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Remarkable progress has been made in the elucidation of ultrafast dynamics and its control driven by femtosecond laser pulses in small molecules, large-scale molecular systems, clusters, nanostructures, surfaces, condensed phase and biomolecules. The exploration of photoinduced ultrafast response, dynamics, reactivity and function in ubiquitous molecular, nanoscale, macroscopic and biological systems pertains to the interrogation and control of the phenomena of energy acquisition, storage and disposal, as explored from the microscopic point of view. [ABSTRACT FROM AUTHOR]

Two laser-plasma processes among several others, namely the formation of relativistic electromagnetic solitons in warm plasmas and the ion acceleration driven by the interaction of ultraintense and ultrafast laser pulses with thin solid targets, have attracted great scientific interest in the last few years, both from a fundamental point of view and also in the light of the expected future technological applications. In this chapter, these two physical phenomena are presented and discussed, and the currently available theoretical descriptions are reviewed. Particular attention is paid to the analytical approaches, which allow to model the underlying physics in a transparent way and to give scaling laws with the laser and the plasma parameters. Emphasis is given to the discussion of the possible different regimes, which can characterize these processes, and to the identification of the most suitable theoretical treatments in the various cases. [ABSTRACT FROM AUTHOR]

Fast electrons are generated in the interaction between intense ultra-short laser pulses and the target. This interaction depends on the laser's intensity, polarization, incident angle, the scale length of the plasma, and the target material. In this review, recent studies carried out at our laboratory on the dependence of the fast electrons on the experimental conditions of the laser and plasma as well as ways to control the fast electrons are presented. [ABSTRACT FROM AUTHOR]

Filamentation is a non-linear propagation regime of ultrashort, high-power laser pulses, in which a dynamic balance is established between Kerr self-focusing and defocusing on the air ionized by the laser pulse itself. It opens the way to various applications in atmospheric sciences, such as white-light Lidar relying on the white light continuum, which can be observed up to high altitudes and allows multicomponent remote sensing. Other applications rely on the ability of the filaments to deliver high intensities and induce non-linear optical effects at remote locations, e.g. the remote sensing of aerosols or the analysis of solid targets. Furthermore, the ionization of the fs-laser-induced filaments permits to control high-voltage discharges, opening the way to laser lightning rods. These applications require a good characterization of filamentation over long distances in the atmosphere. This chapter reviews experimental results about long-range filamentation, the build-up of the white-light continuum and the beam geometry. Emphasis is put on results directly relevant for atmospheric applications. [ABSTRACT FROM AUTHOR]

When ultrashort (fs), high-power laser pulses propagate through the atmosphere, extended plasma filaments form and emit white light in a spectral range spanning from the ultraviolet (230 nm) to the infrared (4.5 µm). This strongly non-linear optical phenomenon results from a dynamical balance between respectively focusing and defocusing Kerr- and plasma-lenses, which are formed by a nonuniform, intensity-dependent refractive index across the laser beam profile. This non-linear propagation regime opens the way to various applications in atmospheric sciences, such as white-light Lidar relying on the white light continuum, which can be observed up to high altitudes and allows multicomponent remote sensing. Other applications rely on the ability of the filaments to deliver high-intensities and induce non-linear optical effects at remote locations, e.g. bioaerosols remote sensing or solid target analysis. Furthermore, the ionization of the fs-laser-induced filaments permits to control high-voltage discharges, opening the way to laser lightning rods. This chapter shall review the basic properties of filamentation, with a particular emphasis on one spectacular feature: Their ability to propagate unperturbed across clouds and fogs. [ABSTRACT FROM AUTHOR]

