Intense few-cycle laser fields: Frontiers of nonlinear optics

The rise time of intense radiation determines the maximum field strength at oms can be exposed to before their polarizability dramatically drops due to the detachment of an outer electron. Recent progress in ultrafast optics h as allowed the generation of ultraintense light pulses comprising merely a few field oscillation cycles. The arising intensity gradient allows electro ns to survive in their bound atomic state up to external field strengths ma ny times higher than the binding Coulomb field and gives rise to ionization rates comparable to the light frequency, resulting in a significant extens ion of the frontiers of nonlinear optics and (nonrelativistic) high-field p hysics. Implications include the generation of coherent harmonic radiation up to kiloelectronvolt photon energies and control of the atomic dipole mom ent on a subfemtosecond (1 fs=10(-15) s) time scale. This review presents t he landmarks of the 30-odd-year evolution of ultrashort-pulse laser physics and technology culminating in the generation of intense few-cycle light pu lses and discusses the impact of these pulses on high-field physics. Partic ular emphasis is placed on high-order harmonic emission and single subfemto second extreme ultraviolet/x-ray pulse generation. These as well as other s trong-field processes are governed directly by the electric-field evolution , and hence their full control requires access to the (absolute) phase of t he 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-ac hieved precision.