Intense few-cycle laser fields: Frontiers of nonlinear optics

TLDR

The rise time of intense radiation limits the maximum field strength atoms experience before polarizability drops, but recent advances in ultrafast optics now produce ultraintense few‑cycle pulses that allow electrons to remain bound at field strengths far above the Coulomb field, extending the frontiers of nonlinear optics and high‑field physics. This review surveys the 30‑year evolution of ultrashort‑pulse laser physics, the emergence of intense few‑cycle pulses, and their impact on high‑field physics, while outlining routes to determine and control the absolute carrier phase. The review focuses on high‑order harmonic emission and single subfemtosecond EUV/X‑ray pulse generation, processes governed by the electric‑field evolution that require access to the absolute carrier phase for full control. The review highlights that these intense few‑cycle pulses enable coherent harmonic radiation up to kiloelectronvolt photon energies and subfemtosecond control of the atomic dipole moment.

Abstract

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.