Rapid progress in recent years in the development of high power ultra-short pulse laser systems has opened up a whole new vista of applications and computational challenges. New experimental developments in the field of extreme nonlinear optics will require more rigorous electromagnetic propagation models beyond those existing in the current literature. In this chapter, we derive a 3D time domain unidirectional vector Maxwell propagator that resolves the underlying optical carrier wave while allowing propagation over macroscopic many-meter distances. Our model allows for extreme focusing conditions down to the order of the wavelength in the material. A novel aspect of our approach is that the pulse propagator is designed to faithfully capture the light-material interaction over the broad spectral landscape of relevance to the interaction. Moreover the model provides a seamless and physically self-consistent means of deriving the many ultra-short pulse propagation equations presented in the literature. Amongst current applications that are most challenging from a computational point of view are those involving critical self-focusing with concomitant explosive growth in the generated light spectrum. Specific application areas chosen for illustration include multiple filament formation during propagation of ultra-intense femtosecond laser pulses in air and nonlinear self-trapping in condensed media. These examples exhibit rather different aspects of intense femtosecond pulse propagation and demonstrate the robustness and flexibility of our recently formulated unidirectional Maxwell propagator. A clear message to emerge from our study is the inadequacy of spectrally local light-material interaction models when nonlinear coupling exists over many hundreds of nanometer frequency bandwidths. More sophisticated, computationally feasible, models of nonlinear dispersion are needed. [ABSTRACT FROM AUTHOR]

Atomic clusters offer a unique area for studying high-intensity laser-matter interactions. When a high-intensity laser pulse interacts with a single cluster, the cluster absorbs most of the incident laser energy into its restricted small volume, producing a high-temperature "cluster plasma" which generates high-energy electrons and ions as well as bright X-rays. When a sufficiently short laser pulse is used, the laser energy is deposited before the cluster can expand, paving the way for developing laser-based debris-free ultrafast X-ray sources for time-resolved diffraction experiments. In this study, in order to understand a fundamental aspect of the laser-cluster interaction, we have carried out systematic investigations of the relativistic cluster plasmas, created by the action of superintense, ultrafast laser irradiation, by the simultaneous measurements of high-resolution X-ray emission spectra and ion energy spectra. It is found that hot electrons produced by a higher-contrast pulse shift the ion balance towards the higher charge states, which enhances both the X-ray line yield of the He-like argon ion and the ion kinetic energy. [ABSTRACT FROM AUTHOR]

X-ray line emissions from ultrashort high-intensity laser-produced plasma were studied in order to clarify the physics of energy transport associated with the generation of ultrashort X-ray pulses for use in various applications. This article reviews two topics. The first is the application of Kα spectroscopy to the study of energy transport in laser-produced plasma. The second topic is the application of X-ray polarization spectroscopy to measurements of the anisotropy of hot electrons generated with ultrashort high-intensity laser pulses. [ABSTRACT FROM AUTHOR]

We show that different schemes can be now followed to produce collimated X-ray radiation using laser systems. By focusing intense femtosecond laser light onto a gas jet, electrons of the plasma can be manipulated to generate ultrafast (femtosecond) X-ray radiation in the forward direction along the laser axis. In this chapter we discuss nonlinear Thomson scattering, betatron emission and Compton scattering. In years to come, the rapid development of laser technology will provide more intense laser systems. We can expect to see the creation of bright X-ray beams with a high degree of collimation (< 1 mrad divergence), as well as even shorter pulse durations, down to attosecond time scales. Such sources will provide multidisciplinary scientific communities with unique tools to probe and excite matter. [ABSTRACT FROM AUTHOR]

The focusability of intense coherent radiation generated by high-order harmonic conversion is investigated. A variety of focusing mirrors are tested and used in the focusing experiments. We demonstrate the focusing of an extremeultraviolet/soft X-ray beam to a 1 µm spot size with a peak intensity as high as 1 × 1014 W/cm2, which is sufficient to stimulate nonlinear optical phenomena in the short wavelength region. [ABSTRACT FROM AUTHOR]

High-order harmonic generation in a long gas jet is controlled in the space and time domains using chirped and self-guided femtosecond laser pulses. Since high-order harmonic generation is intrinsically connected to the ionization process of harmonic generation medium, ionization effects on high-order harmonic generation should be properly understood and taken into account. Here, we present a method to control high-order harmonic generation process by controlling the propagation mode of intense femtosecond laser pulses through the ionizing medium. Experimental results and theoretical analysis show that self-guided and chirped laser pulses can optimize high-order harmonics for achieving high brightness, low beam divergence, and narrow spectral bandwidth. [ABSTRACT FROM AUTHOR]

