The rise time of intense radiation determines the maximum field strength atoms can be exposed to before their polarizability dramatically drops due to the detachment of an outer electron. Recent progress in ultrafast optics has allowed the generation of ultraintense light pulses comprising merely a few field oscillation cycles. The arising intensity gradient allows electrons to survive in their bound atomic state up to external field strengths many times higher than the binding Coulomb field and gives rise to ionization rates comparable to the light frequency, resulting in a significant extension of the frontiers of nonlinear optics and (nonrelativistic) high-field physics. Implications include the generation of coherent harmonic radiation up to kiloelectronvolt photon energies and control of the atomic dipole moment on a subfemtosecond $(1{\mathrm{f}\mathrm{s}=10}^{\mathrm{\ensuremath{-}}15}\mathrm{}\mathrm{s})$ time scale. This review presents the landmarks of the 30-odd-year evolution of ultrashort-pulse laser physics and technology culminating in the generation of intense few-cycle light pulses and discusses the impact of these pulses on high-field physics. Particular emphasis is placed on high-order harmonic emission and single subfemtosecond extreme ultraviolet/x-ray pulse generation. These as well as other strong-field processes are governed directly by the electric-field evolution, and hence their full control requires access to the (absolute) phase of the light carrier. We shall discuss routes to its determination and control, which will, for the first time, allow access to the electromagnetic fields in light waves and control of high-field interactions with never-before-achieved precision.

Intense few-cycle laser fields: Frontiers of nonlinear optics

The advent of ultraintense laser pulses generated by the technique of chirped pulse amplification (CPA) along with the development of high-fluence laser materials has opened up an entirely new field of optics. The electromagnetic field intensities produced by these techniques, in excess of ${10}^{18}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$, lead to relativistic electron motion in the laser field. The CPA method is reviewed and the future growth of laser technique is discussed, including the prospect of generating the ultimate power of a zettawatt. A number of consequences of relativistic-strength optical fields are surveyed. In contrast to the nonrelativistic regime, these laser fields are capable of moving matter more effectively, including motion in the direction of laser propagation. One of the consequences of this is wakefield generation, a relativistic version of optical rectification, in which longitudinal field effects could be as large as the transverse ones. In addition to this, other effects may occur, including relativistic focusing, relativistic transparency, nonlinear modulation and multiple harmonic generation, and strong coupling to matter and other fields (such as high-frequency radiation). A proper utilization of these phenomena and effects leads to the new technology of relativistic engineering, in which light-matter interactions in the relativistic regime drives the development of laser-driven accelerator science. A number of significant applications are reviewed, including the fast ignition of an inertially confined fusion target by short-pulsed laser energy and potential sources of energetic particles (electrons, protons, other ions, positrons, pions, etc.). The coupling of an intense laser field to matter also has implications for the study of the highest energies in astrophysics, such as ultrahigh-energy cosmic rays, with energies in excess of ${10}^{20}\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The laser fields can be so intense as to make the accelerating field large enough for general relativistic effects (via the equivalence principle) to be examined in the laboratory. It will also enable one to access the nonlinear regime of quantum electrodynamics, where the effects of radiative damping are no longer negligible. Furthermore, when the fields are close to the Schwinger value, the vacuum can behave like a nonlinear medium in much the same way as ordinary dielectric matter expanded to laser radiation in the early days of laser research.

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Optics in the relativistic regime

http://absimage.aps.org/image/DAMOP12/MWS_DAMOP12-2012-000066.pdf

Optics in the Relativistic Regime

In this article, we review progress in the development of high peak-power ultrafast lasers, and discuss in detail the design issues which determine the performance of these systems. Presently, lasers capable of generating terawatt peak powers with unprecedented short pulse duration can now be built on a single optical table in a small-scale laboratory, while large-scale lasers can generate peak power of over a petawatt. This progress is made possible by the use of the chirped-pulse amplification technique, combined with the use of broad-bandwidth laser materials such as Ti:sapphire, and the development of techniques for generating and propagating very short (10–30 fs) duration light pulses. We also briefly summarize some of the new scientific advances made possible by this technology, such as the generation of coherent femtosecond x-ray pulses, and the generation of MeV-energy electron beams and high-energy ions.

/pdf/high-power-ultrafast-lasers-20ipbk9mdb.pdf

High power ultrafast lasers

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

The intensity of the XeF C–A transition induced by a subpicosecond KrF laser is shown to have a cubic dependence on KrF laser intensity. The third-order autocorrelation technique for measuring the duration of a single KrF subpicosecond pulse has been developed utilizing this visible transition. A pulse width of 220 fsec was successfully measured with a high contrast of ~10. The visible fluorescence is more useful to researchers than vacuum-UV fluorescences. Furthermore, this simple technique may be applied over a wide UV wavelength region from 204 to 306 nm.

Single-shot measurement of subpicosecond KrF pulse width by three-photon fluorescence of the XeF visible transition.

A 390-fsec pulse with a peak power of 4 TW has been obtained in KrF, yielding an output energy of 1.5 J from an electron-beam-pumped amplifier with an active cross section of 320 cm2. The amplified spontaneous emission is as low as 1.8% in energy. An initial pulse width of 210 fsec is increased to 390 fsec in the amplifier, mainly as a result of linear dispersion in the thick windows and lenses used throughout the system. Nonlinear absorption and self-phase modulation are not significant in the 2.5-cm-thick CaF2 output window at an intensity of 13 GW/cm2.

Multiterawatt subpicosecond KrF laser.

A peak power of 1 TW has been obtained in a XeCl laser with a pulse width of 310 fsec. Amplified spontaneous emission was investigated extensively by changing the gain of wide-aperture discharge amplifiers. As a result, the amplified spontaneous emission content was suppressed to less than 3% in energy through the use of an appropriate spatial filter before the final amplifier and by operating at a low gain level of 4.8%/cm.

Terawatt XeCl discharge laser system.

A peak power of 4 TW has been obtained in KrF with an output energy of 1.5 J in 390 fsec from an electron-beam-pumped amplifier. The energy of the amplified spontaneous emission was as low as 1.8%. The pulse width was measured by the newly developed third-order autocorrelation technique based on the XeF C–A transition. Amplifiers increased the initial pulse width of 210 fsec to 390 fsec. The process of pulse-width broadening was investigated in detail. It was confirmed that the two-photon process contributes to small absorption in CaF2 by the luminescence of self-trapped excitons. A terawatt-class KrF laser system with a moderate repetition rate was developed for applications to multiphoton processes.

Multiterawatt excimer-laser system

A pulse duration of 300 psec has been achieved by direct mode locking of a long-pulse (150-nsec) XeCl laser by using an acousto-optic modulator and a saturable absorber (BBQ). The pulse duration was found to depend considerably on intensity level, and the shortest duration was observed in a train with 4-5 pulses. The time evolution of successive pulses pursued by a streak camera showed that a drastic reduction in the pulse duration occurred after the peak pulse in the train.

A. Endoh

Papers

Single-shot measurement of subpicosecond KrF pulse width by three-photon fluorescence of the XeF visible transition.

Multiterawatt subpicosecond KrF laser.

Terawatt XeCl discharge laser system.

Multiterawatt excimer-laser system

Generation of a 300-psec pulse by direct mode locking of a long-pulse XeCl laser