In the spatial realm between quantum systems like atoms and macroscopic-scale bulk plasmas rest atomic clusters. The response of these nanoscopic-scale atomic assemblages to an intense laser field is the focus of this chapter. After a survey of how clusters can be made, the chapter illustrates how clusters in intense fields can be often described by the collective oscillations of the entire cloud of electrons in clusters of ionized atoms. The cluster behavior can be described in many regimes as a nanoscale “nanoplasma.” How a nanoplasma absorbs intense laser light through the giant dipole resonance is discussed, and this model is employed to explain a range of laser–cluster interactions. The interplay of “inner” ionization and the ejection of freed electrons from the cluster whole in “outer” ionization is explored. Collisional and collisionless absorption of energy by the nanoplasma is considered in linear and nonlinear regimes. Then the subsequent explosion, through electron thermal pressures or Coulomb explosion forces, is detailed. An attempt then is made to integrate these models into regimes of interplay among ionization, absorption and explosion of the clusters.

Before subsequent chapters delve into the physics of matter in intense focused laser fields, this chapter first surveys the laser technologies most commonly employed for experimental investigations of high-intensity laser interaction physics. Chirped pulse amplification (CPA), the pioneering invention that has enabled much of modern research on strong field physics, is explained, and the laser gain media most commonly used with the technique are surveyed. The next sections of the chapter are devoted to presenting the physics and technology behind the various components of a modern CPA laser, starting with mode-locked oscillators for seeds, pulse stretchers and compressors and power amplifier chains. Key topics behind the successful deployment of such lasers, such as pulse temporal contrast control, temporal phase control, and pulse focusing, are briefly considered. Other laser technologies used in strong field physics research are also briefly discussed in the concluding section.

Atoms subject to intense laser light will witness their bound electrons undergo nonlinear oscillatory motion and subsequent field-driven ionization. This nonlinear electron motion will reradiate electromagnatic waves at harmonic multiples of the laser frequency, often to high nonlinear orders. This chapter explores this high-order harmonic generation, considering the single-atom response combined with the coherent addition from many atoms leading to spatially coherent, short-wavelength radiation. After a phenomenological justification of high harmonics in which a quasi-classical three-step model is employed to describe harmonic generation from field-ionizing atoms, the chapter launches into a quantum description of the single-atom nonlinear dipole reponse using the strong field approximation. The following sections then address the effects of propagation and phase-matching through an extended media of nonlinearly driven atoms, assessing the effects of the geometric, intensity-dependent, and plasma-induced phase on the spatial coherence of the generated harmonics. The concluding section looks at the effect of attosecond pulse generation that accompanies harmonic production.

Intense irradiation of solid targets creates an overdense plasma surface which absorbs and reflects incident laser radiation, the subject of this final chapter. Following a survey of the physics of the plasma formation, heat transport and hydrodynamic expansion, the range of effects that absorb the laser energy are each considered, starting with collisional absorption, followed by detailed exploration of collisionless absorption mechanisms including resonance absorption, vacuum heating and JxB heating. The hot electron production and transport that accompanies these absorption mechanisms is explored, followed by assessment of ponderomotive force effects on the plasma surface, with steepening and hole-boring physics elucidated. The reradiation of high harmonic emission from the plasma surface is discussed in the context of the oscillating mirror model. The acceleration of ions from solid targets is then described with particular attention given to the target normal sheath acceleration mechanism. Why and how strong magnetic fields are produced by intense irradiation of solid-target plasmas are answered and some integrated phenomena resulting from all of this physics is surveyed.

Chapter 3 examines in detail the dynamics of motion and energetics of individual free electrons in an intense laser field. It considers first electron dynamics at modest intensity in which the electron motion is nonrelativistic and the magnetic field of the laser field can be neglected. After a definition of the fields and potentials, the concept of a cycle-averaged ponderomotive energy is introduced. Then, the dynamics of free electrons at higher intensity are considered and the nonlinear oscillatory motion that results because of the relativistic mass change of the quivering electron is explored. Next, the radiation that is scattered by a free electron in a relativistic intensity field is considered, and the concept of relativistic Thomson scattering is detailed, exploring its high harmonic spectra and scattered radiation spatial distributions. The next portion of the chapter turns to the kinematics of free electrons in nonuniform laser fields, such as in the spatial profile of a laser focus. The various regimes of ponderomotive ejection of the electron from a focus are explored. A concluding section derives the quantum wavefunctions of an electron in the field.