We demonstrated accurate pulse shaping of amplified femtosecond laser pulses by modulating the amplitude and the phase prior to regenerative chirped-pulse amplification. Based on a spectral interferometer, the pulse shaper was adjusted to compensate for a transfer function of the amplifier consisting of the gain narrowing and self-phase modulation. We performed experiments aiming at selective bond breaking in such as a simple C-C-O structure by utilizing the comprehensive pulse shaping technology, which can produce almost arbitrarily shaped 800-nm laser pulses in the amplitude and the phase at peak intensities up to ∼4 × 1015 W/cm2 range. [ABSTRACT FROM AUTHOR]

An efficient semiclassical optimal control theory for controlling wavepacket dynamics on a single adiabatic potential energy surface applicable to systems with many degrees of freedom is discussed in detail. The approach combines the advantages of various formulations of the optimal control theory: quantum and classical on the one hand and global and local on the other. The efficiency and reliability of the method are demonstrated, using systems with two and four dimensions as examples. [ABSTRACT FROM AUTHOR]

An effective scheme for the laser control of wavepacket dynamics applicable to systems with many degrees of freedom is discussed. It is demonstrated that specially designed quadratically chirped pulses can be used to achieve fast and near-complete excitation of the wavepacket without significantly distorting its shape. The parameters of the laser pulse can be estimated analytically from the Zhu-Nakamura (ZN) theory of nonadiabatic transitions. The scheme is applicable to various processes, such as simple electronic excitations, pump-dumps, and selective bond-breaking, and, taking diatomic and triatomic molecules as examples, it is actually shown to work well. [ABSTRACT FROM AUTHOR]

Laser-induced double and multiple ionization of atoms may proceed either sequentially or nonsequentially in one coherent process. For near-infrared lasers of moderate intensity (1014 to 1015 W/cm2), the physical mechanism of the coherent process is inelastic rescattering of a first-ionized electron off its parent ion. The quantum-mechanical S-matrix description of this process is reviewed. Momentum distributions of the ejected electrons and the doubly charged ion are calculated and compared with the experimental data. Their shape is found to be determined by the effective electron-electron interaction, by which the recolliding first electron ejects the up to this time bound second electron. The significance of the finalstate electron-electron interaction is assessed. The underlying classical dynamics are elucidated. Recent experiments with ultra-short phase-stabilized laser pulses are discussed. Nonsequential multiple ionization is modeled by assuming that the returning electron thermalizes with a certain number of bound electrons, and the corresponding thermalization time is estimated. [ABSTRACT FROM AUTHOR]

We discuss the role of strongly driven electrons in the generation of attosecond laser pulses and in time-resolved experiments that can be performed using attosecond lasers. We show that the formation of attosecond laser pulses in high harmonic generation is an inevitable consequency of the physics of atoms in strong laser fields, combining the sub-cycle dependence of atomic ionization rates with controlled acceleration of the ionized electrons in the oscillatory field of the laser. Molecules undergoing dynamic alignment and/or enhanced ionization, and explosions of large rare gas clusters in an intense laser field are discussed in the context of attosecond electron dynamics. [ABSTRACT FROM AUTHOR]

Ionization of organic molecules irradiated with intense infrared femtosecond pulses is explained from the perspective of intact molecular ion formation. Although fragmentation is more suppressed in general by femtosecond pulse excitation than pico- and nanosecond pulse excitations, molecules are still often heavily fragmented. Among the excitation parameters affecting ionization and fragmentation processes, excitation wavelengths and pulse durations at a fixed laser intensity have been found to drastically change ionization patterns. Intact molecular ions are produced when the wavelengths are non-resonant with the electronic levels of cations, whereas fragmentation proceeds to a large extent when the wave length is resonant with the electronic transitions. An ultimately short pulse presumably leads to the formation of fragment-free ions. Time-of-flight spectra of femtosecond pulse ionization of cyclohexadiene isomers, 2,3-dimethyl-1,3-butadiene, naphthalene, anthracene, and dioxin are presented. The great advantage of femtosecond laser mass spectrometry (FLMS) for the intact molecular ion formation is shown by referring briefly to the results obtained for dioxin. Some details of the experimental methods, such as a method of estimating irradiation intensity, are described. [ABSTRACT FROM AUTHOR]

