We review the phenonomena which occur in multiphoton physics when the electric field of the applied laser radiation becomes comparable with the Coulomb field strength seen by an electron in the ground state of atomic hydrogen. This field is reached at an irradiance of approximately . The normal perturbative photon-by-photon based picture of the interaction of individual electrons with the field is replaced by a tunnelling picture in which, in a time of the order of, or less than one optical cycle, atomic wavepackets are generated which escape the confining Coulomb potential. These wavepackets are strongly influenced by the laser, `quiver' and may be accelerated back to the parent ion in a recollision process. Phase-coherent effects locked to the laser field become important: high harmonics are generated from these recollisions. We discuss the theory of such effects, and review progress in understanding how this quiver motion can be coherently controlled. We discuss ionization dynamics and review mechanisms by which atoms may be stabilized in very strong fields. Finally, we discuss relativistic effects which occur at very high-intensities.

https://iopscience.iop.org/article/10.1088/0034-4885/60/4/001/pdf

Atomic physics with super-high intensity lasers

Engineered photonic waveguides have provided in the past decade an extremely rich laboratory tool to visualize with optical waves the classic analogues of a wide variety of coherent quantum phenomena encountered in atomic, molecular or condensed-matter physics. As compared to quantum systems, optics offers the rather unique advantage of a direct mapping of the wave function evolution in coordinate space by simple fluorescence imaging or scanning tunneling optical microscopy techniques. In this contribution recent theoretical and experimental advances in the field of quantum-optical analogies are reviewed. Special attention is devoted to some relevant optical analogies based on the use of curved photonic structures, including: coherent destruction of tunneling in driven bistable potentials; coherent population transfer and adiabatic passage in laser-driven multilevel atomic systems; quantum decay control and Zeno dynamics; electronic Bloch oscillations and Zener tunneling, Anderson localization and dynamic localization in crystalline potentials.

Quantum-optical analogies using photonic structures

Light propagation and localization in modulated photonic lattices and waveguides

We review both theoretical and experimental advances in the recently emerged physics of modulated photonic lattices. Artificial periodic dielectric media, such as photonic crystals and photonic lattices, provide a powerful tool for the control of the fundamental properties of light propagation in photonic structures. Photonic lattices are arrays of coupled optical waveguides, where the light propagation becomes effectively discretized. Such photonic structures allow one to study many useful optical analogies with other fields, such as the physics of solid state and electron theory. In particular, the light propagation in periodic photonic structures resembles the motion of electrons in a crystalline lattice of semiconductor materials. The discretized nature of light propagation gives rise to many new phenomena which are not possible in homogeneous bulk media, such as discrete diffraction and diffraction management, discrete and gap solitons, and discrete surface waves. Recently, it was discovered that applying periodic modulation to a photonic lattice by varying its geometry or refractive index is very much similar to applying a bias to control the motion of electrons in a crystalline lattice. An interplay between periodicity and modulation in photonic lattices opens up unique opportunities for tailoring diffraction and dispersion properties of light as well as controlling nonlinear interactions.

/pdf/light-propagation-and-localization-in-modulated-photonic-1vo2k2mflc.pdf

Presents an overview of the current understanding of multiphoton ionization of atoms. It begins with an introductory section to explain the background of the subject. Then the article develops the three topics which have been central themes of discussion in multiphoton ionization of atoms these past few years: multiply charged ion production, very high order harmonic generation, and above-threshold ionization, a name given to the absorption of a very large number of photons by an already ionized electron. A large part of the review is devoted to some theoretical aspects of multiphoton ionization of atoms and especially non-perturbative theories. Finally the article considers the very near future prospects of laser-electron interactions and more generally laser-matter interactions at 1018-1019 W cm-2, an intensity range now within reach due to new short pulse laser technology.

https://iopscience.iop.org/article/10.1088/0034-4885/54/10/002/pdf

Multiphoton ionization of atoms

We study the behavior of atomic hydrogen in a monochromatic radiation field of high frequency $\ensuremath{\omega}$ and high intensity $I$, when its structure depends only on the parameter ${\ensuremath{\alpha}}_{0}={I}^{\frac{1}{2}}{\ensuremath{\omega}}^{\ensuremath{-}2}$ a.u., and when multiphoton ionization is quenched. At large ${\ensuremath{\alpha}}_{0}$ the ground-state binding energy undergoes a drastic reduction. This is coupled to an unprecedented stretching of the (oscillating) electron wave function, culminating in its separation into two parts (dichotomy) for ${\ensuremath{\alpha}}_{0}g50$ a.u.