The fate of atoms in strong laser fields in considered in this chapter. The concept of multiphoton ionization (MPI) of multielectron atoms is first considered, and the theory of MPI within the rubric of lowest-order perturbation theory is detailed. The key nonperturbative approach to calculating ionization rates, the strong field approximation (SFA), is then developed. The Keldysh approximation is utilized and various forms of the ionization rate of an atom in the intense laser field are derived from the SFA. The important concept of tunnel ionization is then discussed. Models of tunnel ionization are successively derived, starting with the simple hydrogen atom and concluding with derivation of the PPT/ADK formula for tunnel ionization of complex atoms. The limit of tunneling at high fields in the barrier suppression realm is explored. The phenomena of nonsequential double ionization and above-threshold ionization (ATI) are each considered in turn. Detailed aspects of ATI such as peak suppression, resonant enhancement, and rescattering are scrutinized in the chapter, which concludes with a survey of the physics of ionization stabilization and relativistic effects.

The subject of this chapter is the quantum mechanical analysis of the interaction of electromagnetic radiation with atomic transitions. The analysis is based on the Schrödinger wave equation, and in the first section, the gauge-invariant form of the external electromagnetic field is introduced. The electric dipole interaction and the long-wavelength approximation for the analysis of this interaction are discussed. The perturbative analysis of both single-photon and two-photon electric dipole interactions is presented, and density matrix analysis is introduced. The interaction of radiation with the resonances of atomic hydrogen is then discussed. The analysis is performed for both coupled and uncoupled representations. In the last section of the chapter, the radiative interactions for multielectron atoms are discussed. The Wigner–Eckart theorem and selection rules for transitions between levels characterized by coupling are developed. The effect of hyperfine splitting on radiative transitions is also briefly discussed.

The chapter begins with the introduction of the two-particle Schrödinger wave equation (SWE) and the solution of this equation for the hydrogen atom. The orbital angular momentum of the electron results from the SWE solution. The Pauli spinors are introduced, and the SWE wavefunctions are modified to account for the spin of the electron. The structure of multielectron atoms is then discussed. The discussion is focused on low-Z atoms for which Russell–Saunders or LS coupling is appropriate. Alternate coupling schemes are briefly discussed. Angular momentum coupling algebra, the Clebsch–Gordan coefficients, and 3j symbols are then introduced. The Wigner–Eckart theorem is discussed, and the use of irreducible spherical tensors for evaluation of quantum mechanical matrix elements is discussed in detail.

The classical theory of the interaction of light with the electron clouds of atoms and molecules will be discussed in this chapter. The discussion will begin with the interaction of a steady electric field with a collection of point charges, leading to the development of terms describing the electric dipole and quadrupole moments. The classical Lorentz model is then introduced to describe interaction of an oscillating electric field with the electron cloud of an atom, and the concepts of absorption and emission are introduced. The propagation of a light wave through a medium with electric dipoles is then discussed. Finally, the classical theory of radiation from an oscillating dipole is discussed.

Laser absorption spectroscopy is widely used for sensitive and quantitative detection of trace species. In this chapter, the density-matrix approach is used to introduce laser absorption spectroscopy. Spectroscopic quantities that characterize the absorption process are defined, and the relationships among these quantities are discussed. Broadening processes for spectral line shapes are also discussed, and the Doppler, Voigt, and Galatry profiles are introduced. The chapter concludes with a detailed example calculation featuring NO absorption.

The interaction of electromagnetic radiation with single-photon resonances in diatomic molecules is discussed in this chapter. The properties of the electric dipole moment of the molecule are determined primarily by the electron cloud that binds the two nuclei together, and these properties can be understood by considering a reference frame fixed to the molecule. However, the response of the molecule must be averaged over all possible orientations of the molecule in the laboratory frame. Using irreducible spherical tensors greatly simplifies the orientation averaging of the molecular response. The Born–Oppenheimer approximation is invoked to initially account for the effect of the electronic, vibrational, and rotational modes of the molecule. Corrections are applied to account for the coupling and interactions of the different modes, including Herman–Wallis effects. Tables of rotational line strengths are presented for singlet, doublet, and triplet electronic transitions. These tables incorporate the use of Hund’s case (a) basis state wavefunctions for increased insight into radiative interactions for levels intermediate between Hund’s cases (a) and (b).