Broadband spatiotemporal light springs at relativistic intensities

Spatiotemporal control of laser pulses at relativistic intensities is a longstanding goal with wide-ranging implications in laserplasma acceleration, high-brightness radiation sources, and extreme-field science. Light springs are a class of structured laser pulses that can carry both a multiple spectrum and orbital angular momentum (OAM) modes, resulting in an intensity profile that rotates in time. Previous demonstrations of light springs have been limited to low-power systems due to optical damage thresholds as well as constraints imposed by large-aperture, high-power beamlines. Here, we report the first experimental realization of light springs at relativistic intensities, reaching peak intensities above 1.4×1018 W/cm2 . Our approach spectrally splits a high-power laser pulse in two, imprints distinct helical phases on each beam, and coherently recombines them. We characterize these beams using hyperspectral imaging, off-axis holography, and spectral phase reconstruction, and find excellent agreement with theoretical predictions. By introducing spectral chirp, we further demonstrate temporal evolution of the transverse mode structure during the pulse. These results establish a new platform for generating light at ultra-high intensities and enable new regimes of interaction of relativistic laser-plasma interactions involving time-dependent OAM.