Dichotomy of the hydrogen-atom in superintense, high-frequency laser fields

We study the structure of the hydrogen atom when placed in a high-frequency, superintense laser field, within the framework of a nonperturbative theory recently developed for this purpose. The theory predicts that in the high-frequency limit the atom is stable against decay by multiphoton ionization, and that its structure is determined by a time-independent Schr\"odinger equation containing a ``dressed'' Coulomb potential. The laser frequency \ensuremath{\omega} and the intensity I enter only combined in the parameter ${\ensuremath{\alpha}}_{0}$=${I}^{1/2}$${\ensuremath{\omega}}^{\mathrm{\ensuremath{-}}2}$ a.u. We first analyze the symmetry of the eigenvalue problem for the case of linear polarization under consideration and adopt an appropriate classification scheme for the levels. The small-${\ensuremath{\alpha}}_{0}$ limit of the levels is obtained analytically. In the large-${\ensuremath{\alpha}}_{0}$ limit scaling laws are derived for the ${\ensuremath{\alpha}}_{0}$ dependence of the eigenvalues and eigenfunctions. At finite ${\ensuremath{\alpha}}_{0}$ we have carried out a very accurate numerical computation over an extended range of ${\ensuremath{\alpha}}_{0}$ values (0\ensuremath{\le}${\ensuremath{\alpha}}_{0}$\ensuremath{\le}200 a.u.) for a number of symmetry manifolds, by diagonalization of the Hamiltonian in a Gaussian basis. The correlation diagrams relating the small- and large-${\ensuremath{\alpha}}_{0}$ limits exhibit several avoided crossings. The binding energies show an overall decrease with ${\ensuremath{\alpha}}_{0}$, in some cases preceded by an increase through a maximum. For the ground state this decrease is quite steep. The extreme distortion of the atomic structure accompanying it is studied. It is shown that, with increasing ${\ensuremath{\alpha}}_{0}$, the (oscillating) electronic cloud undergoes radiative stretching, which eventually culminates at large ${\ensuremath{\alpha}}_{0}$ in its splitting into two parts (dichotomy). The consequences of our findings for the experimental energy spectrum of the ejected electrons are considered.

Radiative distortion of the hydrogen atom in superintense, high-frequency fields of linear polarization

According to a general nonperturbative theory that describes atomic behavior in intense, high-frequency radiation fields, the atom becomes stable against decay by multiphoton ionization in the limit of high frequencies if the parameter ${\ensuremath{\alpha}}_{0}$=(I${/2)}^{1/2}$${\ensuremath{\omega}}^{\mathrm{\ensuremath{-}}2}$ (a.u.) (with I the intensity and \ensuremath{\omega} the frequency of the field) is kept constant, although otherwise unrestricted. We show that, under this condition, in the subsequent limit of strong fields (${\ensuremath{\alpha}}_{0}$ large), the Schr\"odinger equation describing the structure of the hydrogen atom in a laser field of circular polarization is separable in toroidal coordinates. Explicit asymptotic expressions are given for its energy eigenvalues and its eigensolutions. They correspond to a rapid decrease of the ionization potential and a drastic increase of the size of the atom with ${\ensuremath{\alpha}}_{0}$. For the binding energy of the ground state we find: \ensuremath{\Vert}${E}_{0}$\ensuremath{\Vert} =(1/2\ensuremath{\pi}${\ensuremath{\alpha}}_{0}$) (ln${\ensuremath{\alpha}}_{0}$+2.654284) (a.u.). A dramatic distortion of the shape of the atom is found, which in the strong field becomes a torus-shaped object. Furthermore, we introduce a classification of its states by strong-field quantum numbers. We show how the levels at low ${\ensuremath{\alpha}}_{0}$, characterized by the weak-field quantum numbers introduced earlier, and the levels at high ${\ensuremath{\alpha}}_{0}$, characterized by the strong-field quantum numbers, are correlated. We find that the energy spectrum in strong fields displays a multiplet structure. A comparison is made between our analytical results and those of a numerical calculation carried out earlier.

M. Pont

Papers

Dichotomy of the hydrogen-atom in superintense, high-frequency laser fields

Radiative distortion of the hydrogen atom in superintense, high-frequency fields of linear polarization

Atomic distortion and ac-Stark shifts of H under extreme radiation conditions.