The nuclear dynamics of polyatomic molecules in intense laser fields (∼1 PW/cm2) is studied through the momentum imaging of the fragment ions produced through Coulomb explosion. Characteristic nuclear dynamics that occur on the multidimensional potential energy surfaces in intense laser fields, such as sequential and concerted bond-breaking and hydrogen migration, are elucidated from momentum correlations among the fragment ions. [ABSTRACT FROM AUTHOR]

In this review, the physics of short-pulse laser-produced plasmas at moderate intensities is described, together with applications to X-ray sources and material processing. [ABSTRACT FROM AUTHOR]

The study of atoms and molecules in strong laser fields is one of the most active research areas in Atomic, Molecular, and Optical Physics. Following extensive study over the past two decades, most phenomena involving strong fieldatom interactions have been relatively well understood. However, there have been a number of observations suggesting that molecules in strong fields behave significantly differently from what we have learned from atoms. Revised concepts are discussed in this article for phenomena as simple as single- and double-electron ionization in molecules. These findings provide a more uniform picture to understand behaviors of atoms and molecules in strong laser fields. [ABSTRACT FROM AUTHOR]

In this chapter the fundamental physical processes underlying the interaction of relativistically intense EM radiation with a preformed collisionless plasma have been discussed, introducing the most appropriate relativistic models which are currently used in the theoretical research: the kinetic approach, the multi-fluid model, and the electron-fluid model in a fixed-ion background. The role of relativistic nonlinearities for focused radiation intensities above Iλ2 ≈ 1018 Wµm2/cm2 has been emphasized, reviewing several physical processes like relativistic laser pulse focusing, compression and splitting, harmonic generation, electron and ion acceleration, quasi-static magnetic field generation, X-ray production, laser-induced nuclear reactions, electron-positron production. All these issues demonstrate that the successful development of more and more intense lasers in the last decade has opened up several new fields of fundamental and applied science, which may results in applications unpredictable ten or twenty years ago. The fundamental aspects of the relativistic dynamic of an electron in the relativistically intense laser pulse have been discussed, starting from the socalled "figure-of-8" trajectory. It is to be emphasized that in such interaction regimes, an appreciable elongation of the electron excursion in the direction of the pulse propagation develops, which may become of the same order as that in the transverse direction. In addition, in an increasingly strong field, the electron parallel momentum scales as a2, while the perpendicular component scales as a. Therefore, in the ultra-relativistic regime the longitudinal electron dynamics can become more important than the transverse one. An important consequence is that a short and intense laser pulse, acting mainly on electrons, is able to create strong charge separations, which are at the basis of the effective charged particle acceleration, both of electrons and ions, and of the formation of relativistic electromagnetic solitons (RES). RES and laser-based ion-acceleration will be discussed in a forthcoming chapter. The ultra-strong laser-pulse interaction in such extreme regimes can be considered as one of the most attractive fields of modern theoretical and experimental research and is destined to produce very interesting applications in the near future.3 [ABSTRACT FROM AUTHOR]

Strong-field processes in the few-optical cycle regime are significantly affected by the carrier-envelope phase of the driving pulses and the control of such parameter is essential for many application, particularly for the generation of isolated and reproducible attosecond pulses. In this chapter we have reviewed, both from an experimental and a theoretical point of view, the influence of CEP on high-order harmonic generation. Moreover the role of CEP on the phase of the individual harmonics has been determined, using a self-referencing technique based on the interference of attosecond light pulses. [ABSTRACT FROM AUTHOR]

We have observed various femtosecond laser-induced phenomena in glasses. We have also observed the photo-oxidation of transition metal ions in a glass sample after the femtosecond laser irradiation. Furthermore, we have demonstrated the 3D optical memory with ultrahigh storage density by using photoreduction phenomenon of the rare-earth ions. We have succeeded in the spaceselective precipitation and control of the nanoparticles in glasses. We have confirmed that the femtosecond laser induced microstructure will open a new possibility in the realization of novel optical functions for a transparent material. [ABSTRACT FROM AUTHOR]

