started. Clinical effectiveness also includes the primary and secondary prevention of these malignancies. The fortifying of body's defense through vaccination as primary prevention with tumor-specific cell surface antigen, has been seen over the past few years, and will lead to health patient's improvement. Data collected from clinical trials and in vitro analysis indicated that the immune system of patients could recognize and eliminate neoplastic cells while sparing normal cells. In B-cell lymphomas and myelomas, the tumor-idiotype (Id) produced by a single B-cell clone, has been used for vaccination. Therapeutic Id vaccination used two types of antigen-presenting cells as natural adjuvants for the induction of antigen-specific T cell response. Dendritic cells (DCs)--based vaccines are under active investigation and are entering clinical evaluation. Current research focuses on optimization of DCs source, choice and loading of antigen, mode of injection, as well as immune monitoring. Some preliminary results were obtained and no significant side effects of dendritic cells vaccination of lymphoma and myeloma patients have so far been reported. The over-estimation of clinical effectiveness, at this moment, is still limited by the small number of patients included in this kind of treatment. The challenge for the future will be to extend these early results to reproducible vaccination strategy according to current standards of good clinical practice (GCP).

Phase-dependent effects of a few-cycle laser pulse.

The Keldysh theory of above-threshold ionization (ATI) is applied to few-cycle laser pulses in order to explore the potential of a recently published new method to measure "carrier-envelope phase difference" phenomena. In this experiment, the carrier-envelope phase difference dependent left-right asymmetry of few-cycle ATI was measured and investigated with a correlation technique. Here, we explore spectral features of the asymmetry, present a theoretical analysis of the experiment, and establish a method to determine the duration of few-cycle pulses whose carrier-envelope phase differences are not controlled.

Feynman's path-integral approach for intense-laser-atom interactions.

Atoms interacting with intense laser fields can emit electrons and photons of very high energies. An intuitive and quantitative explanation of these highly nonlinear processes can be found in terms of a generalization of classical Newtonian particle trajectories, the so-called quantum orbits. Very few quantum orbits are necessary to reproduce the experimental results. These orbits are clearly identified, thus opening the way for an efficient control as well as previously unknown applications of these processes.

The different level of immunological recovery after chemotherapy in leukemia and lymphoma patients.

Analysis of T lymphocyte subsets made a great progress in attempting their important role in host defence against malignant disease. As the major regulatory elements in the immune system, helper (CD4+) and suppressor (CD8+) T cells are required for recognizing a large scale of surface antigen on tumor cells and subsequently, for mediating the regression of cancers. Many data suggest that T cell immunity system is not intact in different malignant diseases. The malignant disorders of hemopoietic tissue are frequently associated with T cell functional impairment, the mechanism(s) of which is still unresolved. In recent years, defects in immunoregulatory T cell network have been repeatedly proposed as the cause of this alteration and might be considered as a parameter of immunodeficiency in those patients. The additional disturbance of T cell immunoregulatory system may be induced by different extrinsic factors. In this context the chemotherapeutic agents with their non-selective action, including an immunomodulatory activity, should be involved in the evaluation of lymphocyte subset changes during treatment. In this brief review we analyzed the data concerning the T cell subset changes, both numerical and functional, before and in course of the treatment of patients with different hematologic malignancies. According to this consideration, it might be suggested that the immune system of these patients have some capacity to recover from the suppressive effect of chemotherapy, although not sufficiently to be normalized. The long-term monitoring of a larger scale of immunological parameters may probably help us to resolve whether T lymphocyte subset changes are related only to the therapy or, in some instance, might be connected with compromised natural host defence mechanisms in oncohematologic patients.

Extreme-ultraviolet harmonic generation near 13 nm with a two-color elliptically polarized laser field.

Theoretical results for high-harmonic generation by a two-color elliptically polarized laser field are presented. Special emphasis is placed on coherent radiation near 13 nm in connection with the development of extreme-ultraviolet lithography. Linearly polarized radiation at this wavelength can be obtained with a linearly polarized bichromatic laser field, whereas circularly polarized radiation can be generated with a bichromatic circularly polarized field with counterrotating coplanar components. In both cases the harmonic emission efficiencies at the saturation intensity are more than 1 order of magnitude larger than for harmonic generation with a monochromatic linearly polarized field.