Relativistic-intensity near-single-cycle light waveforms at kHz repetition rate

The development of ultra-intense and ultra-short light sources is currently a subject of intense research driven by the discovery of novel phenomena in the realm of relativistic optics,such as the production of ultrafast energetic particle and radiation beams for applications.It has been a long-standing challenge to unite two hitherto distinct classes of light sources:those achieving relativistic intensity and those with pulse durations approaching a single light cycle.While the former class traditionally involves large-scale amplification chains,the latter class places high demand on the spatiotemporal control of the electromagnetic laser field.Here,we present a light source producing waveformcontrolled 1.5-cycle pulses with a 719 nm central wavelength that can be focused to relativistic intensity at a 1 kHz repetition rate based on nonlinear post-compression in a long hollow-core fiber.The unique capabilities of this source allow us to observe the first experimental indications of light waveform effects in laser wakefield acceleration of relativistic energy electrons.

Spatial cage solitons——taming light bullets

Multimode nonlinear optics is used to overcome a long-standing limitation of fiber optics,tightly phase locking several spatial modes and enabling the coherent transport of a wave packet through a multimode fiber.A similar problem is encountered in the temporal compression of multimillijoule pulses to few-cycle duration in hollow gas-filled fibers.Scaling the fiber length to up to 6 m,hollow fibers have recently reached 1 TW of peak power.Despite the remarkable utility of the hollow fiber compressor and its widespread application,however,no analytical model exists to enable insight into the scaling behavior of maximum compressibility and peak power.Here we extend a recently introduced formalism for describing mode locking to the analog scenario of locking spatial fiber modes together.Our formalism unveils the coexistence of two soliton branches for anomalous modal dispersion and indicates the formation of stable spatiotemporal light bullets that would be unstable in free space,similar to the temporal cage solitons in mode-locking theory.Our model enables deeper understanding of the physical processes behind the formation of such light bullets and predicts the existence of multimode solitons in a much wider range of fiber types than previously considered possible.