The advent of the ultraintense Tabletop Pulsed Laser Systems (TPLSs) has opened new and thus for unexpected frontiers allowing for the development and the progress of new research areas, in particular in scientific and industrial fields. Usually, these laser systems are used in order to generate plasma on solid or gaseous targets, so-called laser-induced plasma (LIP). The generated plasma behaves like a source of visible, UV and X radiation that can be used for many applications. In particular, X-rays emitted from LIP are used in X spectroscopy, microlithography, microscopy, imaging, radiographies (for example of biological samples). Conversion efficiency studies from laser radiation to X-rays for different targets are central for the energy balance of the source as an important performance parameter and reason of industrial attractiveness. More in generally, TPLSs are used for radiation-matter interaction studies, for fundamental plasma parameter determination, for astrophysical applications, for inertial confinement fusion, for studies in high energy physics or in the compact particle accelerator field, for quantum electrodynamics studies, defense systems, etc. TPLS design and realization are difficult tasks that require interdisciplinary cooperative efforts among researchers from different disciplines: physics, chemistry, engineering, material science, etc. This chapter describes a TPLS that is in operation at the Tor Vergata University laboratories, which is based on multistage pulsed Nd:YAG/Glass laser source (1064 nm/15 ns/10 J/TEM00 emission mode/pulse repetition rate = 1 shot per minute). Finally, some examples of the application of this TPLS are given. [ABSTRACT FROM AUTHOR]

The self-compression phenomena of intense femtosecond pulses in normally dispersive solids were investigated experimentally. Both un-chirped and negatively chirped laser pulses were used as input pulses. It is demonstrated that intense femtosecond laser pulses can be compressed by the nonlinear propagation in the transparent solids, and the temporal and spectral characteristics of output pulses were found to be significantly affected by the input laser intensity, with higher intensity corresponding to shorter compressed pulses. By the propagation in a 3mm thick BK7 glass plate with the laser power of GW level, a self-compression from 50 fs to 20 fs was achieved, with a compression factor of about 2.5. However, the output laser pulse was observed to split into two peaks when the input laser intensity is high enough to generate super-continuum and conical emission. When the input laser pulse is negatively chirped, the spectra of the pulse are reshaped and narrowed due to strong self-action effects, and the temporal pulse duration is found to be self-shortening. With the increase in the input pulse intensity, the resulted self-compressed pulses became even shorter than the input laser pulse, and also shorter than sech2 transform-limited pulse according to the corresponding spectra. [ABSTRACT FROM AUTHOR]

Recent advances of radiative recombination in the presence of strong laser fields are reported. The intense laser radiation is found to introduce new relevant features, among which enhancement and control of the emitted X-ray spectra are the most important. The influence of the plasma medium in which the process generally takes place is considered as well. The results of the recent investigations shed new light on the laser assisted radiative recombination physics and give relevant indications concerning the possibilities to have effective slow electrons and to balance the plasma heating, as needed in important applications. [ABSTRACT FROM AUTHOR]

A concise overview of new optical properties of dense nonequilibrium plasma formed on the solid state target boundary is given. In this chapter, we describe phenomena such as the third harmonic generation in the skin layer, collisionless electron heating in the high frequency skin-effect regime, absorption, and reflection and transmission of radiation by a plasma with anisotropic electron distribution. [ABSTRACT FROM AUTHOR]

We consider dynamical tunneling between two symmetry-related regular islands that are separated in phase space by a chaotic sea. Such tunneling processes are dominantly governed by nonlinear resonances, which induce a coupling mechanism between "regular" quantum states within and "chaotic" states outside the islands. By means of a random matrix ansatz for the chaotic part of the Hamiltonian, one can show that the corresponding coupling matrix element directly determines the level splitting between the symmetric and the antisymmetric eigenstates of the pair of islands. We show in detail how this matrix element can be expressed in terms of elementary classical quantities that are associated with the resonance. The validity of this theory is demonstrated with the kicked Harper model. [ABSTRACT FROM AUTHOR]

A review of several correlation techniques used to extract quantitative information from two-dimensional images obtained with fast-frame CCD cameras is presented. Three examples are discussed that demonstrate the power of these techniques for revealing field-induced dynamics: (1) the relative probability for Coulomb explosions of CO26+ as a function of the angle between the molecular and polarization axes; (2) the relative probability for Coulomb explosion of CO26+ and NO26+ as a function of bond angle and (3) the kinetic energy release and its ability to distinguish enhanced ionization from screening. [ABSTRACT FROM AUTHOR]